de Haas, M.

de Hosson, J.T.M.

Hass, M. de

ARTS repository - University of Groningen

   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 05-12-2023GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords:Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes/Cisolatedimpwill decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a5 10.60 Å, c5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a5 5.32 Å, c5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards theS9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a.S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. Theprobe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by theb9-phase (Mg/Si-ratio;1.3, HCP a57.05 Å, c5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of theb9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation ofb9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusions● The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.● EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by theb9-phase.● The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equationDH 1.67z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit areagab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1z 0210 [3,4] mGrain boundary energyggb 0.41 [9] J m22Molar volume of the matrix phase 1.004z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

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Grain boundary segregation and precipitation in aluminium alloys

University of Groningen

   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords: Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes /Cisolatedimp will decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200 mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200 mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200 mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be ;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be ;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a 5 10.60 Å, c 5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a 5 5.32 Å, c 5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below ;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si ;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards the S9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is ;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a. S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. The probe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by the b9-phase (Mg/Si-ratio ;1.3, HCP a 57.05 Å, c 5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of the b9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation of b9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusionsc The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.c EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by the b9-phase.c The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00 z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equation DH 1.67 z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit area gab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1 z 10210 [3,4] mGrain boundary energy ggb 0.41 [9] J m22Molar volume of the matrix phase 1.004 z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44 z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47 z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

Proceedings - University of Groningen

   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-09-2023GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords:Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes/Cisolatedimpwill decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a5 10.60 Å, c5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a5 5.32 Å, c5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards theS9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a.S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. Theprobe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by theb9-phase (Mg/Si-ratio;1.3, HCP a57.05 Å, c5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of theb9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation ofb9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusions● The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.● EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by theb9-phase.● The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equationDH 1.67z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit areagab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1z 0210 [3,4] mGrain boundary energyggb 0.41 [9] J m22Molar volume of the matrix phase 1.004z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

English

   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 16-07-2023GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords:Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes/Cisolatedimpwill decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a5 10.60 Å, c5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a5 5.32 Å, c5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards theS9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a.S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. Theprobe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by theb9-phase (Mg/Si-ratio;1.3, HCP a57.05 Å, c5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of theb9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation ofb9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusions● The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.● EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by theb9-phase.● The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equationDH 1.67z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit areagab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1z 0210 [3,4] mGrain boundary energyggb 0.41 [9] J m22Molar volume of the matrix phase 1.004z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

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 University of Groningen
Grain boundary segregation and precipitation in aluminium alloys
de Haas, M.; De Hosson, J.T.M.; Hass, M. de
Published in:
Scripta Materialia
DOI:
10.1016/S1359-6462(00)00577-7
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de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation in
aluminium alloys. Scripta Materialia, 44(2), 281 - 286. DOI: 10.1016/S1359-6462(00)00577-7
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Download date: 10-02-2018
GRAIN BOUNDARY SEGREGATION AND PRECIPITATION IN
ALUMINIUM ALLOYS
M. de Hass and J.Th.M. De Hosson
Department of Applied Physics, Materials Science Center and Netherlands Institute for Metals
Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
(Received April 10, 2000)
(Accepted in revised form August 11, 2000)
Keywords: Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopy
Introduction
The main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloys
are considered as a good choice for automotive applications and airframe structure manufacturing.
However, the commercial applications may be restricted by poor characteristics such as resistance to
stress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristics
depend strongly on the size and distribution of grain boundary precipitates, but also on the width of the
precipitate free zone. In this respect, the quench rate is a very important processing parameter. During
industrial processing [1] large bars are water quenched after the solution heat treatment. It is obvious
that the inner regions of these bars are subjected to slower cooling rates than the outer parts, which will
certainly affect grain boundary precipitation during the subsequent ageing treatment.
The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growth
of voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZ
attribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.
Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]
During Quenching
The magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion of
vacancy-impurity complexes occurs. It simply indicates the change in concentration brought about at
the vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is the
solution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosen
because it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumed
to be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink the
ratio Ccomplexes /Cisolatedimp will decrease, whereas this ratio will remain roughly constant in the grain
interior where no vacancy sinks exist. The complexes will diffuse down the concentration gradients
between the grain centers and grain boundaries in an effort to keep the concentration of complexes equal
at all points. Therefore an excess of impurities will be established at the grain boundaries. If it is
assumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tm
on the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary can
be represented as the ratio CGB/CGrain.
Scripta mater. 44 (2001) 281–286
www.elsevier.com/locate/scriptamat
1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.
PII: S1359-6462(00)00577-7
CGB
CGrain
5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)
Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity binding
taken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Further
relevant equations are given in [3,4].
During Ageing
During ageing, the width of the solute-concentrated layer at the GB stops increasing with time. The
diffusion of vacancy-solute complexes down the remaining concentration gradients between the grain
center and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleation
and growth of grain boundary precipitates takes place. For details see [3,4].
Experimental Procedures
The composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy was
homogenized for 6 hours at 565° C and rolled to 200 mm. Discs with a diameter of 3 mm were punched
and a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench was
applied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.
The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy was
homogenized for 6 hours at 530° C and rolled to 200 mm. Discs with a diameter of 3 mm were punched
and a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench was
applied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, the
samples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed to
counteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopy
were prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanol
at 220° C. The voltage was 20 V.
A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe size
of ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Solute
enrichment at the GB in the as-quenched sample was measured by placing the electron probe on and
at different distances from the grain boundary.
Size measurements were made by tilting the foil so that the length of the grain boundary precipitates
was a maximum. For each ageing time, the average length of the ten largest precipitates of three
different random high-angle GB’s was calculated. The largest precipitates were chosen because of the
2D-projection in which the precipitates appear in TEM. The average inter-distance of the grain
boundary precipitates was determined by using four different random high-angle grain boundaries and
inter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction of
precipitate growth) and the transversal inter-distances were measured. Tilting was performed until the
inter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversal
inter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.
Results
Solute Enrichment at the Grain Boundary after the Water Quench in AA7050
Calculations predict that a 200 mm thick disc of AA7050 at 475° C cools down to the water-temperature
of 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The moment
SEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2
that the samples are removed from the furnace is taken as the starting point for the quench and the
quench time is estimated to be ;1 s.
Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB
(S9), the right one of a random high angle GB.
Grain Boundary Precipitate Morphology in AA6061 and AA7050
By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitates
in AA6061 was determined to be ;1.3 for all applied ageing times. The structure could only be
determined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h of
ageing). An HCP-structure (a 5 10.60 Å, c 5 4.04 Å) was found. All precipitates seem to be lenticular
in shape. In AA7050, a phase with a HCP structure (a 5 5.32 Å, c 5 8.79 Å) nucleates on the grain
boundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu next
to Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn is
replaced by Cu.
Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050
Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.
This supports the fact that nucleation only occurs during ageing and not during the rapid water
quenching. For all other applied ageing times grain boundary precipitation could be observed (TEM
micrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundaries
with a misorientation angle below ;3°. Above observations support the theory that there is less or no
segregation towards special and low-angle (,3°) grain boundaries.
As they are the largest in number, only the length of the precipitates with Mg/Si ;1.33 in AA6061
is measured for different ageing times and compared to the predicted values. Input parameters of the
model for AA6061 are given in the appendix.
The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2
precipitates in AA7050 are depicted in Fig. 4 for comparison.
The longitudinal and transversal distances between two separate GB precipitates in AA6061 are
measured for increasing ageing times. The experimental values are compared with the modeled ones
and shown in Fig. 5.
Discussion
As can be seen from Fig. 1 there is no visible segregation towards the S9 boundary of AA7050. The
measured enrichment factor of Mg for the random high-angle grain boundary is ;1.8. This factor is far
Figure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance from
the grain boundary. a. S9 boundary; b. Random high-angle grain boundary.
SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2
lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quench
time of 2 s is 1.4 nm. The probe size is about 1 nm which is entirely within the solute concentrated layer
(SCL). Etching of the grain boundary during sample preparation may be an explanation for the
discrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may then
lie deeper in the sample, which yields an inaccurate composition profile because of much larger
background signal from surrounding Al. A similar discrepancy was found for AA6061. Measured
enrichments at the GB can thus only be interpreted qualitatively.
Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond very
well to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,
especially at small ageing times. This may be explained by the b9-phase (Mg/Si-ratio ;1.3, HCP a 5
7.05 Å, c 5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of the b9-phase
is used.
Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.
Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.
SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2
As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreement
with their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates in
AA7050 with respect to AA6061. This can be explained by the rate-controlling element in the
precipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formation
of Mg(Zn,Cu)2 less Mg is needed than for formation of b9. The predicted values for AA7050 using a
quench time of 2 s are in fair agreement with experimental values.
Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances are
in between the experimentally determined longitudinal and transversal inter-distances. This may be
explained by the model assuming a square collector-plate, from which the inter-distance is derived by
taking the square root of its area. However, the real collector-plates are rectangular in shape, because
the precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance may
thus be larger than the value predicted by a square collector-plate model. The transversal inter-distance
is shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)
collector plates are comparable.
The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical value
for the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.
The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictly
speaking, the model should thus be verified for different misorientations against experimental findings.
Conclusions
c The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only be
understood qualitatively.
c EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of
;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundaries
Figure 4. GB precipitate length vs. ageing time at 160°C in AA7050.
Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.
SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2
of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that the
Q-phase is preceded by the b9-phase.
c The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 and
AA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.
Calculated inter-distances are in between measured longitudinal and transversal inter-distances. This
is explained by the model assuming a square collector plate while a rectangular shape is expected.
Acknowledgments
The authors are indebted to Corus R&D for supplying the material.
References
1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).
2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).
3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).
4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).
5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).
6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).
7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).
8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).
9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).
10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).
11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).
12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).
Appendix
Input Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 Alloy
Parameter Value Unit
Molar volume of precipitate phase Vu 2.00 z 105 [11] M3
Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57
Solution heat treatment temperature Ti 803 °K
Ageing temperature Ta 433 °K
Concentration of solute in the matrix Cg 0.97 At%
Concentration of solute at the grain boundary Xb 99.7 At%
Enthalpy of solution per mole in solubility product equation DH 1.67 z 105 [12] J mol21
Entropy term in solubility product equation S 23.8 [12]
Precipitate matrix interfacial energy per unit area gab 0.32 J m22
GB width within which the solute diffusion is GB diffusion controlled 1 z 10210 [3,4] m
Grain boundary energy ggb 0.41 [9] J m22
Molar volume of the matrix phase 1.004 z 1025 [11] m3
Ratio between nr. of moles of the non-rate controlling element and moles of the
rate-controlling element in precipiate phase
0.75
Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%
Width of the solute-concentrated layer (at a quench time of 1 s) 1.44 z 10-9 m
Width of the solute-depleted layer (at q quench time of 1 s) 1.47 z 1027 m
Vacancy formation energy Ef 1.25 eV
Vacancy-impurity binding energy Eb 0.26 eV
SEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2


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   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 25-04-2023GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords:Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes/Cisolatedimpwill decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a5 10.60 Å, c5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a5 5.32 Å, c5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards theS9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a.S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. Theprobe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by theb9-phase (Mg/Si-ratio;1.3, HCP a57.05 Å, c5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of theb9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation ofb9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusions● The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.● EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by theb9-phase.● The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equationDH 1.67z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit areagab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1z 0210 [3,4] mGrain boundary energyggb 0.41 [9] J m22Molar volume of the matrix phase 1.004z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 06-06-2022GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords:Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes/Cisolatedimpwill decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a5 10.60 Å, c5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a5 5.32 Å, c5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards theS9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a.S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. Theprobe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by theb9-phase (Mg/Si-ratio;1.3, HCP a57.05 Å, c5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of theb9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation ofb9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusions● The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.● EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by theb9-phase.● The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equationDH 1.67z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit areagab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1z 0210 [3,4] mGrain boundary energyggb 0.41 [9] J m22Molar volume of the matrix phase 1.004z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

   University of GroningenGrain boundary segregation and precipitation in aluminium alloysde Haas, M.; de Hosson, J.T.M.; Hass, M. dePublished in:Scripta MaterialiaDOI:10.1016/S1359-6462(00)00577-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2001Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):de Haas, M., de Hosson, J. T. M., & Hass, M. D. (2001). Grain boundary segregation and precipitation inaluminium alloys. Scripta Materialia, 44(2), 281 - 286. https://doi.org/10.1016/S1359-6462(00)00577-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 10-03-2023GRAIN BOUNDARY SEGREGATION AND PRECIPITATION INALUMINIUM ALLOYSM. de Hass and J.Th.M. De HossonDepartment of Applied Physics, Materials Science Center and Netherlands Institute for MetalsResearch, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands(Received April 10, 2000)(Accepted in revised form August 11, 2000)Keywords:Aluminum alloys; Grain boundaries; Segregation; Transmission electron microscopyIntroductionThe main advantage of aluminium alloys is their high strength to weight ratio and therefore these alloysare considered as a good choice for automotive applications and airframe structure manufacturing.However, the commercial applications may be restricted by poor characteristics such as resistance tostress corrosion cracking, intergranular embrittlement and intergranular fracture. These characteristicsdepend strongly on the size and distribution of grain boundary precipitates, but also on the width of theprecipitate free zone. In this respect, the quench rate is a very important processing parameter. Duringindustrial processing [1] large bars are water quenched after the solution heat treatment. It is obviousthat the inner regions of these bars are subjected to slower cooling rates than the outer parts, which willcertainly affect grain boundary precipitation during the subsequent ageing treatment.The intergranular fracture mode mainly consists of microvoid initiation at GB precipitates, growthof voids in the soft PFZ region and final coalescence of the voids. Coarse GB precipitates and wide PFZattribute to this process [2]. This fracture mode seems to be unaffected by deformation temperature.Modeling Segregation and Grain Boundary Precipitation during Processing [3,4]During QuenchingThe magnitude of non-equilibrium segregation (NES) is determined, assuming that only diffusion ofvacancy-impurity complexes occurs. It simply indicates the change in concentration brought about atthe vacancy sink (i.e. the grain boundary) during cooling from temperature Ti to T0.5Tm, where Ti is thesolution treatment temperature and T0.5Tm is half the melting temperature. This temperature is chosenbecause it is assumed that very little diffusion will occur below T0.5Tm. The quenching is now assumedto be sufficiently fast so that the isolated impurity atoms (non-complexed) cannot move. At the sink theratio Ccomplexes/Cisolatedimpwill decrease, whereas this ratio will remain roughly constant in the graininterior where no vacancy sinks exist. The complexes will diffuse down the concentration gradientsbetween the grain centers and grain boundaries in an effort to keep the concentration of complexes equalat all points. Therefore an excess of impurities will be established at the grain boundaries. If it isassumed that during the quench an equilibrium is instantaneously established, corresponding to T0.5Tmon the grain boundary and corresponding to Ti at the grain center, the excess at the grain boundary canbe represented as the ratio CGB/CGrain.Scripta mater. 44 (2001) 281–286www.elsevier.com/locate/scriptamat1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.PII: S1359-6462(00)00577-7CGBCGrain5 ExpSEb 2 EƒkTi 2 Eb 2 EƒkT0.5Tm D EbEƒ (1)Ef is the thermodynamic free energy of vacancy formation and Eb that of vacancy-impurity bindingtaken as 1.25 eV and 0.26 eV for Mg (i.e. the rate controlling element) respectively [5,6]. Furtherrelevant equations are given in [3,4].During AgeingDuring ageing, the width of the solute-concentrated layer at the GB stops increasing with time. Thediffusion of vacancy-solute complexes down the remaining concentration gradients between the graincenter and the GB interface brings the solute towards the grain boundaries. Subsequently, nucleationand growth of grain boundary precipitates takes place. For details see [3,4].Experimental ProceduresThe composition of the AA6061 alloy is 0.97 at% Mg, 0.60 at% Si, 0.14 at% Cu. The alloy washomogenized for 6 hours at 565° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 550° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7h, 21.5 h, 50 h and 100 h.The composition of the AA7050 alloy is 2.7 at% Mg, 2.8 at% Zn and 1.0 at% Cu. The alloy washomogenized for 6 hours at 530° C and rolled to 200mm. Discs with a diameter of 3 mm were punchedand a solution heat treatment at 475° C for 25 minutes was given. Subsequently, a water quench wasapplied and the discs were aged for 3 mins, 1 h, 3 h, 7 h, 23.5 h, 51 h and 99.5 h. From now on, thesamples aged for 3 mins will be referred to as ‘as-quenched’. This small ageing time is performed tocounteract the deleterious effects of natural ageing [7]. Thin foils for transmission electron microscopywere prepared by using a TENUPOL electropolisher with an electrolyte of 30% nitric acid in methanolat 220° C. The voltage was 20 V.A JEOL 2010FEG analytical transmission electron microscope with a minimum electron probe sizeof ; 0.5 nm and equipped with an EDS detector was used to obtain experimental measurements. Soluteenrichment at the GB in the as-quenched sample was measured by placing the electron probe on andat different distances from the grain boundary.Size measurements were made by tilting the foil so that the length of the grain boundary precipitateswas a maximum. For each ageing time, the average length of the ten largest precipitates of threedifferent random high-angle GB’s was calculated. The largest precipitates were chosen because of the2D-projection in which the precipitates appear in TEM. The average inter-distance of the grainboundary precipitates was determined by using four different random high-angle grain boundaries andinter-distances of ten precipitate pairs per boundary. Both the longitudinal (i.e. the direction ofprecipitate growth) and the transversal inter-distances were measured. Tilting was performed until theinter-distance between the precipitates was largest. At grain boundaries inclined to the surface the transversalinter-distance was calculated using the thickness of the specimen and the 2D-projected width of the GB.ResultsSolute Enrichment at the Grain Boundary after the Water Quench in AA7050Calculations predict that a 200mm thick disc of AA7050 at 475° C cools down to the water-temperatureof 20° C in less than 0.1 s. However, the samples have to be taken out of the furnace first. The momentSEGREGATION AND PRECIPITATION IN Al ALLOYS282 Vol. 44, No. 2that the samples are removed from the furnace is taken as the starting point for the quench and thequench time is estimated to be;1 s.Fig 1 shows two composition profiles of as-quenched AA7050. The left profile is of a special GB(S9), the right one of a random high angle GB.Grain Boundary Precipitate Morphology in AA6061 and AA7050By applying EDS-analysis for composition, the Mg/Si-ratio of most of the grain boundary precipitatesin AA6061 was determined to be;1.3 for all applied ageing times. The structure could only bedetermined by electron diffraction after the precipitates were sufficiently large (i.e. after 21.5 h ofageing). An HCP-structure (a5 10.60 Å, c5 4.04 Å) was found. All precipitates seem to be lenticularin shape. In AA7050, a phase with a HCP structure (a5 5.32 Å, c5 8.79 Å) nucleates on the grainboundary. This structure is very similar to that of MgZn2. However, EDS-analysis also reveals Cu nextto Mg and Zn. This could be an indication that Mg(Zn,Cu)2 has formed in which some of the Zn isreplaced by Cu.Grain Boundary Precipitate Nucleation and Growth in AA6061 and AA7050Both as-quenched AA6061 (Fig. 2a) and AA7050 alloys show no obvious grain boundary precipitation.This supports the fact that nucleation only occurs during ageing and not during the rapid waterquenching. For all other applied ageing times grain boundary precipitation could be observed (TEMmicrographs of AA6061 in Figs. 2b-2f), except for the special grain boundaries and the grain boundarieswith a misorientation angle below;3°. Above observations support the theory that there is less or nosegregation towards special and low-angle (,3°) grain boundaries.As they are the largest in number, only the length of the precipitates with Mg/Si;1.33 in AA6061is measured for different ageing times and compared to the predicted values. Input parameters of themodel for AA6061 are given in the appendix.The results are depicted in Fig. 3. Experimental and theoretical values for the length of Mg(Zn,Cu)2precipitates in AA7050 are depicted in Fig. 4 for comparison.The longitudinal and transversal distances between two separate GB precipitates in AA6061 aremeasured for increasing ageing times. The experimental values are compared with the modeled onesand shown in Fig. 5.DiscussionAs can be seen from Fig. 1 there is no visible segregation towards theS9 boundary of AA7050. Themeasured enrichment factor of Mg for the random high-angle grain boundary is;1.8. This factor is farFigure 1. AA7050 solution- heat treated at 475°C and water-quenched. Concentration of Zn, Mg and Cu against distance fromthe grain boundary. a.S9 boundary; b. Random high-angle grain boundary.SEGREGATION AND PRECIPITATION IN Al ALLOYS 283Vol. 44, No. 2lower than the predicted value of 35. The predicted width of the solute-concentrated layer for a quenchtime of 2 s is 1.4 nm. Theprobe size is about 1 nm which is entirely within the solute concentrated layer(SCL). Etching of the grain boundary during sample preparation may be an explanation for thediscrepancy in enrichment factor. As a consequence, the probed area of the grain boundary may thenlie deeper in the sample, which yields an inaccurate composition profile because of much largerbackground signal from surrounding Al. A similar discrepancy was found for AA6061. Measuredenrichments at the GB can thus only be interpreted qualitatively.Structure (after 21.5 h of ageing) and Mg/Si-ratio of the GB precipitates in AA6061 correspond verywell to the Q-phase (Cu2Mg8Si6Al5). However, the Cu-content is lower than the stoichiometric value,especially at small ageing times. This may be explained by theb9-phase (Mg/Si-ratio;1.3, HCP a57.05 Å, c5 4.05 Å, no Cu) preceding the Q-phase [8]. For the model, the molar volume of theb9-phaseis used.Figure 2. Grain boundaries of AA6061 aged for different times at 160°C. a. 3 mins; b. 1 h; c. 3 h; d. 21.5 h; e. 50 h; f. 100 h.Figure 3. GB precipitate length vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS284 Vol. 44, No. 2As shown in Fig. 3, the calculated GB precipitate lengths for AA6061 are in very good agreementwith their experimental counterparts. Fig. 4 clearly shows the faster growth rate of GB precipitates inAA7050 with respect to AA6061. This can be explained by the rate-controlling element in theprecipitation reaction which is Mg (the element with the lowest diffusivity at 160° C [7]). For formationof Mg(Zn,Cu)2 less Mg is needed than for formation ofb9. The predicted values for AA7050 using aquench time of 2 s are in fair agreement with experimental values.Fig. 5 shows that for AA6061 subjected to a quench time of 1 s, the calculated inter-distances arein between the experimentally determined longitudinal and transversal inter-distances. This may beexplained by the model assuming a square collector-plate, from which the inter-distance is derived bytaking the square root of its area. However, the real collector-plates are rectangular in shape, becausethe precipitate grows faster in the longitudinal direction. The measured longitudinal inter-distance maythus be larger than the value predicted by a square collector-plate model. The transversal inter-distanceis shorter than the predicted value, so that the areas of the square (model) and rectangular (experimental)collector plates are comparable.The grain boundary energy used as input parameter for the model is 0.41 J m22 [9]. This is typical valuefor the energy of random high-angle grain boundaries in Al. The model is very sensitive on this parameter.The precipitate growth rate and inter-distance both increase with grain boundary energy [10]. Strictlyspeaking, the model should thus be verified for different misorientations against experimental findings.Conclusions● The solute enrichment at the grain boundary determined by TEM with a nanoprobe can only beunderstood qualitatively.● EDS-analysis in combination with electron diffraction proves that precipitates with a Mg/Si-ratio of;1.3 and a structure similar to the Q-phase (at larger ageing times) are present at the grain boundariesFigure 4. GB precipitate length vs. ageing time at 160°C in AA7050.Figure 5. GB precipitate interdistance vs. ageing time at 160°C in AA6061.SEGREGATION AND PRECIPITATION IN Al ALLOYS 285Vol. 44, No. 2of AA6061. The Cu-content is lower than the stoichiometric value, which may indicate that theQ-phase is preceded by theb9-phase.● The model originally developed for 7XXX-series aluminium alloys is applied to AA6061 andAA7050. Predicted values of the GB precipitate growth are in fair agreement with experimental ones.Calculated inter-distances are in between measured longitudinal and transversal inter-distances. Thisis explained by the model assuming a square collector plate while a rectangular shape is expected.AcknowledgmentsThe authors are indebted to Corus R&D for supplying the material.References1. T. Sheppard, Extrusion of Aluminium Alloys, Kluwer Academic Publishers, Dordrecht (1999).2. G. Itoh, M. Kanno, T. Hagiwara, and T. Sakamoto, Acta Mater. 47, 3799 (1999).3. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1857 (1996).4. H. Jiang and R. G. Faulkner, Acta Mater. 44, 1865 (1996).5. R. G. Faulkner and H. Jiang, Mater. Sci. Technol. 9, 665 (1993).6. R. G. Faulkner, Mater. Sci. Technol. 1, 442 (1985).7. L. Zhen, S. B. Kang, and H. W. Kim, Mater. Sci. Technol. 13, 905 (1997).8. A. Perovic, D. D. Perovic, and G. C. Weatherly, Scripta Mater. 41, 703 (1999).9. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys, Chapman and Hall, London (1990).10. M. A. Cantrell and G. J. Shiflet, Mater. Res. Soc. Symp. Proc. 319, 357 (1994).11. L. F. Mondolfo, Aluminium Alloys, Structure and Properties, Butterworth, London (1979).12. R. C. Dorward and C. Bouvier, Mater. Sci. Eng. A254, 33 (1998) (For solubility product equation).AppendixInput Parameters for the Grain Boundary Precipitation Model for the Case of an AA6061 AlloyParameter Value UnitMolar volume of precipitate phase Vu 2.00z 105 [11] M3Atomic fraction of solute in the nucleus phase Xu (i.e. of Mg) 0.57Solution heat treatment temperature Ti 803 °KAgeing temperature Ta 433 °KConcentration of solute in the matrix Cg 0.97 At%Concentration of solute at the grain boundary Xb 99.7 At%Enthalpy of solution per mole in solubility product equationDH 1.67z 105 [12] J mol21Entropy term in solubility product equation S 23.8 [12]Precipitate matrix interfacial energy per unit areagab 0.32 J m22GB width within which the solute diffusion is GB diffusion controlled 1z 0210 [3,4] mGrain boundary energyggb 0.41 [9] J m22Molar volume of the matrix phase 1.004z 1025 [11] m3Ratio between nr. of moles of the non-rate controlling element and moles of therate-controlling element in precipiate phase0.75Concentration of the non-rate controlling element in matrix (i.e. Si) 0.6 At%Width of the solute-concentrated layer (at a quench time of 1 s) 1.44z 10-9 mWidth of the solute-depleted layer (at q quench time of 1 s) 1.47z 1027 mVacancy formation energy Ef 1.25 eVVacancy-impurity binding energy Eb 0.26 eVSEGREGATION AND PRECIPITATION IN Al ALLOYS286 Vol. 44, No. 2

Grain boundary segregation and precipitation in aluminium alloys

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