As to Al-Zn alloy, the difference between the formation of
G.P.zones and that of clusters was investigated by measurements of electrical resistivity. The results obtained were summarised as follows: (1) G.P.zones formed during the quench and quenched-in vacancies increase greatly as-quenched resistivity P(o) as quenching temperature Tq is raised, and clusters increase slightly P(o) as Tq is lowered. (2) For one Tq and one Ta, the time required to reach P(e)' for Al-1.3at % Zn alloy is longer than that for Al-3.0at % Zn alloy. This is due to the difference of number of zinc atom in the clusters. (3) For one Ta and one concentration of zinc, the time required to reach p'(e) at Tq = 170℃ is longer than that at Tq = 300℃. This is due to the difference in concentration of quenchedin vacancies. As to Al-Cu alloy, the solvus temperature for G.P.zones was determined from the existence of P(m) in ageing curves by measurements of electrical resistivity. Consequently the solvus temperature is between 20℃ and 60℃

Hamamoto, Takao

Ohta, Mutsuo

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering Okayama University Vol. 10, No.1, July 1975G.P.zones and Clustersin Al·Zn Alloy and Al·Cu Alloy* **Mutsuo OlITA and Takao HAMAMOTO(Received June, 6, 1975)SynopsisAs to Al-Zn alloy, the difference between the formation ofG.P.zones and that of clusters was investigated by measurementsof electrical resistivity. The results obtained were summarisedas follows:(1) G.P.zones formed during the quench and quenched-in vacanciesincrease greatly as-quenched resistivity Po as quenchingtemperature Tq is raised, and clusters increase slightly Poas Tq is lowered.(2) For one Tq and one Ta, the time required to reach P~ forAl-l.3at%Zn alloy is longer than that for Al-3.0at%Zn alloy.This is due to the difference of number of zinc atom in theclusters.(3) For one Ta and one concentration of zinc, the time requiredto reach p' at Tq = 170°C is longer than that at Tq = 300°C.eThis is due to the difference in concentration of quenched-in vacancies.As to AI-Cu alloy, the solvus temperature for G.P.zones wasdetermined from the existence of Pm in ageing curves by measure-ments of electrical resistivity. Consequently the solvus temper-ature is between 20°C and 60°C.* Department of Industrial Science** Graduate school of Industrial Science. Now at: Sumitomo Light MetalIndustries Ltd.3940 Mutsuo OHTA and Takao HAMAMOTO1. IntroductionOhta and Hashimoto(l)have determined the solvus temperatures for G.P.zones inAI-Zn alloys of various composition and investigated the change in resistivityduring isothermal annealing at temperatures higher than the solvus (in this paperisothermal annealing and isothermal ageing are distinguished by temperature at whichG.P.zones are formed). According to them, before the precipitation clusters rich insolute were formed at the annealing temperature between solvus for G.P.zones andthat for a-phase in equilibrium diagram. They also reported(2)the increase inresistivity due to clusters which was observed before that due to G.P.zones whenAI-I.7at%Zn alloy was quenched from 340°C into ice-water at O°C and aged at O°C.The reason why clusters were formed earlier than G.P.zones was stated as follows:(1) The concentration of zinc atom in cluster is not so high as that in G.P.zones.(2) The distance between clusters is short.(3) The cluster size is small.However, it has been unknown so far which one of them is applicable. In this paper,some results obtained by measurements of electrical resistivity in regard to clustersdescribed above are reported.There are many studies, also, about ageing phenomena in Al-Cu alloy(3), andsolvus temperature for G.P.zones has been investigated by many. authors (4)-(7) .However, the concentration of copper in AI-Cu alloy used in their experiments was allmore than 1.0wt%, and experimental results were quite different one another. In thispaper, experimental results to determine the solvus temperature for AI-Cu alloy ofrelatively low(0.2Iat%)Cu content were reported also with those of AI-Zn alloy2. Experimental ProcedureThe nominal composition of alloys used were AI-I.3at%Zn, Al-3.0at%Zn, andAI-0.2Iat%Cu and pure metal contained in tAese alloys were 99.996\AI, 99.999%Zn,and99.99%Cu. Alloys were casted into metallic mold in air, and size of produced ingotwas about ISmm in diameter and 100mm in length. These ingots were homogenized at400°C for 48 hrs., forged at the same temperature, and cold-rolled to the plates of0.4mm in thickness. Specimen for electrical resistivity measurements was producedby cutting off from this plate, whose shape and size were illustrated in Fig.l,Each specimen was solution-treated at 500°C for an hour, cooled to quenchingtemperature in the furnace,. and held at this temperature for· an hour ,. :QUerichingwas made by hand quickly by extracting the specimen from the furnace and immergingit into ice-water at O°C, and then it was transferred into liquid nitrogen asquickly as possible.G.P.zones and Clusters in Al-Zn Alloy and Al·Cu Alloy 41Fig.l Specimen(left) Rnd Dummy(right) usedfor measurement of electrical resistivity.Electrical resistivity was measured • ''"' .,,,in this liquid-nitrogen bath whose temper-~ature was continuously corrected by a ~ ~:=r==dummy, illustrated in Fig.l, produced by - ~~___the same alloy as the specimen.The method of electrical-resistivitymeasurement was ordinary potentiometricone using three poteatiometers.Isothermal annealing(ageing) was carriedout in ethyl-alcohl at Ta=O°C, in waterat O°C < Ta ~ 40°C, and in osilicon-oilat Ta > 40°C.500100resistivity P~ after annealingfor long time at Ta.a 200 300 400Tq or Ta ('elFig.2 A plot of Po against quenchingtemperature Tq and metastable05100500AI-13at%Zn-- Tq - quench _0o'c. Po J~ 170'c. 3000c -quench-OOC-- Tacannealingl. pe'Ii 1\ /\. I~.-J3. Results(1) As-quenched resistivityFig.2 shows as-quenched resistivityPo for the Al-l.3at%Zn alloy quenchedfrom various temperatures, Fig.3 showsthat for the Al-3.0at%Zn.alloy, and Fig.4shows that for the Al-0.2lat%Cu alloy.P~ and Pein Fig.2, Fig.3 and Fig.4 aremetastable resistivity after aged(an-nealed) for long time. As to the Al-l.3at%Zn alloy, P gets minimum at Tq=250oC.oFrom this temperature Po increases greatlyas Tq is raised, and increases slightlyas Tq is lowered. As to the Al-3.0at%Znalloy, P gets minimum at 200°C < Tq < 2S0o°c and has the same tendency as Al-l.3at%Zn alloy. As to the Al-0.2lat%Cu alloy,Po gets minimum at 100°C < Tq < 2S0°C.(2) Isothermal annerling(ageing) curvesFig.S shows isothermal annealing curves for Al~1.3at%Zn alloy quenched from170°C and annealed at Ta=O°C, 10°C, 20°C, 30°C, 40°C, SOoC, 60°C,and 70°C. Fig.6sbnws those for Al-l.3at%Zn alloy quenched from 300°C and annealed at Ta=O°C, 10°C,20°C, 30°C,and 40°C.42 Mutsuo OHTA and Takao HAMAMOTOAI-JOat '-Zn--lQ-queodI- o-c. ,.-<>- 170-c -quendl-O'l:1.00 -1a<cmeallng). 1tAI-02Iat%Cu-- iI-Quench-O·c-0- 350-C-quench-O'C- Ta cameallng) •Ie'-- -TaCagelng) • pe0.950900.85o.mE<.>CII=>-•0.425 0o /00 £W IXIlQ or 1lI Coc)o 100 200 300lQ or Ta ("C)400Fig.3 A plot of Po against quenching Fig.4temperature Tq and metastableresistivity P~ after annealing forlong time at Ta.A plot of Po against quenchingtemperature Tq and metastableresistivity p~ and Pe after anneal-ing or ageing for long time at Ta.AI-I.3anZnq,'70t2~J5IC'0500AI-13alxZn"',3OO'Co_1Ilg llmI (MIn)10'Fig.S Isothermal annealing curves atvarious temperatures after quench-ing from 170°C.Fig.6 Isothermal annealing curves atvarious temperaturea after quench-ing from 300°C.Fig.7 shows those for AI-3.0at%Zn alloy quenched from 170°C and annealed at Ta=70°C,BO°C, 90°C, IOO°C, and IIOoC.These quenching temperatures and annealing ones were selected with regard tothe solvus temperature of a-phase in equilibrium diagram and to that for G.P.zonesshown in Fig.B. The former was obtained by extrapolating the curve in theequilibrium diagram by Hansen(B) down to low concentration, and the latter wasG.P.zones and Clusters in A l-Zn Alloy and Al-Cu Alloy 43determined by Ohta and Hashimoto(l)fromthe existence of maximum resistivity Pmin ageing curves. Fig.S, Fig.6 and Fig.7shows that P increases with time andreaches the metastable value. Theincrement of resistivity increases as Tais lowered, and initial increasing-rateof resistivity gets faster with Ta.When the quenching temperature is170°C, the time required to reach themetastable resistivity P~ decreases asconcentration of zinc is lowered, and forone annealing temperature and one concent-ration of zinc, the time decreases withTq.Fig.7 Isothermal annealing curves atvarious temperatures after quench-ing from 170°C.Xl" ...------------------,60S-Zn120L-__~" " ._2 3Atomic perrent line80P~ 40Z~'"Cl.E~ 0-40-tlO0Fig.8 S-GP:Solvus curve for G.P.zonesin the Al-Zn systemS-Zn:Solvus curve for zinc in theAl-Zn systemFig.9 shows isothermal ageing(annealing) curves for Al-0.2lat%Cu alloyquenched from 350°C and aged(annealed) atTa=O°C, 20°C, 60°C, 80°C, 100°C, 120°C,140°C, 160°C, 180°C, 200°C, 220°C, and240°C. This Tq was selected with regardto the solvus temperature calculated byZener's equation (9) into which data fora-phase solvus in equilibrium diagram byHansen(8)was inserted. As is shown inFig.9, at Ta = OOC and 20°C resistivityP increases with time, passes throughmaximum, and then relaxes to a givenvalue. At 60°C ~ Ta ~ 140°C, at first itincreases somewhat with time, then de-creases slowly and relaxes to a givenvalue. At Ta = 160°C, P decreases greatlyfrom the as-quenched value and keeps agiven value after 20 seconds.It would be reasonable to deducethat metastable resistivity P~ for Al-Znalloy and Pefor both alloys lie nearlyon the lines obtained by extrapolatingthe po-Tq curves down to low Tq.44 Mutsuo ,OHTA and Takao HAMAMOTOAI-O.21.at%Cu, Tq -350°cO.42e01..27~, r. LLro~t.......,.., ..,)..<::...-.042:,rJL12L ~l,------'-----7--~-~____'______:_:~~10 102 103 IC~Annealing (Ageing) time (min)Fig.9 Isothermal annealing (ageing) curves at various temperatures after quenchedfrom 350°C.4. DiscussionAI-Zn alloy(1)(a) As-quenched resistivityIncrease of Po at Tq ~ 250°C shown in Fig.2 would be due to G.P.zones formedduring the quench and to quenched-in vacancies, and that at Tq < 200°C can beexplained by Perry's theory(IO)that so-called cluster rich in solute influences onIncrease of Po for AI-3.0at%Zn alloy in Fig.3 can be explained similarly tothat of AI-I.3at%Zn alloy, but due to the difference for concentration of zinc in-crement of Po greatly different between two AI-Zn alloys.(b) Isothermal annealing curvesFig.5, Fig.6, and Fig.7 all show that the time required to reach p' decreases. eas Ta decreases, and from Fig.5 and Fig.6, when Ta is same, the time described aboveat Tq=170°C is longer than that at Tq=300oC. This quite coincides with the for-mation of G.P.zones that when Ta is same, the time required to reach Pm decreases asTq is raised. From this it would be deduced that both G.P:zones and clusters can beformed through diffusion of zinc atom promoted by super-saturated quencbed-in·vacancies.From Fig.5 and Fig.7, however, when Ta=70oC and Tq=170oC, the time required toG.P.zones and Clusters in Al·Zn Alloy and Al·Cu Alloy.----;~~ ~3;F,50,..----~-'1000/ lanop'ePlots of "t" against 1000/Ta(OK),where "t": the time required toreach the metastable resistivity~' 250Fig.IOone Tq,decreasesG. P. zones that for one Ta andthe time required to reach pmas the concentration of zinc increases.reach P~ for AI-I.3at%Zn alloy is longerthan that for Al-3.0at%Zn alloy. Thisrelation is different from the case ofHere should be inferred two causes ofthis as follows:(1) Annihilation of quenched-in vacan-cies stopped the ·development.(2) The alloy reaches to the. pseudo-equilibrium state containing variousCluster-state, which varies with theconcentration of solute atom or thequenching temperature.In the case (1), the lower the concent-ration of zinc atom is, the shorter thetime for .vacancies to k~ep bound to zincatom is, and thus they would annihilateearlier, and consequently the time toreach P~ would become short. However,activation energy for migration of vacan-cies obtained from the slope of lines inFig.IO are approximately O.7eV, muchlarger than that in pure AI, so that it would be reasonable to think that the case(2) account for above phenomenon.(2)(a)AI-Cu alloyAs~quenched resistivityDependence of Po on Tq shown in Fig.4 would be explained by the same interpre-tation as AI-Zn alloy. As compared with Fig.ll which shows Po- Tq curve for AI-O.042at%Cu alloy, Fig.4 shows obviously that increment of Po due to quenched-invacancies is quite different, but on the contrary, that due to ~lusters is onlyslightly different.(b) Isothermal annealing (ageing) curvesWhen specimen is quenched from 350°C, and annealed at Ta =240°C, 220°C, 200°C,180°C, and 160°C, p decreases greatly from as-quenched value after 20 seconds.G.P.zones formed during the quench and quenched-in vacancies would have annihilatedwithin 20 seconds due to high annealing temperature.When specimen is annealed at Ta = 140°C, 120°C, 100°C, 80°C, and 60°C, itwould take longer than at Ta ~ 160°C for G.P.zones and quenched-in vacancies to46 Mutsuo OHTA and Tak~o HAMAMOTO600AI - OQ42an Zn-e- Tq -quench -O"c. 1o-0- 300'c -quench - 0"1:- Ta \annealing). pe'700 r--------~----__,Sol.,. nrna tor 0 ....... .;.. &-'phue.• "'_"01.-"'-~'ru.__"'_"01.........._ .. 01.• ,,"aell" "1"'kJ/0260 /~-h-~e-re0270o 100 20G 30JTq or Ta ('C)400 500 D 2 3 4weight-percent copper5Fig.ll _A plot of Po against quenchingtemperature Tq and metastableresistivity p~ after annealingfor long time at Ta.Fig.12 Solvus curve for 0, and solvusfor G.P.zones in the AI-Cusystem determined by variousworkers.annihilate. Thus as shown in Fig.9, isothermal annealing curves are slightly differ-ent.Aged at Ta = O°C and 20°C, the shape of isothermal ageing curves is differentfrom those at Ta ~ 60°C. That is, p increases with time, and after passes throughmaximluu (indicated by arrows I in Fig.9), then relaxes to.a given value. Decreasein p indicated py an arrow 2 in Fig.9 might correspond to the appearance of meta-stable phase e'.· If the existence of maximUJll in p could be ascribed to the pheno-menon that G.P.zones have attained critical size as in AI-Zn alloy, it would bereasonable to deduce that the highest ageing temperature at which maximum in p wasobserved is the solvus temperature for G.P.zones. Thesolvus ·temperature £or G.P.zones of Al~O.2Iat%Cu alloy is"therefore, between 20°C and 60°C.This solvus temperature was compared with other's data to examine its propriety,Beton and Rollason (5) have determined $olvus ,temperatures forG.P. (I) zones and G.P~(2)zones by measurements of hardness, Meijering(4)by estimating the free energy·.for zones, Hori and Hirano(6)by measurements of electrical resis~ivity in reversion-experiment, and Murakami (12)by X-ray duffuse scattering method. These data areG.P.zones and Clusters in A l-Zn Alloy and AI·Cu Alloy 47plotted together in Fig.12. Data by Beton and Rollason are nearly same as those byMurakami, and data by Meijerirtg are nearly same as those by Hori and Hirano.Fot this reason, it would be thought that Hori and Hirano have identified clustersand G.P.zones according to Gerold(13)who have distinguished coherent precipitateswith matrix and incoherent ones with matrix, and have called clusters and G.P.zones"Entmischungszone" denoted coherent precipitates. On the contrary, Murakami hasdistinguished clusters from G.P.zones in a way that they could be observed.or notby X-ray small angle scattering method. As shown in Fig.12, the solvus temperaturefor G.P.zones deduced in this work is near the solvus curve extrapolated down tolow-concentration of copper which was obtained by Beton, Rollason and Murakami.References(1) Ohta and Hashimoto: J.Japan Inst.Metals, 32(1968), 1097.(2) Ohta and Hashimoto: J.Japan Inst.Metals, 36(1972), 327.(3) for instance, Chalmers: Progress in Materials Science, Pergamon Press, 10(1961)148.(4) Meijering: Rev.Met., ~(1952), 906.(5) Beton and Rollason: J.lnst.Metals, 86(1957-58), 77.(6) Hori and Hirano: J.Japan Inst.Metals, 37(1973), 142.(7) Hirouchi, Liu, and Murakami: J.Japan Inst.Metals, 37(1973), 1120.(8) Hansen: Constitution of Binary Alloys.(9) Zener: Thermodynamics in Physical Metallurgy, Amer.Soc.Metals Seminar(1950).(10) Perry: Acta.Met., 14(1966), 1143.(11) Ohta, Hida, Kanadani and Yamada: Spring Meeting of Japan Inst.Metals, (1973).(12) Murakami, Nagaura, Furuichi and Kanbayashi: Autlffim Meeting of Japan Inst.Metals, (1974).(13) Gerold: Aluminium, 37(1961), 587.

G.P.zones and Clusters in Al-Zn Alloy and Al-Cu Alloy

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