Apps, P. J.

Berta, M.

Prangnell, P.B.

English

The effect of coarse and fine second-phase particles on the formation of ultra-fine grained (UFG) structures have been compared during severe deformation by equal-channel angular extrusion using routes A (no rotation) and BC (+90??? rotation). The presence of coarse particles has been found to increase the rate of grain refinement with route A and the homogeneity of the submicron grain structure formed, but appears less effective using route BC. In contrast, the presence of fine dispersoids inhibits the development of new high-angle grain boundaries and the formation of an UFG structure with both routes. Retardation is far more pronounced with rotation of the sample and the dispersiod-containing alloy processed by route BC contained mainly subgrains. The mechanisms operating in each case are discussed

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Materials Science and Engineering A 410–411 (2005) 381–385Effect of processing route and second phase particles on grainrefinement during equal-channel angular extrusionM. Bertaa, P.J. Appsb, P.B. Prangnella,∗a Manchester Materials Science Centre, The University of Manchester, Grosvenor Street, Manchester M1 7HS, UKb Health and Safety Laboratory, Broad Lane, Sheffield S3 7HQ, UKReceived in revised form 11 April 2005AbstractThe effect of coarse and fine second-phase particles on the formation of ultra-fine grained (UFG) structures have been compared during severedeformation by equal-channel angular extrusion using routes A (no rotation) and BC (+90◦ rotation). The presence of coarse particles has beenfound to increase the rate of grain refinement with route A and the homogeneity of the submicron grain structure formed, but appears less effectiveusing route BC. In contrast, the presence of fine dispersoids inhibits the development of new high-angle grain boundaries and the formation of anUFG structure with both routes. Retardation is far more pronounced with rotation of the sample and the dispersiod-containing alloy processed byr©K1ea(dsmso[cebopvtpclementfor-andse ratenedicles,finethelasticoarset andt cansinginedute0doute BC contained mainly subgrains. The mechanisms operating in each case are discussed.2005 Elsevier B.V. All rights reserved.eywords: ECAE; Severe deformation; Grain refinement; Particles; Dispersiods; Processing route. IntroductionEqual channel angular extrusion (ECAE) is now a well-stablished method of processing metals to ultra-high strains,nd has seen considerable research interest over the last decadee.g. [1–5]). It has been frequently demonstrated that severeeformation by ECAE to strains of the order of 10 can produceubmicron grain structures in metallic alloys[1–5]. Althoughuch research has been conducted on the nature of the graintructures of severely deformed alloys[1–5], and more recentlyn the evolution of the deformation structure to ultra-high strains2–5], there is still some debate about the effects of the pro-essing route and the mechanisms of grain refinement. Forxample, it has been found that rotation of the billet by 90◦etween each extrusion cycle can increase, or reduce, the levelf grain refinement, depending on the die angle, die cornerrofile and material[2,6–8]. To date, the role of the materialariables have not been systematically considered in this con-ext. In previous work it has been shown that, during ECAErocessing without rotation of the billet, second phase parti-les can have a very important effect on the rate and levelof grain refinement achieved[9,10]. Coarse micrometer-scaparticles were found to accelerate the rate of grain refineby generating new high angle boundaries within their demation zones and from particle simulated deformation b[9], whereas nanometer scale dispersiods can retard thof grain refinement by homogenising the slip[10]. Methodsof reducing the strain required to form an ultra-fine grai(UFG) material, such as exploiting the use of coarse partare clearly of industrial importance, while in many casesdispersiods are required in an UFG material to stabilisegrain structure for subsequent applications, such as superpforming.In this paper new results are presented on the effect of cparticles and fine dispersiods on the level of grain refinemenhomogeneity of the sub micrometer grain microstructure thabe produced during ECAE processing with billet rotation, uroute BC (+90◦). The results are compared to data, obtausing the same simple model alloys without billet rotation (roA).2. Experimental∗ Corresponding author. Tel.: +44 161 200 2610; fax: +44 161 200 3586.E-mail address: philip.prangnell@umist.ac.uk (P.B. Prangnell).Three alloys: 8079 (Al–1.3 wt.% Fe–0.09 wt.% Si), Al–0.2 wt.% Sc and Al–0.1 wt.% Mg, were deformed by ECAE atroom temperature by 15 extrusion cycles to a Von Mises strains921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2005.08.026382 M. Berta et al. / Materials Science and Engineering A 410–411 (2005) 381–385of ∼10. A die angle of 120◦ was used with a sharp die cor-ner. Processing was carried out without rotation (Route A) andwith rotation by 90◦ (Route BC) between each extrusion cycle.Prior to deformation, the 8079 and Al–0.2Sc alloys were heattreated to develop a uniform distribution of second phase par-ticles (see references[9,10]). All the materials had a similarinitial grain size of∼300m. The 8079 and Al–0.2Sc alloyscontained coarseθ (Al13Fe4) particles and fine coherent Al3Scdispersiods, with diameters of∼2 and∼20 nm and an inter-particle spacings of the order of 10 and 100 nm, respectively.The high-purity single-phase Al–0.1% Mg alloy was used asa control material. The material’s deformation structures werecharacterized using samples taken from the centre of the of theECAE billet in the ND–ED plane, by high-resolution EBSDusing a FEGSEM fitted with HKL technology EBSD system.The spatial and angular resolution of the system were >50 nmand ∼1◦, respectively. A low misorientation cut-off of 1.5◦was used to minimise misorientation noise. Low angle bound-aries (LABs) have been defined as having between 1.5◦ and15◦ misorientations and high angle grain boundaries (HAGBs)greater than 15◦ misorientation (depicted as black and light greylines).3. Results and discussion3.1. Severe deformation by route ArouteA -t aresEu neds Bs( elon-g llar’Hs ficantdeanbf suredu ndic-u thee tionf deter-m dardd age.I (i ther nota , the8 , hadt bmi-c inedl ariesoFig. 1. Example EBSD maps of the three model alloys deformed to an effectivestrain of 10 by route A: (a) the single phase alloy Al–0.13% Mg, (b) the coarseparticle containing 8079 and (c) the Al–0.2% Sc alloy with fine dispersoids.as well as the highest average boundary misorientation. In com-parison, the dispersiod-containing alloy had the most retainedfibrous grain fragments (arrowed), lowest fraction of HAGBs(64%;Table 1), and the largest grain size, while the single-phasealloy lay somewhere in between (74% HAGBs). The dispersiod-containing alloy also had the widest grain size distribution,reflecting the higher frequency of larger retained fibrous grainfragments.The microstructures of the three materials processed byare shown in EBSD maps inFig. 1. More detailed descripions can be found in [2–4,9,10]. Here, their main featuresummarised after deformation to an effective strain of∼10 (15CAE passes) for comparison purposes with route BC. After thisltra-high strain deformation all three alloys mainly contaiubmicron grains structures with a high fraction of HAGFig. 1a and b). The grain structures tended to be fibrous,ated in the extrusion direction, and develop from a ‘lameAGB structure seen at lower strains[2–4]. Overall the graintructures appear reasonably similar, but there are signiifferences between the three materials.Statistical analysis of the percentage HAGB area, moundary misorientations and grain sizes are shown inTable 1,rom over 5000 grains. The grain sizes have also been measing the mean linear intercept method parallel and perpelar to the main direction of HAGB alignment and fromquivalent circular diameter (ECD) after grain reconstrucrom the EBSD data. The ECD data has also been used toine the width of the grain size distributions from the staneviation normalised with respect to the distribution avert should be noted that the grain length linear interceptλy)s unreliable with aligned SMG microstructures becauseetained longer fibrous grains are frequently bent and doll align perfectly with the reference frame. Nevertheless079 alloy, which contains coarse second phase particleshe most uniform and consistently lowest aspect ratio surometer grain structure. It also had the lowest level of retaarger fibrous grains and lowest fraction of low angle boundf less than 15◦ in misorientation (%HAGBs∼85%; Table 1)M. Berta et al. / Materials Science and Engineering A 410–411 (2005) 381–385 383Table 1Average statistical data of the three model alloys determined from the EBSD maps after deformation to an effective strain of 10 by route A and BC from measurementson∼5000 grainsRoute: material Mean grain size (m) HAGBs Misorientationλx λy λx/λy ECD σS.D./dmA: Al–0.13 Mg 0.48 0.85 1.8 0.87 0.72 74 31◦A: 8079 0.36 0.49 1.4 0.6 0.71 85 35◦A: Al–0.2Sc 0.73 2.2 3.0 0.89 1.39 64 27◦BC: Al–0.13Mg 0.89 0.93 1.1 0.91 0.89 71 29◦BC: 8079 0.63 0.92 1.4 0.78 0.74 65 28◦BC: Al–0.2Sc 2.4 5.4 2.3 1.3 1.84 46 19◦Showing the grain sizes obtained, using the mean linear intercept,λx, parallel and,λy, perpendicular to the main direction of HAGB alignment, grain aspect ratio,equivalent circular diameter (ECD) grain size after grain reconstruction from the EBSD data, the standard deviation of the ECD grain size distributions normalisedwith respect to the mean diameter (σS.D./dm), the percentage of HAGB area, and the mean boundary misorientations.3.2. Severe deformation by Route BCEBSD maps of the three model alloys after deformation toan effective strain of∼10 (15 ECAE passes) with 90◦ rota-tion between each extrusion cycle are shown inF g. 2. Thedeformation structures are aligned with the shear direction fromthe last extrusion cycle. If these maps are compared to thosewithout rotation, it is immediately apparent that the differencesbetween the three alloys are much more obvious. In the caseof the single-phase alloy, although grains are more equiaxed,a more heterogeneous microstructure is seen than for route A,comprised of a mixture of bands of ultra-fine grains and coarsergrain fragments containing mainly subgrains (arrowFig. 2a).Overall this leads to a slightly larger average grain size andlower fraction of HAGBs than for route A (Table 1). There isalso a wider spread in the normalised grain size distribution,reflecting the mixture of bands of very fine and coarse unre-fined regions. The LAGBs within the coarser grain fragmentshave significant misorientations of∼5◦, and are therefore well-defined subgrains, which could easily be mistaken as grains inthe TEM. In comparison the coarse particle containing alloy,8079, appears to have a more homogeneous grain structure,but again contains unrefined regions comprised of low aspectratio submicron grains, which tend to be smaller than in thesingle-phase alloy. The statistical data inTable 1show that thisalloy is more significantly less refined than when processed byrconta ub-g metg r thia sub-s ragbd f subm ed bh nta-t antlya rialwr rlybeen much more significantly affected by the presence of a highdensity of fine dispersiod particles.3.3. Comparison of the grain refinement behaviour of thealloys and processing routesAlthough there is still some debate as to how best to rep-resent the deformation within the dies process zone, when amaterial is deformed by ECAE without rotation of the billet,an element is observed to be continuously sheared, leading topre-existing boundaries becoming rotated and elongated towardsthe extrusion direction forming a fibrous deformation structure.In the single-phase Al–0.1% Mg alloy, the deformation struc-ture evolution during ECAE processing by route A has alreadybeen reported[2–4,9,10]. At low strains dislocations dynam-ically recover into a banded cellular structure divided by lowmisorientation subgrain boundaries. The dense dislocation wallsthat delineate the cell bands are frequently found to be parallelto the dies plane of intersection, or principal shear plane andare thus probably transient features which are destroyed andreform each cycle. Unstable grains are also subdivided on acoarse scale by the formation of deformation bands. Furtherdeformation leads to the formation of micro-shear bands, dueto instability in plastic flow. The new HAGBs formed by thesemicroshear bands and the initial grain and deformation bandboundaries are permanent and rotate and align in the deforma-t trainu upt highsnd ane Thism tendi . Them con-d pre-d hearbfl irec-t ofdoute A.Much more surprising are the results for the dispersiodaining Al–0.2% Sc alloy processed by route BC, which, withHAGB fraction of 46%, is predominantly comprised of srains and cannot be described as containing a sub microrain structure. The average ‘grain size measurements’ folloy in Table 1are therefore somewhat misleading, but aretantially larger than for any of the other samples. The aveoundary misorientation for this material was only 19◦. Theeformation structure contained a few isolated bands oicrometer grains, as well as deformation bands, delineatigher misorientation HAGBs, whereas the LAGB misorieions of the subgrains within such bands were predominround∼3–6◦. The grain refinement process for this mateas already found to be inhibited using route A[10], but withoute BC the rate of formation of new HAGB area has clea-erseyion direction. Their spacing becomes compressed with sntil they form a fine lamellar HAGB structure, which breakso give the submicron-grained structure produced at ultra-trains.In the case of deformation by route BC, which involveshear on alternate planes, the net strain is redundant alement returns to its original shape every four cycles.eans that any pre-existing HAGBs do not necessarily exn area, or compress in spacing, with accumulated strainechanisms by which grain refinement occurs under theseitions are still poorly understood, but are believed to beominantly due to new HAGBs being generated from sands. These are promoted by route BC due to instability inow being encouraged by the reorientation of the shear dion relative to the original principal direction of alignmentense dislocation boundaries formed in previous cycles[2].384 M. Berta et al. / Materials Science and Engineering A 410–411 (2005) 381–385Fig. 2. Example EBSD maps of the three model alloys deformed to an effectivestrain of 10 by route BC (+90◦ rotation): (a) the single phase alloy Al–0.13%Mg, (b) the coarse particle containing 8079 and (c) the Al–0.2% Sc alloy withfine dispersoids.This leads to the heterogeneous deformation structures seein Fig. 2a, comprised of bands of fine grains in regions ofhigh local strain formed by shear bands and retained islands osubgrains.In a coarse particle containing alloy like 8079, the genera-tion of high angle grain boundaries, in local deformation zonesaround coarse second-phase particles and particle stimulatedeformation bands leads to the rapid formation of new HAGBson the scale of the inter-particle spacing[9]. This, combined withheterogeneous plastic flow around the particles, which breaksup lamellar deformation structures at higher strains, has beenfound to lead to the formation of a uniform submicron grainedstructures at much lower strains than in the single phase alloyprocessed by route A[9]. The formation of deformation zonesaround coarse hard particles and the refinement of the spacingof deformation bands may also be beneficial for producing anultra-fine grain structure more rapidly by route BC, but the com-parative rate of grain refinement at low strains has not yet beeninvestigated. In contrast, intense shear bands will be less likelyto develop in the particle-containing alloy, as the slip line lengthwill be reduced to the interparticle spacing and this may explainthe differences observed inTable 1.With route A the level of grain refinement was consider-ably reduced in the dispersoid-containing alloy, compared toa single phase alloy using route, but was far more significantlyaffected when processed by route BC. During processing byroute A, it has been found that the presence of a high den-sity of Al3Sc dispersoids inhibits both the formation of cellsand sharp DDWs, which define the cell band structures seen atmoderate strains in single-phase alloys[10]. The more weaklymisorientated and diffuse dislocation boundaries present in thedispersoid-containing alloy, plus the general homogenisationof slip by the dispersoids, prevents the formation of micros-hear bands which blocks this important mechanism of HAGBf dis-p forem andsb ese,a hapec for-m t ofa . Int non AGBs for-m grains per-s grainr4n thef eenc -e ate ofg grains ctivew ibitst d thef utes,bb genis-i ringdnfdormation. The main source of new HAGB area in theersoid containing alloy at low to medium strains is thereainly restricted to the formation of coarse deformation by texture-orientation instability, and the extension of thnd the original grain boundaries due to the geometric shange of an element with strain. This in turn delays theation of a lamellar HAGB structure and the developmenuniform submicron grained material at ultra-high strainshe case of processing with rotation of the billet, there iset geometric shape change of an element to reduce the Heparation with strain and by reducing the tendency for theation of shear bands, which is the main mechanism ofubdivision with this processing route, the finely spaced disiods appear to have a more dramatic effect on inhibitingefinement.. ConclusionsThe effect of coarse and fine second-phase particles oormation of ultra-fine grain structures by ECAE have bompared using the two processing routes A and BC. The presnce of coarse particles has been found to increase the rrain refinement and the homogeneity of the submicrontructure formed using route A, due, but appears less effeith route BC. In contrast, the presence of fine particles inhhe development of new high-angle grain boundaries anormation of a fine grain structure, by both processing rout this effect is much more pronounced using route BC. Thisehaviour is believed to be caused by the dispersiods homong slip and inhibiting the formation of shear bands dueformation.M. Berta et al. / Materials Science and Engineering A 410–411 (2005) 381–385 385References[1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45(2000) 103.[2] P.B. Prangnell, J.R. Bowen, A. Gholinia, in: A.R. Dinesen, et al.(Eds.), Proceedings of 22nd Risø International Symposium on Scienceof Metastable and Nanocrystalline Alloys, Risø National Laboratory,Roskilde Denmark, 2001, p. 105.[3] P.B. Prangnell, J.R. Bowen, in: Y.T. Zhu, et al. (Eds.), Proceedings ofSecond International Symposium on Ultra-Fine Grained Materials, TMS,Warrendale, PA, USA, 2002, p. 89.[4] P.B. Prangnell, J.R. Bowen, P.J. Apps, Mater. Sci. Eng. 375–377A (2003)178.[5] F. Dalla Torre, R. Lapovok, J. Sandlin, P.F. Thomson, C.H.J. Davies,E.V. Pereloma, Acta Mater. 52 (2004) 4819.[6] A. Gholina, P.B. Prangnell, M.V. Markushev, Acta Mater. 48 (2000)1115.[7] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 45 (1997)4733.[8] l. Dupy, E.F. Rauch, Mater. Sci. Eng. A337 (2002) 241.[9] P.J. Apps, J.R. Bowen, P.B. Prangnell, Acta Mater. 51 (2003) 2811.[10] P.J. Apps, M. Berta, P.B. Prangnell, Acta Mater. 53 (2004) 49.

Effect of Processing Route and Second Phase Particles on Grain Refinement During ECAE

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Effect of Processing Route and Second Phase Particles on Grain Refinement During ECAE

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