Twin nucleation at grain boundaries in Mg–3 wt.% Al–1 wt.% Zn alloy processed by equal channel angular pressing Peter Molna ´r, ⇑ Ales ˇ Ja ¨ger and Pavel Lejc ˇek Institute of Physics of the ASCR, Na Slovance 2, CZ – 182 21, Prague 8, Czech Republic Received 1 March 2012; revised 24 April 2012; accepted 4 June 2012 Available online 9 June 2012 The nucleation of f10 12g twins at grain boundaries was studied in an Mg–3 wt.% Al–1 wt.% Zn alloy processed by equal chan- nel angular pressing at 200 °C. A larger fraction of f10 12g twin boundaries was observed in comparison to the as-rolled Mg–3 wt.% Al–1 wt.% Zn alloy. The preferential sites for the f10 12g twin nucleation were grain boundaries with misorientation angles ranging from 10° to 35°. Growth of the twins into the grain interior was hindered. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloys; Twinning; ECAP; Grain boundaries Magnesium and its alloys with a hexagonal close- packed (hcp) structure are characterized by low activity of the slip systems that can accommodate deformation in the c direction of the hcp structure at room tempera- ture [1,2]. For this reason, f10 12g twinning is easily acti- vated at room temperature and becomes an important deformation mode. It is known that the nucleation cen- ters for twinning are mainly defects in the crystal struc- ture [3]. Grain boundaries represent interfacial defects that may play an important role in twin nucleation. There are numerous studies about the influence of the stress rate [4], grain size [5], and crystallographic orienta- tion of the grain [6] on twinning activity in hcp metals. However, there is a lack of experimental data that con- firm twin nucleation at grain boundaries in magnesium. Beyerlein et al. [7] studied the nucleation of f10 12g twins at the grain boundaries in pure magnesium that was subjected to deformation under compression at room temperature. They found that the f10 12g twins were connected to grain boundaries with lower misori- entation angles, and analyzed the effect of grain size and orientation on the f10 12g twin nucleation. The f10 12g twins had the typical lamellar shape. Similar analysis of twin nucleation at grain boundaries was also performed for zirconium [8]. Atomistic simulations performed by Wang et al. [9] suggested that twin nucleation most likely occurs at grain boundaries. The twin nucleation and growth in- volve the movement of twinning dislocations [10]. The twinning dislocations can be produced via the reaction of grain boundary dislocations with incoming lattice dis- locations, or by their dissociation under local stress into a small number of twinning partial dislocations [9]. Currently much effort is being put into the prepara- tion of metallic materials with a grain size lower than 1 lm, i.e. ultrafine-grained (UFG) materials. Such mate- rials can be prepared by severe plastic deformation meth- ods, e.g. equal channel angular pressing (ECAP). ECAP imposes large shear strains on processed materials and, as a result, a high dislocation density is present in the de- formed material [11]. This may influence the twin nucle- ation in hcp metals processed by this technique. Most of the experimental observations on twin nucle- ation from grain boundaries in hcp metals have been ob- tained after uniaxial loading at room temperature or below (e.g. [7,8]). Characterization of the twin nucle- ation at the grain boundaries in a magnesium alloy pro- cessed by ECAP (under stress conditions different from uniaxial loading, i.e. mostly simple shear) at higher tem- perature has not been done yet. The main goal of this work is to study the nucleation of f10 12g twins at grain boundaries in Mg–3 wt.% Al– 1 wt.% Zn (AZ31) magnesium alloy processed by ECAP at temperature of 200 °C using electron backscattered diffraction (EBSD). For the first time, we statistically evaluate the grain boundaries from which the f10 12g twins were directly nucleated. Moreover, the different 1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.06.004 ⇑ Corresponding author. Tel.: +420 266 052 896; fax: +420 286 890 527; e-mail: [email protected]Available online at www.sciencedirect.com Scripta Materialia 67 (2012) 467–470 www.elsevier.com/locate/scriptamat
Transcript
Available online at www.sciencedirect.com
Scripta Materialia 67 (2012) 467–470
www.elsevier.com/locate/scriptamat
Twin nucleation at grain boundaries in Mg–3 wt.% Al–1 wt.% Znalloy processed by equal channel angular pressing
Peter Molnar,⇑ Ales Jager and Pavel Lejcek
Institute of Physics of the ASCR, Na Slovance 2, CZ – 182 21, Prague 8, Czech Republic
Received 1 March 2012; revised 24 April 2012; accepted 4 June 2012Available online 9 June 2012
The nucleation of f1012g twins at grain boundaries was studied in an Mg–3 wt.% Al–1 wt.% Zn alloy processed by equal chan-nel angular pressing at 200 �C. A larger fraction of f1012g twin boundaries was observed in comparison to the as-rolled Mg–3 wt.%Al–1 wt.% Zn alloy. The preferential sites for the f10 12g twin nucleation were grain boundaries with misorientation angles rangingfrom 10� to 35�. Growth of the twins into the grain interior was hindered.� 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Magnesium and its alloys with a hexagonal close-packed (hcp) structure are characterized by low activityof the slip systems that can accommodate deformationin the c direction of the hcp structure at room tempera-ture [1,2]. For this reason, f1012g twinning is easily acti-vated at room temperature and becomes an importantdeformation mode. It is known that the nucleation cen-ters for twinning are mainly defects in the crystal struc-ture [3]. Grain boundaries represent interfacial defectsthat may play an important role in twin nucleation.There are numerous studies about the influence of thestress rate [4], grain size [5], and crystallographic orienta-tion of the grain [6] on twinning activity in hcp metals.However, there is a lack of experimental data that con-firm twin nucleation at grain boundaries in magnesium.
Beyerlein et al. [7] studied the nucleation of f1012gtwins at the grain boundaries in pure magnesium thatwas subjected to deformation under compression atroom temperature. They found that the f1012g twinswere connected to grain boundaries with lower misori-entation angles, and analyzed the effect of grain sizeand orientation on the f1012g twin nucleation. Thef1012g twins had the typical lamellar shape. Similaranalysis of twin nucleation at grain boundaries was alsoperformed for zirconium [8].
Atomistic simulations performed by Wang et al. [9]suggested that twin nucleation most likely occurs at
1359-6462/$ - see front matter � 2012 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2012.06.004
grain boundaries. The twin nucleation and growth in-volve the movement of twinning dislocations [10]. Thetwinning dislocations can be produced via the reactionof grain boundary dislocations with incoming lattice dis-locations, or by their dissociation under local stress intoa small number of twinning partial dislocations [9].
Currently much effort is being put into the prepara-tion of metallic materials with a grain size lower than1 lm, i.e. ultrafine-grained (UFG) materials. Such mate-rials can be prepared by severe plastic deformation meth-ods, e.g. equal channel angular pressing (ECAP). ECAPimposes large shear strains on processed materials and,as a result, a high dislocation density is present in the de-formed material [11]. This may influence the twin nucle-ation in hcp metals processed by this technique.
Most of the experimental observations on twin nucle-ation from grain boundaries in hcp metals have been ob-tained after uniaxial loading at room temperature orbelow (e.g. [7,8]). Characterization of the twin nucle-ation at the grain boundaries in a magnesium alloy pro-cessed by ECAP (under stress conditions different fromuniaxial loading, i.e. mostly simple shear) at higher tem-perature has not been done yet.
The main goal of this work is to study the nucleationof f1 012g twins at grain boundaries in Mg–3 wt.% Al–1 wt.% Zn (AZ31) magnesium alloy processed by ECAPat temperature of 200 �C using electron backscattereddiffraction (EBSD). For the first time, we statisticallyevaluate the grain boundaries from which the f101 2gtwins were directly nucleated. Moreover, the different
Figure 1. Scheme of the ECAP die with reference axes (a). Misorien-tation angle distribution in AZ31 alloy for: (b) the as-received (as-rolled) state; (c) a sample after 1 � 200 �C ECAP; and (d) a sampleafter 2 � 200 �C ECAP. Misorientation axis distribution in the rangeof 85�–89� for AZ31 after (e) 1 � 200 �C and (g) 2 � 200 �C ECAP. (f)Distribution of the grain boundary misorientations, as characterizedby the appearance of f1012g twins.
468 P. Molnar et al. / Scripta Materialia 67 (2012) 467–470
sites of nucleation and the evolution of the morphologyof the f1012g twins in the AZ31 alloy after ECAP areanalyzed.
The material used in this work was a 13 mm thick,rolled plate of magnesium alloy AZ31 (nominal compo-sition Mg–3 wt.% Al–1 wt.% Zn) produced by AMTS,Israel. The wrought AZ31 alloy has a texture that istypical of hot-rolled magnesium alloys, with basalplanes oriented perpendicular to the normal directionof the plate. The average grain size of 20 lm was mea-sured by EBSD. The billets, with dimensions 10 �10 � 70 mm3 and their largest dimension parallel tothe rolling direction (RD), were machined from therolled plate. The billets were oriented with the rollingplane (RD–transverse direction) perpendicular to theextrusion direction (ED) and the RD parallel to the in-sert direction (ID) (Fig. 1a). The ECAP die had an innerangle of U = 90�, which dictated the total strain imposedduring the ECAP pass, and an outer angle of W = 45�,which mainly influences the uniformity of the strain[11]. The billet was sequentially lubricated with molyb-denum disulphide and graphite powder, exposed for3 min and extruded. AZ31 alloy was processed byECAP using route A (no rotation between the passes),with two passes at 200 �C (2 � 200 �C). The pressingspeed for all passes was 5 mm min�1. The average grainsize after processing was 4 lm. The microstructure wasstudied using EBSD in a dual-beam FEI Quanta 3DFEG microscope (working voltage 20 kV). EBSD mapswere measured at steps of 150 and 80 nm. The analysisof the EBSD data was performed with TSL 5.3 OIManalysis software. The indexing quality of the EBSDmaps estimated through the confidence index was higherthan 97% of the scan. All microstructural observationswere performed on the plane defined by ED and ID.The samples for the EBSD analysis were mechanicallyground to 2400 grit using SiC paper, and were polishedwith diamond suspensions (3 and 1 lm) and a colloidalsilica solution. To obtain samples with a high-qualitysurface, ion beam polishing was performed using a Ga-tan-PECS. In total, 300 f10 12g twins were evaluatedfrom the AZ31 alloy after 2 � 200 �C ECAP. Each ana-lyzed twin emanated from a specific grain boundary anddid not cross the whole grain. This enables unambigu-ous characterization of the grain boundary misorienta-tion angle.
Figure 1b–d shows the misorientation angle chartsfor the initial as-rolled AZ31 alloy and for AZ31 after1 � 200 �C and 2 � 200 �C ECAP, respectively. If themisorientation angle of two neighboring points exceeded2�, then the grain boundary was located between themusing OIM analysis software. The initial as-receivedAZ31 alloy had a distinct peak around the misorienta-tion angle of 30� and a small fraction of low-angle grainboundaries (LAGBs). The misorientation angle distribu-tions for the AZ31 alloy after 1 � 200 �C and 2 � 200 �CECAP are similar and show a high fraction of LAGBs,with distinct peaks around misorientation angles of 30�and 86�. The latter peak could correspond to thef101 2g twin boundaries. The f101 2g twin boundaryis characterized by a 86.3� misorientation angle aroundthe h1210i axis. Since consideration of the misorienta-tion angle histogram alone is not sufficient for the
unambiguous determination of the f10 12g twin bound-aries, Figure 1e and g shows the misorientation axisdistribution for misorientation angles ranging from 85�to 89� for samples after 1 � 200 �C and 2 � 200 �CECAP, respectively. The misorientation axes are indeedmainly grouped around the h1 210i direction. Withincreasing misorientation angle, a significant numberof misorientation axes appeared around the h1010idirection. The combination of these misorientation an-gles with h10 10i axes did not represent the f10 12g twinboundaries. The area fraction of f101 2g twins presentin the AZ31 alloy in the as-received state and after1 � 200 �C and 2 � 200 �C ECAP was estimated fromEBSD maps to be 0.3%, 1.1% and 2%, respectively. De-spite low area fractions of twins for AZ31 alloy afterECAP, there is a relatively high number fraction off1012g twin boundaries with a misorientation angleof 86.3� h1210i (Fig. 1c and d). This difference is ratio-nalized by the large number of relatively small f1 012gtwins, which has a significant influence on the misorien-tation angle distribution for grain boundaries. A similarmisorientation distribution of grain boundaries afterECAP at 250 �C was found for both AZ31 alloy [12]and pure magnesium [13].
Figure 2 shows the EBSD boundary maps taken fromAZ31 alloy after 2 � 200 �C ECAP. High-angle grainboundaries (HAGBs), with misorientations greater than10�, are marked in blue while LAGBs, with misorienta-tion angles less than 10�, are colored red. The f1 012gtwin boundaries satisfying the 5� tolerance around amisorientation angle of 86.3� and axis h1210i are drawnin green. These twin boundaries have a wavy shape anddo not appear as typical twin lamellae crossing the graininterior (Fig. 2b–d). A similar morphology of f10 12gtwins was observed in the microstructure of AZ31 alloyafter 1 � 200 �C ECAP, but the area fraction was lower.For this reason, the analyzes was performed from dataobtained from AZ31 after 2 � 200 �C ECAP.
Ma et al. [14] observed that a portion of the dynam-ically recrystallized grains after extrusion of AZ61magnesium alloy at 450 �C satisfy the 86.3� h1210i
Figure 2. Grain boundary map obtained by EBSD after 2 � 200 �CECAP (a) showing the f1012g twin boundaries in detail (b, c, d). TheHAGBs are colored blue, the LAGBs are colored red and the f1012gtwins are colored green. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of thisarticle.)
P. Molnar et al. / Scripta Materialia 67 (2012) 467–470 469
misorientation for f10 12g twins. New recrystallizedgrains that possessed the f1012g twin orientation havealso been observed in pure magnesium after ECAP at250 �C [13]. Xu et al. [15] found in Mg–5.0Zn–0.9Y–0.16Zr (in wt.%) alloy that, after extrusion at 300 �C,grains with a f1 012g twin relationship were presentalong the original grain boundaries. In all these cases,twins did not have a lamellar morphology. Unfortu-nately the volume fraction of recrystallized grains witha f1012g twin relationship was not estimated.
To show the morphology of the f1 012g twins in de-tail after 2 � 200 �C ECAP, an inverse pole figure map(IPF) taken with 80 nm steps is shown in Figure 3a.The f1 012g twins nucleated at the HAGBs in the grainsnumbered 1 and 2. In order to identify the f1012g twinplane, a trace analysis was performed. Green lines repre-sent twin boundaries with the 86.3� h1210i misorienta-
Figure 3. (a) IPF map after 2 � 200 �C ECAP. (b) f1012g twin nucleiin areas with a high density of dislocations. (c) A f1012g twinnucleated at a particle (arrow) and an HAGB. (d) f1012g twinnucleation at an LAGB (red), where a number of twin nuclei areformed along the LAGB that can subsequently coalesce into a singlelarge twin. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)
tion. The trace of the f1012g twin plane is drawn in red(Fig. 3a). Only a small part of the whole twin boundary,which is almost perpendicular to the HAGB, is drawn inred, representing the trace of the f1012g plane (Fig. 3a,arrow in grain 1). This small part represents the coherentf10 12g twin boundary. The incoherent twin boundary(green color) also fulfills the misorientation condition86.3� h121 0i; though it does not correspond to the traceof the f1012g plane. The twin boundary in grain 2 is alsoincoherent. All these incoherent twin boundaries can beconsidered as a twin front that should easily propagateinto the grain interior. However, it can be observed that,instead of growing into the grain interior, the twins prop-agate laterally, i.e. along the HAGBs at which they werenucleated. For twins to grow laterally, new twinning dis-locations must be nucleated. This implies that the condi-tions for twin nucleation are more favorable then fortwin growth.
The movement of the twinning dislocations and con-sequently twin growth could be hindered due to the highdensity of dislocations present in the grain interior as aresult of the severe plastic deformation produced byECAP [16]. Serra and Bacon [17] found that the interac-tion of the f101 2g twin boundary with high dislocationloop density suppresses twinning. Twin growth mightalso be inhibited by small particles present in AZ31 al-loy. Recently, Robson et al. [18] found that the particlesin an Mg–5 wt.% Zn alloy inhibited f1012g twingrowth and increased twin nucleation.
To explore whether there are preferred grain bound-aries for f1012g twin nucleation, a statistical analyzeswas performed on AZ31 alloy after it had undergonethe 2 � 200 �C ECAP procedure. The resulting distribu-tion of grain boundary misorientations showed thatmost of the f1012g twins were nucleated at grainboundaries with misorientation angles ranging between10� and 35� (Fig. 1f). Similar experimental results wereobtained by Beyerlein et al. [7,19] for pure magnesiumafter compression at room temperature. Moreover, thepreference for f1012g twin nucleation at grain bound-ary misorientation angles lower than 28� was obtainedfrom atomistic simulations [9]. As suggested by Wanget al. [9], those grain boundaries may contain grainboundary dislocations that can support f1 012g twinnucleation. Our experimental observation is in agree-ment with this suggestion.
HAGBs are not the only nucleation sites for f10 12gtwins. Figure 3b–d shows the different nucleation placesfor f1012g twins observed in AZ31 alloy after2 � 200 �C ECAP. f1012g twin nuclei were found atsites with a high dislocation density, represented byLAGBs (red) inside the grain (Fig. 3b). The misorienta-tion angle for these boundaries is approximately 7�. Thelarge number of small f1012g twin nuclei (green) cansubsequently coalesce into a single large twin. Thef10 12g twin nucleates at the particle surface, which rep-resents a volume defect in the structure (see arrow inFig. 3c). The twin boundary is wavy and propagatesto grain interior from the particle surface (Fig. 3c).The same figure also shows a f1012g twin nucleatingat an HAGB with a misorientation angle of 34�. Figure3d shows the nucleation of a f1012g twin at an LAGB(red) with a misorientation angle of 10�. Arrows 1 and 2
470 P. Molnar et al. / Scripta Materialia 67 (2012) 467–470
in Figure 3d show both ends of the LAGB. The left partof the LAGB is already a f1 012g twin (Fig. 3d, arrow1). Twin boundaries (green) that satisfy a 5� tolerancearound a misorientation angle of 86.3� and axish1210i are located in grain A. The HAGB (blue) thatis located between the f1012g twin and grain B hasan 84.6� ½1419333� misorientation, which is slightly be-yond the 5� tolerance around the 86.3� h1210i misorien-tation. This is caused by the presence of dislocations inthe LAGB, which locally changes the misorientation ofthis HAGB (blue). A small nucleus is formed at theLAGB (Fig. 3d, arrow 3). It can be supposed that thistwin nucleus will coalesce with the rest of the f1012gtwin. In this way, the LAGB will transform into af101 2g twin. Small nuclei present at the LAGB supportthe mechanism of the f1012g twin formation obtainedby atomistic simulation [9].
The observed results imply that, even at a tempera-ture of about 200 �C, f1 012g twin nucleation occursin AZ31 alloy processed by ECAP. Although the twinsize is relatively small, thus facilitating the grain refine-ment of AZ31, it is known that twinning causes strongcrystallographic anisotropy, which may significantlyinfluence the subsequent deformation of this alloy.
In this paper, f101 2g twin nucleation at grainboundaries in Mg–3 wt.% Al–1 wt.% Zn alloy after2 � 200 �C ECAP was investigated. It was shown thatthe f1 012g twins nucleate at different defects of thecrystal structure, i.e. at (i) LAGBs, (ii) HAGBs, (iii) dis-location pile-ups in the grain interior and (iv) particlesurfaces. Grain boundaries with misorientation anglesbetween 10� and 35� were preferential places forf101 2g twin nucleation. Experimental observation im-plies that f1012g twin nucleation includes the coales-cence of small f1012g twin nuclei into a single largetwin. A large number of f10 12g twins with a somewhatirregular shape, with wavy boundaries, was observed in-stead of typical lamellar twins. The growth of twins intothe grain interior was hindered, the f1012g twins prop-agating instead laterally along the grain boundary.
P.M. acknowledges the Czech Science Founda-tion (Grant P108/12/P054) and A.J. and P.L. acknowl-edge the Czech Science Foundation (Grant P108/12/G043) for financial support.
[1] A. Chapuis, J.H. Driver, Acta Mat. 59 (2011) 1986.[2] T. Obara, H. Yoshinga, S. Morozumi, Acta Metall. 21
