Influence of Grain Size and Texture on the Yield Asymmetry of Mg-3Al-1Zn Alloy

J. Mater. Sci. Technol., 2011, 27(1), 29-34. Influence of Grain Size and Texture on the Yield Asymmetry of Mg-3Al-1Zn Alloy S.M. Yin 1), C.H. Wang 1), Y.D. Diao 1), S.D. Wu 2) and S.X. Li 2) 1) Shenyang University of Chemical Technology, Shenyang 110142, China 2) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received January 20, 2010, in revised form April 9, 2010] The yield asymmetry between compression and tension of magnesium alloy Mg-3Al-1Zn (AZ31) with different grain sizes and textures has been studied by tensile and compressive testing of as-cast, as-extruded and equal channel angular pressed (ECAPed) specimens. The significant yield asymmetry (the ratio of yield strength between compression and tension σ yc /σ yt is 0.44) was found in as-extruded specimens and the corresponding microstructure evolution during deformation revealed that {10 12} tensile twinning is the underlying reason for the large yield asymmetry. Strong texture and grain size are influential factors for large yield asymmetry. The separate contributions of grain size and texture on yield asymmetry were investigated. KEY WORDS: Magnesium alloys; Yield asymmetry; Texture; Grain size; Equal channel angular pressing (ECAP) 1. Introduction The application of magnesium alloys for lightweight structural components has increased significantly during the last decade, which is mainly due to energy and environment problems over the world [1 3]. However, most of magnesium alloys with HCP structure are easy to form texture during processing; in turn, the texture will influence the mechanical properties of magnesium alloys [4 8], especially on the yield asymmetry between compression and tension. The strong yield asymmetry limits the use of magnesium alloys as structural components, such as beams, since the compressive side of a beam will plastically deform long before the tensile side does. Yield asymmetry is also a disadvantage of wrought magnesium alloy for further drawing and manufacturing [9 11]. Besides, most of magnesium wrought alloy components in modern vehicles and consumer goods subject to reverse loading during service. Different part of components usually has different textures and will exhibit Corresponding author. Ph.D.; Tel: +86 13804213518; E-mail address: yin shuming@yeah.net (S.M. Yin). quite different cyclic deformation behavior which result in different fatigue life [12,13]. Strong yield asymmetry of wrought magnesium alloy has hindered the application of magnesium alloy. An explanation for the issue is that twinning dominates deformation in compression while slip dominates deformation in tension and the stress needed to activate twinning is lower than that of slip. Previous studies [14,15] indicate that temperature and grain size influence twinning which leads to yield asymmetry. However, the separate influence of grain size and texture on yield asymmetry is not very clear. In this paper the texture and grain size of the material are modified by using equal channel angular pressing (ECAP) and the respective influence on the yielding asymmetry is investigated. 2. Experimental The materials used in the present study are commercial magnesium AZ31 alloy with as-cast ingot and as-extruded bar as well as extruded bar treated by ECAP. These materials have the same chemical composition as shown in Table 1. ECAP was carried out

30 S.M. Yin et al.: J. Mater. Sci. Technol., 2011, 27(1), 29 34 Table 1 Chemical composition of AZ31 (wt%) Mg Al Zn Mn Si Fe Cu Ni Balance 2.87 1.01 0.3162 0.0293 0.0019 0.0016 0.008 Fig. 1 Microstructures of AZ31 alloy: (a) as-cast; (b) as-extruded; (c), (d) and (e) ECAP 1, 4 and 8 passes, respectively on the extruded bar through a die of 90 with route Bc [16] to achieve different grain sizes and to modify textures. The first pass of ECAP was conducted at 523 K and the following passes were conducted at 503 K. Three tensile specimens with a gauge section of 6 mm 5 mm 20 mm were sparkle cut from a cast ingot; the other tensile specimens with a gauge section of 5 mm 2 mm 15 mm were sparkle cut along the extruded direction (ED). Compressive specimens with dimensions of 4 mm 4 mm 8 mm were also cut with the same direction as that of tensile specimens. Tensile and compressive tests were conducted on a MTS A/T machine with a strain rate of 10 3 s 1 at ambient temperature. The microstructure evolutions of as-extruded AZ31 alloy during ECAP were investigated by using an optical microscope (OM) and a scanning electronic microscope (SEM). Specimens for electron backscatter diffraction (EBSD) were prepared by mechanical polishing with silicon carbide abrasive paper followed by electro-polishing using a solution comprised of 40 ml HClO 4 and 360 ml ethanol at 273 K. The textures of as-extruded bar as well as ECAP processed specimens for one, four and eight passes were measured by EBSD with a SEM of LEO- Supra35. 3. Results and Discussion Figure 1 shows the microstructures of AZ31 alloy. The average grain sizes for the cast and the asextruded magnesium alloys are 126 µm and 74 µm, respectively. The grain size of AZ31 alloy ECAP-1 pass ranges from 50 µm to 4 µm, and the average grain size is about 13 µm. The grain sizes are more uniform for the samples subjected to ECAP-4Bc passes and ECAP-8Bc passes and their average sizes

S.M. Yin et al.: J. Mater. Sci. Technol., 2011, 27(1), 29 34 31 Fig. 2 {0002} pole figures of AZ31 alloy: (a) as-extruded; (b), (c) and (d) ECAP 1, 4 and 8 passes, respectively / MPa 400 300 200 100 0 Cast-t Cast-c Extru-t Extru-c ECAP1-c ECAP1-t ECAP4-c ECAP4-t ECAP8-c ECAP8-t 0.0 0.1 0.2 0.3 0.4 0.5 Figure 3 shows tensile and compressive curves of the material with different grain sizes and textures. The ratios of compressive yield strength to tensile yield strength, σ yc /σ yt, are about 1, 0.44, 0.72, 0.83 and 1 for as-cast, extruded, ECAP 1, 4, and 8 passes specimens, respectively. In order to clearly express the yield asymmetry, we therefore define the yield asymmetry as 1 σ yc /σ yt. From this definition, the yield asymmetries for as-cast, extruded, ECAP 1, 4, and 8 passes specimens are about 0, 0.56, 0.28, 0.17 and 0, respectively. 4. Discussion Fig. 3 True stress-true strain curves of AZ31 alloy. ( -t and -c denote tension compression respectively.) are 4 and 3.4 µm, respectively. The textures of specimens are shown in Fig. 2 except for as-cast samples since there is basically no preferred orientation in cast materials with equiaxial grains. It is clearly show that basal planes in most grains in extruded bar are parallel to ED, while basal plane of most grains tilt about 20, 40 and 45 from ED after ECAP for 1, 4 and 8 passes, respectively, which is consistent with the previous studies [17 19]. 4.1 Tensile twinning Since the as-extruded AZ31 alloy exhibits the largest yield asymmetry among all the samples, investigation of the microstructure evolution during deformation is of great importance for revealing the underlying reason of yield asymmetry. The microstructures evolution of as extruded samples during deformation is shown in Fig. 4. It is revealed that almost no twin was found in the tensile samples at yielding (Fig. 4(a)), while significant amount of twins appeared in the compressive samples (Fig. 4(b)). The

32 S.M. Yin et al.: J. Mater. Sci. Technol., 2011, 27(1), 29 34 (b) Fig. 4 Microstructures evolution of as-extruded AZ31 alloy: (a) yield in tension; (b) yield in compression Misorientation angle / deg. 80 60 40 20 T1 T2 T3 T4 type of the twins is also investigated by EBSD technique. Figure 5(a) shows the orientation map of a typical zone at compressive strain of 1.8% and Fig. 5(b) shows the reorientations of basal plane for the twins in grain A as marked in Fig. 5(a). It is clearly shown that the basal planes reorient 86.3±0.5. Thus it can be identified as tensile twin [15,20,21]. Similar results were also obtained in other grains. It is verified that twinning plays an important role in the yield asymmetry of the material. Twinning is usually a pole mechanism, only allowing simple shear in one direction rather than both forward and backward directions like dislocation slip, which may cause yield asymmetry in textured magnesium alloys. If twinning dominates in compression and slip dominates in tension, the stress needed to activate slip is larger than that of twinning, the yield asymmetry will be generated. In the case of as-extruded AZ31 alloy, Schmid factors of different slip systems were obtained from the orientation map. The Schmid factors for twinning and basal <a> slip are 0.43 and 0.12, respectively, while that for prismatic <a> slip system and pyramid <a> slip system are about 0.48 and 0.4, respectively. The critical resolved shear stresses (CRSS) τ for twinning, basal slip, prismatic slip as well as pyramidal slip are 30, 20, 75 and 75 MPa, respectively [22 25]. Based on the Schmid law (σ= τ M, σ is the yield stress and M is the Schmid factor) the stresses needed to activate twining, basal slip, prismatic slip and pyramidal slip are about 69, 167, 156 and 187 MPa, respectively. Previous study [26] showed that twinning can be activiated rather easily when compression is 0 0 10 20 30 40 50 60 70 80 Distance / m Fig. 5 (a) Euler angle contrast maps of specimen at compressive strain of 1.8%; (b) misorientation profile of twins in grain A perpendicular to c-axis or tension is along c-axis but not in the opposite loading directions. In conventional extruded bar, basal planes of most grains are parallel to ED (Fig. 2(a)), in other words, the c-axis of HCP lattice in most grains are perpendicular to ED i.e. the loading direction. Therefore, twinning is suppressed during tension and tensile deformation may be dominated by prismatic slip because it needs less stress (156 MPa) than basal slip (167 MPa) and pyramidal slip (187 MPa). However, twinning is apt to occur in compression and may become a dominated deformation mechanism because the stress needed to activate twinning is less than that needed to activate slip. Therefore, the as-extruded AZ31 alloy exhibits large yield asymmetry. The ratio of calculated twining stress over prismatic slip stress, 69/156, is 0.44 which is very close to the experimental results. 4.2 Texture Randomly textured casting samples exhibit almost no yielding asymmetry, which is mainly due to two reasons. First, random textures result in the volume fraction of grains favouring tensile twinning in compression, which is almost the same as that in tension in random textured AZ31.

S.M. Yin et al.: J. Mater. Sci. Technol., 2011, 27(1), 29 34 33 1 yc / yt _ 0.8 0.6 0.4 0.2 0.0 Yield asymmetry of material Yield asymmetry caused by texture 10 20 30 40 50 States of material 10 (as-cast);20 (as-extruded) 30 (ECAP 1Pass);40 (ECAP 4Pass);50 (ECAP 8Pass) Fig. 6 AZ31 alloy used in this work and the corresponding yield asymmetry. 10: as-cast (grain size 126 µm); 20: as-extruded (grain size 74 µm); 30: ECAP-1 pass (grain size 13 µm); 40: ECAP-Bc4 passes (grain size 4 µm); 50: ECAP-Bc8 passes (grain size 3.4 µm) Yield asymmetry induced by grain refining 0.6 0.5 0.4 0.3 0.2 0.1 0.0 (0.8,0.56) 1 10 100 Grain size, d / m Experiment data Fig. 7 Yield asymmetry variations contributed solely by grain refining, which are about 0.4, 0.26, 0.24 and 0 for grain sizes of 3.4, 4, 13 and 74 µm respectively Second, in cast AZ31 with random texture, the average stress needed to activate basal <a> is lower than that for tensile twin. Therefore, basal <a> slip is easy to be activiated compared with tensile twinning both in tension and in compression. (Basal <a> slip and tensile twinning obey Schmid law. Random texture results in almost the same Schmid factors for the two deformation modes in the casting AZ31 alloy. But the CRSS for basal <a> slip is lower than that for tensile twinning.) Therefore, magnesium alloys with random textrue will exhibits yield symmetry, no mater what temperature and grain size are. The as-extruded AZ31 alloy with strong fiber texture exhibits the largest yield asymmetry when loading is along ED. Two reasons are responsible for the issue. First, there are significant difference in the volume fraction of grains favoured tensile twinning between compression and tension as shown in Fig. 4. Second, there is significant difference of the stress needed to activate tensile twinning and slip as discussed above. However, strong texture doesn t mean large yield asymmetry. Loading direction and grain size as well as temperature are influential factors [15,28,29]. Therefore, strong texture is just a prerequisite for large yield asymmetry for magnesium alloys. 4.3 Grain size Not only the texture of material but also the grain size has great influence on the yield asymmetry which was shown in Figs. 6 and 7. In terms of Hall-Patch relationship (σ=σ 0 +kd 1 2 ), the grain size has great influence on the yield stress for both slip and twinning. But the slope (k Twin ) for twinning is larger than that for slip (k Slip ), which means the stress needed to activate twinning will be larger than that of slip if grain size is refined to some extent [15]. Under the circumstance, slip will dominate the deformation no matter whether it is in compression or in tension and the yield asymmetry will no longer exist. The critical grain size for the magnesium AZ31 alloy will be estimated later. In this work the yield asymmetry decreased drastically after ECAP for one pass. This is due to the significant change of grain size and texture which impedes the occurrence of twinning and promotes slip. Further grain size refining by ECAP technique will result in the yield asymmetry decrease with the increase of passes, but not so markedly as the first pass because the grain size does not decrease very markedly in the following passes. When the grain size was refined to 3.4 µm (8 passes) the yield asymmetry between compression and tension almost vanished. It should be noted that ECAP changes the initial texture and grain size of the material simultaneously, so it is necessary to evaluate the influence of the texture and grain size on the yield asymmetry separately. A rough estimation is provided as follows. Assuming the grain size of as-extruded AZ31 alloy (74 µm) is large enough to ignore grain size influence on the yield asymmetry, or in other words, the contribution of yield asymmetry is totally attributed to strong fiber texture; one can see from Fig. 6 strong fiber texture induced the largest yield asymmetry of 0.56. Further, it is assumed that the evolution of texture is mainly involved in the change of basal plane orientation in most grains with respect to the loading direction. They are about 20, 40 and 45 (see Fig. 2) [16,17] for ECAPed 1, 4 and 8 passes of AZ31 alloy respectively. Therefore, the yield asymmetry resulted solely from texture evolution might be estimated as 0.52 (=0.56 cos20 ); 0.43 (=0.56 cos40 ) and 0.40 (0.56 cos45 ) for the above mentioned materials, respectively. It is shown by open symbols in Fig. 6. The difference between an experimental datum (solid symbol) and an estimated datum (open symbol) is caused solely by grain refining, which is

34 S.M. Yin et al.: J. Mater. Sci. Technol., 2011, 27(1), 29 34 about 0.24 (=0.52 0.28), 0.26 (=0.43 0.17) and 0.40 (=0.40 0) for the above mentioned materials (dash line), respectively. It means that the finer the grain size is, the larger influence on the variation of σ yc /σ yt will be and then less yield asymmetry there is. We now estimate the critical grain size below which no yield asymmetry appears, no matter whether the texture exists or not. The yield asymmetry induced by grain refining solely is shown in Fig. 7. In this figure the yield asymmetry of about 0.4, 0.26, 0.24 and 0 is shown for the grain size of 3.4, 4, 13 and 74 µm, respectively. From this figure, one can see that when the grain size decreases to about 0.8 µm, the yield asymmetry induced by grain refining solely is about 0.56 (see Fig. 7) which is just the value of yield asymmetry resulted by extrusion as shown in Fig. 6, in other words, the critical grain size can be estimated as 0.8 µm for AZ31 alloy, below which no yield asymmetry appears, no matter whether the texture exists or not. 4. Conclusion Yield asymmetry of magnesium alloys can be alleviated by weakening texture or refining the grain size. The critical grain size can be estimated as 0.8 µm for magnesium AZ31 alloy below which no yield asymmetry appears. Acknowledgements This work is financially supported by the National Natural Science Foundation of China under Grant Nos. 50471082 and 50571102. REFERENCES [1 ] Y. Kojima, T. Aizawa, S. Kamado and K. Higashi: Mater. Sci. Forum, 2003, 419-422, 3. [2 ] S. Kamado, J. Koike, K. Kondoh and Y. Kawamura: Mater. Sci. Forum, 2003, 419-422, 21. [3 ] H. Friedrich and S.J. Schumann: Mater. Process. Technol., 2001, 117, 276. [4 ] S.M. Yin, H.J. Yang, S.X. Li, S.D. Wu and F. Yang: Scripta Mater., 2008, 58, 451. [5 ] S.M. Yin, F. Yang, X.M. Yang, S.D.Wu, S.X. Li and G.Y. Li: Mater. Sci. Eng. A, 2008, 494, 397. [6 ] W.J. Kim, C.W. An, Y.S. Kim and S.I. Hong: Scripta Mater., 2002, 47, 39. [7 ] A. Styczynski, C. Hartig, J. Bohlen and D. Letzig: Scripta Mater., 2004, 50, 943. [8 ] T. Liu, Y.D. Wang, S.D. Wu, L. Peng, C.X. Huang, C.B. Jiang and S.X. Li: Scripta Mater., 2004, 51, 1057. [9 ] C. Davies and M. Barnett: JOM, 2004, 56, 22. [10] C. Bettles and M. Gibson: JOM, 2005, 57, 46. [11] J. Lévesque, K. Inal, K.W. Neale and R.K. Mishra: Int. J. Plasticity, 2010, 26, 65. [12] S. Hasegawa, Y. Tsuchida, H. Yano and M. Matsui: Int. J. Fatigue, 2007, 29, 1839. [13] H. Zenner and F. Renner: Int. J. Fatigue, 2002, 24, 1255. [14] Y. Chino, K. Kimura, M. Hakamada and M. Mabuchi: Mater. Sci. Eng. A, 2008, 485(1-2), 311. [15] M.R. Barnett, Z. Keshavarz, A.G Bee and D. Atwell: Acta Mater., 2004, 52, 5093. [16] M. Furukawa, Y. Iwahashi, Z. Hortia, M. Nemoto and T.G. Langdon: Mater. Sci. Eng. A, 1998, 257, 328. [17] W.J. Kim and H.J. Jeong: Mater. Sci. Forum, 2003, 419-422, 201. [18] S.R. Agnew, J.A. Horton, T.M. Lillo and D.W. Brown: Scripta Mater., 2004, 50, 377. [19] S. Hidetoshi and M. Toshiji: Scripta Mater., 2006, 54, 633. [20] B.C. Wonsiewicz and W.A. Backofen: Trans. TMS- AIME, 1967, 239, 1422. [21] L Jiang, J.J. Jonas, A.A Luo, A.K Sachdev and S. Godet: Scripta Mater., 2006, 54, 771. [22] J. Koike and R. Ohyama: Acta Mater., 2005, 53, 1963. [23] S.R. Agnew, C.N. Tome, D.W. Brown, T.M. Holden and S.C. Vogel: Scripta Mater., 2003, 48, 1003. [24] S.B. Yi, Brokmeier, R.E. Bbormaro, K.U. Kainer and T. Lippmann: Scripta Mater., 2004, 51, 455. [25] S.R. Agnew and O. Duygulu: Int. J. Plasticity, 2005, 21, 1161. [26] Y.N. Wang and J.C. Huang: Acta Mater., 2007, 55, 897. [27] Y.N. Wang and J.C. Huang: Mater. Chem. Phys., 2003, 81, 11. [28] S. Kleiner and P.J. Uggowitzer: Mater. Sci. Eng. A, 2004, 379, 258. [29] E.A. Ball and P.B. Prangnell: Scripta Metall. Mater., 1994, 31, 111.