Acta Metall Sin  2019, Vol. 55 Issue (8): 976-986    DOI: 10.11900/0412.1961.2019.00050
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Microstructure and Texture Evolution of AZ31 Mg Alloy Processed by Multi-Pass Compressing Under Room Temperature
Liping DENG1,Kaixuan CUI2,Bingshu WANG2(),Hongliang XIANG1,Qiang LI2
1. School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350108, China
2. College of Materials Science and Engineering, Fuzhou University，Fuzhou 350108, China
Abstract

Mg alloy has hexagonal structure and exhibits poor workability at room temperature, which is attributed to the difficulty in activating a sufficient number of independent slips to accommodate the deformation. Twinning plays an important role in plastic deformation of Mg alloys during low and medium temperature to accommodate the imposed strain, especially the strain along the c-axis. Therefore, the microstructure and texture evolutions of AZ31 Mg alloy during multi-pass compressions at room temperature were investigated by EBSD technology. The results show that the microstructure and texture evolutions are mainly controlled by tension twinning during multi-pass compression. And the more the strain passes, the severer the texture transformation. The c-axes of the grains are almost rotated to the compression direction by tension twins. The twins generated during multi-directional compression can separate grains and then refine them. However, the de-twinning can rotate the grains back to the initial orientations, which is against the texture weakening. The Schmid law governs the characteristics of {10$1ˉ$2} twinning, and thus controls the texture evolution. Both the residual matrix and the pre-deformation induced twins intersect with the twins generated during subsequent deformation. And this can separate the grains and weaken the texture strength. The number and morphology of the activated twin behavior during multi-pass compression would be influenced by the pass reductions, consequently affecting the grain refinement.

 ZTFLH: TG146.22
Fund: National Natural Science Foundation of China((Nos.51301040 and 51601039));China Postdoctoral Science Foundation(No.2016M590591)
Corresponding Authors:  Bingshu WANG     E-mail:  bswang@fzu.edu.cn
 Fig.1  Orientation map (a), and (0001), (10$1ˉ$0) pole figures (b) of the as-received AZ31 Mg alloy sheet and schematic of samples used for compression testing (c) (ND—normal direction, TD—transverse direction, RD—rolling direction) Fig.2  Orientation image maps (a, c, e) and boundary structure maps (b, d, f) for TD3.0% sample (a, b), TD3.0%-RD3.0% sample (c, d) and TD3.0%-RD3.0%-ND3.1% sample (e, f) (Insets show {0001} pole figures. The red lines indicate {10$1ˉ$2} twin boundaries, and the blue lines indicate boundaries between two {10$1ˉ$2} twin variants within a single grain) Fig.3  Orientation image maps (a, c, e) and boundary structure maps (b, d, f) for TD5.5% sample (a, b), TD5.5%-RD5.0% sample (c, d) and TD5.5%-RD5.0%-ND5.2% sample (e, f) (Insets show {0001} pole figures. The red lines indicate {10$1ˉ$2} twin boundaries, and the blue lines indicate boundaries between two {10$1ˉ$2} twin variants within a single grain) Table 2  Evolution of twin volume fraction and the length of twin boundary per area along TD5.5%-RD5.0%-ND5.2% strain paths Table 1  Evolution of twin volume fraction, the number of twin lamellae per grain and the length of twin boundary per area along TD3.0%-RD3.0%-ND3.1% strain paths Fig.4  Texture evolution during multi-pass compression in TD3.0% sample (a), TD3.0%-RD3.0% sample (b), TD3.0%-RD3.0%-ND3.1% sample (c), TD5.5% sample (d), TD5.5%-RD5.0% sample (e) and TD5.5%-RD5.0%-ND5.2% sample (f)Color online Fig.5  Schmid factor analyses on {10$1ˉ$2} twinning system of the samples under compression path of TD5.5% (a~c), TD5.5%-RD5.0% (d~f) and TD5.5%-RD5.0%-ND5.2% (g~i)Color online(a) the angle between a-axis and the compression direction in the basal plane is 30°, the matrix (Ma, black box) and six possible twin variants (red and blue boxes)(b) the angle between a-axis and the compression direction in the basal plane is 0°, the matrix and six possible twin variants (red and blue boxes)(c) the orientations of twinning variants with the highest Schmid factors in 1st compression(d) the new twin orientations (red boxes) generated from the matrix (Md1~Md4, black boxes) in 2nd compression(e) the new twin orientations (red boxes) generated from the matrix (Md5 and Md6) in 2nd compression(f) the orientations of twinning variants with the highest Schmid factors in 2nd compression(g) the new twin orientations (red boxes) generated from the matrix (Mc1~Mc4, black boxes) in 3rd compression(h) the new twin orientations (red boxes) generated from the matrix (Mf1~Mf8, black boxes) in 3rd compression(i) the orientations of twinning variants with the highest Schmid factors in 3rd compression Fig.6  Analyses of the twins refine grains with EBSD orientation mapping (a) and {0001} pole figure (b) (M— matrix, TTD—twin generated from M in 1st compression, TRD1—twin generated from M in 2nd compression, TRD2 and TRD3—twins generated from TTD in 2nd compression, TND—twin generated from TRD3 in 3rd compression)Color online Fig.7  Analyses of the twins refine grains with EBSD orientation mapping (a, c) and {0001} pole figures (b, d) of TD5.5%-RD5.0% (a, b) and TD5.5%-RD5.0%-ND5.2% (c, d) samples (TTD, TTD1 and TTD2—twins generated from M in 1st compression, TRD—twin generated from TTD in 2nd compression, TND1—twin generated from TRD in 3rd compression, TND2—twin generated from TTD in 3rd compression)Color online