Effects of initial texture on deformation behavior during cold rolling and static recrystallization during subsequent annealing of AZ31 alloy

doi:10.1016/j.jmst.2020.04.074

Abstract

Abstract:

This study demonstrates that the initial texture of Mg alloy significantly affects the microstructure developed during cold rolling and the recrystallization behavior during subsequent annealing. In a sample with a texture oriented toward the normal direction (ND sample), thick and large-sized shear bands are intensively formed during cold rolling and the deformation is strongly localized along these bands. In contrast, in a sample with a texture oriented toward the transverse direction (TD sample), many {10-12} twins are formed at the early stage of cold rolling, and then, numerous small-sized shear bands are formed in these twins. Results of nanoindentation tests reveal that the cold-rolled ND sample has internal strain energy that is substantially concentrated along the shear bands, whereas the cold-rolled TD sample has a large amount of internal strain energy that is homogeneously distributed throughout the material; this latter behavior is strongly related to the extensive {10-12} twinning and the resultant microstructural variations in the TD sample. During subsequent annealing, recrystallization occurs locally along the shear bands in the ND sample, which leads to the formation of a bimodal grain structure comprising fine recrystallized grains and coarse unrecrystallized grains. In contrast, during subsequent annealing of the TD sample, static recrystallization occurs homogeneously throughout the material, which results in the formation of a uniform grain structure that mostly comprises equiaxed recrystallized grains.

Key words: Magnesium alloy, Cold rolling, Initial texture, Twinning, Static recrystallization

Sang Won Lee, Gukin Han, Tea-Sung Jun, Sung Hyuk Park. Effects of initial texture on deformation behavior during cold rolling and static recrystallization during subsequent annealing of AZ31 alloy[J]. J. Mater. Sci. Technol., 2021, 66: 139-149.

Figures/Tables 12

Fig. 1. (a) Inverse pole figure map of initial hot-rolled AZ31 plate, (b) schematic illustration depicting ND and TD samples machined from hot-rolled plate for cold rolling, and (c) dimensions and coordinate system of machined ND and TD samples and their (0001) pole figures. davg denotes the average grain size.

Fig. 2. (a, d) Optical micrographs, (b, e) inverse pole figure maps, and (c, f) (0002) pole figures of cold-rolled (a-c) ND and (d-f) TD samples.

Fig. 3. (a, b) Twin boundary maps and (c, d) misorientation angle distributions of cold-rolled (a, c) ND and (b, d) TD samples.

Fig. 4. Variations in area fraction of grains with angle θ between their basal poles and ND for (a) ND and (b) TD samples before and after cold rolling.

Fig. 5. Optical micrographs of cold-rolled and subsequently annealed (a-c) ND and (d-f) TD samples with annealing times of (a, d) 5, (b, e) 30, and (c, f) 90 min.

Fig. 6. Inverse pole figure maps of cold-rolled and subsequently annealed (a-c) ND and (d-f) TD samples with annealing times of (a, d) 5, (b, e) 30, and (c, f) 90 min. davg denotes the average grain size.

Fig. 7. Inverse pole figure maps of recrystallized regions of cold-rolled and annealed (a-c) ND and (d-f) TD samples annealed for (a, d) 5, (b, e) 30, and (c, f) 90 min. fRX and dRX denote, respectively, the area fraction and average size of recrystallized grains.

Fig. 8. Variations in (a) area fraction of recrystallized (RXed) grains and (b) average grain sizes of recrystallized and total (i.e., unrecrystallized + recrystallized) regions of ND and TD samples with annealing time.

Fig. 9. (0002) Pole figures of cold-rolled ND and TD samples and those of ND and TD samples after subsequent annealing.

Fig. 10. Variations in average Vickers hardnesses of ND and TD samples with annealing time.

Fig. 11. (a) Nanoindentation hardness profiles of cold-rolled samples, which were measured along the RD at the mid-thickness. Kernel average misorientation (KAM) maps of cold-rolled (b) ND and (c) TD samples. KAMavg denotes the average value of the KAM.

Fig. 12. Optical micrographs of cold-rolled (a) ND and (c) TD samples. (b, d) Nanoindentation hardness maps, which were obtained by nanoindentation measurements in the red rectangular areas marked in (a) and (c), respectively, superimposed with corresponding optical micrographs.

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