In-situ investigation on nucleation and propagation of {10-12} twins during uniaxial multi-pass compression in an extruded AZ31 Mg alloy

In current study, microstructure and texture evolutions, the mechanism of variant selection for {10-12} twins, the nucleation and propagation of twins during uniaxial multi-pass compression in an extruded AZ31 Mg alloy were investigated by in-situ EBSD technology. The specimen was compressed along ND to different strains of 1.9%, 3.4% and 6% at a strain rate of 1 mm/min at room temperature. The results show that the yield stress maintains increased during the uniaxial multi-pass compression. Due to the growth and propagation of the pre-generated twins, the ultimate stress of last compression is approximately equal to the yield stress of next recompression along ND. It is found that the mechanism of variant selection for {10-12} tension twinning is dominated by both high Schmid factor (SF) and high strain compatibility factor m' in extruded Mg alloys. Meanwhile, the strain compatibility plays an important role in the propagation of twins in common grain boundaries. Two types of grains were discussed in the paper: grains containing one twin variant (one-twin grains) and grains containing two twin variants (two-twin grains). In the one-twin grains, the twinning growth speed shows scattered distribution after three-pass compression. Twin growth and propagation are affected by the maximum length of activated twins. In addition, the misorientation of twinning boundaries can affect the expanding of pre-generated twins. While in the two-twin grains, the variants with para-position (PP) relationships (7.4 degrees < 1-210 >) can promote the twin growth and coalescence. More nucleation sites and close mis-orientations between two variants lead to the high speed of twinning growth and propagation in those two-twin grains. This is the reason why two-twin grains can achieve higher volume fraction of twins compared with that in one-twin grains.