Strain-rate effects on twinning, dynamic recrystallization and their competition of casting AZ31 Mg alloys during the plane strain compression

Plane strain compression tests (PSC) were conducted on as-cast AZ31 Mg alloy samples at the temperature of 400 ?C with different strain rates of 10 s?1, 80 s?1 and 160 s?1. The twinning and recrystallization behaviors close to and away from crack boundaries were characterized using Electron Backscattered Diffraction technology. The results show: (1) the twinning within the grains close to crack boundaries is more active than that far away from crack boundaries due to the local stress concentration. In addition, the rate-sensitive twinning proliferates with increasing strain rate; (2) the {10-11}-{10-12} double twinning-induced dynamic recrystallization (TDRX) can form a necklace-like structure especially within the grains close to crack boundaries. In addition to twinning, the TDRX also presents the rate sensitivity and becomes more active at the higher strain rate due to the higher flow stress; (3) the failure strain of PSC tests first decreases and then increases with increasing strain rate. This phenomenon strongly depends on the strain-rate controlled competition between twinning hardening and TDRX softening.