A new annealing hardening mechanism in pre-twinned Mg-3Al-1Zn alloy

In the present study, a pre-strained Mg AZ31 plate (7% cold rolling and 2% compression along the transverse direction (TD)) was annealed at 200 °C, 250 °C, 350 °C for 0.5 h and 3 h. An interesting annealing hardening phenomenon appears during recompression along the normal direction (ND). All the as-used annealing treatments increase the yield strengths by about 20-27 MPa. Microstructure examination shows that a large number of {101̅2} twins in the pre-strained samples are completely removed by the annealing treatments, except that at 200 °C for 0.5 h. Disappearance of the pre-existing twins leads to a transition from a detwinning predominant deformation to a basal slip predominant one during ND-compression. This change of deformation mechanism accounts for the annealing hardening phenomenon.