Improved stretch formability of AZ31 sheet via grain size control

It is reported that stretch formability of Mg alloys enhances with increasing grain size. In order to understand the effects of grain size, the evolution of microstructure during tensile test was investigated comparing with mechanical properties such as work hardening coefficient n-value) and Lankford value (r-value). An increase in grain size promotes the activation of {10 12} tensile twins and most of the tensile twins formed in the grains whose basal poles tilted to the rolling direction (RD) by around 20 degrees during tensile loading toward RD. Promoting tensile twinning contributes to an increase of n-value induced by the interaction between tensile twins and basal dislocations, and a decrease of r-value associated with releasing stress concentration on grain boundaries continuously activating basal dislocation. In the present study, the high formability is associated with tensile twinning up to a certain grain size and then in coarser grain size, it deteriorates by changing the matrix grains into hard orientations.