Effects of tensile twinning on the stretch formability of Mg

This study examined the influences of twinning during the Erichsen test. {10-12} tensile twinning plays the critical role in Mg alloys. Alloy elements and grain size are important factors that determine the formation of {10-12} tensile twins at room temperature. Mg-6.0Zn(Z6) and Mg-6.0Zn-0.3Ca(ZX60) alloys were fabricated and their grain size was varied under different annealing conditions. Tensile twinning is promoted by the addition of Ca, as assessed from measurements of the microstructure and the viscoplastic self-consistent calculations. The coarse-grain Ca-containing alloy showed the largest amount of tensile twinning. However, the stretch formability increased with grain size up to a certain point and then decreased. It can be inferred that microstructures with large grains activated the tensile twin, which became origins of cracks. The results of the small Erichsen test showed that tensile twinning contributes to high stretch formability releasing the stress concentration in the grain boundaries but the interaction between twin and slip causes cracks as the grain size increases.