Observation and analysis of the coexistence of two “opposite” twin modes in a Mg-Al-Zn alloy

• The uncommon coexistence of two “opposite” twin modes inside the same grain were observed.
• The activation of contraction twins was favored by the external stress state.
• Extension twins were likely induced by the pile-up of basal slip in parent grains.

Understanding twinning mechanism is crucial to effectively tailor twin-related properties of HCP metals. Here, we reported an uncommon twin phenomenon during rolling of tilted Mg alloy sheets: coexistence of {10−12} extension twin (ET) and {10−11} contraction twin (CT) inside the same grain. 26 such twinning grains containing 40 ETs and 33 CTs were identified via crystallographic analysis to explore the coexistence mechanism in terms of Schmid law and local strain accommodation effects. The statistical results show that the geometrical relationship between stress state during rolling and grain orientations is favorable for the activation of CTs but not for ETs. Further examination indicates that ETs are very likely to be induced by pile-up of basal slip in their respective parent grains.

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