Mechanical anisotropy induced by the competition between twinning and basal slip of AZ31 magnesium alloy under biaxial tension

The mechanical behavior of a hot-rolled Mg AZ31 plate was studied under biaxial tensions along two perpendicular loading axes of x and y in the ND-TD plane, with the x-axis aligned at a 45° counterclockwise angle from the TD (ND and TD refer to the normal direction and transverse direction). A complex strain partitioning in two loading axes is observed with the variation of stress ratio σ x x : σ y y . For σ x x : σ y y = 1:1 and 1:1.5, the strains along the x- and y-axes, ε x x and ε y y , are both positive. However, With the stress ratio of σ x x : σ y y = 1:3, ε y y remains positive, while ε x x becomes negative. Interestingly, for σ x x : σ y y = 1:2, ε x x changes from negative during the early stage of loading to positive upon failure. Crystal plasticity simulations and theoretical calculations reveal that the observed strain transition. The reasons for the strain transition with stress ratio is related to the competition between { 10 1 ¯ 2 } twinning and basal slip. For { 10 1 ¯ 2 } twinning, the accommodated strains by most of the twins are ε x x >0 and ε y y >0 for all the stress ratios considered. For basal slip, most of the basal variants accommodate a positive y-axis strain, while for loading cases with low stress ratios, most of the basal variants result in a negative x-axis strain. As the stress ratio decreases, basal slip becomes more active, while the activity of { 10 1 ¯ 2 } twinning gradually decreases, leading to the observed strain transition. For σ x x : σ y y = 1:2, the transition from negative to positive is related to the increasing activity of { 10 1 ¯ 2 } twinning with strain. Theoretical calculations reveal that the competition of basal slip and { 10 1 ¯ 2 } twinning under biaxial tension is determined by the variation of their Schmid factors with σ x x : σ y y .