Structural characteristics of {1¯012} non-cozone twin-twin interactions in magnesium

Twin-twin interactions form twin-twin boundaries (TTBs) which can prevent twin propagation, inhibit direct twin transmission, retard detwinning, and facilitate secondary twins. The current work studies the microstructure and interaction mechanisms of non-cozone twin-twin junctions by combining electron back-scatter diffraction observations and atomistic simulations. Non-cozone twin-twin interactions are defined as the intersecting line of the two twins isn't parallel to one <12¯10> zone axis (a-axis) and include two types, Type II(a) (T₂→T₁) and Type II(b) (T₃→T₁), according to the crystallography of two interacting twins. For Type II(a) interaction, both statistical results of experimental observations and interfacial energy calculation confirm TTB formation on the obtuse side of the incoming twin instead of the acute side. However, for Type II(b) interaction, the growth of twins on both sides is impeded, although the TTB on the acute side possesses the lowest interfacial energy. Atomistic simulation demonstrates that, for Type II(a) twin-twin interactions, positive resolved shear stresses on the obtuse side favor T₁ and T₂ twinning, while negative resolved shear stresses on the acute side impede T₁ and T₂ twinning. For Type II(b), negative resolved shear stresses on both the acute and obtuse sides result in impediment of twinning on both sides. These results can be used in developing micro/macro-scale predictive models that deal with the role of multiple twins and twin variants during mechanical processing. The analytical and simulation methods can be generalized and applied to atomistic analysis in different material systems to further explain the hardening mechanisms associated with twin-twin interactions.