Abnormal Twinning Behavior and Constitutive Modeling of a Fine-Grained Extruded Mg–8.0Al–0.1Mn–2.0Ca Alloy under High-Speed Impact along Various Directions

To understand the mechanical and twinning behaviors of a fine-grained extruded Mg–8.0Al–0.1Mn–2.0Ca alloy under high-speed impact, impact tests were carried out using a split Hopkinson pressure bar, and microstructures at strains of 0.05, 0.10 and 0.20 were obtained using a series of stop rings manufactured by high-strength steel. The stress response and twinning behavior are closely related to loading direction and applied strain rate. The true stress–true strain curves are s-shaped in extrusion direction (ED) and c-shaped in transverse direction (TD), showing apparent anisotropy, while the yield strength is insensitive to loading direction. Almost identical strain-rate sensitivity is demonstrated by the stress in ED and TD. Interestingly, de-twinning is apparent as the applied strain increases to 0.20, and it is enhanced with increasing the applied strain rate. In contrast, the twin density in ED samples is clearly higher than that in TD samples. By modifying the terms of strain hardening and strain rate hardening in the classical JC model, an optimized model is built, which can accurately predict the stress response behavior of the studied alloy under high-speed impact along ED and TD. The correlation coefficient (R) and average absolute relative error (AARE) are 98.63% and 0.0199 for ED, and 96.88% and 0.0202 for TD, respectively.