Modeling hetero-deformation induced stress partitioning revealing non-basal slip activity in bimodal-grained ZK60 Mg alloy

The excellent strength-ductility combination of hetero-grained Mg alloys has been reported to stem from pronounced hetero-deformation induced (HDI) stress. This stress alters the internal stress state of various slip systems and triggers significant activity of non-basal slips. However, the HDI stress state of different slip systems, and the mechanisms underlying the selective activation between basal and non-basal slips remain unclear to date. This study develops a novel HDI stress partitioning framework that in-situ calculates the crystallographic parameters and geometrical information of each datapoint within grains, aiming to reveal the correlation between HDI stress partitioning on individual slip systems and localized deformation model in the case of bimodal-grained ZK60 alloy. The framework demonstrates that HDI stress shows a strong dependence on the density of geometrically necessary dislocations (GNDs) and slip-system-level grain size, while exhibiting a relatively weaker correlation with equivalent-circle size of the hetero-grains. Given the close relation between the stress partitioning and the physical parameters, the framework can accurately predict the single and multiple slip activity fields obtained from high-resolution digital image correlation (HR-DIC). This holds even for slip systems with low Schmid factors, which are theoretically difficult to activate. Using this framework, it is found that HDI stress plays a more prominent role in diminishing the effective resolved shear stress (RSS) of basal 〈a〉 and prismatic 〈a〉 (i.e., 〈a〉 component) dislocations, while having a negligible effect on pyramidal 〈c + a〉 slips. Benefiting from the increased ratio of RSS_{<c + a>}/RSS_<a>, pyramidal 〈c + a〉 dislocations are extensively activated, leading to excellent strength-ductility combination in the bimodal-grained ZK60 alloy.