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.

In this study, the effect of the processing route using a friction stir processing (FSP) method on the microstructure and mechanical behavior of a Mg-9Li-1Zn alloy was systematically investigated. In the FSP method, the odd-numbered (1st and 3rd) process directions and even-numbered (2nd and 4th) passes were alternated to distribute the strain throughout the whole processed zone uniformly. Consequently, the processed zone had a much more uniform microstructure and hardness distribution than the processed zone obtained using the conventional FSP method. Using this method, the grain size of a Mg-9Li-1Zn sheet alloy was refined from ~31 µm to ~0.21 µm with uniformly distributed α and β phases. The processed alloy exhibited a high strength-ductility synergy with an ultimate tensile strength (UTS) of 220.1 MPa and total elongation of 70.0% at a strain rate of 10⁻³ s ^{− 1}, overwhelmingly higher than those of the base metal, 155.6 MPa in UTS and 36.0% in elongation. The in-situ SEM-DIC analysis and TEM observation demonstrated that such an outstanding ductility with moderate strength is caused by grain boundary sliding, the dominant deformation mechanism of the ultra-fine-grained sample after FSP. The processing route with reverse processing direction was proven to be efficient in producing the ultrafine grain size microstructure and improving the mechanical properties of superlight Mg-9Li-1Zn alloy.