Revealing the underlying slip/twinning mechanisms of tension-compression asymmetry and anisotropy in Mg-Gd-Y-Zn-Zr alloys with heterostructure

This study investigates the underlying slip/twinning mechanisms in an extruded Mg-Gd-Y-Zn-Zr alloy with a fiber coarse grain-ultrafine grain (FCG-UFG) heterostructure under various strain paths, combining experimental characterization and Visco-Plastic Self-Sonsistent modeling incorporating Predominant Twin Reorientation (VPSC-PTR). The material constants in the VPSC-PTR model were inversely calibrated using Schmid factor (SF)-corrected grain reference orientation deviation (GROD) data. The VPSC-PTR model was skillfully employed to quantitatively distinguish deformation mechanisms and texture evolution between coarse and fine grains in bimodal Mg alloys, demonstrating its strong potential for broader applications in heterostructured alloys. The results show that under both tension and compression, the UFG region initially accommodates plastic strain mainly through basal 〈a〉 slip. This produces a pronounced strain gradient at FCG grain boundaries, stimulating adjacent FCG grains—initially in hard orientations for basal 〈a〉 slip—to activate prismatic 〈a〉 slip and pyramidal 〈c + a〉 slip. The divergence in deformation mechanisms between FCG and UFG regions and the resulting mechanical incompatibility act synergistically to enhance hetero-deformation induced (HDI) strengthening, leading to simultaneous improvements in strength and ductility. Furthermore, the alloy exhibits an uncommon reversed tension-compression yield asymmetry (${\sigma }_y^C$/${\sigma }_y^T$ > 1). This behavior originates from the high critical stress required for kinking deformation of LPSO phases under compression, coupled with the suppression of conventional {10–12} extension twinning, which collectively reverse the typical twin-dominated yield asymmetry seen in conventional Mg alloys. Owing to its weak basal texture, the UFG region deforms mainly via basal 〈a〉 slip under various strain paths, contributing little to compressive anisotropy. In contrast, the orientation-dependent competition between basal and non-basal 〈a〉 slip within FCG grains, along with the distribution characteristics of LPSO phases, governs compressive mechanical anisotropy. GROD analysis further indicates that {10–12} extension twins promote the activation of pyramidal 〈c + a〉 slip. The introduction of twins and associated 〈c + a〉 dislocations effectively alleviates local stress concentrations and enhances plasticity in FCG regions, thereby delaying fracture. These findings provide new insights into the deformation mechanisms of heterostructured Mg alloys under multi-directional loading and will facilitate the design of high-performance Mg alloys with reduced tension-compression asymmetry and mechanical anisotropy.