Strength-ductility synergy in AZ31 magnesium alloy via HDI strengthening and texture softening during composite inclined die extrusion

Addressing the common challenges of limited ductility and the “strength-ductility” trade-off caused by strong basal textures in traditionally extruded magnesium (Mg) alloys, this study proposes and designs a novel composite inclined die extrusion (CIDE) process. Through a sophisticated design of the die tooling structure, the evolution of gradient microstructures and the mechanisms of heterostructure-induced (HDI) strengthening and texture softening in AZ31 Mg alloys were systematically investigated. The results demonstrate that the CIDE process successfully creates a typical gradient heterostructure: the inner side exhibits a coarse-grained zone (CGZ) with a strong basal texture (average grain size, AGS ≈ 48.46 μm), while the outer side evolves into a fine-grained zone (FGZ) with significantly randomized texture (AGS ≈ 13.86 μm). Microstructural characterization and quantitative calculations indicate that geometrically necessary dislocations (GNDs) induced by asymmetric shear significantly pile up at the gradient interface, resulting in a dislocation density gradient as high as 1.83 × 10¹⁴m⁻², which effectively activates a powerful HDI stress strengthening mechanism. Mechanical test results indicate that the extruded Mg alloy achieves an exceptional strength-ductility synergy, with the true stress and true strain reaching 348.7 MPa and 19.8%, respectively, while the basal texture intensity is significantly softened from 18.87 to 8.90. This performance optimization is primarily attributed to the “strain gradient–back stress–texture synergy” effect induced by the gradient microstructure, which effectively delays the onset of necking and improves deformation compatibility. This work provides new scientific guidance and a theoretical basis for the development of high-strength and high-ductility heterostructure magnesium alloys.