Microstructure and anisotropic mechanical behavior of the high-strength and ductility AZ91 Mg alloy processed by hot extrusion and multi-pass RD-ECAP

A high strength, good ductility and cost-effective Mg alloy is highly desirable for the light-weighting application. Here, a combined processing route of hot extrusion with multi-pass rotary die equal channel angular pressing (RD-ECAP) was applied to AZ91 Mg alloy with the aim of simultaneously improving its strength and ductility. The anisotropic mechanical behavior of the ECAP alloy was investigated in comparison to the extruded alloy. The evolution of the microstructure and deformation texture was further examined as a function of the ECAP pass to construct the microstructure/texture-anisotropic properties link. The results indicate that the ECAP alloy exhibits a simultaneous improvement in both strength and ductility along three orthotropic directions compared to the as-cast alloy, and more ECAP passes continue this improvement trend. Thus, the optimal mechanical properties with the yield strength of 214.3–279.9 MPa, the ultimate tensile strength of 321.0–382.0 MPa, and the elongation of 8.2–15.5% along three tensile directions are achieved in the ECAP alloy after 12 passes RD-ECAP. This also implies a considerable anisotropy in the ECAP alloy, but it is significantly weakened compared to the extruded alloy. Moreover, it is found that the low ECAP pass (4 passes) is enough to achieve a homogenous microstructure, which is characterized by the uniform and fine equiaxed grains (~7 μm) with the fine β precipitates (~1.5 μm) along the grain boundaries. Therefore, the initial extrusion can accelerate the grain refinement and microstructure homogenization in subsequent ECAP processing. Increasing the ECAP passes over 4, there are no discernible changes in the grain and precipitate microstructures, but the texture migration is observed. The high strength of the ECAP alloys not only stems from the well-known refinement strengthening, but also for texture strengthening. The contribution of the texture strengthening is enhanced with the increase of the ECAP pass.