Enhancing Strength and Reducing Yield Asymmetry in Extruded AZ91 Alloy through Combined Ca and Sr Additions

In the present work, the effects of combined Ca and Sr additions on the microstructure, texture and mechanical properties of hot-extruded AZ91 alloy were investigated. Moreover, a detailed characterization of the recrystallization mechanisms governing the formation of new grains and final texture in extruded base AZ91 alloy and extruded AZ91-1Ca-xSr alloys was conducted. The evolution of Al-Ca and Al-Sr precipitates and their thermal stability at 400 degrees C extrusion temperature is predicted by thermodynamic calculations using Thermo-Calc software. The extruded microstructure of the base AZ91 alloy reveals the presence of discontinuous dynamic recrystallized grains alongside deformed grains. In contrast, extruded AZ91-1Ca-xSr alloys exhibit fully recrystallized microstructure consisting of intermetallic Al-Ca and Al-Sr stringers elongated in the extrusion direction, forming a neckless structure of stringers. In AZ91-1Ca-xSr extrusions, Al2Ca and Al4Sr precipitate effectively function as sites for particle-stimulated nucleation (PSN), thereby introducing localized strain energy variations. PSN leads to the formation of nuclei with random orientations, consequently reducing the overall sharpness of the texture. Ultimately, combined addition of Ca and Sr leads to improvements in both tensile and compressive strengths, with a reduction in tension-compression yield asymmetry. The enhancement of strength of extruded AZ91-1Ca-xSr alloys is primarily attributed to precipitation strengthening and grain size reduction resulting from the addition of Ca and Sr. Compared to the existing literature on the individual addition of Ca and Sr to extruded AZ91, the combined addition of both elements demonstrates superior tensile and compressive properties.