Effect of different extrusion parameters on the microstructure and mechanical properties of Mg-4Sm-3Gd-2Yb-0.5Zr alloy

The effect of different extrusion parameters on the microstructure and mechanical properties of Mg-4Sm-3Gd-2Yb-0.5Zr (SGY2) alloy was investigated. It was observed that under different extrusion parameters, unDRXed grains of SGY2 alloy exhibited a pronounced basal plane texture, specifically <0110>//ED, while the texture of DRXed grains was relatively dispersed. Under the condition of 420 °C and an extrusion ratio of 9.4 (420 °C-ER9.4), the basal plane texture intensity of unDRXed grains in SGY2 alloy was the highest. Furthermore, SGY2 alloy at different extrusion parameters exhibited recrystallization mechanisms mainly characterized by continuous dynamic recrystallization (CDRX), with some DRXed grains and deformed grains experiencing discontinuous dynamic recrystallization (DDRX). Additionally, at the 420 °C-ER9.4, the second phase particles in the as-extruded SGY2 alloy were smaller in size and exhibited a dispersed distribution. Under this condition, a significant amount of dislocation accumulation, dislocation bypassing, and dislocation tangling phenomena were observed in the SGY2 alloy. The primary deformation mechanism of unDRXed grains in the SGY2 alloy at the 420 °C-ER9.4 may involve prismatic plane 〈a〉, pyramidal plane 〈a〉, and pyramidal plane 〈c + a〉 slip, thereby activating a significant amount of dislocations. Compared to other extrusion conditions, this condition is more prone to activate non-basal plane slip. The as-extruded SGY2 alloy exhibited superior mechanical properties, with ultimate tensile strength (UTS) and yield strength (YS) of 332 MPa and 278 MPa, respectively. This is mainly attributed to the extremely fine grains, with many DRXed grains having grain sizes smaller than 1 µm, and higher density grain boundaries produced under this condition. Additionally, the unDRXed grains contain a high density of dislocations with small Schmid factor (SF), thus effectively inhibiting basal plane slip and strengthening the alloy to some extent. Similarly, the increased presence of second phase particles will also contribute to strengthening the alloy matrix through precipitation hardening.