Effects of Al addition on microstructure and mechanical properties of extruded Mg–3Bi alloy

Abstract

Effects of Al addition to a Mg–Bi binary alloy on its microstructural characteristics and tensile properties after extrusion are investigated via extrusion of Mg–3Bi–xAl (x = 0, 1, and 2 wt%) billets and analysis of the extruded materials. The Al addition negligibly affects the second-phase particles of the extruded alloy; however, an increase in the Al content causes significant decreases in the average grain size and maximum texture intensity of the extruded material owing to an increase in the area fraction of dynamically recrystallized (DRXed) grains. The Al addition improves the strength of the extruded alloy; this improvement is attributed to the enhanced grain-boundary hardening and solid-solution hardening effects induced by grain refinement and Al solute atoms, respectively. As the Al content increases from 0 wt% to 1 wt% and 2 wt%, the tensile elongation increases substantially from 2.8% to 9.4% and 16.9%, respectively. The reduction in the number and size of unDRXed grains with increasing Al content suppresses the formation and coalescence of cracks in the unDRXed grains during tension, which results in a significant improvement in the tensile ductility of the extruded material. During tensile deformation, large undesirable twins that act as crack initiation sites are locally formed in the unDRXed grains of the Mg–3Bi alloy, whereas relatively smaller twins are uniformly formed in both the DRXed and the unDRXed grains of the Mg–3Bi–2Al alloy. Consequently, the extruded Mg–3Bi–2Al alloy has a substantially higher tensile yield strength–elongation product (2924 MPa%) than the extruded Al-free B3 alloy (381 MPa%).