Microstructural evolution and mechanical properties of binary Mg–xBi (x = 2, 5, and 8 wt%) alloys

The microstructures and mechanical properties of Mg–xBi (x = 2, 5, and 8 wt%) were investigated compared with pure Mg. The as-cast Mg–Bi billets consist of dendritic α-Mg grain matrix, divorced eutectic Mg3Bi2 phase and secondary precipitated Mg3Bi2 phase. After homogenization, secondary precipitated Mg3Bi2 particles were all dissolved into the matrix, while most of the divorced eutectic Mg3Bi2 intermetallic compounds retained. All the as-extruded samples exhibit fully dynamic recrystallization (DRX) and the average grain size decreases and the amount of nano-scale Mg3Bi2 precipitates increases with increasing Bi content. Additionally, Bi alloying has little influence on the texture of the as-extruded pure Mg, with all the samples showing typical basal texture. As the results of grain refinement and precipitation hardening, the strengths of the as-extruded samples increase under both tensile and compressive tests. Besides, the yield asymmetry significantly decreases with Bi content. Moreover, all the as-extruded samples represent similar ductility under compression, while the tensile elongation first increases and then decreases with more Bi added. The as-extruded Mg–5Bi alloy demonstrates good combination of strength and ductility. However, Mg–8Bi alloy displays lower tensile ductility than that of Mg–5Bi alloy due to the presence of abundant undissolved coarse Mg3Bi2 particles, which act as cracking sources during tensile test. Furthermore, the strain hardening rate Θ, strain hardening exponent n, and hardening capacity Hc are greatly reduced by Bi addition, mainly owing to the enhanced synergy effects of grain refinement and numerous Mg3Bi2 precipitates.