Effects of Al addition on the microstructure and mechanical properties of as-extruded Mg-Bi-Si-Mn based alloy

This study investigates the effect of aluminum addition on the microstructure and mechanical properties of the Mg-5Bi-0.5Si-0.5Mn-1Al (BSMA5001) alloy. The results demonstrate that the BSMA5001 alloy exhibits excellent comprehensive mechanical properties, with a yield strength of ~ 232 MPa, an ultimate tensile strength of ~ 280 MPa, and an elongation to failure of ~ 13.6%. Compared with Mg-5Bi-0.5Si-0.5Mn (BSM500) alloy, the tensile strength is increased by ~ 40 MPa, and its elongation is increased obviously from ~ 4.3% to ~ 13.6%. Microstructural characterization of the Mg-Bi-Si-Mn-(Al) alloy reveals the precipitation of a high density of micro- and nano-sized second phases, including Mg 3 Bi 2 and Al-Mn particles. Micron-sized second-phase particles promote recrystallization via the particle-stimulated nucleation mechanism, while high-density Al-Mn nano-phases can effectively impede the migration of dynamically recrystallized grain boundaries to achieve grain refinement. As a result, the BSMA5001 alloy formed a fully recrystallized microstructure with an average grain size of only ~ 2.03 μm, contributing to the remarkable strengthening effect. Concurrently, the densely dispersed nano-scale Al-Mn phases effectively hinder dislocation gliding and exert a strong pinning effect, increasing the residual dislocation density and providing substantial dislocation strengthening. These findings offer valuable theoretical insights and a foundational reference for the development of low-cost Mg alloys with high strength and good ductility.