Effects of Zn/Al mass ratio on microstructure evolution and mechanical properties of Mg–Sn based alloys

Effects of Zn/Al mass ratio on the microstructure evolution and mechanical properties of the Mg–3Sn–5Zn-xAl (x=0,1,2,3 wt%) alloy were systematically investigated in the present study. The secondary phases of as-cast alloys were transformed from MgZn2/α-Mg eutectics and Mg2Sn to icosahedral (I-) phase and Mg2Sn with decreasing of Zn/Al mass ratio. After extrusion, almost completely recrystallized fine grains were obtained in TZ35-xAl alloys, but the volume fraction of recrystallized region was slightly reduced by the addition of Al. This phenomenon was chiefly ascribed to the particle-stimulated nucleation (PSN) mechanism and Zener grain-boundary pinning effect induced by enormous numbers of fine Mg–Zn phase, I-phase, Mg2Sn and newly formed ϕ-phase (related to I-phase) particles. As-extruded alloys exhibited superior strength and ductility balance. Specifically, Mg–3Sn–5Zn–1Al alloy got the highest ultimate tensile strength and elongation, reaching 340 MPa and 20.1%, respectively. Moreover, tensile yield strength of as-extruded alloys rises dramatically from 169 MPa to 212 MPa along with rising of the Al level. Improved tensile yield strength is primarily attributed to the synergistic reinforcement mechanism of grain refinement and dynamic precipitation of fine secondary particles. The excellent ductility for all as-extruded alloys owes a great deal to the activation of non-basal slip systems, high volume fraction of fine DRXed grains and weakened texture.