Effects of heat treatment on microstructure and mechanical properties of Mg-6Zn-4Sm-0.4Zr alloy

A new rare earth magnesium alloy（Mg-6 Zn-4 Sm-0.4 Zr, wt.%） was prepared by permanent mould casting. The microstructure and mechanical properties of the alloy sample in as-cast and various heat treatment situations were characterized with an optical microscope（OM）, X-ray diffractometer（XRD）, scanning electron microscope（SEM） equipped with energy dispersive spectroscope（EDS）, transmission electron microscope（TEM） and mechanical tests at room temperature, respectively. The experimental results show that the as-cast alloy mainly consists of α-Mg, eutectic Mg2Zn3, MgZnSm and Mg41Sm5. These eutectic phases with continuous or semicontinuous morphology principally distribute along grain boundaries. Almost all the eutectic compounds dissolve in α-Mg and the grains have no obvious growth trend after optimum solution treatment at 490 °C for 18 h. Meanwhile, the ultimate tensile strength（UTS） of 229 MPa and elongation（EL） to rupture of 9.78% can be achieved through the optimal solution treatment, which increase by 37 MPa and 57.74%, respectively, compared with that of the as-cast alloy. Further aging treatments at 200 °C for different durations lead to the conspicuous increment of mechanical properties and prominent age-hardening response. Peak-aged alloy（treated at 200 °C for 12 h） reveals better mechanical properties（UTS 258 MPa, EL 9.42%, hardness 73.4 HV） compared with the same alloy treated in other aging conditions, which is mainly ascribed to precipitated Mg2Zn3 and MgZn2 phases. Fracture analysis demonstrates that the as-cast alloy belongs to inter-granular and cleavage fracture patterns, while the solutionized alloy（treated at 490 °C for 18 h） reveals trans-granular and quasi-cleavage fracture modes. For the peak-aged alloy, the fracture pattern obeys the mixture of trans-granular and cleavage modes.