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

Abstract

A new rare earth magnesium alloy (Mg-6Zn-4Sm-0.4Zr, 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 a-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.