Grain-Refinement of ZK60Mg Alloy by MDFing under Decreasing-Temperature Conditions and Change in Mechanical Properties

Hot-extruded ZK60Mg alloy was processed by multi-directionally forging under decreasing-temperature conditions (dMDFing). A homogeneous ultrafine-grained structure was successfully obtained by dMDFing up to a cumulative strain of Σ∆ε = 6.4. The coarse initial grains with an average grain size of 84.8 µm was subdivided into ultrafine equiaxed grains below 0.83 µm by mechanical twinning, kinking and dynamic recrystallization. The ultrafine-grained structure developed by dMDFing possessed excellent mechanical properties with very high hardness of 950 MPa, ultimate tensile strength of 300 MPa and plastic strain to fracture of 29%. Strain-rate dependence of tensile behavior resulted in a large value of strain-rate sensitivity at room temperature, which suggested that the grain boundary sliding contributed to the superior ductility in the ultrafine-grained ZK60Mg alloy. Hardness was not increased by aging at 413 K after dMDFing to Σ∆ε = 4.0.