Effect of heat-treatment on microstructure and deformation behavior of Mg-8.5Li-6.6Zn-1.5Y alloy

Abstract

The Mg-8.5Li-6.6Zn-1.5Y (wt%) as-cast alloy exhibits a (β+α) duplex phase structure. Interspersed eutectics, primarily reticular I-phase, predominantly form along phase and grain boundaries, enhancing the strength but leading to a reduction in ductility due to the brittle nature of the I-phase. This study focuses on modifying the alloy's microstructure through heat treatment to simultaneously improve both strength and ductility. Heating the alloy at 450 °C/6 h results in the dissolution of continuous reticular I-phase and the massive α-Mg. Subsequent slow cooling facilitates the reintroduction of α-Mg, with the cooling rate directly impacting the mean size of the α-Mg phase. The slower the cooling, the larger the α-Mg phase. The strength and ductility of LZW861 alloy are simultaneously enhanced by heat treatment, particularly in the air-cooled (450 °C/6 h-AC) alloy, while the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) increases from 147 MPa, 189 MPa, and 17.8% to 192 MPa, 242 MPa, and 22.3%, respectively, compared to the as-cast state. This increase in strength can be attributed partly to the precipitation of fine needle-like α-Mg uniformly dispersed in the β-Li matrix. Additionally, the increase in dispersed (Li,Mg)₃Zn nanoparticles contributes to matrix strengthening. The enhancement of ductility after 450 °C/6 h-AC heat treatment is ascribed to the dissolution of reticulated I-phase and the refinement of α-Mg phase, which enhance interphase deformation compatibility and weaken crack initiation at dispersed β/α interface.