Effect of extrusion and rotary swaging on the microstructural evolution and properties of Mg-5Li-5.3Al-0.7Si alloy

High-strength Mg–5Li-5.3Al-0.7Si alloys were prepared by applying a combination of conventional extrusion with an extrusion ratio of 25:1 at 320 °C and subsequent rotary swaging (RS) with 0.2 mm reduction per pass in diameter at room temperature. The microstructural evolution was characterized and mechanical properties were investigated by tensile tests. The EBSD results showed that the grain size of the alloy decreased remarkably after extrusion and RS. Meanwhile, the yield strength (YS) of the as-extruded alloy was significantly improved from 161 MPa to 301 MPa by 2 passes RS, with only a slight reduction in ductility. The grain refinement during RS should be attributed to the activation of twins and continuous dynamic recrystallization (CDRX). The texture evolution during RS was investigated by EBSD. The number of grains with <0001> parallel to extrusion direction (ED) decrease significantly after RS. These grains are subjected to multi-directional stress and deformation, which rotate and develop a stronger texture component with <101‾0> - <112‾0> parallel to ED during RS, leading to the optimization of grain orientation. The phase evolution was investigated by SEM, EDS and TEM. Extrusion and RS effectively crush and disperse the second-phase particles of Mg₂Si and Al_0.89Li_0.11, leading to their homogenization and refinement. The main mechanisms for strengthening the as-swaged alloy include grain refinement strengthening, dislocation strengthening, and second-phase strengthening. Grain refinement strengthening plays a dominant role in the as-swaged alloy.