Effect of annealing temperature on microstructure, work hardening and softening behavior of cold-rolled Mg-8Li-3Al-0.3Si alloy

Abstract

In this article, the effect of annealing temperature on the microstructure and mechanical performance of cold-rolled dual phase Mg-8Li-3Al-0.3Si (LAS830) alloy was systemically investigated. Meanwhile, the regulation of annealing temperature on the work hardening and softening behavior of the cold-rolled LAS830 alloy was discussed. The results show that the dislocation density significantly decreases and the nanoscale MgLi₂Al phase disappears for the cold-rolled LAS830 alloy after annealing at 200 °C for 1 h, causing a decrease in the yield strength (YS) and ultimate tensile strength (UTS). However, the MgLi₂Al phase precipitates again when the annealing temperature increases to 240, 280 and 320 °C. The volume fraction of the MgLi₂Al phase increases with increasing annealing temperature from 240 to 320 °C, while the volume fraction of AlLi phase gradually decreases due to the dissolution into β-Li phase, which results in an improvement of YS and UTS as a result of solution strengthening and secondary phase strengthening. The work hardening rate of the cold-rolled LAS830 alloy in the stage Ⅲ significantly decreases after annealing at 200 °C due to the reduction of dislocation density, but it gradually increases with the improvement of annealing temperature, this is mainly ascribed to the growth of recrystallized grains and an increase in the volume fraction of MgLi₂Al phase. Moreover, the high density of dislocations is generated in the LAS830 alloy after rolling at room temperature, which provides a significant driving force for static recovery to occur, thereby improving the softening effect of alloy. The softening effect of the cold-rolled LAS830 alloy significantly decreases after annealing at 200 °C, and it further decreases with increasing the annealing temperature, which is mainly related to the grains size and secondary phase content in the alloy.