Role of twinning on the stress and strain behaviors during reverse loading in rolled magnesium alloy sheets

Abstract

Reverse loading from compression to tension was performed on a rolled AZ31 magnesium alloy sheet to systematically study the effects of twinning and detwinning on the stress and strain behaviors after stress reversal. A crystal plasticity finite-element simulation was also conducted to study the underlying deformation mechanisms. This paper consists of the following three findings. (1) A sigmoidal curve occurred irrespective of compressive strain, while the sigmoidal curve became less pronounced as the compressive strain increased. The simulation results showed that the shift from detwinning-dominated to slip-dominated deformation became more gradual as the compressive strain increased; thus, the sigmoidal curve became less pronounced. (2) The yield stress after stress reversal increased when the sheet was annealed prior to reverse loading. The simulation results suggested that one of the mechanisms was that the residual stresses generated during compression acted as back stresses to detwinning activity; thus, the yield stress was increased by removing the residual stresses by annealing. (3) The Lankford value after stress reversal was relatively small and further decreased during the first work-hardening stage, whereas it increased during the second stage. The simulation results indicated that the thickness strain was composed of both slip and detwinning activities in the first stage, while only slip activity contributed toward the thickness strain in the second stage, which resulted in the difference in the evolution of the Lankford value between the first and second stages.