The strain accommodation of grain boundary and deformation mechanism of directionally solidified Mg alloy

Abstract

The Mg93.72Gd5.85Y0.43 alloy with columnar grains and preferred orientation of [224(_)5] was selected in this paper, and its deformation mechanism was investigated by room-temperature tensile test, quasi-in-situ observation and Electron Back Scatter Diffraction (EBSD). The results showed that all the columnar grains had the “soft” orientation ([224(_)5]) for both basal <a> slip system and {101(_)2} tensile twinning system. At the initial deformation stage (ε ≤ 3%), basal <a> dislocations started to move and crossed grain boundary, forming the basal-basal slip system pairs (B/B). As the deformation increased (ε ≤ 50%), {101(_)2} tensile twins selectively nucleated in single columnar grain and then expanded and merged rapidly, forming the bands of tensile twins and the basal slip system + tensile twinning-basal slip system pairs (BT/B). At the later deformation stage (ε ≤ 74.5%), the orientation transition brought by tensile twinning triggered the activation of {101(_)1} compression twins, leading to severe stress concentration and crack nucleation near the large-angle grain boundary and twinning boundary. The evolution of B/B pairs to BT/B pairs driven by grain boundary compatibility stress resulted in relaxation of the stress concentration at grain boundaries and guaranteed the excellent plasticity of the experimental alloy. A modified grain boundary compatibility stress calculation was proposed in this paper and was proved to be more effective in predicating the nucleation of tensile twins than the geometric compatibility factor.