Strain rate-dependent tension-compression asymmetry in cast Mg-Gd-Y alloy: Insights into slip and twinning mechanisms

Tension-compression asymmetry is a critical concern for magnesium (Mg) alloys, particularly in automotive crash structures. This study systematically examines the tension-compression asymmetry of a cast Mg-Gd-Y alloy at various strain rates. Experimental results indicate symmetric yielding stress under both tension and compression at all strain rates, along with a reduction in the tension-compression asymmetry of ultimate stress and plastic strain as the strain rate increases. This trend arises from an unusual strain rate-dependent tension-compression asymmetry, characterized by strain rate toughening in tension and negligible strain rate effect in compression. The differing behavior is linked to the distinct twinning mechanisms under tension and compression. The suppression of twinning under tension contributes to the positive strain rate dependence of pyramidal slip, whereas the activation of abundant twins during compression means that pyramidal slip is unnecessary to accommodate c-axis strain, leading to the absence of a strain rate effect in compression. Abundant twins nucleate consistently from yielding to 2% strain, but only after basal and prismatic <a> slip have mediated microplasticity, suggesting that these slip systems reduce the nucleation stress for twinning during compression, resulting in a lower activation stress for twinning compared to tension. This study provides new insights into micromechanisms of the tension-compression asymmetry in cast Mg-Gd-Y alloys and offers practical guidance for the application of these materials in critical components that must endure both tension and compression under varying strain rates.