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.

中文翻译：

---

铸造 Mg-Gd-Y 合金中应变速率依赖性的拉压不对称性：滑移和孪晶机制的见解


拉伸-压缩不对称性是镁 （Mg） 合金的一个关键问题，尤其是在汽车碰撞结构中。本研究系统地研究了铸造 Mg-Gd-Y 合金在不同应变速率下的拉伸-压缩不对称性。实验结果表明，在所有应变速率下，拉伸和压缩下均对称屈服应力，并且随着应变速率的增加，极限应力和塑性应变的拉伸-压缩不对称性减小。这种趋势源于一种不寻常的应变速率依赖性拉伸-压缩不对称性，其特征是拉伸时的应变速率增韧和压缩时的应变速率效应可以忽略不计。不同的行为与拉伸和压缩下不同的孪生机制有关。在拉伸下对孪晶的抑制有助于锥体滑移的正应变率依赖性，而压缩过程中大量孪晶的激活意味着锥体滑移对于适应 c 轴应变是不必要的，从而导致压缩中不存在应变率效应。丰度双胞胎从屈服到 2% 应变始终成核，但仅在基部和棱柱形 〈a〉 滑动介导了微可塑性之后，这表明这些滑动系统减少了压缩过程中孪晶的成核应力，导致孪晶的激活应力低于张力。本研究为铸造 Mg-Gd-Y 合金中拉伸-压缩不对称的微观机制提供了新的见解，并为这些材料在必须在不同应变速率下承受拉伸和压缩的关键部件中的应用提供了实用指导。