Abstract Molecular dynamics simulations were performed to investigate the cyclic deformation of magnesium single crystalline nano-pillars in the first three cycles. To study the tension–compression asymmetry, two symmetric uniaxial cyclic loading paths (applied strain amplitude = 10%) were employed, i.e., a triangle wave loading path started by tension and a triangle wave loading one started by compression. The influence of crystalline orientation on the cyclic deformation of magnesium nano-pillars was studied by changing the angle θ between the loading direction and the c-axis of the nano-pillars (e.g., 0°, 45°, and 90°). Under the cyclic loading path started by tension, the full deformation process in each cycle is controlled by twinning- detwinning-pyramidal slip-retwinning at θ = 0° with significant tension–compression asymmetry observed, basal slip + twinning-detwinning-basal slip + retwinning at θ = 45° without any tension–compression asymmetry, and double twinning-detwinning-retwinning-detwinning at θ = 90° with significant tension–compression asymmetry. Under the cyclic loading one started by compression, on the other hand, the deformation process is dominated by pyramidal slip-twinning-detwinning at θ = 0°, basal slip-twinning-detwinning-retwinning at θ = 45°, and prismatic slip-double twinning-detwinning in the first two cycles and twinning-detwinning-retwinning in the third cycle at θ = 90°, while tension–compression asymmetry is observed in the cases at θ = 0° and 90° only due to different deformation modes in the tension and compression stages. Especially, for the loading path started by compression at θ = 90°, cyclic hardening is seen in the tension stage, but softening is observed in the compression stage only in the third cycle due to the change of deformation modes. The current work provides new insights for understanding the mechanical property of Mg crystals subjected to a cyclic loading.

中文翻译：

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镁单晶循环变形的分子动力学模拟

摘要 进行了分子动力学模拟以研究镁单晶纳米柱在前三个循环中的循环变形。为了研究拉压不对称性，采用了两个对称的单轴循环加载路径（应用应变幅度 = 10%），即由张力开始的三角波加载路径和由压缩开始的三角波加载路径。通过改变加载方向与纳米柱c轴的夹角θ（例如0°、45°和90°），研究了晶体取向对镁纳米柱循环变形的影响。在由张力开始的循环加载路径下，每个循环的完整变形过程由 θ = 0° 的孪生-解孪-锥体滑移-重缠控制，观察到显着的拉-压不对称性，基底滑移 + 孪生-解缠-基底滑移 + 在 θ = 45° 处重缠，没有任何拉压不对称性，在 θ = 90° 处双孪生-解缠 - 重缠 - 解捻，具有显着的拉压不对称性。另一方面，在由压缩开始的循环加载下，变形过程以 θ = 0° 处的金字塔滑移-解孪生、θ = 45° 处的基底滑移-孪生-解孪-重缠和棱柱滑移-解缠为主。 θ = 90°时，前两个循环中出现双孪生-去孪生，第三个循环中出现孪生-去孪生-重缠，而在 θ = 0°和 90° 的情况下仅由于不同的变形模式观察到拉压不对称性拉伸和压缩阶段。特别是，对于 θ = 90°的压缩开始的加载路径，在拉伸阶段可以看到循环硬化，但由于变形模式的变化，仅在第三个循环的压缩阶段观察到软化。目前的工作为理解受到循环载荷的镁晶体的机械性能提供了新的见解。