Abstract In this study, the tension-compression asymmetry of single-crystalline and nanocrystalline NiTi shape memory alloys (SMAs) was investigated by molecular dynamics (MD) simulations. Compound twinning martensite variants were simulated via thermally-induced martensitic transformation. The characteristics of the forward and reverse martensitic transformations were derived using atomic structural evolution. The tension-compression asymmetry of single-crystalline NiTi was attributed to different stress-induced martensitic variants and different deformation modes, which led to stress asymmetry and strain asymmetry, respectively. However, the phenomenological tension-compression asymmetry in nanocrystalline NiTi was mainly attributed to stress-induced martensitic variants, and no clear martensitic reorientation occurred under tension and compression loads. The corresponding atomic-scale structural evolution also explained the shorter tensile stress plateau in the nanocrystalline NiTi when compared to that in the single crystal samples. Moreover, the nanocrystalline tension-compression asymmetric behaviour was simulated under a broad temperature range to obtain the critical stress-temperature relation. Finally, the tension-compression asymmetry mechanisms of single-crystalline and nanocrystalline NiTi SMAs were numerically derived at the atomic scale.

中文翻译：

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单晶和纳米晶镍钛形状记忆合金的拉压不对称性：原子尺度研究

摘要 在这项研究中，通过分子动力学 (MD) 模拟研究了单晶和纳米晶 NiTi 形状记忆合金 (SMA) 的拉压不对称性。通过热诱导马氏体转变模拟复合孪晶马氏体变体。使用原子结构演化推导出正向和反向马氏体转变的特征。单晶 NiTi 的拉压不对称性归因于不同的应力诱导马氏体变体和不同的变形模式，分别导致应力不对称和应变不对称。然而，纳米晶 NiTi 中的现象学拉压不对称性主要归因于应力诱导的马氏体变体，并且在拉伸和压缩载荷下没有发生明显的马氏体再取向。与单晶样品相比，相应的原子级结构演变也解释了纳米晶 NiTi 中较短的拉伸应力平台。此外，在较宽的温度范围内模拟了纳米晶拉伸-压缩不对称行为，以获得临界应力-温度关系。最后，在原子尺度上数值推导了单晶和纳米晶 NiTi SMA 的拉压不对称机制。在较宽的温度范围内模拟纳米晶拉伸-压缩不对称行为以获得临界应力-温度关系。最后，在原子尺度上数值推导了单晶和纳米晶 NiTi SMA 的拉压不对称机制。在较宽的温度范围内模拟纳米晶拉伸-压缩不对称行为以获得临界应力-温度关系。最后，在原子尺度上数值推导了单晶和纳米晶 NiTi SMA 的拉压不对称机制。