Abstract We present a combined ab initio and classical molecular dynamics study of the stability and structure of the two B19′ compound twin boundaries in NiTi: ( 100 ) and ( 001 ) . The role of these compound twins in suppressing the martensitic transition temperature in this material was also considered. At T = 0 K, DFT computations showed that the ( 001 ) twin is unstable to relaxation to B33. We determined from ab initio molecular dynamics simulations, however, that this twin is stabilized at finite temperature due to entropic and anharmonic effects. To our knowledge, this is the first example of finite temperature stabilization of such a defect phase. The evolution of both twin structures with temperature was also investigated, showing distortion of the lattice parameters near the boundaries. Free energy calculations obtained for the ( 100 ) twin showed a suppression of the martensitic transition temperature of up ∼ 100 K depending on twin density. A smaller degree of suppression was seen for the ( 001 ) twin. This suppression is shown to be driven mainly by the internal energy of the interface, although at high twin densities and high temperatures, entropy also can play a significant role.

中文翻译：

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NiTi 中 B19' 复合孪晶对马氏体转变温度的稳定性、结构和抑制：从头算和经典模拟

摘要 我们对 NiTi 中两个 B19' 复合孪晶界的稳定性和结构进行了从头算和经典分子动力学的结合研究：(100) 和 (001)。还考虑了这些复合孪晶在抑制该材料中的马氏体转变温度方面的作用。在 T = 0 K 时，DFT 计算表明 (001) 孪晶对于松弛至 B33 是不稳定的。然而，我们从 ab initio 分子动力学模拟中确定，由于熵和非谐效应，这个孪生体在有限温度下是稳定的。据我们所知，这是这种缺陷相的有限温度稳定的第一个例子。还研究了两种孪晶结构随温度的演变，显示了边界附近晶格参数的畸变。为 (100) 孪晶获得的自由能计算表明，取决于孪晶密度，马氏体转变温度被抑制了约 100 K。对于（001）孪晶，观察到较小程度的抑制。这种抑制主要由界面的内能驱动，尽管在高双密度和高温下，熵也可以发挥重要作用。