The primary objective of this study was to explore the crystal orientation-dependent fracture mechanism of nickel–titanium (NiTi) alloys by using molecular dynamics simulations. We discovered that the stress concentration ahead of the crack tip induced the formation of martensite with a herringbone twin or mixed HCP/OTHER atom morphology, while reverse martensite transformation occurred behind the crack tip due to stress relaxation. Interestingly, typical brittle fracture by cleaving along the high-energy domain boundary, quasi-brittle fracture as a result of the transformation-induced plasticity, as well as ductile fracture caused by a combination of phase transition, stacking fault, and atomic slip were observed in the samples with (1_00) [010], (110) [11_0], and (1_1_2) [111] crack orientations, respectively. The findings demonstrated that the crack growth behavior in each orientation was closely correlated to the microstructure evolution around the crack tip. Moreover, the relationship between the traction force and separation was achieved from the fracture process zone near the crack surface. The reported orientation dependency of the traction–separation curves was decisively driven by the anisotropy of cracking characteristics and microstructure morphologies. The results of this work offer comprehensive insights into the importance of crystal orientations on the atomic-scale fracture performance of NiTi alloys.

本研究的主要目的是通过分子动力学模拟探索镍钛 (NiTi) 合金的晶体取向相关断裂机制。我们发现，裂纹尖端前的应力集中导致形成具有人字形孪晶或混合 HCP/OTHER 原子形态的马氏体，而由于应力松弛，裂纹尖端后面发生反向马氏体转变。有趣的是，观察到典型的沿高能域边界解理的脆性断裂、相变诱导塑性导致的准脆性断裂，以及相变、堆垛层错和原子滑移共同导致的延性断裂在分别具有 (1_00) [010]、(110) [11_0] 和 (1_1_2) [111] 裂纹方向的样品中。研究结果表明，每个方向的裂纹扩展行为与裂纹尖端周围的微观结构演化密切相关。此外，牵引力与分离之间的关系是从裂纹表面附近的断裂过程区获得的。所报告的牵引分离曲线的方向依赖性是由开裂特性和微观结构形态的各向异性决定性驱动的。这项工作的结果为了解晶体取向对 NiTi 合金原子级断裂性能的重要性提供了全面的见解。牵引力与分离之间的关系是从裂纹表面附近的断裂过程区获得的。所报告的牵引分离曲线的方向依赖性是由开裂特性和微观结构形态的各向异性决定性驱动的。这项工作的结果为了解晶体取向对 NiTi 合金原子级断裂性能的重要性提供了全面的见解。牵引力与分离之间的关系是从裂纹表面附近的断裂过程区获得的。所报告的牵引分离曲线的方向依赖性是由开裂特性和微观结构形态的各向异性决定性驱动的。这项工作的结果为了解晶体取向对 NiTi 合金原子级断裂性能的重要性提供了全面的见解。