Atomistic Simulation on the Relation between Amorphization and Crystalline Transformation in Ni-Ti Alloy
Atomistic Simulation on the Relation between Amorphization and Crystalline Transformation in Ni-Ti Alloy
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DOI：
10.1299/jmmp.4.1061
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发表时间：
2010
期刊：
影响因子：
--
通讯作者：
K. Saitoh;K. Kubota
中科院分区：
文献类型：
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作者：
K. Saitoh;K. Kubota
镍钛合金是典型的形状记忆材料。马氏体晶相与奥氏体晶相之间的原子尺度相变是造成这种作用的原因。然而，据报道，这些合金在剧烈变形下有时也会出现非晶化和纳米晶化。非晶化机制应与马氏体相变（MT）相竞争。本文主要研究了Ni-Ti合金中非晶化与MT之间的原子关系。利用MEAM框架上的简化势能进行分子动力学模拟。主要通过发展共邻分析（CNA）方法来识别和追踪晶体状态的变化。在B2立方结构的非晶化条件下，检测到转换原子周围的CNA簇的特征拓扑变化。通过对周期性试样进行简单剪切，发现先出现马氏体相，然后形成非晶相。非晶态相的成核与剪切面（如{100}、{110}或{111}）的选择密切相关。当剪切方向与固有滑移系取向不匹配时，非晶态相相对容易形核，并长期存在。认识到位错滑移（塑性变形）、MT和非晶化在原子尺度上密切相关。除{111}剪切面具有“预非晶态”结构外，相变方向主要为“B2（奥氏体）”→“B19’（马氏体）”→“非晶态”。发现“预非晶”一旦形成就会消失，前期的马氏体相或主相和后期的非晶相形成。
Ni-Ti alloys are typical shape-memory materials. It is suggested that the atomic-scale phase transformation between martensite and austenite crystalline phases is responsible for such function. However, it is reported that these alloys sometimes show also amorphization together with nanocrystal under severe deformation. The mechanism of amorphization should compete against martensitic (crystalline) transformation (MT). This study focuses on the atomistic relation between amorphization and MT in Ni-Ti alloys. Molecular dynamics simulation is performed by using our simplified potential built on the MEAM framework. The change in crystalline state is identified and traced mainly by developing common neighbor analysis (CNA) method. Under amorphization from B2 cubic structure, the characteristic topological change in CNA cluster around a transforming atom is detected. By applying simple shear in periodic specimen, it is found that the martensite phases appear first then amorphous phases are formed. The nucleation of amorphous phases is strongly dependent on choice of shear plane, e.g. {100}, {110} or {111} plane. When the shear direction does not match to the orientation of inherent slip system, amorphous phase is nucleated with relative ease and remains for a long time. It is recognized that dislocation slip (plastic deformation), MT and amorphization are closely related in atomic scale. The direction of phase transition is mostly "B2(austenite)"→"B19'(martensite)"→"amorphous", except for the case of {111} shear plane which possesses "pre-amorphous" structures. It is found that the "pre-amorphous" is once formed and then disappears, preceding martensite phase or principal and later formation of amorphous phase.