王云志：马氏体相变的相场模拟走向非线性
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要想了解材料结构演化过程的细节，就需要为其晶体结构在此过程的复杂变化建模。虽然这可用传统的相场微弹性理论来实现，但之前人们做原子模拟时就发现这个方法并不完善：现有模型没有纳入更加复杂的非线性相变路径，因而可能错失许多重要的物理现象。来自俄亥俄州立大学的王云志教授及其同事开发了一种模拟固体晶体结构复杂变化的新的相场方法，能更精确地描述马氏体转变期间的晶体结构重排。他们扩展了相场微弹性理论，纳入了各种复杂相变路径，并发现在非线性耦合情况下，马氏体相的临界晶核构型和激活能均与相场微弹性理论所描述的有很大差异。这一模型可能改变人们之前对金属和陶瓷结构转变过程的理解，论文近期发表于npj Computational Materials 3:19 (2017)；doi:10.1038/s41524-017-0022-2，英文标题与摘要如下，点击阅读原文可以下载论文PDF。
Effect of nonlinear and noncollinear transformation strain pathways in phase-field modeling of nucleation and growth during martensite transformation
Pengyang Zhao, Chen Shen, Ju Li & Yunzhi Wang
The phase-field microelasticity theory has exhibited great capacities in studying elasticity and its effects on microstructure evolution due to various structural and chemical non-uniformities (impurities and defects) in solids. However, the usually adopted linear and/or collinear coupling between eigen transformation strain tensors and order parameters in phase-field microelasticity have excluded many nonlinear transformation pathways that have been revealed in many atomistic calculations. Here we extend phase-field microelasticity by adopting general nonlinear and noncollinear eigen transformation strain paths, which allows for the incorporation of complex transformation pathways and provides a multiscale modeling scheme linking atomistic mechanisms with overall kinetics to better describe solid-state phase transformations. Our case study on a generic cubic to tetragonal martensitic transformation shows that nonlinear transformation pathways can significantly alter the nucleation and growth rates, as well as the configuration and activation energy of the critical nuclei. It is also found that for a pure-shear martensitic transformation, depending on the actual transformation pathway, the nuclei and austenite/martensite interfaces can have nonzero far-field hydrostatic stress and may thus interact with other crystalline defects such as point defects and/or background tension/compression field in a more profound way than what is expected from a linear transformation pathway. Further significance is discussed on the implication of vacancy clustering at austenite/martensite interfaces and segregation at coherent precipitate/matrix interfaces.
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