王雲志：馬氏體相變的相場模擬走向非線性

海歸學者發起的公益學術平台

分享信息，整合資源

交流學術，偶爾風月

要想了解材料結構演化過程的細節，就需要為其晶體結構在此過程的複雜變化建模。雖然這可用傳統的相場微彈性理論來實現，但之前人們做原子模擬時就發現這個方法並不完善：現有模型沒有納入更加複雜的非線性相變路徑，因而可能錯失許多重要的物理現象。來自俄亥俄州立大學的王雲志教授及其同事開發了一種模擬固體晶體結構複雜變化的新的相場方法，能更精確地描述馬氏體轉變期間的晶體結構重排。他們擴展了相場微彈性理論，納入了各種複雜相變路徑，並發現在非線性耦合情況下，馬氏體相的臨界晶核構型和激活能均與相場微彈性理論所描述的有很大差異。這一模型可能改變人們之前對金屬和陶瓷結構轉變過程的理解，論文近期發表於npj Computational Materials 3:19 (2017)；doi:10.1038/s41524-017-0022-2，英文標題與摘要如下，點擊閱讀原文可以下載論文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.