γ-α相变中不同晶界特征下铁素体生长形貌的相场模拟

Phase-field modeling of ferrite morphology in austenite-to-ferrite transformation with considering anisotropic effects

利用多相场模型模拟了奥氏体(γ)-铁素体(α)相变过程中不同晶界特征下铁素体晶粒的形貌与生长动力学.模型中通过能量梯度系数和耦合项系数的协同变化定量表达晶界能与晶界迁移率的各向异性,同时固定相场界面宽度来保证计算精度.模拟结果显示:随着原奥氏体晶界能与铁素体-奥氏体晶界能比值σ_(γ,γ)/σ_(α,γ)的增加,三叉相界面处的平衡角β减小,铁素体晶粒沿原奥氏体晶界与垂直于奥氏体晶界方向的生长速率差变大.铁素体与奥氏体晶粒间的晶粒取向越接近,铁素体生长越缓慢.模拟结果可描述铁素体晶粒生长形貌的多样性,与实验结果符合.

The morphology of proeutectoid ferrite in steels has attracted much attention in view of its close correlation with the fundamentals about the phase transformation theory as well as its potential practice relating to the final microstructure and properties of the steel product. With the recent development of mesoscale microstructure-based transformation models, the approach to integrated microstructural simulation is ideally suited to provide a more in-depth insight into the mechanism and morphology complexity for this problem. Among the various mesoscopic models, the phase-field method can readily be used to simulate the complex morphological phenomena during the austenite-to-ferrite transformation in steels in view of its convenience to include the material properties, especially the grain boundary properties, in a phenomenological way, and thus to model the microstructural process in an anisotropic system. In this study, a modified multi-phase-field（MPF） model that takes into account various anisotropic interfacial conditions is developed to simulate the growth morphology of ferrite during the austenite-to-ferrite transformation in a Fe-C-Mn alloy. In this model, a quantitative relation between the MPF model parameters and the physical anisotropic interfacial properties, including the grain-boundary energy and the mobility, is carefully considered, which allows the identical width of the diffuse interface regarding arbitrary interfacial anisotropies in the MPF simulations. In this way, both the accuracy and the numerical stability of the model can be ensured. Using this model, the effects of the grain boundary anisotropy on the ferrite growth are studied. The simulation results indicate that apart from the interfacial energy of σ_（α,γ）, the grain boundary energy between the initial austenite grains, σ_（γ,γ）; does also significantly influence the growing morphology of ferrite. The ferrite growth along the initial austenite grain boundaries is facilitated when increasing the ratio of σ_（γ,γ）/σ_（α,γ）,and hence leading to a smaller equilibrium angle at the triple junction. The results also indicate that misorientationdependent grain boundary energy and mobility play important roles in determining the ferrite growth behavior. The growth of ferrite with a low misorientation α/γ interface is greatly inhibited. The ferrites nucleated at the triple junctions of the initial austenite grains present different growth scenarios while assigning different orientation relationships. Finally,the simulated ferrite morphologies in a polycrystalline structure are compared with the optical micrograph and are found that they are in good consistence with each other. This MPF model can replicate the morphology diversity of the ferrite grains in the austenite-to-ferrite transformation.