Phase-field modeling of ferrite morphology in austenite-to-ferrite transformation with considering anisotropic effects
Authors: Zhang Jun  , Chen Wen-Xiong  , Zheng Cheng-Wu  , Li Dian-Zhong  ,   School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China;,   Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Published: December 2020

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 <em>σ</em><sub><em>α</em>, <em>γ</em></sub>, <sub></sub>the grain boundary energy between the initial austenite grains, <em>σ</em><sub><em>γ</em>, <em>γ</em></sub>; 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 <em>σ</em><sub><em>γ</em>, <em>γ</em></sub>/<em>σ</em><sub><em>α</em>, <em>γ</em></sub>, and hence leading to a smaller equilibrium angle at the triple junction. The results also indicate that misorientation-dependent grain boundary energy and mobility play important roles in determining the ferrite growth behavior. The growth of ferrite with a low misorientation <em>α</em>/<em>γ</em> 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.