相场法模拟Fe-C合金中奥氏体-铁素体等温相变过程

MODELING OF ISOTHERMAL AUSTENITE TO FERRITE TRANSFORMATION IN A Fe-C ALLOY BY PHASE-FIELD METHOD

运用相场法研究了Fe-C合金在临界区等温过程中发生的奥氏体-铁素体相变过程.通过分析铁素体生长过程中C的扩散行为,发现奥氏体-铁素体相变表现为混合控制生长的特征,奥氏体/铁素体相界面处于非平衡状态.进一步研究了不同等温温度(1010,1048和1087 K)下奥氏体-铁素体相变的微观组织和C浓度场的演化情况.结果表明,随着等温温度的降低,铁素体形核率增加,铁素体相变平衡体积分数增加,但奥氏体内部C浓度分布的不均匀程度加剧,1010 K等温时的微观组织呈现为不规则细小铁素体晶粒围绕分散分布的残余奥氏体的两相结构.随着等温温度的降低,奥氏体-铁素体相变过程表现出由扩散控制生长模式向界面控制生长模式转化的趋势.

Austenite-to-ferrite transformation in modem steels is a key metallurgical phenomenon as it can be exploited to produce microstructures that are closely associated with significant improvement of their properties. Both experimental and theoretical studies of this transformation have received much attention. In particular, in recent years, considerable efforts have been directed to the development of numerical models for adequate quantitative descriptions of the nucleation and growth of ferrite grains as well as the overall transformation kinetics. In this work, a modified multi-phase field model has been developed to simulate the isothermal γ-α transformation in a Fe- C alloy. This model takes both the effect of a finite interface mobility and a finite diffusivity into account, which hence enables a clear description of the mixed-mode nature of the transformation. In contrast to the diffusioncontrolled phase transformation model, the carbon concentration in front of the moving γ-α interface is found to be non-equilibrium under this circumstance. In order to study the microstructural behavior and kinetics over the entire temperature range of the phase transformation, three different isothermal transformation processes have been simulated. The simulation results indicate that the nucleation density of ferrite increases with decreasing the temperature, which thus leads to a larger volume fraction of ferrite. However, the heterogeneous distribution of carbon in the untransformed austenite is intensified. The final microstructural product of the transformation at low temperature of 1010 K consists of fine residual austenite islands surrounded by fine polygonal ferrite. The simulation results also indicate that the transformation mode from austenite to ferrite varies from essentially diffusioncontrolled at high temperature towards interface-controlled at low temperature.