Nucleation barrier of fcc(γ) → hcp(ε) martensitic transformation in Fe-based alloys

摘要

Based on the dislocation theory and Olson's stacking fault model, a model describing the nucleation of an hcp(ε) martensite embryo at low-angle grain boundary is proposed with the influence of external stress field taken into account. The dependences of temperature (T), shear stress (τ) and dislocation density at grain boundary on the martensite nucleation in FeMnSi based alloy, as an example, are numerically simulated. It has been shown that there exist the subcritical and critical embryos during the course of ε-phase nucleation. The free energy difference between them is just the energy barrier of embryo growth. Depending on T and τ, the characteristic embryo sizes may vary in wide ranges and decrease with increasing σ and decreasing T. The energy condition of martensitic transformation at M_s and critical shear stress (τ_c) is discussed from the viewpoint of kinetics and thus the TEM observed result that stacking fault energy is not zero at M_s temperature is reasonably explained. Besides, it is predicted that the high dislocation density at grain boundary can promote the nucleation of fcc → hcp transformation in Fe-based alloys.