FFT-Based Modelling of Transformation Plasticity in Polycrystalline Materials during Diffusive Phase Transformation

Abstract

During phase transformation of steels, when stress is applied, significant large strain can be observed even though the applied stress is smaller than their yield stresses. This phenomenon is called Transformation Plasticity or TRansformation Induced Plasticity (TRIP). Transformation plasticity is known to play an important role during steel producing processes. Although its importance, the phenomenon is not fully understood because of complicated coupled effect of metallurgical, thermal and mechanical behaviour during phase transformation. There are several explanations which account for the phenomenon. Among those, Greenwood-Johnson effect appears to be appropriate explanation especially for diffusive phase transformation. According to Greenwood-Johnson effect, volume change during phase transformation causes locally heterogeneous stress variation and it results in the macroscopic strain together with small applied stress. Along with the notion, Leblond et. al. developed an analytical model which describes well the phenomenon of transformation plasticity. On the other hand, the authors have developed a micromechanical model of polycrystalline materials using discrete FFT (Fast Fourier Transform) method with diffusive phase transformation. In this study, volume expansion along with phase transformation (Greenwood-Johnson effect) is taken into account in the model in order to evaluate the transformation plasticity and micromechanical behaviour during phase transformation. The results by FFT confirm linear relation between applied stress and transformation plastic strain, only if the applied stress does not exceed a half the value of yield stress of the parent phase. In contrast, if applied stress is relatively large (more than half of yield stress of weaker phase), the linear relation is never satisfied. The numerical results are compared with those of experimental and of Leblond model. Furthermore, pre-deformation (deformation just before phase transformation) effect on transformation plasticity is investigated. As a first step, uniaxial tensile followed by phase transformation simulation is carried out. Back stress develops in the course of tensile process and thus the material will be macroscopically anisotropic. It is found that the pre-deformation causes anisotropic dilatation during phase transformation. The mechanism of this anisotropic dilatation will be discussed in the micromechanical point of view.