Phase Field Study of the Diffusional Paths in Pearlite-Austenite Transformation

In the development of advanced steel, accurate and detailed knowledge about the kinetics of phase transformations and microstructure formation is critical. The critical issue in pearlite-austenite transformation is the consideration of diffusional paths of the alloy element. Simulation has been an available method to study the diffusion of alloy elements and the migration rate of the phase boundary in the complex morphological evolution of austenite growth. The isothermal pearlite-austenite transformations at 720 and 740 degrees C in Fe-0.6%C-2%Mn (mass fraction) alloy were studied by phase-field methods based on MICRESS. At different temperatures, the effects of diffusional paths on the austenite transformation were discussed. To achieve a semiquantitative verification of the simulated results, the migration rates of the austenite/pearlite boundary at 720 and 740 degrees C were estimated from the experimental kinetics curves by fitting the JMA equation. By measuring the Mn profile in austenite, the modes of the austenization at 720 and 740 degrees C can be verified as partitioned local equilibrium (PLE) and non-partitioned local equilibrium (NPLE) modes. The heterogeneous distribution of Mn in austenite at 740 degrees C can be observed with STEM-EDS. However, the homogenous distribution of Mn can be found near the pearlite/austenite boundary in austenite at 720 degrees C. The cases considering the gamma, alpha, and interface-diffusional paths were simulated by phase-field methods to compare with the migration rates of the austenite/pearlite boundary. Because carbon is an interstitial element in steel and has an interstitial diffusional mechanism, it can be speculated reasonably that the diffusion of C mainly proceeded in austenite and ferrite through the considerations of the atom-size of carbon and the experimental results. Phase-field methods were used to study Mn diffusion in the lamellar pearlite-austenite transformation. With the analysis of the experimental estimations, the interface-diffusional path was observed as the dominant path for the Mn diffusion. It is because the Mn atoms have greater diffusivity in interfaces than in gamma or alpha-diffusional paths. Furthermore, the diffusional activation energy is closely related to the diffusivity of Mn at the interface. Moreover, compared with the gamma-diffusional path, the diffusional flux of Mn in ferrite is much larger than that in austenite. Thus, it can be concluded that the contribution of the alpha-diffusional path to the migration rate of the pearlite/austenite boundary is larger than that of the gamma-diffusional path. As a result, considering the alpha-diffusional path in the thermodynamics analysis under NPLE mode makes more sense. However, ignoring the interface- and alpha-diffusional path, which is different from the traditional cognition in PLE mode, will result in a magnitude error for the thermodynamics analysis.

Automated summary

In the study of pearlite-austenite transformation, the investigation of diffusional paths and their effects on the transformation process is important. This research focused on the diffusional paths in Fe-0.6%C-2%Mn alloy at 720 and 740 degrees C and found that the interface diffusional path is dominant for Mn diffusion. The results also showed different partitioning modes of Mn distribution in austenite at different temperatures.