Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels
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I. INTRODUCTION
IN recent years, considerable effort has been made to better understand the austenite to ferrite ( : ) transformation in steels,[1–15] which is perhaps the most important transformation in terms of its commercial significance. Important detailed information about the transformation process has been discovered with novel experimental techniques such as synchrotron X-ray diffraction[1] and electron backscattered diffraction.[2] As a consequence of such progress, a few new models have been proposed to describe the growth of ferrite,[3,4] which is important in developing new simulations to describe transformation kinetics more precisely. Compared with experimental observation, progress in computer simulation of the : transformation in steels has been limited. For example, nucleation site saturation is assumed in most simulations,[5–13] which is not very realistic. Moreover, electron-backscatter diffraction (EBSD) observation has confirmed that grain coarsening behind transformation fronts is significant,[2] which is the major factor limiting grain refinement in thermomechanical processing. However, grain coarsening behind these transformation fronts has seldom been considered in simulations, most of which have employed the assumption that every nucleus becomes a grain. In this simulation, the authors abandon the site saturation assumption, and take into consideration grain coarsening behind the transformation fronts, as well as the possibility of nucleation off original austenite grain boundaries. The effects of these factors are studied carefully here. Compared with experimental observations, computer simulation can actually better demonstrate how such grain coarsening influences the process of microstructure evolution (which is difficult or even impossible to observe experimentally). CHENG-JIANG HUANG, Newman Scholar, and DAVID J. BROWNE, Senior Lecturer, are with the School of Electrical, Electronic and Mechanical Engineering, Engineering and Materials Science Centre, University College Dublin, Dublin 4, Ireland. Contact e-mail: [email protected] Manuscript submitted June 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
The phase-field method is chosen for this study for its excellent capability of simulating interface evolution of any complexity. Phase-field models have no difficulty in simultaneously simulating nucleation, grain growth, and coarsening. It has been known that there are two possible mechanisms governing grain coarsening: coarsening by grain boundary migration and coalescence by grain rotation. (According to Harris et al.,[15] grain rotation is more significant when the involved grains are small, which is exactly the case at the early stage of : transformation. Thus, it is necessary to take both coarsening mechanisms into consideration.) The phase-field method is capable of simulating both coarsening mechanisms, thanks to the recent development of this method by Kobayashi et al.[16,17] and Warren et al.[18] Another important advantage of the phase-field method is its s
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