The kinetics of grain-boundary nucleated phase transformations: Simulations and modelling

The kinetics of nucleation-and-growth phase transformations for which nucleation occurs on the parent-phase grain boundaries was studied using mesoscopic microstructure simulations. The influences of the parent microstructure (notably grain size and grain-size distribution) and the parent -> product transformation mechanism were determined and discussed. Subsequently, several mesoscopic kinetic models were investigated with respect to their ability to describe the simulated transformation kinetics, namely the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, (i.e., assuming random nucleation), previously proposed empirical extensions of the JMAK model in the framework of the modular kinetic model approach, the Cahn model (assuming nucleation on randomly distributed planes) and a new empirical extension of the Cahn model proposed here. None of the models exactly predicts the real phase transformation kinetics. The classical JMAK model is unable to describe the simulated grain-boundary nucleated transformation kinetics. The Cahn model leads to erroneous results on fitting to simulated transformation curves. It is concluded that the modified Cahn model proposed here is best suited to infer correct values of kinetic material parameters from experimentally obtained data on the kinetics of grain-boundary nucleated phase transformations. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.