Solid-state phase transformations involving solute partitioning: modeling and measuring on the level of individual grains

A simplified grain growth model is presented for the transition from non-overlapping to overlapping diffusion fields of growing neighboring grains during partitioning solid-state transformations in polycrystalline materials. The model is based on unique observations on the austenite decomposition into ferrite and pearlite in medium-carbon steel with the three-dimensional X-ray diffraction microscope. The model explains three types of observed pro-eutectoid ferrite grain growth and austenite grain decomposition, and the indirectly observed carbon exchange between decomposing austenite grains. A direct comparison of the model and the experiment at the level of individual grains shows that the growth of ferrite grains is strongly related to the local carbon concentration and the local density of nuclei. Unusual observations of a non-stationary austenite grain size prior to the transformation, and oscillatory ferrite growth are reported.