THE EVOLUTION AND GROWTH-KINETICS OF PRECIPITATE PLATES GROWING BY THE LEDGE MECHANISM

The evolution and growth kinetics of precipitate plates growing by the ledge mechanism during solid-solid phase transformations have been studied by employing a previously developed finite-difference-based diffusional growth model. In particular, the effects of ledge density, ledge nucleation kinetics and the presence of multiple precipitates (in parallel groups) on solute buildup in the matrix, morphological evolution, and lengthening kinetics of precipitate plates are analyzed. For solute-poor precipitates, higher ledge densities along the broad faces of precipitate plates produce an increased solute buildup at the board faces which causes ledge coalescence and can result in decreased plate lengthening rates due to diffusional interaction with the solute field at the plate tip. Two types of ledge nucleation kinetics were considered. One, in which the time interval between ledge nucleation events increases parabolically with time produces precipitate morphologies and plate lengthening kinetics similar to those observed experimentally for proeutectoid ferrite plates. The other, steady state ledge nucleation kinetics, yields growth behavior which, at high undercoolings, does not adequately match experimental observations. For groups of closely spaced precipitates arranged in a periodic array, small precipitate spacings (less than about 100 ledge heights) can lead to decreased plate lengthening kinetics at high supersaturations (OMEGA0 > 0.55).