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 st udied by employing a previously developed finite-difference-based diff usional growth model. In particular, the effects of ledge density, led ge nucleation kinetics and the presence of multiple precipitates (in p arallel groups) on solute buildup in the matrix, morphological evoluti on, and lengthening kinetics of precipitate plates are analyzed. For s olute-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 plat e lengthening rates due to diffusional interaction with the solute fie ld at the plate tip. Two types of ledge nucleation kinetics were consi dered. One, in which the time interval between ledge nucleation events increases parabolically with time produces precipitate morphologies a nd plate lengthening kinetics similar to those observed experimentally for proeutectoid ferrite plates. The other, steady state ledge nuclea tion kinetics, yields growth behavior which, at high undercoolings, do es not adequately match experimental observations. For groups of close ly spaced precipitates arranged in a periodic array, small precipitate spacings (less than about 100 ledge heights) can lead to decreased pl ate lengthening kinetics at high supersaturations (OMEGA0 > 0.55).