RAPID SOLIDIFICATION UNDER LOCAL NONEQUILIBRIUM CONDITIONS

The effects of local nonequilibrium solute diffusion on a solute conce ntration field, solute partitioning, interface temperature, and absolu te stability limit have been considered. The model incorporates two di ffusive speeds, V-Db, the bulk-liquid diffusive speed, and V-Di, the i nterface diffusive speed, as the most important parameters governing t he solute concentration in the liquid phase and solute partitioning. T he analysis of the model predicts a transition from diffusion-controll ed solidification to purely thermally controlled regimes, which occurs abruptly when the interface velocity V equals the bulk liquid diffusi ve speed V-Db. The abrupt change in the solidification mechanism is de scribed by the velocity-dependent effective diffusion coefficient D = D(1 - V-2/V-Db(2)) and the generalized partition coefficient K. If V > V-Db, then D = 0 and K* = 1. This implies an undistributed diffusi on field in the liquid (diffusionless solidification) and complete sol ute trapping at V > V-Db.