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The effects of local nonequilibrium solute diffusion on a solute concentration field, solute partitioning, interface temperature, and absolute stability limit have been considered. The model incorporates two diffusive speeds, V Db , the bulk-liquid diffusive speed, and V Di , the interface diffusive speed, as the most important parameters governing the solute concentration in the liquid phase and solute partitioning. The analysis of the model predicts a transition from diffusion-controlled solidification to purely thermally controlled regimes, which occurs abruptly when the interface velocity V equals the bulk liquid diffusive speed V Db . The abrupt change in the solidification mechanism is described by the velocity-dependent effective diffusion coefficient D*ϭD(1ϪV 2 /V Db 2 ) and the generalized partition coefficient K*. If VϾV Db , then D*ϭ0 and K*ϭ1. This implies an undistributed diffusion field in the liquid ͑diffusionless solidification͒ and complete solute trapping at VϾV Db . ͓S1063-651X͑97͒08504-8͔

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