Theory for the trapping of disorder and solute in intermetallic phases by rapid solidification

Abstract

A theory is developed to predict the long range order parameter, composition and temperature at the interface of a chemically ordered phase as a function of interface velocity and liquid composition during rapid crystal growth. It extends the solute trapping theory of Aziz to a solid phase consisting of two sublattices. The engulfment of atoms randomly on the two sublattices by the rapidly moving liquid-solid interface is balanced against the interdiffusion across the interface that attempts to restore local equilibrium. With increasing interface velocity the theory predicts a progression from the solidification of a phase with equilibrium long range order parameter and with equilibrium solute partitioning to the solidification of a disordered crystalline phase with the same composition as the liqiud. Predictions for solids with free energy functions in which the order disorder transition is first or second order show that the decrease of order parameter to zero with increasing interface velocity will be discontinuous or continuous respectively. Also solute trapping can occur at either a higher or a lower growth rate than disorder trapping depending on the free energy function.