SOLUTE TRAPPING AT A RAPIDLY MOVING SOLID/LIQUID INTERFACE FOR A LENNARD-JONES ALLOY

Non-equilibrium Molecular Dynamics simulation methods have been used to study the trapping of "impurities" in an A(85)B(15), Lennard-Jones alloy where the B atoms are 10% bigger in diameter than A. The observation of surface melting in this system is used to calculate an equilibrium interfacial segregation coefficient. Simulations of rapid melting and resolidification were performed for the (100) and (1 11) orientations at two different substrate temperatures (0.65 T-m and 0.97 T-m) for each orientation. Solute impurity atoms are shown to have been trapped in the solid at greater concentrations than are obtained under equilibrium conditions. An apparent non-equilibrium segregation coefficient is calculated from these results. We observe that segregation of impurities occurs when the interface velocity is below the calculated diffusive speed of impurities away from the interface. Above this velocity impurities are unable to segregate into the liquid and are trapped in the rapidly-growing solid. Partitionless solidification is observed when the regrowth velocity greatly exceeds the diffusive velocity. This result is in agreement with the theoretical predictions of Aziz. The orientation of the crystal is found to have an important effect on the regrowth velocity. As expected, growth on the (111) face is less than half that on the (100) for identical substrate temperatures, but this does not enhance segregation over trapping due to the quite different solidification mechanism involved. Growth at the (111) face is found to proceed by the nucleation of a (1 11) surface ledge and its subsequent rapid passage. The (111) face is also found to involve a greater undercooling than (100) in agreement with the prediction of White et al.