NONEQUILIBRIUM PARTITIONING DURING RAPID SOLIDIFICATION OF SI-AS ALLOYS

The velocity dependence of the partition coefficient was measured for rapid solidification of polycrystalline Si-4.5 at% As and Si-9 at% As alloys induced by pulsed laser melting. The results constitute the fir st test of partitioning models both for the high velocity regime and f or non-dilute alloys. The continuous growth model (CGM) of Aziz and Ka plan fits the data well, but with an unusually low diffusive speed of 0.46 m/s. The data show negligible dependence of partitioning on conce ntration, also consistent with the CGM. The predictions of the Hillert -Sundman model are inconsistent with partitioning results. Using the a periodic stepwise growth model (ASGM) of Goldman and Aziz, an average over crystallographic orientations with parameters from independent si ngle-crystal experiments is shown to be reasonably consistent with the se polycrystalline partitioning results. The results, combined with ot hers, indicate that the CGM without solute drag and its extension to l ateral ledge motion, the ASGM, are the only models that fit the data f or both solute partioning and kinetic undercooling interface response functions. No current solute drag models can match both partitioning a nd undercooling measurements.