HETEROGENEOUS NUCLEATION IN SOLIDIFYING ALLOYS

A variety of solution models are used to determine driving forces for nucleation in solidifying alloys. The resulting data are then used to analyse heterogeneous nucleation experiments in which liquid alloy droplets solidify within a surrounding solid matrix. By comparing these experiments with homogeneous nucleation experiments on pure metals, the analysis makes it possible to determine solid-liquid-matrix contact angles and therefore quantify the catalysing effect of the matrix on the nucleation process. This approach is shown to be more useful than a simple comparison of undercooling data. In almost all cases, the liquid alloy droplets are found to solidify by heterogeneous nucleation with contact angles less than 180°. A careful analysis of the results supports previous suggestions that Turnbull's classic experiments on solidification of pure metals did not show true homogeneous nucleation. © 1979.