Athermal heterogeneous nucleation of solidification

For heterogeneous nucleation of solidification, the conditions for initiation of solid grains on nucleant areas of defined size are analysed. Calculations of the work of formation show that the solid formed on a nucleant area has both metastable- and unstable-equilibrium configurations, which permit analysis of the competition between thermal nucleation (a stochastic process) and athermal nucleation, a deterministic process in which the number of nucleation events depends on the undercooling, but not on time nor on the mechanism (adsorption, wetting or nucleation) of the initial formation of a thin solid layer on the nucleant area. It is concluded that for potent nucleants, initiating grains at small undercooling, athermal nucleation dominates. Thermal nucleation in advance of athermal is negligible, but athermal nucleation can be delayed when solute restricts solid growth.