Heterogeneous grain initiation in solidification

Abstract

Grain refinement in casting of aluminium alloys can be quantitatively modelled by assuming that the barrier for grain initiation on an inoculant particle is that for free growth from the particle rather than for the initial formation of the solid. This leads to deterministic nucleation behaviour in which the number of grains is dependent on undercooling and not on time. Taking grain refinement of aluminium alloys as an example, the conditions for such athermal nucleation are analysed for a variety of nucleant particle shapes. The relevance for other cases, for example the action of ice-nucleating agents in living systems, is explored. In general, it is shown that for potent heterogeneous catalysis of solidification, the finite size of the nucleation sites renders classical nucleation theory inapplicable. The concept of nucleation in such a case is briefly discussed.