Heterogeneous nucleation on potent spherical substrates during solidification

For a spherical-cap nucleus to become a transformation nucleus, the linear dimension (d) of the flat substrate must exceed the critical nucleus size (2r*). This Turnbull criterion (d >= 2r*) defines a minimum undercooling for grain formation on, and effective inoculation with, flat nucleating substrates. However, for nucleation on potent substrates the spherical-cap model is no longer tenable. The free growth model has in general considered the growth of a two-dimensional nucleus on a potent flat substrate. Inspired by the particle-core structures observed in magnesium alloys after inoculation with nearly spherical zirconium particles, a model has been proposed, on the basis of an adsorption and surface diffusion mechanism, for heterogeneous nucleation and grain formation on potent spherical substrates of d >= 2r*. The critical undercooling required is found to be approximately the same as that defined by Turnbull's patch nucleation theory. The model shows excellent agreement with experiments compared from different perspectives. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.