Heterogeneous nucleation theory revisited: Effect of triple junction line energy

• New experimental information on grain boundary triple junction line energies.
• New data has been applied to heterogeneous nucleation in a nickel based alloy.
• Energy needed is dominated by the GB triple junction line energy for small nuclei.
• Role of precipitate-matrix interfacial energy for small nuclei is questioned.

This paper will re-consider the basic aspects of solid state classical heterogeneous nucleation theory. In particular it will show how recent experimental work that has been able to estimate grain boundary triple junction line energy can be used to supplement the understanding of the energy balance operating during nucleation. This implies that the energy of the newly created interface needs to be recalculated, assuming that the additional line energy of the line surrounding the precipitate in the plane of the boundary is taken into account. Examples will be given of the application of this revised approach to GB precipitate nucleation of cap-shaped particles of chromium carbide in nickel based alloys. It will be shown that original assumed values of the precipitate-matrix interfacial energy for this transformation have to be revised downward for critical nucleus sizes of less than about 10 nm.