Characterisation of critical nucleus/matrix interface: application to Cu–Co alloys and microalloyed austenite

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Abstract
The semicoherent interface is characterised between the matrix and a critical nucleus which has the same crystal structure and orientation, but a different lattice spacing. A coherency loss parameter C is introduced and is related to the interfacial energy γ and volume strain energy ∆Gɛ accompanying the formation of a second phase particle. It is found that the C value for a critical nucleus C* depends on both the driving force for nucleation ∆Gchem and the lattice mismatch between the precipitate and the matrix. For a given alloy system, C* is a sole function of ∆Gchem. The method is employed to calculate the C* values pertaining to the interface between an fcc Co nucleus and Cu in Cu–Co alloys and between a TiC nucleus and austenite. It is indicated by the results that there is a transition value of ∆Gchem above which a fully coherent nucleus/matrix interface is obtained. This value is 1 GJ m−3 for Co rich nuclei in Cu–Co alloys and 18 GJ m−3 for TiC in austenite. These predictions are consistent with experimental observations. MST/813