AbstractThe 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 energyγand volume strain energy∆Gɛaccompanying 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∆Gchemand 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∆Gchemabove which a fully coherent nucleus/matrix interface is obtained. This value is 1 GJ m−3for Co rich nuclei in Cu–Co alloys and 18 GJ m−3for TiC in austenite. These predictions are consistent with experimental observations.MST/813