Non-equilibrium thermodynamics were used to define the chemical potential for each species in the grain and inter-phase boundaries of a polycrystalline diffusion couple. The resultant equation for the chemical potential led to a characteristic decay length that described the spatial variation of the chemical potential. From the values of the latter, a time-dependent so-called free energy curve was calculated for the interface and was used to define the effective driving force for product phase nucleation as a function of time. By using this approach, product nucleation in thin film reactions was seen to be similar to precipitation from a bulk homogeneous solid solution. However, the early values of the effective driving force were found to be considerably lower than when the interfaces were in local equilibrium. This resulted in the possibility of heterogeneous nucleation, even of the first phase, and of a new approach to phase selection.

K.R.Coffey, K.Barmak: Acta Metallurgica et Materialia, 1994, 42[8], 2905-11