Non-classical nucleation theory of ordered intermetallic precipitates - Application to the Al-Li alloy

The continuum non-classical nucleation theory of Cahn and Hilliard for isostructural decomposition has been extended to the case in which the precipitate and matrix not only have different compositions, but also have different structures. The particular example of precipitation of ordered intermetallics from a disordered matrix is considered. It is found that a critical nucleus consists of fluctuations of both composition and long-range order parameter profiles. It is shown that only when the composition of the initial disordered matrix is near the phase boundary of the disordered phase, are the composition and order parameter values inside the critical nucleus close to those of the equilibrium ordered phase, and that the critical profiles become increasingly diffuse as the composition of the disordered matrix approaches the ordering instability line. Based on the non-classical nucleation theory, the size of critical fluctuations, the critical free energy change and the nucleation rate are estimated for the specific case of the precipitation of metastable δ′ ordered phase from a disordered matrix in the Al Li alloy.