Thermodynamic analysis of phase separation in the Al-Cu binary fcc phase

A thermodynamic analysis of the Al-Cu binary system has been performed to consider the metastable phase equilibria of the face-centered cubic (fcc) phase. The total energies of the ordered structures were obtained using first-principles calculations based on the fcc lattice. The cluster expansion method was applied to the results, and the free energies were calculated for the solid solution at finite temperatures. The results were analyzed together with some experimental data, and the equilibrium phase diagram of the Al-Cu binary system was calculated based on the CALPHAD (Calculation of Phase Diagrams) method. The results suggested a metastable two-phase separation of the fcc phase in the Al-rich region. The coherent spinodal line was calculated according to Cahn's treatment and revealed that the critical temperature decreased by about 100 K because of the coherency of the separated phases in the fcc matrix. In consideration of both Cahn's treatment and the effect of vacancies, it was suggested that the initial formation step of the Guinier-Preston zone might be relevant to the metastable coherent spinodal decomposition in the fcc solid solution.