The application of radiation diffuse scattering to the calculation of phase diagrams of F.C.C. substitutional alloys

By using quantitative information about the radiation diffuse-scattering intensity of the disordered f.c.c. substitutional alloy Me′1-cMe″c (c—concentration) the Fourier component, w∼k, of mixing energies of Me′ and Me″ atoms may be estimated. We have to use the measurement data of the diffuse-scattering intensities at the corresponding reciprocal-space points k of the disordered phase and then determine the parameter w∼(k). The statistical thermodynamics of the non-ideal solid solution is determined by these energy parameters {w∼(k)}. Therefore, one can obtain the configuration free energy of an alloy, F=U-TS (U—internal energy, S—entropy), and then determine its fundamental thermodynamic characteristics, including not only its phase diagram, but also the concentration-dependent order–disorder transformation temperature, temperature and concentration long-range order parameter dependences, chemical activity, heat capacity etc. Some thermodynamic properties are calculated within the framework of the statistical-thermodynamic approach for f.c.c.-Ni–Fe alloy. The diffuse-scattering intensity values are taken from data in the literature.