Thermodynamic modeling of fcc order/disorder transformations in the Co–Pt system

The present work reports on a thermodynamic modeling of the Co–Pt system with ordered fcc phases of L10 and L12 structures by means of the CALPHAD method. The liquid, hcp and fcc phases have been modeled as substitutional solutions where the interaction parameters are composition dependent in the form of the Redlich–Kister polynomial. The disordered and ordered fcc phases have been modeled in terms of the compound energy formalism with a single Gibbs energy function. The obtained phase equilibria and activities of Co and Pt agree well with the available experimental data. First-principles calculations are performed to obtain the enthalpies of formation for the ordered fcc phases at 0 K. These calculated enthalpies of formations for the ordered phases are less negative than the enthalpies of the disordered state at low temperatures determined from the CALPHAD modeling. The Fe–Pt and Ni–Pt systems exhibit the same feature as that in the Co–Pt system, which is discussed in terms of the total magnetic moment of ordered fcc phases.