Experimental Determination and Thermodynamic Assessment of Phase Equilibria in the Co–Mo System

Phase equilibria of the solid phases including the magnetic and martensitic transformation temperatures in the Co­Mo system were investigated using two-phase alloys, the diffusion couple technique, differential scanning calorimetry, and vibrating sample magnetometry. Furthermore, ab initio calculations of D019-Co3Mo and several fcc-base ordered structures, including metastable compounds, were carried out to estimate the formation energy. Based on these results, a thermodynamic assessment using the CALPHAD method was performed. A foursublattice model was used for the fcc-base phase to describe the order­disorder phase transformation. For the ® phase, both a three and a foursublattice model were applied. The set of thermodynamic values describing the Gibbs energy of the Co­Mo system reproduces the experimental phase diagram well. The four-sublattice model for the ® phase reproduces the site fractions as well as the phase boundaries better than the threesublattice model. The calculated metastable fcc-base phase diagram considering chemical and magnetic ordering is also reasonable. This is important for estimating the phase stability of the L12 phase in Co-base £/£A superalloys. [doi:10.2320/matertrans.M2012149]