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 Co-Mo system were investigated using two-phase alloys, the diffusion couple technique, differential scanning calorimetry, and vibrating sample magnetometry. Furthermore, ab initio calculations of D0(19)-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 four-sublattice model was used for the fcc-base phase to describe the order-disorder phase transformation. For the mu phase, both a three and a four-sublattice model were applied. The set of thermodynamic values describing the Gibbs energy of the Co-Mo system reproduces the experimental phase diagram well. The four-sublattice model for the mu phase reproduces the site fractions as well as the phase boundaries better than the three-sublattice 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 L1(2) phase in Co-base gamma/gamma' superalloys. [doi:10.2320/matertrans.M2012149]