Experimental determination and thermodynamic assessment of phase equilibria in the como 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 D0 ₁₉-Co ₃Mo 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 μ 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 μ 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 L1 ₂ phase in Co-base γ/γ' superalloys.