Abstract First principles calculation reveals that the temperature-dependent heat capacities of Ir and four Ir3X (X = Ti, Nb, Zr, and Hf) phases at a certain temperature are very close to each other due to their similar densities of states of phonons, and for each phase, the difference between CP and CV is also very small. In addition, it is found that for Ir/Ir3X superalloys, composition change should have much more contribution to temperature-dependent lattice misfit than thermal expansion, and that the coherence as well as lattice misfit should be divided into two groups, i.e., the coherent Ir/Ir3Ti and Ir/Ir3Nb interfaces with constrained lattice misfit, and the semi-coherent Ir/Ir3Zr and Ir/Ir3Hf interfaces with unconstrained lattice misfit. The calculated results are compared with available experimental results in the literature and the agreements between them are fairly good.