First principles calculation reveals that ideal mechanical strengths of L1 2 Ir 3Nb are higher than those of L1 2 Ir 3Hf, and that Hf and Nb atoms have different effects on the brittle behavior of Ir, i.e., the Ir 3Hf bulk seems less brittle than f c c Ir, while Ir 3Nb becomes more brittle. Densities of states suggest that the chemical bonding formed in Ir 3Nb should be stronger and more directional than that in Ir 3Hf. Calculation also shows that compared with pure Ir bulk, the bond strengths of (1 0 0)-I and (1 1 0) Ir/Ir 3X (X = Hf, Nb) interfaces have increased, while the bond strengths of (1 0 0)-II and (1 1 1) Ir/Ir 3X interfaces have decreased, due to the formation of the interface. In addition, it is found out that the (1 0 0)-I interfaces are energetically favorable with negative interface energies, which is good agreement with similar experimental observations in the literature.