First-principles study on the thermodynamic properties of Nb, Cr₂Nb and Nb₅Si₃ alloys

Nb-Si-Cr alloys have shown great potential as the aviation materials because of their good mechanical properties and high temperature oxidation resistance. The thermodynamic properties of Nb, Cr₂Nb, α-Nb₅Si₃, β-Nb₅Si₃, as the main constituent phases of Nb-Si-Cr alloys, were calculated by first-principles methods combined with quasi-harmonic Debye model. The calculated results are in agreement with available experimental data. Compared the thermo-dynamic properties of different phases, it is found that the thermal expansions of these phases are similar and have the same trend with change of temperature. Thermal expansion coefficient of these phases are dependent with increases with temperature and decreases with pressure, respectively, while the bulk moduli decreases with increasing temperature. At last, the micro stress created by the differences in coefficients of thermal expansion between Nb and Cr₂Nb upon cooling was evaluated. The result showed that the stress is too small to cause cracking at the interface between Nb and Cr₂Nb.