Thermochemistry of the Ni-Hf system—Intermetallic phases

Abstract

The vaporization of alloys of the Ni-Hf system was investigated in the temperature range between 1200 and 1650 K by Knudsen effusion mass spectrometry. The different compositions of the 16 alloy samples investigated covered the complete homogeneity range of the Ni-Hf system. The partial pressure of Ni was determined over all samples. The thermodynamic activities of Ni and Hf in the alloys were evaluated from these pressures and by Gibbs-Duhem integration. In addition, Gibbs energies of formation, enthalpies of formation, and entropies of formation resulted for the nine intermetallic phases of the Ni-Hf system. Beside similar thermodynamic data for the evaporation reactions were studied. The data obtained are discussed and a method for distinguishing the congruent melting compounds from the peritectic ones by defining stability factors calculated from the Gibbs energies of formation is suggested.