THERMODYNAMIC STUDY OF THE LIQUID AND SOLID ALLOYS OF THE NICKEL-RICH PART OF THE AL-NI PHASE-DIAGRAM INCLUDING THE ALNI3 PHASE

The vaporization of liquid and solid alloys of the Al - Ni system in the range χNi = 0.7 to 1 has been extensively investigated by the use of Knudsen effusion mass spectrometry in the temperature range between 1389 and 1734 K. 15 alloy samples of different compositions of the complete concentration range were studied in the course of 25 runs and their Al and Ni partial pressures determined. The vapour pressure measurements in the course of the individual runs were carried out over temperature ranges of up to about 300 K covering the solid and the liquid state for many of the alloys investigated. Chemical activities and chemical potentials for Al and Ni as well as Gibbs energies were evaluated at 1600 K for the solid and at 1728 K for the liquid state. Two independent methods were used for the determination of Ni activities of the solid alloys yielding consistent results. The large temperature range of the partial pressure measurement for the solid AlNi3 phase with χNi = 0.750 rendered possible the determination of a complete set of partial and integral thermodynamic functions (ΔG, ΔH, ΔS) for the formation of this phase. The values obtained are discussed. The partial and integral enthalpies of formation are compared with the enthalpies of dissociation of the gaseous species Ni2(g), Al2(g), and AlNi(g). The integral enthalpy of formation, obtained for the first time by vapour pressure measurements, agrees well with the results of a recent calorimetric study.