Systematic vaporisation studies on the Ni –Al system with 0.32 ≤ x(Ni) ≤ 0.50 were carried out in the temperature range 1077 – 1592 K by Knudsen effusion mass spectrometry. While Al(g) was the vapour species observed over all the samples, Ni(g) was also detected over samples with the compositions x(Ni) = 0.50 and 0.49. From the p(Al) –T relations, activities of Al, and subsequently activities of Ni (by Gibbs–Duhem integration) were obtained. Moreover, partial and integral molar enthalpies and Gibbs energies of formation were evaluated. The variation of activities and other thermodynamic properties with composition are discussed. The enthalpy and Gibbs energy of formation of Ni_0.5Al_0.5(s) at 298.15 K were obtained as Δ_fH^o_m = – 62.1 ± 4.2 kJ mol^–1 and Δ_fG^o_m = –57.7 ± 1.2 kJ mol^–1, respectively. This is the first time that a systematic mass spectrometric investigation has been carried out on the aluminium rich region of Ni –Al system where the vacancy concentration in the NiAl phase can be as high as 10 %.

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