Thermodynamic properties and diffusion thermodynamic factors in B2-NiAl

The vaporization of Ni-Al alloys has been investigated in the temperature range 1178 to 1574 K by Knudsen effusion mass spectrometry (KEMS). Thirteen different compositions have been examined in the composition range 38 to 57 at. pct Al. The partial pressures and thermodynamic activities of both Ni and Al have been evaluated both directly from the measured ion intensities for a component in both the alloy and the pure element, I-M(+)/I-M(+o), and also from the ion intensity ratios of the alloy components, I-Al(+)/I-Ni(+), by means of a Gibbs-Duhem integration. Reliable partial molar enthalpies and entropies for both components have been obtained by mass spectrometry for this system for the first time. Both properties are found, to be nearly temperature independent over the wide temperature range investigated. Two separate component diffusion thermodynamic factors have also been evaluated for the first time by taking into account the large vacancy concentrations in these alloys. The enthalpy and Gibbs energy of mixing of stoichiometric Ni0.5Al0.5 at 1400 K, evaluated using the Gibbs-Duhem ion intensity ratio (GD-IIR) method, are -78.4 +/- 1.2 and -49.0 kJ/mol, respectively, with Al(liquid) and completely paramagnetic Ni(fcc, cpm) as reference states.