Thermodynamic investigation of the A2/B2 region of the Fe-Al system by Knudsen effusion mass spectrometry

The vaporization of Fe-Al alloys has been investigated in the temperature range of 1140 to 1600 K by Knudsen effusion mass spectrometry. Eleven different compositions were examined in the composition range between 30 and 51 at. pct Al, mostly in the 132 region of the phase diagram. The partial pressures and thermodynamic activities of both Fe and Al were evaluated: 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-Fe(+), by means of a Gibbs-Duhem integration. Reliable partial and integral molar enthalpies and entropies of mixing have been obtained by mass spectrometry for this system for the first time. Nearly temperature-independent integral enthalpies and entropies of mixing over the wide temperature range investigated were found, with the mixing entropies being large and negative.