Thermodynamic activity measurements in the B2 phases of the Fe–Al and Ni–Al systems

The vaporisation of Fe-Al and Ni-Al alloys has been investigated in the temperature range 1140-1600 K and 1178 to 1574 K, respectively, by Knudsen effusion mass spectrometry (KEMS). Eleven different Fe-Al and also eleven Ni-Al compositions have been investigated in the composition ranges 30-51 at.% Al and 38-53 at.% Al, respectively. The Fe-Al samples have been investigated mostly in the B2 region of the phase diagram. The partial pressures and thermodynamic activities were evaluated directly from the measured ion intensities formed from the equilibrium vapour over the alloy and the pure element. From the temperature dependence of the activities the partial and integral molar enthalpies and entropies of mixing have been obtained. These are the most accurate data obtained by mass spectrometry on Fe-Al and Ni-Al systems so far. 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.