THERMODYNAMIC STUDY OF INTERMETALLIC PHASES IN THE HF-AL SYSTEM

The vaporization thermodynamics of the aluminum-rich portion of the Hf-Al system have been investigated by Knudsen cell-mass spectrometry in the temperature range 1280-1680 K. The aluminum vapor pressures were measured in the two-phase regions in the composition range 50%-75% at. Al, and the enthalpy changes of the decomposition reactions were determined by second-law and third-law methods. Hence, the enthalpies of formation of the intermetallic compounds were derived: HfAl3, - 44.7 kJ g(-1) atom(-1); Hf2Al3, - 40.8 kJ g(-1) atom(-1). The results of the vaporization experiments which yielded a residue not assignable to a definite phase were tentatively submitted to thermodynamic analysis, by introducing the formation of HfAl or Hf5Al4, and their enthalpies of formation were derived.