Vacancies in FeAl and NiAl

The B2 compound NiAl extends substantially into Al-rich compositions where it forms constitutional vacancies abundantly, but the corresponding FeAl does not. However, FeAl forms thermal vacancies abundantly. This difference is traced to the smaller heat of formation of FeAl, so that the enthalpy of formation of constitutional vacancies, from the stoichiometric compound in the presence of excess Al, is favourable for NiAl but slightly unfavourable for FeAl. At high temperatures, the effect of mixing entropy enables FeAl to overcome this small enthalpy disadvantage, despite the competition of the next phase (FeAl2) for the excess Al. A corresponding calculation of the NiAl and Ni2Al3 equilibrium enables the Al-rich phase limit of NiAl to be estimated. The energy of formation of triple defects is calculated to be appreciably lower in FeAl than in NiAl, so that these become abundant at high temperatures in stoichiometric FeAl. (C) 1997 Elsevier Science Limited.