Atomic vacancies in intermetallic compounds were characterized, by means of positron lifetime measurements, after electron irradiation. In the case of TiAl, Ti3Al and Ni3Al, no structural vacancies were observed at room temperature. This confirmed that, in these compounds, slight deviations from stoichiometry were compensated by antisite atoms. In the case of Al-poor B2 alloys (FeAl, NiAl), vacancies persisted due to the high thermal equilibrium vacancy concentrations and their low diffusivities. The kinetics of vacancy elimination in FeAl and NiAl were considered. An appreciable temperature dependence of the positron lifetime in vacancies was detected in close-packed intermetallics. This was attributed to increased atomic relaxation or partial positron de-trapping at high temperatures. On the other hand, the temperature dependence of the positron lifetime in vacancies was small in the case of the open-structured B2 aluminides. The lifetimes of free delocalized positrons in transition-metal aluminides and in NiZr and NiTi could be related to those of the pure components; taking account of the densities of valence electrons. Annealing studies of B2-FeAl, after electron irradiation, yielded time constants (for the disappearance of vacancies) which were identical to those that were deduced for the equilibration of thermal vacancies. In the case of electron-irradiated Ti aluminides, annealing processes at 250 and 450K were observed which were tentatively attributed to the long-range migration of vacancies.

R.Würschum, K.Badura-Gergen, E.A.Kümmerle, C.Grupp, H.E.Schaefer: Physical Review B, 1996, 54[2], 849-56