CHARACTERIZATION OF RADIATION-INDUCED LATTICE VACANCIES IN INTERMETALLIC COMPOUNDS BY MEANS OF POSITRON-LIFETIME STUDIES

In the present paper a characterization of atomic vacancies in interme tallic compounds is given by means of positron-lifetime measurements a fter electron irradiation and comparison with the states after prepara tion, after long-time annealing, or in high-temperature equilibrium. I n TiAl, Ti3Al, and Ni,AI no structural vacancies (detection limit C-V= 10(-6)) are observed at ambient temperature. This confirms that in the se compounds slight deviations from stoichiometry are compensated by a ntisite atoms. In the Al-poor B2 alloys FeAl and NiAl, on the other ha nd, remnant vacancies exist due to the high thermal equilibrium vacanc y concentrations and their slow diffusivities. The kinetics of vacancy elimination in FeAl and NiAl is discussed. A substantial temperature dependence of the positron lifetime in vacancies is detected in close- packed intermetallics which is attributed to an increased atomic relax ation or partial positron detrapping at high temperatures. In contrast to that, the temperature dependence of the positron lifetime in vacan cies is small in the open-structured B2 aluminides. The lifetimes tau( f) of free delocalized positrons in transition-metal aluminides and in NiZr and NiTi can be correlated to those of the pure components, taki ng into account the densities of valence electrons. For the positron l ifetimes tau(1) Of vacancies in intermetallic compounds, values of tau (1)/tau(f)=1.5-1.7 are observed similar as in the pure metals. Anneali ng studies of B2-FeAl after electron irradiation yield time constants for the disappearance of vacancies identical to those deduced recently for the equilibration of thermal vacancies. In electron-irradiated Ti aluminides annealing processes at 250 K and 450 K are observed where the latter process is tentatively attributed to long-range migration o f vacancies.