Thermal Vacancy Formation in Ni3Al and γ-TiAl Compounds Studied by Positron Lifetime and Nearest-Neighbour Bond Models

High temperature positron lifetime experiments for studying thermal vacancy formation were performed on the close packed intermetallic phases gamma-TiAl and Ni3Al up to 1400 K and 1550 K, respectively. Neither in Ti(48.5)Al(51.5) nor in Ni(74.1)Al(25.9) and Ni77Al23 can structural vacancies be detected at room temperature. Atomic concentrations of thermal vacancies of about 10 exp -4 at the melting temperature are deduced and effective vacancy formation enthalpies of 1.4 +/- 0.06 eV for gamma-TiAl and of 1.73 +/- 0.06 eV to 1.84 +/- 0.06 eV for Ni3Al in the L1(2)-phase are determined. A fit of nearest-neighbor models to the experimental data demonstrates predominant formation of thermal vacancies on the Ti or on the Ni-sublattice, and the thermal concentrations of antisite defects appear to be higher than those of vacancies. (Author)