High-temperature positron lifetime experiments for studying thermal vacancy formation were performed on the close-packed intermetallic phases γ-TiAl and Ni3Al up to 1400 and 1550K, respectively. Neither in Ti48.5Al51.5 nor in Ni74.1Al25.9 and Ni77Al23 structural vacancies could be detected at room temperature. Atomic concentrations of thermal vacancies of about 10-4 at the melting temperature were deduced and effective vacancy formation enthalpies of 1.4eV for γ-TiAl and of 1.73 to 1.84eV for Ni3Al in the Ll2-phase were determined. A fit of nearest-neighbour models to the experimental data demonstrates predominant formation of thermal vacancies on the Ti or on the Ni sub-lattice and the thermal concentrations of antisite defects appear to be higher than those of vacancies.

Thermal Vacancy Formation in Ni3Al and γ-TiAl Compounds Studied by Positron Lifetime and Nearest-Neighbour Bond Models. Badura, K., Brossmann, U., Wuerschum, R., Schaefer, H.E.: Materials Science Forum, 1995, 175-178[1], 295-8