Studies of vacancies and dislocations in TiAl by positron annihilation

Abstract

Polycrystalline specimens of Ti 51Al 49, Ti 48Al 52 and Ti 44Al 56 compounds have been subjected to electron irradiation, proton irradiation and deformation in compression, and then vacancies, vacancy clusters and dislocations in the specimens have been investigated using positron lifetime spectroscopy. It has been found that deviations from the stoichiometric composition in TiAl are compensated by anti-site atoms and there are no structural vacancies on either side of stoichiometry. On isochronal annealing after electron irradiation, two major recovery stages have been observed; the first stage around 250 K is attributed to the migration of vacancies while the second stage between 400 and 800 K is attributed to the annealing-out of secondary defects. The intrinsic secondary defects formed by excess vacancies in TiAl are small dislocation loops or collapsed planar defects. Proton irradiation, however, causes the formation of microvoids as secondary defects in TiAl because of the existence of implanted hydrogen atoms. All such radiation damage is annealed-out below 800 K. The mean magnitude of the Burgers vector of dislocations in TiAl seems to decrease with increasing deformation temperature and to be generally smaller in titanium-rich two-phase TiAl than in aluminum-rich single-phase TiAl.