THERMAL FORMATION OF VACANCIES IN TIAL

The formation of thermal vacancies, which is a key issue with respect to the high-temperature properties of ordered intermetallic compounds, was studied in gamma-TiAl by means of positron lifetime spectroscopy between ambient temperature and 1400 K. An estimate of the atomic concentration of thermal vacancies yields a value similar as in pure fcc metals and an effective vacancy formation enthalpy of 1.41+/-0.06 eV in good agreement with nearest-neighbor-bond model calculations. A comparison of these data with the results of recent self-diffusion studies suggests a low mobility of vacancies as typical for ordered intermetallic compounds. No evidence for structural vacancies at ambient temperature could be observed.