Thermal vacancies and positron-lifetime measurements inFe76.3Al23.7

The formation of thermal vacancies in the intermetallic compound ${\mathrm{Fe}}_{76.3}$ ${\mathrm{Al}}_{23.7}$ was investigated between 20 and 1060 °C by positron-lifetime measurements. The positron lifetime ${\mathrm{\tau }}_{\mathit{f}}$=112 ps at 20 °C indicates that no structural vacancies can be detected. The increase of the mean positron lifetime τ¯ due to thermal vacancy formation starts at relatively low temperatures (${\mathit{T}}_3$=475 °C). The fit of a simple two-state trapping model to the temperature variation of τ¯ yields an effective vacancy formation enthalpy ${\mathit{H}}^{\mathit{F}}$=1.18±0.04 eV, which suggests a thermal vacancy concentration at the melting temperature of several atomic percent which is much higher than in pure metals and similar to that found in some intermetallic compounds with a B2 structure. From a comparison with self-diffusion results, a vacancy migration enthalpy of ${\mathit{H}}^{\mathit{M}}$=1.4–1.6 eV is deduced. The phase transitions in the present alloy are discussed in terms of vacancy formation and migration.