Thermal vacancies in Fe3Al studied by positron annihilation

Vacancies in Fe3Al intermetallic alloys were investigated by positron annihilation spectroscopy combined with X-ray diffraction. Two complementary techniques of positron annihilation were employed: positron lifetime spectroscopy and coincidence Doppler broadening. The experimental results were combined with ab initio theoretical calculations of positron characteristics. It was found that Fe3Al quenched from 1000 deg. C exhibits a mainly disordered A2 structure. Defects in the as-quenched sample were identified as Fe vacancies on the A sub-lattice surrounded by Al nearest neighbors. The concentration of quenched-in vacancies was determined from the positron lifetime results. The samples were subsequently subjected to isochronal annealing which enabled an investigation of the recovery of quenched-in vacancies and the formation of thermal vacancies in B2 phase. It was found that recovery of quenched-in vacancies is accompanied by ordering to the D03 structure. The enthalpy and entropy of formation were determined for vacancies in the B2 phase.