A positron annihilation study of two Fe-Al alloys in the B2 region

In order to investigate the nature of defects in the B2(h) and B2(l) regions of Fe–Al phase diagram, two alloys with aluminum contents of 40 at.% and 45 at.% have been quenched from temperatures in the range 950 °C–600 °C. Positron annihilation spectroscopy and Vickers microhardness have been used to follow the changes produced by the quenching treatments. Two different behaviours could be distinguished in the evolution of the positron lifetime for both alloys. Between 950 °C and 800 °C the lifetime remains constant and it drastically falls down at 750 °C. Between 600 °C and 750 °C the lifetime monotonically increased with the quenching temperature. A dependence with the Al content was also observed; higher positron lifetimes were obtained for the alloy with a 45 at.% of Al over all the temperature range. The constant lifetime value obtained between 800 °C and 950 °C has been assigned to the presence of a triple defect formed by two Fe vacancies bound to a Fe on an Al site as main defect. The stage from 600 °C to 750 °C has been assigned to the presence of Fe vacancies and an increasing number with the quenching temperature of triple defect. The detected step between 750 °C and 800 °C is in good agreement with the solvus line between B2(h) and B2(l) regions.