Vacancy Formation in Fe-Al of B2 and $DO_3$ Alloys

The positron lifetime spectroscopy is employed to study vacancy formation in intermetallic phases of $DO_3$ and B2 structures from Fe-Al system as a function of Al concentration, ternary additive (Cr) and their thermal treatment. Lifetime spectra were fitted en block by a simple trapping model encoded directly to the software (computer program LT-9). In the investigated range of Al concentration (28, 38, 42, and 45 at%) only two types of defects are found. In $DO_3$ region a single type of defects (characterized by positron lifetime $τ_1$=170±2 ps) is detected and indicated as vacancies in the Fe sublattice $(V_{Fe})$. In B2 region a small amount of an additional type of defects (characterized by positron lifetime $τ_2$=214±13 ps) appears. Supposedly, these are vacancies in Al sublattice $(V_{Al})$. For Fe28Al and Fe28Al5Cr samples changes in $V_{Fe}$ concentration are determined as a function of the sample composition, annealing time at 1000°C and quenching the samples to air and oil. The defect concentration increases with increase in Al content. For FeAl with Al above 38 at%, the total concentration of defects is so high that positrons are exclusively trapped by defects. Therefore the concentrations $V_{Al}$ and $V_{Fe}$ cannot be determined separately. However, the ratio of $V_{Al}$ concentration to $V_{Fe}$ concentration is estimated as a function of Al content.