Vacancy formation in Fe-Al of B2 and DO3 alloys

Abstract

The positron lifetime spectroscopy is employed to study vacancy formation in intermetallic phases of DO3 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 DO3 region a single type of defects (characterized by positron lifetime ¿1 = 170 § 2 ps) is detected and indicated as vacancies in the Fe sublattice (VFe). In B2 region a small amount of an additional type of defects (characterized by positron lifetime ¿2 = 214 § 13 ps) appears. Supposedly, these are vacancies in Al sublattice (VAl). For Fe28Al and Fe28Al5Cr samples changes in VFe 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 VAl and VFe cannot be determined separately. However, the ratio of VAl concentration to VFe concentration is estimated as a function of Al content.