Point defect study in Fe₇₂Al₂₈ alloy as a function of thermal treatment by positron lifetime spectroscopy

The defect structure of Fe28Al samples is examined with the Positron Annihilation Lifetime Spectroscopy. The studies are carried out for samples in as-cast state and after heat treatments: annealing for 24 hours at 900°C (or 1050°C) and either slow cooling with furnace or quenching to oil. The PALS spectra are analyzed using two-state trapping model. Only one type of defects is detected. The positron lifetime in these defects (τV) suggests that they are quenched-in Fe-monovacancies (V_Fe). The vacancy concentration strongly depends on the rate of cooling. Besides, τV also depends slightly on the rate of cooling of the material. This fact suggests, according to the predictions of latest theoretical calculations, that τV is sensitive to the atomic configuration in the nearest neighborhood of V_Fe, which give hope to estimate the degree of atomic ordering in alloys by the PALS technique.