The defect concentrations and mechanical properties of B2-structured samples were investigated at room temperature as a function of the Al concentration. Mechanical testing of large-grained Fe-Al samples showed that the yield strength decreased rapidly with small (Fe-rich) deviations from the stoichiometric composition, but was almost independent of composition between 40 and 45at%Al. The use of X-ray diffractometry revealed the lattice parameter, the degree of long-range order, the vacancy concentration and the fraction of lattice sites which was occupied by antisite atoms. It was found that the lattice parameter increased with increasing Al concentration for Fe-rich and stoichiometric compositions, but that the lattice parameter of the 51at%Al alloy was almost the same as that of the 50at%Al alloy. This indicated that one Fe-Al phase boundary was situated near to the stoichiometric composition. The antisite atom concentration was shown to decrease, while the vacancy concentration increased, with increasing Al content. Comparisons of the antisite atom and vacancy concentrations, with the concentration dependence of the yield strength, suggested that vacancies controlled the room-temperature mechanical properties.

H.Xiao, I.Baker: Acta Metallurgica et Materialia, 1995, 43[1], 391-6