Influence of point defects on the hardness and reduced modulus of B2-ordered FeAl

B2-ordered FeAl exhibits a wide homogeneity range (from 23.5 to 53 at.% Al) and a strong sensitivity of its mechanical properties to point defects, but the individual roles of vacancies and anti-site atoms remain insufficiently clarified. In this work, we investigate how these defects influence hardness and reduced modulus across the full single-phase composition range of B2-ordered FeAl. For this purpose, diffusion couples between two Fe-Al alloys with 30 and 53 at.% Al were prepared and mechanical properties were studied by nanoindentation. Diffusion bonding at 1000 °C followed by various heat treatments at 400–1000 °C is employed to systematically vary the concentration of quenched-in vacancies, while anti-site defect concentrations are primarily dictated by composition. Nanoindentation along the composition gradient reveals that low-vacancy states exhibit a non-monotonic hardness–composition behavior with a distinct minimum in hardness, whereas high-vacancy states show a monotonic increase in hardness with increasing Al content. Comparison of differently heat-treated conditions demonstrates that such trends can be explained by the combined effects of vacancies and anti-site defects, where vacancy hardening is known to be most pronounced near stoichiometric B2-ordered FeAl, while anti-site defect strengthening dominates on the Fe-rich side of the composition range. ... mehrThe composition dependence of the reduced modulus likewise changes with heat treatment, indicating that point defects also affect elastic stiffness. Together, these results provide a coherent experimental understanding of how vacancies and anti-site defects jointly control the mechanical response of B2-ordered FeAl and help rationalize previously reported anomalies in the composition-dependent properties.