Unraveling the causes of the unusual mechanical behavior of B2 FeAl

Adding 30 to 50 at.% aluminum to iron results in single-phase alloys with an ordered bcc-based crystal structure, so-called B2-ordered FeAl. Within the extended composition range of this intermetallic phase, the mechanical behavior varies in a very particular way. When increasing the Al content to >40 at.%, an unusual, abrupt change in e.g. yield strength and hardness is observed and the brittle-to-ductile transition temperature strongly increases, while the ordered structure remains unaffected. To achieve a better understanding of this effect, which is still not well explained, the current project adopts a novel experimental approach that allows for a much more systematic investigation of this phenomenon than previously reported in the literature. By preparing diffusion couples of an Al-poor and an Al-rich alloy (e.g., with 30 and with 50 at.% Al), extended concentration profiles are produced covering the entire composition range. Micromechanical testing methods such as nanoindentation and micropillar compression then allow the systematic investigation of mechanical properties as a function of composition and temperature over the entire homogeneity range of this phase in only one sample under conditions that are identical for all compositions.