Grain-Size-Dependent Plastic Behavior in Bulk Nanocrystalline FeAl

While the deformation behavior of nanocrystalline ductile metals and alloys is extensively studied, there is little understanding for brittle intermetallic alloys with very small grain sizes. Herein, B2-ordered FeAl with different grain sizes is produced and deformed by high-pressure torsion. At a grain size of 120 nm, conventional dislocation processes remain dominant, resulting in a disordered saturation structure with highly defected grains of around 100 nm. The situation is different for an initial grain size of 30 nm; grain-boundary-mediated processes appear along with dislocation processes and deformation shows a tendency toward extreme localization in the form of thin bands. Interestingly, the saturation structure is not reached after severe plastic deformation. The nanocrystals remain ordered with a grain size of 30 nm; only within the deformation bands, some degree of disordering and an increase of the dislocation density are revealed by profile analysis using selected electron diffraction. This result demonstrates an extreme stability of ordered FeAl at very small grain sizes, and indicates that the deformation behavior in brittle intermetallics can strongly depend on the grain size.