Unexpected grain size reduction by heating in bulk nanocrystalline FeAl

The fact that heating can lead to a reduction of the grain size in an already grain-refined alloy is unexpected as the typical behavior would be grain growth. In contrast the grain size of bulk nanocrystalline intermetallic FeAl can be reduced further considerably by heating. The study is based on transmission electron microscopy investigations demonstrating an effect on the microstructural scale of fundamental importance in materials physics. Prior to heating the intermetallic alloy FeAl was deformed by severe plastic deformation, yielding a disordered nanocrystalline structure. By heating, the structure changes to an ordered nanocrystalline one with a 10 times reduced dislocation density and what is most striking with a grain size reduction by 50%. This reduction occurs by converting small-angle grain boundaries into large-angle ones. In addition, the bulk annealing results were confirmed by in situ heating in the transmission electron microscopy.