Direct observation of a coincident dislocation- and grain boundary-mediated deformation in nanocrystalline iron

The vast majority of our understanding about the deformation mechanisms in nanocrystalline materials is limited to information gained from experimental and theoretical characterization of FCC materials. Related behavior in nanocrystalline BCC materials is not as frequently studied, and thus outstanding questions remain regarding deformation regimes and Hall-Petch trends. Using in situ TEM, we investigate the deformation mechanisms of nanocrystalline BCC iron films with an average grain size of 35 nm produced by physical vapor deposition. The tensile experiments showed that fracture resulted after strains of about 5%. Crack propagation occurred primarily by separation of grain boundaries at the crack tip, which was accompanied by localized intragranular ductile (often superplastic) fracture. Deformation at the crack tip was accommodated by dislocation motion, grain rotation, and grain growth. No evidence was observed of twinning in nanocrystalline BCC iron. The concurrent nature of the grain rotation and dislocation motion indicates that grain rotation occurs at fairly large grain sizes and there is no sharp transition from dislocation-mediated to grain boundary sliding mechanisms as grain size is decreased in BCC iron.