Grain-size dependence of plastic deformation in nanocrystalline Fe

Plastic deformation of nanocrystalline Fe was investigated by nanoindentation. Samples, synthesized by mechanical attrition, consisted of powder particles with diameters greater than 30 μm. The average grain diameters within the particles of different samples ranged from 10 nm to 10 μm. To avoid potential artifacts, samples were prepared without use of heat treatment, and measurements were conducted at a depth significantly smaller than the powder particle size. Corrections were made for the indentation-size effect and for pileup or sink in around the indent. The volume-averaged grain size was used in the analysis. The Hall-Petch relation is obeyed for grain sizes above about 18 nm, and slight softening occurs at smaller grain sizes. The strain-rate sensitivity increases monotonically with decreasing grain size. The results are consistent with grain-boundary sliding.