Effect of grain size and grain-boundary structure on plasticity in nanocrystalline iron

In the present study, plasticity of nanocrystalline Fe and its grain boundary structure have been studied systematically using nanoindentation and HREM, respectively.; The effect of grain size of nanocrystalline Fe on plasticity was investigated. Samples with various grain sizes were synthesized by high-energy and low-energy ball milling at different milling times and milling amplitude (low-energy ball milling). Grain size and rms strain were determined by Warren-Averbach analysis of x-ray Bragg peak broadening. Hardness and strain-rate sensitivity were determined using nanoindentation. It is found that the hardness increases with decreasing grain size down to 18 nm (Hall-Petch relation), but decreases with decreasing grain size further below this value, behavior that has been termed inverse Hall-Petch relation. It is also found that the strain-rate sensitivity increases monotonically with decreasing grain size.; Motivated by the fact that the strain-rate sensitivity of sintered nanocrystalline material is lower than that in the as-milled state for the same grain size, the effect of grain-boundary relaxation on plasticity of nanocrystalline Fe was investigated. To obtain samples with the same grain size, but different degree of grain-boundary relaxation, the as-milled nanocrystalline Fe samples were annealed at 80°C and 100°C for various times. Using nanoindentation, it was found that hardness variation with annealing time was slight, but strain-rate sensitivity changed significantly. The strain-rate sensitivity peaks as a function of time, suggesting two competing processes: one is responsible for the increase of the SRS and the other for the decrease. The process for the decrease of the strain-rate sensitivity is likely to be grain-boundary relaxation, but further study is required to understand what is responsible for the increase of SRS. Grain-boundary structure evolution during annealing was studied using HREM. It was found that disconnected lattice fringes at grain boundary of as-milled sample gradually changed to continuous lattice fringes with regularly spaced grain-boundary dislocations during annealing. The latter structure is suggested to be more relaxed.