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

Abstract

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.