Deformation twinning in nanocrystalline materials

The Hall-Petch breakdown in nanocrystalline metals: A crossover to glass-like deformation

Grain size, strain rate, and temperature dependence of flow stress in ultra-fine grained and nanocrystalline Cu and Al: Synthesis, experiment, and constitutive modeling

Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 106 s−1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d>1 μm, 1 μm

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Grain-size dependent mechanical behavior of nanocrystalline metals

Two experimental techniques of nanoindentation and tensile testing were used at room temperature to investigate the strain rate sensitivity of an electrodeposited Ni with a mean grain size (d) of 20 nm, respectively. It was found that the nanocrystalline (nc) Ni possessed a higher strain rate sensitivity exponent (m) during nanoindentation than during tensile testing. Furthermore, a higher m was accompanied by a smaller activation volume (V ). It is believed that the higher stress concentration could activate a shorter dislocation line length (L), which should be responsible for the higher m value during the nanoindentation. Based on a model of dislocation nucleation or bowing-out mechanism, the relationship between m and d for Ni and its alloys was investigated. In the end, a simple and straightforward equation relating m to d was proposed in aid of a simple assumption associating L with d, which implied that the enhanced m in nc Ni and its alloys with d> ∼ 6 nm should be due to the reduction of the dislocation line length. (Some figures in this article are in colour only in the electronic version)

http://iopscience.iop.org/article/10.1088/0022-3727/40/23/027/pdf

Experimental and modelling investigations on strain rate sensitivity of an electrodeposited 20 nm grain sized Ni

The deformation mechanism of nanocrystalline Ni (with grain sizes in the range of 30–100 nm) at ultrahigh strain rates (>107s−1) was investigated. A laser-driven compression process was applied to achieve high pressures (20–70 GPa) on nanosecond timescales and thus induce high-strain-rate deformation in the nanocrystalline Ni. Postmortem transmission electron microscopy examinations revealed that the nanocrystalline structures survive the shock deformation, and that dislocation activity is a prevalent deformation mechanism for the grain sizes studied. No deformation twinning was observed even at stresses more than twice the threshold for twin formation in micron-sized polycrystals. These results agree qualitatively with molecular dynamics simulations and suggest that twinning is a difficult event in nanocrystalline Ni under shock-loading conditions.

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H. Jamarkani

Papers

Deforming nanocrystalline nickel at ultrahigh strain rates