Nanoscale polycrystalline metals typically exhibit increasing hardness with decreasing grain size down to a critical value on the order of 5 to 30 nm. Below this, a plateau or decrease is often observed. Similar observations arc made for nanoscale multilayer thin films. There, TEM observations and modeling suggest that the hardness peak may be associated with the inability of interfaces to contain dislocations within individual nanoscale layers. This manuscript pursues the same concept for nanoscale polycrystalline metals via an analytic study of dislocation nucleation and motion within a regular 2D hexagonal array of grains. The model predicts a hardness peak and loss of dislocation confinement in the 5 to 30 nm grain size regime, but only if the nature of dislocation interaction with grain boundaries changes in the nanoscale regime.
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