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

The deformation behavior of nanocrystalline Ni-W alloys is evaluated by nanoindentation techniques for grain sizes of 3-150 nm, spanning both the range of classical Hall-Petch behavior as well as the regime where deviations from the Hall-Petch trend are observed. The breakdown in strength scaling, observed at a grain size of 10-20 nm, is accompanied by a marked transition to inhomogeneous, glass-like flow (i.e. shear banding) at the finest grain sizes approaching the amorphous limit. As a consequence of this mechanistic crossover, additional inflections arise in the mechanical properties; maxima are observed in both the rate and pressure dependence of deformation at approximately the same grain size as the onset of the Hall-Petch breakdown. These data experimentally connect the mechanical properties of nanocrystalline alloys to the well-known behavior of amorphous metals.