ABSTRACT
E-mail: rona@umd.eduThe strength and strain-rate sensitivity properties of nanopolycrystals are attributed, at effective low temperatures, to a combination of Taylortype, dislocation-density-based deformation within the grain volumes and Hall-Petch (H-P)-characterized obstacle resistance of the grain boundaries, consistent with results on conventional-grain-size materials but now scaled downward even to single-dislocation loop behavior. Because of the generally substantial grain-boundary resistances associated with the yield point behaviors of body-centered cubic (BCC) metals, only their grain volume deformation is thermally activated. For face-centered cubic (FCC) metals, there is thermal activation both of controlling dislocation intersections within the grain volumes and, especially, of needed cross-slip at higher local stresses in the grain-boundary regions, thus providing an increasingly larger strain-rate sensitivity at smaller grain sizes. Certain hexagonal close-packed (HCP) metals follow a BCC-like behavior, but for others, the thermally activated grain-boundary resistances and enhanced nanopolycrystal strainrate sensitivities are FCC-like and controlled by the local operation of prism or pyramidal slip systems. Under effective high temperature, creep-like conditions, an inverse H-P grain size dependence occurs and, then, the strain-rate sensitivity is predicted to be smaller at smaller grain size.
