R. W. Armstrong* and G. D. Hughes**
*Department of Mechanical Engineering
University of Maryland
College Park, MD 20742
**Laboratory for Physical Sciences
8050 Greenmead Drive
College Park, MD
20740.
ABSTRACT
Consideration is given to a number of exciting reasons for
investigating the strength properties of ultrafine grain size materials,
for example, as carefully pursued by J. R. Weertman and colleagues. One
reason relates to the increased importance of the interfacial energy versus
volume free energy of crystals at nanoscale grain sizes, now suggested to
cotinect with reported observations of greater grain boundary and grain
volume disorder in molecular dynamics modeling of nanocrystalline
material. A second reason relates to the proposed transition from gain
volume deformation mechanisms to localized grain boundary shearing at the
nanoscale, connecting with a number of predictions obtained from the
dislocation pile-up model for the Hall-Petch (H-P) reciprocal square root
of grain size dependence for strength values when small numbers of
dislocations are present in the pile-ups. Model limitations on strength
properties are described and an estimate is given of the local pile-up
stress influence on Coble-type diffusional creep at nanocrystal boundaries.