ABSTRACT
Although some understanding seems to be emerging on the influence of grain size on the strength of nanocrystalline (NC) materials, it is not presently possible to accurately model or predict their deformation, fracture, and fatigue behavior, as well as the relative trade-offs of these responses with changes in the microstructure. Even empirical models predicting deformation behavior do not exist due to lack of reliable data. Also, atomistic modeling has been of limited utility in understanding behavior over a wide range of grain sizes ranging from a few nanometers (~5 nm) to hundreds of nanometers due to inherent limitations on the computation time step, leading to unrealistic applied stresses or strain-rates, and scale of calculations. Moreover, the sole modeling of the microstructures is hindered by the need to characterize defect densities and understand their impact on strength and ductility. For example, NC materials processed by ball milling of powders or extensive shear deformation (e.g., equal channel angular extrusion [ECAE]) can have high defect densities, such as voids, and considerable lattice curvature. Accordingly, NC materials are often highly metastable and are subject to coarsening. Recently, processing techniques such as electrodeposition have
advanced to the point to allow the production of fully dense, homogeneous, and low-defect material that can be used to measure properties reliably and reduce uncertainty in modeling associated with initial defect densities (Nieh and Wang 2005).
