Mechanical and Other Properties of Nanocrystalline Materials

Properties, especially mechanical properties of nanocrystalline metals and alloys are described along with other behavior related to nanocrystal structures which create quantum confinement at dimensions <6 nm. The transition from the larger length scales in the characterization of engineering polycrystalline materials through the nanoregime to the amorphous state is discussed in relation to polycrystal grain size D in relation to the grain boundary or grain boundary phase width, Δt: D > > Δt (coarse-grain, micron regime), D > Δt (nanograin regime), and D ∼ Δt (amorphous phase regime). Over these length scales, a Hall–Petch relationship is shown to be generally applicable up to a plateau which converges to the amorphous state in common metals such as Cu (fcc) and Fe (bcc). Deformation mechanisms also change from slip (or dislocation generation, glide, and pileup) to grain boundary sliding, to mixtures of these. In nanocrystalline metals, stacking-fault and twin boundary free energies are suppressed and partial dislocation emission and associated twin formation can occur. Deformation in small samples can favor surface sources in contrast to grain boundary dislocation sources depending upon grain boundary structure and the number of grains in the specimen thickness. There is a critical specimen thickness, Δ-to-grain size, D, ratio for nanocrystalline metals similar to engineering test samples having coarse-grain sizes, where Δ/D > 8.