Nanocrystalline metals (grain size less than 100 nm) have been
the subject of considerable interests due to many unique mechanical
properties, such as increased strength/hardness, improved
toughness and enhanced diffusivity compared to coarse grained
counterparts [1,2]. The strength/hardness has been found and verified
to increase with decreasing grain size down to a critical value
(10–20 nm) by several classical experiments, following the wellknown
Hall–Petch (H-P) relation. The increased strength/hardness
has been attributed to the increased area fraction of grain boundaries
(GBs), which act as strong barriers to dislocation motions.
However, this relation has been questioned by several investigations
which indicate the decrease of hardness below a critical grain
size [3–5], both experiments and simulations [6–8] have also shown
that the strength/hardness decreases with further grain refinement
below the critical value (10–20 nm), suggesting a shift in the
dominated deformation mechanisms from dislocation-mediated
plasticity to grain-boundary-associated plasticity such as grainboundary
sliding, grain-boundary diffusion and grain rotation.