Although deformation processes in submicron-sized metallic crystals are
well documented, the direct observation of deformation mechanisms in
crystals with dimensions below the sub-10-nm range is currently lacking.
Here, through in situ high-resolution transmission electron
microscopy (HRTEM) observations, we show that (1) in sharp contrast to
what happens in bulk materials, in which plasticity is mediated by
dislocation emission from Frank-Read sources and multiplication, partial
dislocations emitted from free surfaces dominate the deformation of
gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid
factor) is not the only factor in determining the deformation mechanism
of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase
transformation from a face-centered cubic to a body-centered tetragonal
structure after failure. These findings provide direct experimental
evidence for the vast amount of theoretical modelling on the deformation
mechanisms of nanomaterials that have appeared in recent years.