  The search for new ultra strong materials has been a very active research
area. With relation to metals, a successful way to improve their strength is by
the creation of a gradient of nanograins (GNG) inside the material. Recently,
R. Thevamaran et al. [Science v354, 312-316 (2016)] propose a single step
method based on high velocity impact of silver nanocubes to produce
high-quality GNG. This method consists of producing high impact collisions of
silver cubes at hypersonic velocity (~400 m/s) against a rigid wall. Although
they observed an improvement in the mechanical properties of the silver after
the impact, the GNG creation and the strengthening mechanism at nanoscale
remain unclear. In order to gain further insights about these mechanisms, we
carried out fully atomistic molecular dynamics simulations (MD) to investigate
the atomic conformations/rearrangements during and after high impact collisions
of silver nanocubes at ultrasonic velocity. Our results indicate the
co-existence of polycrystalline arrangements after the impact formed by core
HCP domains surrounded by FCC ones, which could also contribute to explain the
structural hardening.
