Gradient nanograins (GNG) creation in metals has been a promising approach to obtain ultra-strong materials. Recently, R. Thevamaran et al. (Science 354:312 in 2016) proposed a single-step method based on high-velocity impacts of silver nanocubes (SNC) to produce almost perfect GNG. However, after certain time, these grains spontaneously coalesce, which compromises the induced hardening and other mechanical properties. To better understand these processes, a detailed investigation at the atomic scale of the deformation/hardening mechanisms are needed, which is one of the objectives of the present work. We carried out fully atomistic molecular dynamics (MD) simulations of silver nanocubes at high impact velocity values using realistic structural models. Our MD results suggest that besides the GNG mechanisms, the observed SNC hardening could be also the result of the existence of polycrystalline arrangements formed by HCP domains encapsulated by FCC ones in the smashed SNC. This can be a new way to design ultra-strong materials, even in the absence of GNG domains.