Crystal plasticity simulations reveal cooperative plasticity mechanisms leading to enhanced strength and toughness in gradient nanostructured metals

Heterogeneous deformation is thought to provide the strengthening in gradient nanostructured metals, but the underlying plasticity mechanisms and optimal gradient structures for attaining improved mechanical performance remain elusive. Through crystal plasticity simulations of three-dimensional heterogeneous nanostructures, we reveal the heterogeneous-deformation-induced plasticity mechanisms which evoke increased flow strength and strain hardening. We have developed a synthetic microstructure generation algorithm which replicates both the microstructural features and external geometry of experimentally characterized samples—providing a direct comparison between the mechanical response recorded in simulations and experiments. Aligning well with experiments, the simulations show synergistically enhanced mechanical properties. High strain hardening rates directly correlate to large stress gradients that emerge due to heterogeneous deformation. Samples with smooth grain size gradients exhibit the most extreme stress gradients and correspondingly higher strain hardening rates. Large increases in dislocation density—especially within larger grains—and high intra- and inter-granular crystal rotation are plasticity mechanisms common to samples which exhibit high mechanical performance. Although the nanostructural gradients have pronounced influence over the mechanical performance, samples with the same grain size and dislocation density gradients exhibit varied mechanical properties. We show that the distribution of initial grain orientations and the microstructural constraints placed on larger grains also influence the active plasticity mechanisms providing improved mechanical properties.