Revealing the effects of concomitant grain coarsening and refinement on the internal variable evolution and mechanical properties of gradient nanostructured nickel

The outstanding mechanical properties of gradient nanostructured (GNS) metals and alloys such as a superior combination of strength and ductility can be attributed to two important deformation mechanisms: hetero-deformation induced (HDI) strengthening and strain hardening, as well as grain growth. Recent experimental evidence indicates that GNS Ni specimens undergo concomitant grain coarsening and refinement during tensile deformation. Nevertheless, how such grain growth behaviors affect the evolution and distribution of their in-ternal variables such as dislocation density and HDI hardening, and how such effects are manifested in their mechanical properties, are still unknown. Therefore, this work strives to answer these questions by incorporating a modified strain gradient plasticity formulation taking into account the saturation of geometrically necessary dislocations (GND) accumulation due to the dynamic equilibrium between dislocation source formation and deactivation into a crystal plasticity finite element framework, in conjunction with two other models accounting for the evolution of statistically stored dislocations (SSD) and HDI stress, respectively, to explore the effects of grain coarsening and refinement on the internal variable evolution and mechanical properties of GNS Ni spec-imens with different grain size gradients under deformation. It is revealed that SSD density increases throughout the samples as a result of reduced dynamic recovery in the fine grains induced by grain coarsening as well as enhanced dislocation storage in the coarse grains due to their refinement. In contrast, grain coarsening and refinement leads to a decrease in GND density and HDI stress in the fine grains and an increase in the coarse grains as a result of the corresponding changes in strain gradient intensity. Last but not least, the overall effect of grain growth on the mechanical behaviors of GNS Ni is the outcome of the synergy and competition between its individual effects on the Hall-Petch strengthening, forest dislocation hardening and HDI hardening, respectively, and the different grain size gradients of the GNS Ni specimens give rise to the opposite variations in their me-chanical behaviors with respect to grain growth.