Higher plastic flow strength can be achieved by reducing the grain size in polycrystal metals—a well-known size effect termed Hall-Petch strengthening [1,2]. This is a classic example of a structure-property relation, where changes in the initial microstructure influence the material’s mechanical properties. While reducing the average grain size typically increases the flow strength, this often comes at the cost of lower toughness. In a nanograined material, the numerous grain boundaries in close proximity are effective interceptors of dislocation motion, which can result in strengthening as higher stresses are required for plasticity to ensue [3]. However, if the appropriate plasticity mechanisms capable of accommodating the deformation cannot be activated, catastrophic failure can occur [4]. A few nanocrystalline materials have been shown to achieve high flow strengths without brittle failure—facilitated by sustained intergranular plasticity in the presence of high dislocation densities [5], [6], [7]. Both high plastic flow strength and ductility must be attained for nanocrystalline metals to be used in structural engineering applications [8].

Since coarse grained metals are typically more ductile but weaker than their nanograined counterparts, a tradeoff between plastic flow strength and toughness is inevitable. However, recent advancements show that gradient nanostructured metals have the potential to bridge this flow strength-toughness barrier [9]. These materials are processed in such a way to produce a spatial gradient in a nanostructural feature. By incorporating structural heterogeneities, properties can be improved or maintained in comparison to those of the homogeneous components [10]. For example, gradient nanograined (GNG) metals contain a spatial gradient of increasing/decreasing grain size. Many GNG metals have shown improved flow strength with maintained or improved ductility compared to the bulk coarse-grained samples [11], [12], [13], [14], [15], [16], [17]. Using the same concept of incorporating a range of grain sizes to employ the Hall-Petch effect, bimodal grain structures contain large coarse grains surrounded by nanograins and typically exhibit improved ductility, but reduced flow stress compared to their nanograined counterparts [10,18,19].

Both GNG and bimodal metals exhibit heterogeneous deformation, which provides material strengthening. Coarse grains yield at lower flow strengths than nanograins, causing a mechanical incompatibility which is accommodated by geometrically necessary dislocations. Pile-up of dislocations at the boundary between hard and soft domains will induce back stress in the soft domain (appearing harder) and forward stress in the hard domain [[11], [12], [13],15]. These heterogeneous deformation induced (HDI) stresses cause strengthening of the material. While HDI strengthening can be present in both GNG and bimodal structures, the stress and strain distributions differ: In bimodal structures, high stresses and high strains are discretely localized in the nanograins and coarse grains, respectively [19,20]. In contrast, smooth stress and strain gradients develop within GNG materials which mirror the structural gradients [21], [22], [23], [24], [25]. The optimal stress and strain distributions for maximal mechanical performance are unknown.

Due to the challenges in experimentally examining the role of HDI stresses and corresponding activated plasticity mechanisms across the heterogeneous sample, crystal plasticity simulations of GNG structures have often been performed in attempt to gain an understanding of the mechanisms leading to improved mechanical performance. To reduce computational cost, the nanostructures are commonly simplified to be two dimensional or consist of artificial layers or representative volume elements with different material properties to represent the grain size gradient [[21], [22], [23],26]. These studies have shown that gradient plasticity occurs through the resultant stress and strain gradients that develop during loading [21], [22], [23], [24]. While these studies clearly demonstrate gradient plasticity, they lack the realistic nanostructures or crystal plasticity framework necessary to study the plasticity mechanisms activated during deformation which contribute to improved properties.

We have recently demonstrated synergistically improved strength and toughness in heterogeneous nanostructures formed through impact of initially single crystal silver microcubes [17]. Distinct nanostructures are formed depending on the orientation of the single crystal cube upon impact: cubes impacted along the [100] direction produce a smooth gradient nanograined structure, while [111] and [110] impacted samples only undergo minimal recrystallization and more closely represent bimodal crystal structures [27]. Studying the nanostructures of similar uncompressed and compressed samples revealed that gradient plasticity occurs—with high GND density at interface regions clearly indicating the role of dislocation pile up and resultant HDI stresses [27]. Here, we use crystal plasticity simulations to investigate the structure-property relations of the heterogeneous nanostructures formed through impact of single-crystal Ag microcubes. This is the first crystal plasticity study of three-dimensional synthetic microstructures directly informed by experimental data of gradient nanograined structures performed at the same scale. Replicating the experimental microstructures—including the same size and geometry—is extremely important to accurately assess the deformation mechanisms in further detail with validation provided by the experiments. As a direct comparison to experiments, the crystal plasticity simulations elucidate the role of heterogeneous deformation and the associated plasticity mechanisms in improving mechanical properties.