The strategy of units ordering offers a new approach to overcoming the conventional trade-off between strength and plasticity in metallic materials. Nano-twinned copper (Cu) serves as an ideal model system for constructing ordered materials and unraveling the mechanisms underlying their enhanced mechanical properties due to its stable microstructure, precisely controllable microstructures, and well-understood dislocation mechanisms during plastic deformation. Based on the existing research on ordered nanotwinned Cu, this review systematically elucidates the effects of two typical ordering configurations (i.e., layered and gradient structures) on the mechanical properties, deformation behavior, and plastic deformation mechanisms. The aim is to reveal the intrinsic mechanisms by which the units ordering regulates the mechanical performance. Also, this review outlines the existing opportunities and challenges in this field to provide a reference for the development of high-performance metallic materials.

The strength and ductility as critical mechanical properties of metallic materials often exhibit a trade-off relationship. This phenomenon becomes a formidable bottleneck in conventional strengthening strategies that rely on solid solution, second-phase particles, grain refinement, and dislocations, with homogeneous or randomly microstructure. Fortunately, the ordering architecture of units offers a promising pathway to overcome this limitation, attracting recent attention. Typical ordered configurations include laminated and gradient structures, where the units consist of common microstructural features such as grains, twins, dislocations, and phases. Numerous studies demonstrate that the laminated and gradient architectures both can achieve a superior combination of strength and ductility, thereby mitigating the strength-ductility trade-off. The underlying mechanism is often qualitatively attributed to heterogeneous plastic deformation across different sections and the associated formation of geometrically necessary dislocations (GNDs), promoting extra strengthening and work hardening through long-range back stress. However, a quantitative understanding of how the specific ordering strategy affects the mechanical properties, and the strengthening mechanisms remains elusive. In this review, we systematically compare the mechanical properties and deformation mechanisms of laminated nano-twinned (LNT) Cu and gradient nano-twinned (GNT) Cu to address this important issue.

LNT and GNT Cu both exhibit extra strengthening and enhanced work hardening, as demonstrated by their strengths, and the work-hardening rates at small strains exceed the corresponding rule-of-mixture predictions. These effects are further amplified via increasing the structural gradient, defined as the hardness variation per unit depth. GNT Cu demonstrates superior enhancements in both strength and work hardening, compared to LNT Cu, highlighting the advantage of gradient ordering over laminated ordering. Compared to conventional homogeneous or randomly heterogeneous structures, LNT and GNT architectures both achieve an enhanced strength-ductility synergy. In particular, the GNT structure maintains a considerable ductility with only a minor reduction as the strength increases. GNT Cu with the maximum structural gradient exhibits strength exceeding that of its strongest constituent unit, while retaining appreciable ductility. These results demonstrate that gradient ordering can induce stronger strengthening and superior strength-ductility synergy, compared to laminated ordering.

LNT and GNT Cu both display a characteristic gradient plastic deformation across their sections, which promotes the accumulation of GNDs and thereby enhances strengthening and work hardening. In LNT Cu, plastic strain gradients and GNDs concentrate mainly at interfaces. In contrast, in GNT Cu, they are distributed more homogeneously throughout all thesections, forming bundles of concentrated dislocations within grain interiors. Consequently, the dislocation density in GNT Cu is nearly an order of magnitude higher than that in its homogeneous counterparts. Moreover, these dispersed GNDs interact more frequently with other dislocations, generating higher back stresses, compared to the interface-accumulated GNDs in LNT Cu. This GND-distribution-dominated strengthening mechanism is responsible for the superior strength-ductility synergy achieved through gradient ordering.