1 Introduction
The principal objective in the development of high performance metallic materials is to achieve superior strength—which serves as the key design benchmark—with maintaining high ductility, fracture toughness and fatigue resistance to ensure structural reliability and service safety [
1]. In practice, however, enhancing the strength of metals typically compromises other complementary properties, mostly notably ductility [
2–
5]. This inherent trade-off, particularly the conflict between strength and ductility, has become a fundamental bottleneck in the field [
6,
7]. Resolving this enduring “strength–ductility” paradox remains an outstanding challenge.
Traditional design strategies for structural materials have long been relied on the established principles such as alloying, compositional and structural homogenization, and the controlled introduction of high-density defects [
1,
8–
10]. These approaches grounded in the premise that non-uniform microstructures and heterogeneous defects distribution generally leads to performance degradation. While heterogeneous phase reinforcement strategies can achieve linear increases in strength and elastic modulus with the addition of reinforcing phases, overall performance remains inherently limited by the volume fraction and characteristics of those added phases [
2,
4,
11]. In the context of growing resource scarcity and environmental concerns, conventional design paradigms—centered on alloying and homogenization—are increasingly inadequate to meet the dual demands of high performance and sustainable development.
2 Concept and Definition of Unit-Ordering Metallic Materials
Refined through millions of years of evolution, natural materials generally exhibit multiscale and hierarchically organized architectures, offering valuable insights for overcoming performance bottlenecks in engineered systems [
12–
14]. Inspired by this, we propose a novel design paradigm, “Multi-scale Unit–Ordering–Transformative Performance”, for metallic materials. By precisely controlling the spatial ordering of heterogeneous units, this approach breaks the conventional linear limitations of property enhancement and enables “1 + 1 > 2” synergistic effects in metals. This original design concept provides a theoretical foundation for developing a new generation of metallic materials with breakthrough performance.
Units are microstructural elements that can be reliably identified as distinct, contiguous entity using a standard experimental or computational segmentation procedure. They include conventional grains units, and defect units, such as subgrains, twins, domains, and second-phase precipitates. Isolated point defects and individual dislocations are not considered units unless they organize into larger-scale patterns (e.g., dislocation cells or short-range order clusters above a minimum size threshold). Spatial ordering describes the multiscale arrangement of these units, defined by both configuration types and structural parameters. Composite units consist of different-sized grains or combinations of grain- and defect-type units (middle of Fig. 1).
Ordering configurations refer to gradient, layered, and core–shell structures—multilevel, multiscale spatial arrangements that may be periodic, continuous, or discontinuous. As shown in the middle of Figure 1, the three-dimensional gradient structure, in which the grain size, twin thickness or density of the units varies in a continuous or discontinuous gradient spatially. Structural parameters quantitatively define these architectures based on unit types and configuration modes.
It is essential to note that not all ordering structures yield significant performance advantages. Optimal properties typically depend on three critical factors: selection of units, design of spatial ordering, and precise control of key parameters. In metallic systems, the core goal of ordering design is to induce cross-scale and hierarchical coupling among units, surpassing the limitations of individual elements and enabling remarkable improvements in overall performance, as illustrated in the right of Figure 1.
Units at a single scale exhibit distinct mechanical behaviors, with significant differences across scales. However, their size variation is inherently limited, making free-standing or localized ordering insufficient to achieve effective enhancement. To address this, we introduce the concept of combined units—the integration of grain- and defect-type units—greatly expanding the tunable structural scale across 7–8 orders of magnitude, from sub-nanometer to centimeter scales. By implementing spatially gradient ordering throughout the entire sample, we fully exploit the mechanical potential of various defect units, enabling effective coupling, interaction, and synergy among units [
15]. This approach enhances macroscopic and overall performance significantly. Due to space limitations, this paper only discusses examples of gradient-ordering metals with various units, as shown in Figure 1.
3 Proof of Concept and Mechanism
The concept of metallic materials with ordering units has been demonstrated in pure Cu featuring a gradient nanotwinned (GNT) structure, wherein both grains and nanoscale twins serve as the combined units. As the key ordering parameter—the structural gradient, defined as the increase in hardness per unit thickness along the gradient direction—increased, both the strength and work hardening rate of GNT Cu improved simultaneously, exceeding the average values predicted by conventional rule-of-mixtures calculations [
16]. This observation indicates the presence of significant additional strengthening and hardening effects. Remarkably, when the gradient magnitude was sufficiently large, the strength of GNT Cu surpassed even that of the strongest individual unit within the gradient structure—a unique strength–ductility synergy not observed in other homogeneous or heterogeneous materials [
16,
17].
Subsequently, we further validated the universality of the unit ordering strategy for enhancing the comprehensive performance of metallic materials. By employing a cyclic torsion gradient plastic deformation technique with precise control over torsion angle, total cycles, and deformation temperature, we quantitatively achieved a gradient architecture of dislocation cell units spanning from the surface to the interior in stainless steel [
19] and high-entropy alloys [
18,
20], while preserving the original grain characteristics, including grain size, orientation and shape (Fig. 2A–C). This controlled, cross-scale distribution of high-density dislocation configurations (such as dislocation cells and walls) within the grains as shown in Figure 2D enabled a transformative enhancement of the mechanical properties.
The mechanical test results revealed that the yield strength of the gradient dislocation cell-structured (GDS) alloys at room temperature was 2–3 times higher than that of the coarse-grained counterparts before treatment, without significant loss of ductility [
18], as shown in Figure 2E. The strength-ductility synergy achieved in this GDS alloy significantly outperforms that of homogeneous or gradient-structured materials with the same composition reported in the literature [
18,
20], and also successfully resolves the longstanding trade-off between high strength and high ductility. Furthermore, the gradient dislocation structures imparts exceptional mechanical performance at cryogenic temperatures (Fig. 2E), characterized not only by superior strength and ductility but also by an ultrahigh strain-hardening capacity (Fig. 2F) [
18]. Remarkably, the strain-hardening rate even exceeds that of coarse-grained counterparts, challenging the conventional understanding that coarse-grained structures possess the highest work-hardening capability [
18].
In contrast to the classical full dislocation-dominated plasticity mechanisms of conventional face center cubic (FCC) metals, the superior mechanical properties and plastic deformation of gradient dislocation-cell structured alloy are dominated by the generation of dislocations and the extensive atomic-scale stacking faults (SFs, planar fault) at room temperature (Fig. 3A, B). With increasing the tensile strain, the density of planar SF interfaces increase in volume fraction, prevailing in the grain interior (Fig. 3A, B), about 4.4 nm on average. These unique SFs structures serve as geometrically necessary dislocations, which not only accommodate plastic deformation and suppress strain localization, but also govern the strengthening, toughening, and strain-hardening mechanisms in gradient dislocation-cell structured alloys—a mechanism fundamentally distinct from the full-dislocation-dominated work-hardening behavior observed in traditional homogeneous and heterogeneous metallic materials [
15,
20].
When the deformation temperature is lowered to 77 K [
18], the mutually intersected planar interfaces notably proliferate, further subdividing the microsized topmost surface grains into nanometer sized mosaics (Fig. 3C, D). The initial equiaxed dislocation structures in grain interiors have disappeared; instead, abundant mosaic-shaped substructures prevail, containing extremely fine SFs and twins are separated primarily by low-angle misorientation (< 15°) , with a mean size of ~50 nm (Fig. 3C). These mosaics are further refined by individual tiny mosaics, which contain extremely fine SFs, about 1.8 nm on average and are separated primarily by low-angle misorientation (Fig. 3D). This unique configuration-the combination units of dislocation cells and grains units, spatial gradient structure, and cryogenic environment—gives rise to ultrahigh-density two-dimensional planar fault domains. These structures effectively impede the dislocation motion while simultaneously enabling the storage of even higher defect densities, thereby contributing to exceptional cryogenic strain-hardening capability. The emergent cryogenic mechanical behavior and associated strain-hardening mechanisms induced by this unit ordering are fundamentally distinct from those governed by dislocations, twinning, or phase transformation in conventional homogeneous or heterogeneous metallic materials [
18].
These findings preliminary demonstrate that the effective combination of grain-based units and defect-based units can substantially broaden the microstructural size spectrum. When coupled with rational spatial gradient ordering, this strategy enables strong constraint interactions across multiple length scales, ranging from sub-nanometer to micrometer to millimeter scales, and across different hierarchical levels, giving rise to a pronounced gradient amplification effect. This opens up new frontiers for achieving high mechanical performance in structural metallic materials and for the future reverse engineering of new materials by design.
4 Perspective
Systematic studies on the structure–property relationships, transformative performance, and underlying mechanical principles of ordering metals with combined units are still in their infancy. Existing structural materials science—from design and fabrication to characterization and strengthening theory—has been largely built upon conventional models assuming homogeneous structures and continuous media. In contrast, gradient-ordering metals, characterized by spatial heterogeneous and multiscale microstructures, present a wide array of new challenges across the entire research chain, including design principles, fabrication techniques, characterization methods (both macro and micro), strengthening mechanisms, and fracture mechanics theories. Next, taking gradient ordering as an example, we will highlight several key challenges and opportunities for metallic materials with ordering units.
Diversity and complexity of combined units and ordering configurations. Grain-based and defect-based units exhibit diverse configurations (e.g., grain boundaries, twin boundaries, dislocations, secondary phases, and domains etc.) and structural characteristics (e.g., size, orientation, density, texture phase composition, and distributions). In addition, the parameters governing ordering architecture are inherently complex. Key design parameters include the magnitude of structural gradient, gradient sequence, volume fraction, linear vs. nonlinear distribution, and spatial dimensionality. At present, the challenge of designing, coupled with the difficulty of achieving precise and controllable fabrication, of gradient-architected metallic materials with well-defined functional units remains unresolved.
Our design paradigm is not limited to gradient architectures alone. A key aspect of its generality lies in the unit ordering strategy, which governs how microstructural features are spatially arranged to achieve transformative property combinations. By tuning the ordering of grains, dislocation cells, texture, or twin boundaries, one can independently regulate strength, ductility, and strain hardening.
Diverse structure-property relationships in gradient architectures. Because gradient-ordering metals have such complex internal structures, they exhibit a wide variety of mechanical responses and integrated performance. Simply put, changing the combination of structural units or adjusting their arrangement parameters can lead to significant changes in the material’s overall performance—sometimes resulting in a synergistic effect where “1 + 1 > 2”, but other times leading to unexpected weakening.
This presents a core challenge: currently, we still know very little about how these structural units interact and work together across different scales. In other words, while we can observe the relationship between the input (structural design) and the output (performance), what happens in between—the mechanisms through which these units “communicate” with one another—remains an unsolved puzzle. This is precisely the key direction that requires further clarification and deeper investigation in current research.
Lack of theoretical frameworks for non-homogeneous systems. While traditional materials science has long been grounded in homogeneous structures and continuum mechanics, gradient-ordering metals present a fundamentally different paradigm. As inherently heterogeneous systems, they raise unresolved questions regarding their strengthening, toughening, and fracture mechanisms. A comprehensive theoretical framework for these materials has yet to be established. To address this gap, there is an urgent need for integrated research that combines multiscale experimental characterization, theoretical modeling, and computational simulations. Such efforts are essential to constructing a four-dimensional coupling model that captures the spatiotemporal relationships among “unit-ordering–performance and physical mechanisms”.
Unexplored rules for unit selection and architectural optimization. Fundamental questions remain regarding how to select and combine structural units, define key gradient parameters, and establish optimal design criteria. Given the complexity of ordering design, two major challenges emerge: First, it is unclear how to precisely control multiscale, multilevel gradients to activate size effects, interface coupling, and performance amplification. Second, achieving synergistic enhancement, the “1 + 1 > 2” effect, and unlocking emergent properties requires deeper understanding. Addressing these core scientific issues demands a multidisciplinary approach that integrates materials science, solid mechanics, condensed matter physics, and physical chemistry, through both experimental innovation and theoretical breakthroughs.
Addressing the above scientific challenges requires integrated interdisciplinary research spanning materials science, mechanics, physics, and chemistry. Such efforts aim to establish correlations among structural parameters across multiple scales—from nano to micro to macro—and to clarify how different scales couple and interact under complex constraints, ultimately influencing individual performance responses and their underlying mechanisms. Furthermore, there is an urgent need to develop new mechanisms and theories for designing fundamental ordering structures based on units.
5 Concluding Remarks
On the basis of the proposed “Multi-scale Unit– Ordering–Transformative Performance” paradigm, the ordering design of heterogeneous units presents a promising pathway to overcome the long-standing strength-ductility trade-off in metallic materials. This innovative approach is expected to transcend the performance limits of traditional materials, offering new theoretical frameworks and technological avenues for the development of multifunctional and transformative metallic materials. Future research should focus on establishing precise processing methodologies and robust theoretical frameworks to fully realize the potential of ordering design, paving the way for next-generation metallic materials with tailored, transformative properties.
The Author(s). This article is published by Higher Education Press.
PDF
(5236KB)
