Strong and tough metallic materials are desired for light-weight structural applications in transportation and aerospace industries. Recently, heterostructures have been found to possess unprecedented strength-and-ductility synergy, which is until now considered impossible to achieve. Heterostructured metallic materials comprise heterogeneous zones with dramatic variations (> 100%) particularly in mechanical properties. The interaction in these hetero-zones produces a synergistic effect wherein the integrated property exceeds the prediction by the rule-of-mixtures. More importantly, the heterostructured materials can be produced by current industrial facilities at large scale and low cost. The superior properties of heterostructured materials are attributed to the heterodeformation induced (HDI) strengthening and strain hardening, which is produced by the piling-up of geometrically necessary dislocations (GNDs). These GNDs are needed to accommodate the strain gradient near hetero-zone boundaries, across which there is high mechanical incompatibility and strain partitioning. This paper classifies the types of heterostructures and delineates the deformation behavior and mechanisms of heterostructured materials.

Heterostructured metals typically exhibit excellent mechanical properties, such as high strength, plasticity, and fracture toughness, which are not present in conventional homogeneous materials. This is primarily due to the synergistic effects arising from the interactions between the internal components including the stress/strain gradients, geometrically necessary dislocations, and unique interfacial behavior. This study focuses on two typical heterogeneous nanostructures (laminated and nanotwinned) by reviewing the recent progress in their strengthening and toughening mechanisms. The analysis highlights the effects of the properties and sizes of the individual components, interfaces, and loading directions on the macroscopic strengthening and toughening behavior.

Key components in nuclear engineering serve as a security barrier, ensuring the smooth development of nuclear power technology, as well as safe and efficient operation of the nuclear power system in China. Metallic multilayers are novel nanostructured materials based on interface self-healing theory, which exhibit broad nuclear application due to their high-density heterogeneous interfaces. They can not only effectively hinder dislocation movement to enhance material strength but also obviously absorb irradiation-induced defects and promote their annihilation or recombination to improve material irradiation damage tolerance. Considering the recent domestic and international studies on irradiation characteristics of metal/high-entropy alloy multilayers, this study reviewed the evolution of microstructure and mechanical properties, and their underlying mechanisms in metal/high-entropy alloy multilayers before and after irradiation. Furthermore, it also explored strategies to enhance multilayers irradiation tolerance. The development of nanostructured multilayered materials with high tolerance to radiation damage were also proposed.

Metallic structural materials have a wide range of industrial applications (including in the aviation, aerospace, navigation, military industry, nuclear power, chemical industry, construction, and bridge-building fields) due to their unique properties (such as heat resistance and high strength and toughness). At present, there are development opportunities for metallic structural materials, but these materials are also facing challenges due to the gradual substitution of carbon fiber composites and the increasing shortage of metal mineral resources. China's metallic structural material industry is facing development roadblocks and opportunities. Nanostructured metals and alloys have a wide range of potential industrial applications in the field of aviation, aerospace, navigation, military industry with requirements for energy conservation and weight reduction due to their high strength, but their low fracture elongation is a major limitation. The low ductility of nanostructured metals is caused by their low strain hardening rate; the strain hardening rate is caused by the difficulty of dislocation accumulation. This is because the small grain size limits dislocation propagation and reaction. After more than 20 years of research, the low ductility of nanostructured metals has been improved by tailoring the metal microstructures, such as by introducing nano-precipitation, twin boundaries, multi-scale grain distribution, twinning, or phase transformation, nano-gradient structure, and heterogeneous structure, or by lowering dislocation density, etc. These toughening schemes improve the dislocation accumulation capacity and strain hardening rate of nanostructured metals, and ultimately improve their toughness. The tensile properties of nanostructured metals are closely related to their microstructures and deformation temperature, strain rate, tensile sample size, and loading state.

The design and manufacture of heterostructured metallic materials for the balanced improvement of strength and ductility by interior microstructure construction have been the research frontiers and focus in mechanical engineering and materials science. Recently, the understanding of multiple hardening mechanisms in heterostructured metallic materials has progressively advanced. Although establishing quantitative relationships between hardening effects and microstructural parameters and further instructing the research and development of manufacturing for a superior combination between strength and ductility will be of significant value to the design theory, the manufacturing processes and property characterization of heterostructured metallic materials are crucial. In this article, the research progress on the theoretical foundations of designing microstructures and manufacturing processes for heterostructured metallic materials was reviewed. First, the heterostructured metallic materials from the perspective of their microstructural regulation method were categorized. Second, the theoretical foundations for the microstructural regulation of heterostructured materials were reviewed. Third, the manufacturing process for heterostructured materials was classified in terms of the up-bottom and bottom-up approaches as well as reviewed. Finally, the challenges and future development of the design and manufacture of heterostructured metallic materials were addressed.

The mechanical properties of metal matrix composites depend on not only the content of the reinforcements but also the composite architecture (shape, size, and spatial distribution). This paper focuses on the heterogeneous architecture design of metal matrix composites, including the heterogeneous architecture design of reinforcements and the intrinsic heterogeneous design of the matrix. In addition, it summarizes the development process of scale refinement, size quantification, and structural parameter diversification of metal matrix composite architecture design. The future development direction of architectural composite and the strengthening and toughing design of metal matrix composites based on the energy dissipation theory is also proposed.

The concurrent enhancement of strength and ductility is an unremitting pursuit in metallic material research. Recently, by deliberately controlling the spatial distribution of domains with substantially different mechanical properties, heterostructured architecture has overcome the limitation of strength-ductility synergy in metallic materials. Mainstream theories, such as hetero-deformation-induced hardening, strain partition, premature local necking delay, and interface affected zone, have provided crucial guidance for the designing of preferable heterostructured metallic materials. These theories suggest that the domains of heterostructured metallic materials present unique local stress and strain characteristics upon loading, accompanying deformation and fracture behaviors that deviate from the predictions of classical theories. In this study, the evolutions of local stress and strain during the early deformation, plastic deformation, and fracture stages of heterostructured metallic materials were reviewed. Moreover, interactions between deformation or fracture behaviors and local stress or strain as well as their effects on mechanical properties are summarized, presenting a new perspective for designing and developing high-performance heterostructured metallic materials.

High-entropy alloys overcome the limitations posed by traditional alloys due to features such as high strength, toughness, high wear resistance, and corrosion resistance. These alloys are novel metallic materials with excellent application potential; however, typically an inverse relationship is observed between the strength and ductility of a metal, which includes high-entropy alloys. Therefore, the design and development of high entropy alloys with high strength and high ductility have become a limitation in current research. Recently, heterostructure design has achieved great success in strengthening and toughening traditional metallic materials. Heterostructured and high-entropy alloys has garnered much attention and research interest to realize the strength and toughness of high-entropy alloys with high strength and high ductility. This study reviews the existing design models for heterostructures from the heterostructure scale perspective. Furthermore, the effects of different heterostructures on the strengthening and toughening mechanism and mechanical properties were analyzed, and future microstructural designs with high strength and toughness were anticipated.

Heterostructured materials (HSMs) can be created by introducing differently sized constituent components to enhance their performance by disentangling conflicting materials' properties, through the synergistic coupling effect of the constituents. This strategy has been successfully applied to structural materials to overcome the trade-off between strength and ductility and achieve superior mechanical properties; however, it remains less explored for functional materials. Beyond the random distribution of the constituents in HSMs, the ordering of constituents, e.g., grains, phases, and domain structures, can further enhance their coupling effect, thus leading to improved material properties or even transformative new functionalities. In this short perspective article, permanent magnetic materials are used as examples to review the recent progress in achieving enhanced properties and/or creating new physical mechanisms by building HSMs with ordered structures. This paper demonstrates that high-performance or revolutionary functional materials can be achieved by creating ordered HSMs.

The trade-off relationship between the strength and the electrical conductivity has been the bottleneck restricting the development of conductive metallic materials with high strength and high electrical conductivity. In this study, commercially pure Al wires and commercially pure Cu wires with various grain characteristics were prepared by the cold-drawing process to investigate the influencing mechanisms of grain on strength and electrical conductivity. Surprisingly, the synchronous increase in strength and electrical conductivity may be achieved both for the commercially pure Al wires and commercially pure Cu wires in the later stage of cold-drawing deformation, which shatters the traditional constrictive relationship between the strength and the electrical conductivity. Additionally, the microstructure investigation demonstrates that with the increase of drawing deformation, the axial grains were lengthened, the radial grains were increasingly polished, and the radial <001> texture was transformed to <111> texture. Finally, the heterogeneous microstructures, including heterogeneous grain formation and heterogeneous crystal orientation were formed. The theoretical analysis reveals that the grain width and texture mainly influence the strength, and the grain length primarily influences the electrical conductivity. Consequently, the axial long grain, the radial fine grain, and radial hard orientation texture are proved to be the primary mechanisms causing the synchronous improvement of strength and electrical conductivity of commercially pure Al wires and commercially pure Cu wires. This suggests that the heterogeneous grain design may be considered a useful method to create conductive metallic materials with high strength and high electrical conductivity.

To improve the comprehensive performance of Ti matrix composites for defense applications such as aviation and aerospace, as-sintered TiBw/TC18 composites with different reinforcement contents were hot extruded and heat-treated. The composites were characterized and analyzed by OM, SEM, and TEM. The mechanical properties of the composites were measured using an electronic universal testing machine. By extruding in the β single-phase region, the β grain size of TiBw/TC18 was reduced from 70 μm to about 40 μm. After the subsequent triple-annealing or solution aging heat treatment, α phase with different sizes was precipitated and distributed in the β phase. The elongation of the as-extruded composites significantly showed improvement, but the strength decreased by about 17%. After applying the triple-annealing heat treatment, the tensile strength and elongation of 2.0%TiBw/TC18 (volume fraction) reached 1200 MPa and 21.7%, which are higher by 5.5% and 189%, respectively, than those in the sintered state. Moreover, after applying the solution aging heat treatment, the as-extruded 2.0%TiBw/TC18 exhibited tensile strength and elongation of 1389 MPa and 9.9%, which are higher by 22.2% and 32%, respectively, than those exhibited by as-sintered 2.0%TiBw/TC18. Consequently, the hot extrusion can effectively reduce the grain size of as-sintered TiBw/TC18, and the tensile properties of the extruded TiBw/TC18 can be modified to meet the requirements of different service conditions through different subsequent heat treatments.

As the lightest structural metallic materials, Mg alloys have immense development potential in the automotive, aerospace, medical, and electronic industries. However, the low strength and the poor ductility of Mg alloys limit their engineering applications. Recent investigations have shown that heterostructured Mg alloys exhibit significantly improved strength and ductility. This work applies accumulative roll-bonding and subsequent annealing to a Mg-3Gd alloy to produce layered heterostructures composed of alternating recovered and recrystallized layers of varying thicknesses. These heterostructures exhibit higher strength than homogeneous grain structures at a similar tensile ductility. They also show a continuous flow behavior desired for metal forming. A high density of the <c + a> dislocations is activated at the interfaces between the layers to accommodate the deformation incompatibility, which contributes to dislocation multiplications and accumulations and enhances work hardening rate and ductility.

Efforts to develop high-strength and heat-resistant Al alloys have been ongoing to reduce the weight of automobiles and achieve transportation with low emissions. Traditional heat-resistant Al alloys are difficult to use at temperatures higher than 300^oC because of the strength loss from precipitate coarsening behavior. This study examined the microstructure, mechanical properties, and thermal stability of a heterostructured Al nanocomposite reinforced by AlN nanoparticles using FESEM, TEM, EBSD, tensile test, and thermal exposure experiments. The heterogeneous lamellar structure of Al-AlN nanocomposite was composed of alternate distributed particle-rich and particle-free zones. Ultrafine Al grains formed in the particle-rich zone, whereas coarse Al grains formed in the particle-free zone. The mechanical tests of the Al-AlN nanocomposite showed no visible microhardness or loss of tensile strength after severe thermal exposure at 500^oC for up to 100 h. The outstanding thermal stability and tensile strength combination were much better than the data in the literature. It is believed that the intergranular AlN nanoparticles pinned the Al grain boundaries and contributed to the superior thermal stability and strength. Furthermore, an abnormal increase in strength at the initial stage of the thermal exposure tests was revealed. A thermal exposure temperature resulted in a greater increase in strength and hardness, which was rationally interpreted in view of grain boundary relaxation strengthening.

Aluminum alloys have been widely used in fields such as automotive and aerospace industries, owing to their excellent mechanical properties, lightweight, and low recycling costs. However, aluminum alloys processed by selective laser melting (SLM) typically suffer from insufficient strength and fracture toughness. To tackle this issue, a new strategy that integrates eutectic composition design and grain refinement has been adopted to create a heterogeneous structure that can improve strength and toughness of SLMed Al-Fe-Zr alloys. The SLMed AlFe₅ alloy consists of high-volume-fraction of coarse and columnar grains and low-volume-fraction of fine grains, and no obvious heterogeneity is visible across the microstructure length scale. With the addition of Zr, the volume fraction of fine grains significantly increases, leading to the heterogeneous distribution of coarse and fine grains in the SLMed AlFe₅Zr₁ alloy. Meanwhile, both AlFe₅ and AlFe₅Zr₁ alloys show a nanoscale cellular structure. This type of a nanosized cellular structure, together with supersaturated Fe and high-density dislocations, contributes to a high yield strength of 400 MPa for the SLMed AlFeZr alloys. The heterogeneous structure can further improve the strain strengthening capability, enabling a tensile strength as high as 450 MPa for the AlFe₅Zr₁ alloy. Furthermore, the heterogeneous structure promotes crack deflection and crack tip blunting, which can effectively increase crack growth resistance and impart superior fracture toughness to the AlFe₅Zr₁ alloy.

Twinning-induced plasticity (TWIP) steel has received significant research attention because of its superior mechanical properties, including uniform elongation, ultimate tensile strength, and fracture toughness. However, it has a relatively low yield stress, which limits its industrial application. Increasing the dislocation density has been proved to be an effective method for enhancing the yield stress. In this work, a simple warm rolling (WR) route was applied at 700^oC to manufacture partially recrystallized TWIP steel with a high yield stress (1250 MPa), good total elongation (24%), and exceptional fracture toughness (K_JIC of approximately 125 MPa·m^1/2). The steel manufactured using WR was characterized using SEM, EBSD, and TEM at different length scales. Compared to the steel microstructure obtained after hot rolling or cold rolling (CR), this WR TWIP steel exhibits a distinct heterogeneous structure. The matrix has numerous dislocations with twinned coarse grains (approximately 75%) and nearly defect-free recrystallized fine grains (approximately 25%), which form during the reheating period of the WR process. The in situ tensile tests of the WR and CR steels show that the deformed coarse grains provide high yield stress with negligible deformation, whereas the recrystallized fine grains can undergo considerable plastic deformation, which results in a good work hardening capacity during tensile deformation. The fracture toughness tests of the compact tension (C(T)) samples indicate that the recrystallized grains in the WR steel can enhance the crack tip blunting and deflect cracks, which enhance the crack-growth resistance. Alternatively, these toughening mechanisms are not observed in the homogeneous CR steel. Therefore, this heterogeneous structure, which is induced by the high temperature WR process, provides the TWIP steel with excellent strength and toughness.