Strength-ductility Synergy Research Articles

Constructing heterogeneous micro/nano multilayer structure in Al alloys via solute grain boundary segregation for superior strength-ductility synergy

A heterogeneous micro/nano multilayered (HM/NMed) Al/Al-0.5 wt%Cu composite, composed of alternating coarse-grained pure Al layers and nanostructured Al-0.5 wt%Cu alloy layers containing a nanolamellar structure (NLS) with lamellar thickness as small as ∼60 nm, was developed by accumulative roll bonding (ARB) followed by low temperature annealing. Deformation induced Cu atoms grain boundary (GB) segregation was observed in Al-0.5 wt%Cu layer, which is responsible for the formation and stabilization of the NLS in the Al-0.5 wt%Cu alloy layers. This resultant HM/NMed Al/Al-0.5 wt%Cu composite with dramatic difference in microstructure and strength/hardness between neighboring pure Al and Al-0.5 wt%Cu alloy layers can achieve an exceptional strength-ductility combination: a yield strength close to the NLSed Al-0.5 wt%Cu alloy, and a tensile ductility comparable to coarse-grained pure Al. Strong hetero-deformation-induced (HDI) strengthening associated with the activation of stacking faults, are responsible for this exceptional strength-ductility synergy. The results presented here demonstrate a general design strategy based on solute GB segregation that enables the fabrication of HM/NMed Al-based materials with multi-scale hierarchical microstructures and improved mechanical properties.

Enhanced Strength–Ductility Synergy of Mg–6Zn Alloy Wire by Inducing Guinier–Preston Zone Precipitation

This article designs a strategy of introducing ultrafine precipitates into single‐phase microstructure to enhance Mg–6Zn alloy wire. Three combined processes including equal channel angular pressing (ECAP), direct extrusion, and multipass drawing are used to produce 0.3 mm‐diameter wire. The results show that an isothermal process of ECAP, extrusion, and drawing at 32 °C (named route B) produces a near‐fully solid solution microstructure and causes a very high elongation of 21% as well as an ultimate tensile strength of 24 MPa. Furthermore, on this basis, the addition of an aging treatment before drawing induces the precipitation of the Guinier–Preston zone in the near‐single‐phase Mg matrix and improves the tensile elongation to 28%, and the ultimate tensile strength also is increased to 290 MPa.

Crack inhibition and mechanical property enhancement of a CM247LC alloy fabricated by laser powder bed fusion through remelting strategy

High cracking susceptibility arising from the high thermal gradients is a critical issue of high γ'-fraction Ni-base superalloys fabricated by laser powder bed fusion (LPBF). Here in, laser remelting was used to inhibit cracking and enhance the strength-ductility synergy of additively manufactured CM247LC alloy by LPBF. The effects of laser remelting with different energy densities and scan strategies on microstructure, cracking behavior and mechanical properties of LPBF-processed CM247LC superalloy were systematically investigated. The crack inhibition mechanisms of laser remelting during the LPBF process were revealed. Low energy density of remelting and scan strategy with a rotation of 90° between remelting and prior melting were promising for decreasing the crack density. The crack inhibition caused by laser remelting could be attributed to the partial elimination of defects (e.g., lack-of-fusion pores), the mitigation of the segregation of elements, the backfilling of the grain boundaries and liquid film, and the reduction of the residual stress and the high-angle grain boundaries (HAGBs). Enhanced mechanical strength and ductility (ultimate tensile strength of 1280 MPa, yield strength of 885 MPa, and elongation of 11.9%) of LPBF-processed CM247LC samples were obtained by optimized-remelting parameters. This work demonstrated the potential of laser remelting in improving the printability of high γ'-fraction Ni-base superalloys fabricated by LPBF.

Cracks-free yet ultrafine-grained 7050 aluminium alloys by selective laser melting

Eliminating hot cracking and achieving a good strength-ductility synergy in selective laser melted (SLMed) high-strength aluminium alloys remain challenging. We here inoculated 7050 high-strength aluminium alloy with 2 wt% Ti nanoparticles to eliminate hot cracking during SLM. Furthermore, ultrafine equiaxed grains with random textures were achieved in SLMed 7050 – 2 wt% Ti alloy, in contrast to the textured columnar grains observed in SLMed 7050 alloy. A high tensile yield strength of 393 MPa and an elongation of 11.6 % were achieved in SLMed 7050 – 2 wt% Ti alloy.

Bimodal-grain structure enables superior strength-ductility synergy of pure Al

Developing materials with a strength-ductility synergy was eternally desirable in structural materials. Here, we prepared the bimodal grain structures through the designed powder assembly approach without introducing other elements, which utilized stable dislocations and substructures to regulate fine-grained structures. During the strain, geometrically necessary dislocations formed at fine/coarse grain boundaries to coordinate deformation induced by plastic incompatibility. The stress field induced by these previously accumulated geometrically necessary dislocations interacted with the subsequently formed dislocations. The dislocation movement was thus dynamically coordinated, which in return enlarged the work-hardening region of the materials. Compared with the cast-rolled pure Al, the ultimate tensile strength of the prepared pure aluminum was 212±2 MPa, which was increased by about 102 % and maintained the desirable elongation of 13.5±0.7 %.

Toward strength-ductility synergy in Al–Mg alloys: augmenting solute Mg concentration and strain-hardening ability via melt spinning and hot extrusion

Toward strength-ductility synergy in Al–Mg alloys: augmenting solute Mg concentration and strain-hardening ability via melt spinning and hot extrusion

High Performance Mg Alloy with Designed Microstructure and Phases.

A high strength and ductile Mg-Gd-Y-Zn-Zr alloy was designed and fabricated. The local strain evolution of the alloys during plastic deformation was analyzed using high-resolution digital image correlation (DIC). The results showed that the β particles, nano-sized γ' phases, and LPSO phases were distributed in the as-extruded alloy and a bimodal microstructure was exhibited, including elongated un-dynamic recrystallized grains and fine dynamic recrystallized grains. With increasing extrusion ratio, the grain size remained, with the volume fraction of dynamic recrystallization of the as-extruded alloy increasing from 30% to 75%, and the as-extruded alloy exhibited a high strength-ductility synergy, which is attributed to the grain refinement, extensive β particles, and elongated block-shaped LPSO phases. The strain evolution analysis showed that a strain-transfer from un-DRXed regions to adjacent DRXed regions and LPSO phases can promote uniform plastic deformation, which tends to improve the ductility of the alloy.

A new high-pressure die casting Mg−Gd−Sm−Al alloy with high strength-ductility synergy

A high-pressure die casting (HPDC) Mg−5Gd−1.5Sm−0.7Al alloy was newly developed and exhibits outstanding strength-ductility synergy, with the yield strength and the tensile elongation to fracture being approximately 200 MPa and 8.5%, respectively. This alloy has two types of α-Mg grains: coarse α1-Mg ((46 ± 18) μm) and fine α2-Mg ((9.2 ± 2.3) μm) grains, and various Al-GS (GS = Gd and Sm) particles located at grain boundaries while clear solute-atom segregation near grain boundaries with limited or free intermetallic particles. Characterizations using Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) indicate the crystal structures of Al-GS phases. After aging, dense β′ precipitates and chain-shaped β’’-like structures precipitated near grain boundaries while a high density of ultrafine β’’-(Mg,Al)3Sm precipitates and Al-GS clusters formed in grain center. Relatively fine grains, Al-GS primary particles, solute-atom segregation near grain boundaries, and/or multiple precipitates contribute to the high strength of the studied alloy, while the multi-scale α-Mg grains, variety of intermetallic particles but discontinuous skeleton, and the multi-typed precipitated lead to its satisfactory ductility.

Tuning generalized planar fault energies to enable deformation twinning in nanocrystalline aluminum alloys

As deformation twins have a profound impact on the plastic flow and mechanical properties of metallic materials, enhancing deformation twinning in face-centered cubic (FCC) metallic materials has long served as a unique microstructure design strategy to attain an extraordinary strength-ductility synergy. Deformation twinning, however, rarely occurs in pure FCC Al and its alloys since its generalized planar fault energies (GPFEs) are almost unaffected by most soluble alloying elements such as Mg, Zn and Cu. Here we successfully tune the GPFEs of a nanocrystalline Al-Mg alloy by alloying with Zr, Fe or Y element, and enable deformation twinning in the Zr-, Fe- and Y-containing alloys. Based on a combined analysis of microscopic observations, modeling and ab initio calculations, we find a strong grain-size-dependent twinning (i.e., twinning occurs in preferable grains having sizes in the range ∼20–40 nm), as well as only one single twinning plane (i.e., twinning occurs in single, parallel atomic planes) for twin formation rather than intersecting twinning planes (i.e., twinning occurs in multiple, unparallel atomic planes) usually observed in coarse-grained FCC materials. This interesting twinning behavior is further observed to be accompanied by grain rotations, producing defective twin boundaries. Our experimental results extend the current understanding of the plastic deformation mechanisms in nanograined metallic materials, and will guide microstructure design of twinnable nanograined Al alloys with an improved strength-ductility synergy.

Ultrastrong and ductile medium-entropy alloys via hierarchical ordering.

Long-range ordered phases in most high-entropy and medium-entropy alloys (HEAs/MEAs) exhibit poor ductility, stemming from their brittle nature of complex crystal structure with specific bonding state. Here, we propose a design strategy to severalfold strengthen a single-phase face-centered cubic (fcc) Ni2CoFeV MEA by introducing trigonal κ and cubic L12 intermetallic phases via hierarchical ordering. The tri-phase MEA has an ultrahigh tensile strength exceeding 1.6 GPa and an outstanding ductility of 30% at room temperature, which surpasses the strength-ductility synergy of most reported HEAs/MEAs. The simultaneous activation of unusual dislocation multiple slip and stacking faults (SFs) in the κ phase, along with nano-SF networks, Lomer-Cottrell locks, and high-density dislocations in the coupled L12 and fcc phases, contributes to enhanced strain hardening and excellent ductility. This work offers a promising prototype to design super-strong and ductile structural materials by harnessing the hierarchical ordered phases.

Ultrahigh strength-ductility synergy via heterogeneous grain structure and multi-scale L12-γ′ precipitates in a cobalt-based superalloy GH159

Achieving excellent strength-ductility synergy through convenient techniques is highly desirable for modern engineering applications. In this study, we constructed a heterogeneous grain structure consisting of large residual deformed grains and fine recrystallized grains, along with multi-scale L12-γ′ precipitates, in a commercial cobalt-based superalloy GH159. This was achieved via short-term annealing followed by subsequent aging. The mechanical properties were significantly enhanced, demonstrating an exceptional ultimate tensile strength of 1611 MPa, and outstanding uniform and total elongations of 23.5 % and 34.7 %, respectively. We investigated the evolution of the dual heterogeneous structures and the corresponding strengthening mechanisms during deformation. Hetero-deformation induced (HDI) strengthening, L12-γ′ precipitation strengthening, along with nanotwins, L-C locks, and 9R phases contribute significantly to the excellent strength-ductility synergy in GH159. These findings offer a strategy for developing GH159 as a structural material for various engineering applications, providing accessible and cost-effective processing methods that pave the way for large-scale industrial production. Furthermore, this study provides design insights for enhancing the mechanical properties of cobalt-based precipitation-strengthened superalloys.

Advanced interstitial high-entropy alloy engineered for exceptional wear resistance via dual nanoprecipitation-induced heterogeneous structure

Despite achieving notable strength and wear resistance in precipitation-strengthened Cantor high-entropy alloy (HEA), the inherent brittleness of precipitates leads to a loss of ductility. In this study, we propose a novel approach to address this issue by introducing nanosized M23C6-combined with Cr2N, termed dual-nanoprecipitation, and heterogeneous structure in a C and N co-doped interstitial HEA (iHEA). The abundant nanoscale M23C6 and Cr2N particles serve not only as reinforcing agents to enhance strength and wear resistance but also hinder the recrystallization process, resulting in a heterogeneous structure containing recrystallized and non-recrystallized zones. This heterogeneity triggers additional strengthening and strain hardening mechanisms, thereby enhancing the deformability of iHEA. Furthermore, the heterogeneous structure effectively mitigates strain localization on the sliding surface during wear, thus improving tribological properties. By overcoming the challenge of poor ductility while maintaining exceptional strength and wear resistance, the dual-nanoprecipitation-induced heterogeneous structure offers promising avenues for the development of alloys with superior strength-ductility synergy and wear resistance.

Enhancing strength-ductility synergy in 316L stainless steel through pre-straining at 4.2K

In this study, we introduce a novel approach to enhance the strength of face-centered cubic (FCC)-structured 316L austenitic stainless steel (ASS) while preserving its tensile ductility. The proposed method involves subjecting the 316L ASS to a pre-strain of 3.5 % at a cryogenic temperature of 4.2 K, resulting in the formation of multiple-fine-defects, including mechanical twins, stacking faults, hexagonal close-packed phases, and body-centered cubic (BCC) phases, which act as BCC martensite seeds. The formation of multiple-fine-defects in 316L ASS through pre-straining at 4.2 K leads to a substantial increase in the yield strength of approximately 45 %, while also maintaining tensile ductility during the subsequent tensile testing at room temperature. Moreover, these pre-existing defects can effectively suppress cross-slip, resulting in a high fraction of planar BCC martensite, which lead to a higher strain-hardening ability than that of the as-annealed 316L ASS. These findings suggest that the mechanical properties of metastable FCC alloys with low to medium stacking fault energies can be effectively enhanced by facilitating martensite seeds via pre-straining at cryogenic temperatures.

Strength-ductility synergy and anisotropy evolution of Cu-Be sheets with different Be contents

This study examines the evolution of mechanical properties and texture of hot rolled Cu-Be alloy sheets with varying Be content. The Cu-3.0Be alloy exhibits superior performance attributed to its fine grain structure, weaker texture, and lower dislocation density.

Nanocarbon architecture-intervened interface design of Cu matrix composites towards necking-delayed fracture and toughening

In-situ formed interfacial carbides always sacrifice the structure integrity of reinforcement in nanocarbon/metal composites. Herein, leaf-like carbon nanotube-graphene nanoribbon (CNT-GNR) hybrids, consisting of CNT midrib and GNR margin, was synthetized to solve the above dilemma in nanocarbon-CuCr system. Results demonstrate that high-density nano-sized Cr7C3, which had specific orientation relationship related to the CuCr matrix ([0 0 1]Cr7C3//[011]CuCr, (2‾ 20) Cr7C3//(11 1‾)CuCr), was in-situ formed at GNR-Cu interface via “heterogeneous nucleation-growth” pattern. Meanwhile, the integrity of CNT midrib was well-retained. In-situ SEM tensile results show that such designed interface effectively prevents crack propagation, makes crack blunt via coordinated plastic deformation of local matrix, and guarantees the failure of CNT-GNR by breakage mode, being crucial for exerting the toughening and promoting strength-ductility synergy of CNT-GNR/CuCr.

Revealing the exceptional cryogenic strength-ductility synergy of a solid solution 6063 alloy by in-situ EBSD experiments

A solid solution 6063 aluminium alloy features an exceptional combination of strength and ductility at 77 K. Here, the deformation mechanisms responsible for superior strength-ductility synergy and excellent strain hardening capacity at a cryogenic temperature of the alloy were comparatively investigated by in-situ electron backscatter diffraction (EBSD) observations coupled with transmission electron microscopy (TEM) characterization and fracture morphologies at both 298 and 77 K. It is found that kernel average misorientation (KAM) mappings and quantified KAM in degree suggest a higher proportion of geometrically necessary dislocations (GNDs) at 77 K. The existence of orientation scatter partitions at 77 K implies the activation of multiple slip systems, which is consistent with the results of potential slip systems calculated by Taylor axes. Furthermore, dislocation tangles characterized by brief and curved dislocation cells and abundant small dimples have been observed at 77 K. This temperature-mediated activation of dislocations facilitates the increased dislocations, thus enhancing the strain hardening capacity and ductility of the alloy. This research enriches cryogenic deformation theory and provides valuable insights into the design of high-performance aluminium alloys that are suitable for cryogenic applications.

Achieving the strength-ductility synergy in ultra-fined grained CNT/2024Al composites via a low-temperature aging strategy

Synchronous enhancement of strength and ductility is a persistent challenge in the development and application of carbon nanotube (CNT)-reinforced aluminum matrix composites. This study proposed a low-temperature aging strategy to induce the nanoscale precipitates and evade the strength and ductility trade-off dilemma. The composites under various aging conditions were characterized in detail at the macro, micro, and nano scales. The precipitation behavior and strengthening mechanism were investigated systematically. Results indicated that the composite exhibited a better mechanical performance when aged at 100 °C. Compared to as-extruded composites, the yield and ultimate tensile strength of CNT/2024Al composites increased by 82.7 % and 64.8 %, respectively, whereas the elongation decreased by only 1.1 %. The results of microstructure and theoretical estimation suggested the dense nanoscale GP zones were primarily responsible for achieving the strength-ductility synergy. This present study on tailoring precipitate evolution could provide fundamental insights and references to enhance the mechanical properties of aluminum matrix composites.

Achieving strength-ductility synergy in Al-4.5 wt%Cu binary alloy through semisolid isothermal treatment coupled with ECAP

Bulk nanostructured (NS) aluminum alloys typically have high strength but with extremely limited ductility. An advanced two-step thermomechanical processing, i.e. semisolid isothermal treatment coupled with equal-channel angular pressing (ECAP), was adopted to achieve high strength (413±2.5 MPa) and high elongation (12.0±0.6 %) in bulk NS Al-4.5 wt%Cu binary alloy. The broken nanoscale Al2Cu particles (∼50–80 nm) from lamellar eutectic, dynamic precipitations Al2Cu (∼30–60 nm), refined grains, and high-density dislocations are believed to be key contributors for the high strength. The bimodal grain structure, which caused a well-balanced work hardening and a good deformation compatibility, is critical for the enhanced strength-ductility synergy. This work demonstrates a new approach for improving strength-ductility synergy in interdendritic-eutectic type aluminum alloys.

Constructing high-density dislocations by primary (Nb,Ti)(C,N) to induce massive secondary precipitations in austenitic heat-resistant cast steel

The microstructural evolution and the high-temperature tensile properties of a novel Nb-Ti-N alloyed austenitic heat-resistant cast steel were investigated during the isothermal aging at 900 °C for up to 1000 h. The mechanisms of enhancing strength-ductility synergy and the high-temperature tensile fracture behaviors after the aging process were analyzed in detail through tensile tests at 900 °C. Obtained results show that the high-density dislocation regions and the Cr segregation, formed around the intragranular primary (Nb,Ti)(C,N), provided numerous nucleation sites for submicron-sized secondary M23C6 precipitates. The density of secondary M23C6 significantly increased, while the coarsening rate was notably reduced during the aging process. Simultaneously, the uniform dispersed precipitation of nano-sized secondary (Nb,Ti)C was promoted by massive dislocations, which exhibited high resistance to coarsening during long-term aging. The improvement of high-temperature strength was attributed to the precipitation strengthening effects induced by the submicron-sized M23C6 and nano-sized (Nb,Ti)C particles. Additionally, the enhancement of ductility during aging was mainly due to the more effective stress transmission and the ameliorated interface relationship between primary (Nb,Ti)(C,N) and austenitic matrix, which was facilitated by the abundant precipitation of secondary phases. The current findings indicate that high-density dislocations constructed by the intragranular primary (Nb,Ti)(C,N) induce the extensive dispersion of secondary strengthening phases, which enhanced both ultimate tensile strength of 158 MPa and elongation of 45.3% at 900 °C after isothermal aging for 1000 h.

Overcoming strength-ductility tradeoff with high pressure thermal treatment

Conventional material processing approaches often achieve strengthening of materials at the cost of reduced ductility. Here, we show that high-pressure and high-temperature (HPHT) treatment can help overcome the strength-ductility trade-off in structural materials. We report an initially strong-yet-brittle eutectic high entropy alloy simultaneously doubling its strength to 1150 MPa and its tensile ductility to 36% after the HPHT treatment. Such strength-ductility synergy is attributed to the HPHT-induced formation of a hierarchically patterned microstructure with coherent interfaces, which promotes multiple deformation mechanisms, including dislocations, stacking faults, microbands and deformation twins, at multiple length scales. More importantly, the HPHT-induced microstructure helps relieve stress concentration at the interfaces, thereby arresting interfacial cracking commonly observed in traditional eutectic high entropy alloys. These findings suggest a new direction of research in employing HPHT techniques to help develop next generation structural materials.