Excellent Strength-ductility Synergy Research Articles

Excellent strength-ductility synergy in a novel medium manganese steel containing 2.4Al-2.9Mn-4.1Ni realized through limited thermo-mechanical processing steps

In this study, the effect of microstructural and micro-textural evolution on tensile properties in hot forged (HF) and hot rolled (HR) medium-Mn steel containing ∼4 wt% Ni is systematically investigated. The microstructures of HF and HR specimens comprise of martensite (α′), nanosized NiAl (B2) precipitates and retained austenite (RA). The excellent tensile properties in HF and HR specimens have been achieved through minimal thermomechanical processing routes. The HR specimen exhibits better strength (∼1868 ± 69 MPa), and ductility (∼16.5 ± 1%) as compared to the HF specimen (∼1765 ± 32 MPa, and ∼12 ± 2%). The prior austenite grain (PAG) size of HR specimen is lower than HF, leading to lower width of lath martensite which contributes to achieving higher strength in the former specimen. Apart from the lath width, the presence of twin martensite along with RA, higher fraction of high angle grain boundaries (HAGBs), and ∑3 boundaries improve the strength-ductility synergy in HR specimen as compared to HF condition. The development of higher intensity γ-fiber <111>//ND during hot rolling has resulted in better ductility in HR specimen, whereas the presence of high intensity Cube texture {001}<100> has deteriorated the tensile properties of HF specimen. Further, visco-plastic self-consistent (VPSC) simulation has been carried out to understand the influence of primary {110}<111> and secondary {112}<111> slip systems activation on strain hardening response of the specimens. It has been observed that the higher activation of primary slip system {110}<111> in HR specimen leads to better strain hardening response at stage IV as compared to HF specimen.

Simultaneously increasing mechanical and corrosion properties in CoCrFeNiCu high entropy alloy via friction stir processing with an improved hemispherical convex tool

It generally shows the trade-off for the mechanical properties and corrosion resistance of high entropy alloys (HEAs). In this study, friction stir processing (FSP) with an improved hemispherical convex tool was applied to process the as-cast CoCrFeNiCu HEA, which simultaneously increased the mechanical properties and corrosion resistance of the HEA by a large degree. After FSP, the grains were significantly refined from 6 μm to ∼300 nm, with 91% of high angle grain boundaries and 18% of nanometer twins, leading to a remarkable increase in the yield strength from ∼194 MPa to ∼935 MPa. More importantly, the FSP HEA showed an excellent strength-ductility synergy with yield strength of ∼695 MPa and a uniform tensile elongation of ∼17.5% after annealing, which was mainly attributed to the comprehensive role of equiaxed ultrafine microstructure, second phase and the high fraction of annealing twins (27%). In addition, the FSP HEA showed higher corrosion resistance than the base material by electrochemical test, which was mainly attributed to the improvement of the self-corrosion potential and the stability of the passivation film caused by the mitigation of Cu segregation and grain refinement. This study provides a new way to simultaneously increase the mechanical properties and corrosion resistance in HEAs.

Strength-ductility synergy and grain refinement mechanisms in a Co-Cr-Ni medium-entropy alloy with novel analogous harmonic structure

A novel strategy is implemented to enhance the strength-ductility synergy in the Co-Cr-Ni-based medium-entropy alloy (MEA) by designing analogous harmonic structures (AHS), where the soft face-centered cubic (FCC) matrix is surrounded by hard multiphase constitutes. The high yield strength of 1028 MPa and ultimate tensile strength of 1558 MPa coupled with considerable uniform elongation of 20% are achieved for the AHS-MEA. Upon uniaxial tension, a large number of the nanograins appear in the core near the interface meanwhile the hard σ precipitates are penetrated by the mobile dislocations in the shell. A finite-element based model is developed to elucidate the intrinsic mechanisms underlying microstructure evolution during plastic deformation. Similar to the equal channel angular pressing/extrusion and high pressure torsion, the coexistence of large tensile hydrostatic stresses and shear stresses plays a crucial role in the grain refinement. The study provides new insights into the nanocrystalline formation mechanism and excellent strength-ductility synergy.

Excellent strength-ductility synergy in a ultra-high strength Ni-W-Co-Ta medium-heavy alloy by orthogonal rolling

Excellent strength-ductility synergy in a ultra-high strength Ni-W-Co-Ta medium-heavy alloy by orthogonal rolling

Low-density nano-precipitation hardened Ni-based medium entropy alloy with excellent strength-ductility synergy

The development of advanced structural materials with a synergistic combination of strength and ductility is currently one of the thrust areas of research for material scientists. An alloy design strategy for such materials takes into consideration the ease of processing, density reduction, and economic viability along with unprecedented mechanical properties. The present study reports a low-density, low-cost medium entropy alloy, designed based on the CALPHAD approach, for structural applications. The designed Ni-Fe-Cr-Al based alloy was cast, homogenized and cold rolled to 75% thickness reduction. The cold-rolled full hard material was isothermally annealed at 900°C for varying duration with a quest for an in-depth understanding of static recrystallization mechanisms. The specimens annealed at 900°C for varying durations were aged at 600°C for 5 h. Based on the systematic understanding of recrystallization and precipitation mechanisms involved, a suitable thermo-mechanical process route was judicially selected for the designed alloy, which contributed to an excellent combination of high tensile strength of > 1 GPa accompanied by an appreciable total elongation of > 50%. This superior strength-ductility synergy surpassed the strength-ductility combination of many existing medium and high entropy alloys and can be investigated further to assess its potential applications in challenging existing solid solution (SS) strengthened superalloys.

Enhancing strength–ductility synergy in a Mg–Gd–Y–Zr alloy at sub-zero temperatures via high dislocation density and shearable precipitates

The strength–ductility trade-off dilemma is hard to be evaded in high-strength Mg alloys at sub-zero temperatures, especially in the Mg alloys containing a high volume fraction of precipitates. In this paper, we report an enhanced strength–ductility synergy at sub-zero temperatures in an aged Mg–7.37Gd–3.1Y–0.27Zr alloy. The tensile stress–strain curves at room temperature (RT), −70 °C and −196 °C show that the strength increases monotonically with decreasing temperature, but the elongation increases first from RT to −70 °C then declines from −70 °C to −196 °C. After systematic investigation of the microstructure evolutions at different deformation temperatures via synchrotron X-ray diffraction, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), it is found that a high dislocation density with sufficient <c+a> dislocations promotes good tensile ductility at −70 °C, which is attributed to the minimized critical resolved shear stress (CRSS) ratio of non-basal <c+a> to basal <a> dislocations. In addition, more shearable precipitates can further improve the ductility via lengthening the mean free path of dislocation glide. The present work demonstrates that an excellent strength–ductility synergy at sub-zero temperatures can be achieved by introducing a high dislocation density and shearable precipitates in high-strength Mg alloys.

Achieving ultrahigh specific strength of an ultrafine grained Mg–9Li–1Al alloy via the combined processing of ECAP with repeated annealing and rolling

A super-high strength Mg–9Li–1Al (LA91) alloy with ultimate tensile strength (UTS) of 312 MPa and specific strength of ∼215 kNmkg−1 was successfully achieved via a combined processing of multi-pass equal-channel angular pressing (ECAP) with repeated annealing and room temperature (RT) rolling. Significant grain refinement after ECAP led to the formation of ultrafine-grain (UFG) α-Mg phase with grain size of ∼350 nm. Further refinement of the α-Mg grains reaching ∼200 nm was achieved by further repeated annealing and RT-rolling with high dislocation density. The formation of strong basal texture of the α-Mg phase of the combined-processed alloy also contributed to the achieved excellent strength-ductility synergy. The 1 wt% Al (aluminum) in the Mg–Li dual-phase matrix acted as a high-efficient strengthening element, and was the major contributory factor to the 48% improved specific strength of the alloy compared to the UFG Mg–9Li binary alloy in our former study. The Al element distributing throughout the alloy phases without segregation contributed to the enhanced refinement of the α-Mg grains during the combined processing and improved strain-hardening rate during the tension plastic deformation, allowing for the achievement of a high strength of the LA91 alloy.

Tuning the microstructure and strengthening mechanism of the CoCrFeNi high-entropy alloy via doping metalloid and interstitial elements

The synergistic strengthening and toughening of metalloid and interstitial elements were demonstrated for the first time in the CoCrFeNi high-entropy alloy (HEA) by the sequential addition of Si, B, and N elements. The addition of Si simultaneously enhances the strength and ductility of the CoCrFeNi HEA, further, the doping of B promotes the formation of Cr2B precipitates and the grain refinement, and the addition of N further refines the grain size and increases the lattice distortion and the degree of chemical short-range order (SRO). The strengthening effect to yield strength of the addition of Si, B, and N elements is well described by the strengthening model. Doping Si, B, and N elements can regulate the stacking fault energy (SFE), lattice distortion, and short-range order of the alloy, and consequently induce the multiple twins and nanoscale transformation from the face-centered cubic (FCC) to the hexagonal close-packed (HCP) phase upon room-temperature tension, which enhances the work-hardening capability of the alloy. Finally, a dual heterostructure of grain size and precipitate phase distribution is formed in CCFN-Si-B-N-700 HEA by regulating heat treatment. The yield strength of the current alloy is further enhanced up to 1.2 GPa, and the ductility is maintained at 13 %. This study not only provides new insights into the strengthening and toughening mechanism of metalloid and interstitial elements but also a new paradigm for the development of HEAs with excellent strength-ductility synergy.

Achieving excellent strength-ductility synergy via cold rolling-annealing in Al-containing refractory high-entropy alloys

Most refractory high-entropy alloys (RHEAs) with Al possess excellent strength but low elongation. Low ductility is generally induced by the formation of brittle phases after long-term heat treatment. Here, we demonstrate a new strategy to optimize strength-ductility in RHEAs through cold rolling-annealing treatments. The Zr35Ti30Nb20Al10Ta5 RHEA was cold rolled followed by annealing. A high tensile yield strength of ∼997 MPa with an elongation of ∼20% was obtained. Several factors together optimize the mechanical properties. First, the high-density dislocation entanglements introduced by cold rolling and short-time annealing are beneficial to uniform strain hardening and ductility. Second, short-term annealing can prevent the generation of the brittle LAVES phase in Al-containing RHEAs. Third, the B2 nanoprecipitates are dispersed in the BCC matrix phase, strengthening the alloys. Fourth, the fine grain size induced by cold rolling and short annealing times contributes to the strength.

Hierarchical precipitates facilitate the excellent strength-ductility synergy in a CoCrNi-based medium-entropy alloy

An interstitial carbon-doped CoCrNiSi0.3 medium-entropy alloy (MEA) has been developed, which exhibits excellent strength-ductility synergy. This has been achieved through the implementation of a three-level hierarchical-precipitate-microstructure design. The CoCrNiSi0.3C0.048 MEA displays an ultra-high yield strength of 1.34 GPa, accompanied by a uniform tensile elongation of 8.62%, surpassing the performance of most carbon/silicon-doped MEAs. The three-level hierarchical precipitate structure comprises the primary Cr23C6 carbides (2–10 μm), the secondary SiC precipitates (200–500 nm) surrounding the grain boundaries, and the tertiary SiC precipitates (∼50 nm) within the grains. The strengthening and toughening behavior of the current MEAs are attributed to multiple mechanisms, including crack branching and blunting in Cr23C6 carbides, dislocation-bypass mechanism around the non-shearable SiC precipitates, hetero-deformation-induced strengthening caused by the interface between matrix/three-level hierarchical precipitates, as well as stacking-fault networks and dense nanotwins activated by precipitates in the matrix. These findings provide valuable insights into the development of high-performance alloys for engineering applications in the future.

Enhancement on mechanical properties of CoCrNi medium entropy alloy via cold spray additive manufacturing associated with sintering

Equimolar CoCrNi medium entropy alloy (MEA) has a broad potential for industrial applications due to its excellent strength-ductility synergy. In this study, the emerging solid-state cold spray additive manufacturing (CSAM) approach was applied to fabricate bulk CoCrNi MEA deposits for the first time. However, due to its nature, CSAM normally results in limited metallurgical bonding and weak interfacial bonding between deformed particles, particularly for high-strength alloys such as the CoCrNi MEA used in this work. Therefore, a post-sintering treatment with temperatures ranging from 1200 °C to 1350 °C was applied after CSAM to improve the interparticle bonding and strengthen the as-fabricated MEA deposits. During the sintering process, recovery, recrystallization, sintering necks and grain migration for polycrystalline growth were observed at different stages. The sintering treatment significantly enhanced the tensile strength and ductility of the as-fabricated sample from ∼240 MPa and ∼2 % to ∼660 MPa and 43 %. This study validates the potential of the united CSAM-sintering strategy in additive manufacturing of CoCrNi MEA.

Additive manufacturing of interstitial-strengthened high entropy alloy: Scanning strategy dependent anisotropic mechanical properties

A non-equiatomic interstitial-strengthened high entropy alloy (iHEA), Fe49.5Mn30Co10Cr10C0.5 (at.%), is manufactured by laser powder-bed fusion (LPBF) with stripe and chessboard scanning strategies. The present study highlights the correlation between the laser scanning strategies with resulting microstructure, textures, and anisotropic mechanical properties in as-built iHEA. The results show that the LPBF processed iHEA exhibits an excellent strength-ductility synergy due to the combined deformation mechanisms of dislocation slip, martensite phase transformation- and nano twinning-induced plasticity. The samples printed by the stripe scanning strategy show more evident mechanical anisotropy than that of the chessboard-scanned samples. The difference in the degree of mechanical anisotropy is mainly attributed to the heterogeneous grain morphology and crystallographic texture resulted from different scanning strategies.

Towards strength-ductility synergy in nanosheets strengthened titanium matrix composites through laser power bed fusion of MXene/Ti composite powder

ABSTRACT A Ti3C2 MXene/Ti composite powder was developed through surface modification and powder coating for producing a high-performance titanium matrix composites (TMCs) by laser power bed fusion. Introducing MXene rarely changed the original sphericity and flowability of Ti powder while it slightly enhanced the laser absorption capacity of Ti powder. The printing conditions in turn affected the morphology and distribution of MXene in as-printed composites. At optimised printing parameters, majority of MXene with high structural integrity and uniform distribution remained along the grain boundaries of as-printed composites, and a combination of high surface quality, geometrical accuracy and densification was obtained. The remaining MXene played the synergy of pulling-out effect and load transfer role, which increased the yield strength from 530 to 710 MPa, while retaining good ductility (elongation of 19.8%). Compared with ASTM standard cast Ti64 alloy, our MXene/Ti composites also exhibited superior comprehensive tensile properties and excellent strength-ductility synergy.

Microstructure evolution and deformation mechanism of coherent L12-strengthened high-entropy alloy during sliding wear

Face-centered-cubic high-entropy alloys (HEAs) strengthened with coherent ordered nanoprecipitates have demonstrated excellent strength-ductility synergy, even at elevated temperature. However, there still lacks fundamental understanding on their microstructure evolution and deformation mechanisms during dry sliding wear. Herein, we systematically investigated the friction and wear behaviors of CoCrNi2(Al0.2Nb0.2) alloy with high-density coherent L12 nanoprecipitates during sliding at room and elevated temperatures, with particular focus on wear-induced microstructure evolution. The alloy shows a low wear rate of 1.80 × 10−5 mm3/(N⋅m) at room temperature (RT) and even an ultralow wear rate of the order of 10−6 mm3/(N⋅m) at 600 °C. Detailed TEM analyses reveal that sliding-induced stacking faults (SFs) and dislocation cells play important roles in the formation of the gradient microstructure at RT and 600 °C, respectively. The superior wear resistance at RT is mainly attributed to the precipitation strengthening of high-density coherent L12 phase and the dynamic work-hardening of SF networks near the sliding surface. However, at 600 °C, the reduced wear rates and coefficients of friction are associated with the formation of glaze layer and the high resistance to thermal softening. This work provides significant insight into the sliding-induced microstructure evolution and deformation mechanism of L12-strengthened high-entropy alloys during sliding wear.

Excellent strength-ductility synergy and corrosion resistance in a metastable high entropy alloy via heterogeneous structure design

For metastable high entropy alloys (HEAs), the balance between mechanical properties and corrosion resistance is rarely achieved. Here, metastable HEAs with a heterogeneous structure (HS) were fabricated by innovatively combining cold rolling and partial recrystallization annealing, showing excellent corrosion resistance and strength-ductility synergy. HS exhibits heterogeneous distributions of dislocation density and grain size. Extraordinary strength-ductility is mediated by prominent hetero-deformation-induced strengthening, high-density dislocations, grain refinement, and transformation-induced plasticity. Such HS samples show higher corrosion resistance than the homogeneous coarse-grained sample, which is due to the low coincidence site lattice grain boundaries, substructures and the formation of stable passive films.

Microstructure evolution and tensile behaviour of fine-grained 6082 Al wire with high ultimate strength and high work hardening by friction stir extrusion of bulk Al sheet

Driven by the highly enhanced demand for metallic materials with excellent strength-ductility synergy, fine-grained alloy fabricated by severe plastic deformation (SPD) is a long-lasting research interest. In this study, a fine-grained aluminium (Al) alloy wire with high ultimate tensile strength (316.8 MPa) and high work hardening capability (n = 0.34) was developed via a novel bulk-consolidation friction stir extrusion (FSE) enabled by CoreFlow™ process based on FSE processed 6082-T6 Al sheet. Grain refinement of 200 times occurred in CoreFlowed Al alloy, which is rarely observed in FSEed Al alloy counterpart based on milling metal chips. The undesirable feature of coarse and preferred secondary phase particles in the initial 6082-T6 Al billet was well tailored during CoreFlow™ process, which led to an attractive characteristic of dispersed and refined particle phases in CoreFlowed Al alloy. Texture components of S (V = 8.24%) and Goss (V = 6.86%) were observed, however, the overall micro-texture intensity of CoreFlowed Al wire was significantly weakened. Due to the cooperative deformation modes consisting of dislocation slip and grain boundary sliding, as well as the positive compatibility effect of fine-grains on deformation, CoreFlowed Al specimens demonstrated an excellent ductility (EI = 19.3%) and acceptable yield strength (YS = 182 MPa) compared to initial 6082-T6 Al sheet (EL = 10.4%, and YS = 225 MPa). Benefiting from the higher work hardening capability, the final ultimate tensile strength (UTS) of CoreFlowed Al (316.8 MPa) reached almost the same level as the 6082-T6 Al sheet (324.6 MPa). Furthermore, after one simple post-processing one-step heat treatment (175 °C for 10 h), the wire's yield strength was improved to 250 MPa, although only maintaining 9% elongation. This low elongation could be attributed to micro-cracks induced by coarsening of the second phases, while high yield strength resulted from fine grain size and precipitate strengthening. The wide range of variations in the mechanical properties of CoreFlowed Al wire under different conditions provides significant freedom in tailoring the mechanical properties of alloy wire in applications.

Simultaneously enhancing strength-ductility synergy and strain hardenability via Si-alloying in medium-Al FeMnAlC lightweight steels

We studied the initial microstructures, mechanical responses and deformation-induced microstructures of the medium-Al Fe-21Mn-6Al-1C-xSi (x = 0, 1.5, 3 wt.%) lightweight steels both in their solution-treated and aged states. Our results reveal that nano-sized long-range ordered domains exist in the solution-treated steels, which feature L12 type ordering in the 0Si and 1.5Si steels and L′12 type ordering in the 3Si steel. Aging at 550 °C for 20 h has little effect on the microstructures of the 0Si steel, but leads to κ′-carbide precipitation in the 1.5Si steel. For the 3Si steel, aging at 550 °C for 1 h already results in the precipitation of uniformly-distributed nano-sized κ′-carbides (4.2 nm). The strength, ductility and strain hardenability of the solution-treated steels all enhance with increasing Si content. The yield strength (YS) of the aged steels with κ′-carbides enhances with increasing aging time, accompanied by the reduction of ductility. The short-time (1 h) aged 3Si steel exhibits excellent strength-ductility synergy, with YS > 900 MPa and total elongation > 50%. The increase in YS with Si addition originates from the grain boundary, solid-solution and order strengthening. For the 0Si steel, the strain hardening is governed by the evolution of high-density dislocation walls and microbands. The occurrence of both dynamic slip band refinement (DSBR) and twinning-induced plasticity effects in the 3Si steel accounts for the increase of strain hardenability with Si-alloying. The DSBR effect still exists and the progressively-formed slip bands are homogeneously distributed in the short-time aged 3Si steel, explaining its high ductility.

Spark plasma forging and network size effect on strength-ductility trade-off in graphene reinforced Ti6Al4V matrix nanocomposites

Architecting tightly bonded interface microstructure is the principal guarantee for increased mechanical properties of titanium matrix composites (TMCs). Adopting an effective technique to modulate interface structure is desirable to break through the trade-off of strength-ductility. Herein, a strategy is applied to establish interlocking interface in multilayer graphene (MLG) reinforced Ti6Al4V (TC4) matrix TMCs by spark plasma sintering (SPS) with subsequent spark plasma forging (SPF). By this technique, 3D network-structured composites of 0.35 wt% MLG/TC4 with tailored network size are fabricated via varied TC4 particle sizes (2–15 μm, 15–53 μm, 63–97 μm, 120–150 μm), then their microstructure and mechanical properties are explored systematically. Results depict that enhanced interface bonding and oriented configuration of reinforcements are generated in inhomogeneous network architecture with diminished internal defects by minimizing agglomerations after deformation, simultaneously enhancing the strength and ductility. Among the different network sizes, SPFed composite with 15–53 μm TC4 powder displays the better maintained tensile ductility (ε, 10.2%) with remarkably higher yield strength (YS, σ0.2) of 1089 MPa (27.52% higher than TC4), resulting in excellent strength–ductility synergy. Strengthening mechanisms and corresponding contributions are further discussed systematically. The work provides a valuable and facile pathway to prepare high-performance TMCs with outstanding interface microstructure.

Physical metallurgy of medium-Mn advanced high-strength steels

ABSTRACT Steels with medium manganese (Mn) content (3∼12 wt-%) have emerged as a new alloy class and received considerable attention during the last decade. The microstructure and mechanical response of such alloys show significant differences from those of established steel grades, especially pertaining to the microstructural variety that can be tuned and the associated micromechanisms activated during deformation. The interplay and tuning opportunities between composition and the many microstructural features allow to trigger almost all known strengthening and strain-hardening mechanisms, enabling excellent strength-ductility synergy, at relatively lean alloy content. Previous investigations have revealed a high degree of microstructure and deformation complexity in such steels, but the underlying mechanisms are not adequately discussed and acknowledged. This encourages us to critically review and discuss these materials, focusing on the progress in fundamental research, with the aim to obtain better understanding and enable further progress in this field. The review addresses the main phase transformation phenomena in these steels and their mechanical behaviour, covering the whole inelastic deformation regime including yielding, strain hardening, plastic instability and damage. Based on these insights, the relationships between processing, microstructure and mechanical properties are critically assessed and rationalized. Open questions and challenges with respect to both, fundamental studies and industrial production are also identified and discussed to guide future research efforts.

Combination of annealing and laser shock peening for tailoring microstructure and mechanical properties of laser directed energy deposited CrMnFeCoNi high-entropy alloy

In this study, a post-treatment method combining annealing and laser shock peening (LSP) is used to tailor the microstructure and mechanical properties of CrMnFeCoNi high-entropy alloy (HEA) prepared by laser directed energy deposition (LDED). The microstructure, microhardness, residual stress and tensile property of the as-built, annealed, LSP and annealed + LSP specimens are compared. Results indicate that LSP produces greater plastic deformation on the surface of annealed specimen compared to the pristine as-built specimen, as manifested by a thicker plastically-affected layer. Recrystallization, dislocation network annihilation and thermal stress relaxation caused by annealing contribute to an increase in the plasticity of LDED-prepared specimens, which provides great strengthening conditions for LSP. Gradient strain hardening and compressive residual stress are gained in the surface layer of the annealed specimen subjected to LSP. Moreover, plastic deformation induces a gradient microstructure consisting of ultra-fine grains, high-density mechanical twins and slip bands in order. An excellent strength–ductility synergy is achieved in LDED-prepared CrMnFeCoNi HEA after combined post-treatment of annealing and subsequent LSP.