Combination Of Strength Ductility Research Articles

Grain growth kinetics, phase evolution during annealing and strain rate sensitivity of TRIP-assisted, dual-phase, non-equiatomic high entropy alloy

Transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) belong to a recently developed class of HEAs. TRIP-DP-HEAs have been reported to exhibit superior strength-ductility combination at room temperature as well as at high temperatures. Small amounts of carbon additions to TRIP-DP-HEAs are known to further improve the strength as a result of interstitial solid solution and nanoprecipitate strengthening effects. Current investigation compares the grain growth kinetics in a metastable, dual-phase (FCC and HCP), Fe49.5Mn30Co10Cr10C0.5, carbon-containing high entropy alloy (C-DP-HEA) with that in the corresponding carbon-free variant, Fe50Mn30Co10Cr10 HEA (DP-HEA). The alloys after being subjected to cold rolling, were annealed at 900 °C, 1000 °C and 1100 °C for 3, 30, 60 and 120 min, followed by water quenching. Slower grain growth kinetics was observed in C-DP-HEA due to Zener pinning by carbide particles. The activation energy for grain growth in C-DP-HEA was determined to be 339.71 kJ/mol, which is larger than that of DP-HEA (282.68 kJ/mol). The microstructures of the C-DP-HEA samples, after annealing at 900 °C for different durations of times and quenching, were investigated. The HCP phase fraction in the microstructure initially decreased with increase in annealing time/grain size and later increased with further increase in annealing time/grain size. This is explained based on the counteracting effect of grain boundary density and back stresses acting in samples having increased grain size. Strain rate sensitivity (SRS) parameter, ‘m’, was evaluated for both C-DP-HEA and DP-HEA. The strain rate jump tests were conducted at two quasi-static strain rates, 1 × 10−3 s−1 and 5 × 10−3 s−1, and at room temperature as well as at higher temperatures, 900 °C, 1000 °C and 1100 °C. The ‘m’ value was found to depend on the grain size, test temperature, and the presence of carbon in the alloy.

Achieving excellent strength–ductility combination of Al-aided Mg–Gd–Al–Zr–Zn alloys fabricated by cold metal transfer-based wire-arc directed energy deposition via uniform equiaxed grains and multiple precipitates

Limited ductility consistently poses a challenge for high-strength Mg–RE (rare earth) structures prepared by additive manufacturing. Particularly following heat treatment, while the strength of the Mg–RE alloy experiences a substantial increase, there is a noteworthy decrease in its ductility. Herein, a novel Mg–Gd–Al–Zr–Zn thin wall was successfully fabricated by cold metal transfer-based wire-arc directed energy deposition (CMT-WA-DED) and Al alloying. The microstructural evolution and mechanical properties of deposited and heat-treatment alloys were elucidated systematically. By adding Al element, Al2Gd and Al–Zr phases with thermal stability were precipitated in the deposited alloy. During the solidification or heat treatment stage, Al2Gd and Al–Zr phases could pin grain boundaries to restrict grain growth. The as-deposited alloys exhibited fully equiaxed grains at different heights and orientations. Thus, the thin wall exhibited good isotropic performance. After the heat treatment, the grain size only slightly grew and no abnormal grain growth was observed. Various morphologies of β precipitation phases were discovered along grain boundaries and within the grains. Additionally, a high density of β' and β1 phases within the grains and β phases along the grain boundary played pivotal roles in enhancing the mechanical strength of the Mg–Gd–Al–Zr–Zn alloy. The alloys exhibited an excellent combination of strength and ductility with ultimate tensile strength of 340 ± 7 MPa, yield strength of 212 ± 5 MPa, and elongation of 12.7 ± 0.9 %. This work provides theoretical and practical reference value for the Mg–RE structures obtaining an excellent combination of strength and ductility prepared by additive manufacturing.

Heterostructure enables anomalous improvement of cryogenic mechanical properties in titanium

The pursuit of strong and ductile structures for cryogenic applications has fueled a persistent interest in the microstructure design of metals and alloys. While heterostructure design has recently demonstrated efficacy in achieving superior strength-ductility combinations at room temperature, its potential remains less explored in the cryogenic regime. In this report, we unveil anomalously improved cryogenic mechanical properties of a pure titanium with harmonic heterostructure that matches or even surpasses those of commercial titanium alloys. Through detailed comparison to room-temperature deformation behavior, we reveal that the superior performance of heterostructure at cryogenic temperature is fundamentally underpinned by the amplified inter-zone mechanical incompatibility and the suppression of dislocation cross-slip. The former promotes the generation of abundant geometrically necessary dislocations, and the latter optimally configures them. Collectively, these two factors culminate in the hetero-deformation-induced effect, delivering the anomalous improvement of cryogenic mechanical properties in titanium. These findings not only significantly advance our understanding of cryo-deformation behaviors in heterostructured materials but also offer the potential for developing high-performance materials for cryogenic applications.

Revealing the Enhancement Mechanism of Laser Cutting on the Strength–Ductility Combination in Low Carbon Steel

The strength–ductility mechanism of the low-carbon steels processed by laser cutting is investigated in this paper. A typical gradient-phased structure can be obtained near the laser cutting surface, which consists of a laser-remelted layer (LRL, with the microstructure of lath bainite + granular bainite) and heat-affected zone (HAZ). As the distance from the laser cutting surface increases, the content of lath martensite decreases in the HAZ, which is accompanied by a rise in the content of ferrite. Considering that the microstructures of the LRL and HAZ are completely different from the base metal (BM, ferrite + pearlite), a significant strain gradient can be inevitably generated by the remarkable microhardness differences in the gradient-phased structure. The hetero-deformation-induced strengthening and hardening will be produced, which is related to the pileups of the geometrically necessary dislocations (GNDs) that are generated to accommodate the strain gradient near interfaces. Plural phases of the HAZ can also contribute to the increment of the hetero-deformation-induced strengthening and hardening during deformation. Due to the gradient-phased structure, the low carbon steels under the process of laser cutting have a superior combination of strength and ductility as yield strength of ~487 MPa, tensile strength of ~655 MPa, and total elongation of ~32.7%.

Good strength-ductility synergy achieved in a Fe62Ni16Co9Mn9Ti3Si1 Fe-high-entropy alloy with heterogeneous structures

Developing metal structural materials with a good combination of strength-ductility is of great significance for the rapid development of the aerospace industry and the energy industry. High-entropy alloys (HEAs) have attracted widespread attention due to their excellent mechanical properties, especially the excellent ductility and toughness of face-centered cubic (FCC) high-entropy alloys. However, similar to traditional alloys, there is a trade-off between strength and ductility in HEAs. In this paper, we report the successful preparation of Fe-HEA (Fe62Ni16Co9Mn9Ti3Si1 (atomic percentage, %)) with high strength-ductility synergy at room temperature by cryo-rolling and annealing at 600 °C for 10 min (CR600), which increases the yield strength by 59% compared to room temperature rolling (RR600), while the uniform elongation decreases by only 20%. The ultra-high yield strength and the not significant reduction in uniform elongation are mainly attributed to the formation of high-density dislocations, dual-phase bimodal ultra-fine grain heterostructures, (NiMn)3-xTix and (Fe, Ni)2SiTi composite nanoprecipitates, precipitation-induced chemical composition heterogeneity, and BCC→HCP phase transformation during tensile deformation during CR and subsequent annealing at a reduced temperature (600 °C). In addition, by increasing the annealing temperature of the alloy after CR to 800 °C, its uniform elongation can be increased to 53%, which is mainly due to the excellent strain hardening ability induced by changes in heterogeneous structures, nanoprecipitates, and martensitic phase transformation. Compared with other similar alloys, our CR alloy exhibits higher strength-ductility synergy. The results of this study provide a new perspective for developing HEAs with higher strength-ductility synergy.

Strengthening complex concentrated alloy without ductility loss by 3D printed high-density coherent nanoparticles

Metallic 3D printing enables fast fabrication of net-shaped components for broad engineering applications, yet it restrains the use of most mechanical processing methods for strengthening alloys, e.g. forging, rolling, etc. Here, we proposed a new strategy for enhancing the strength of 3D printed complex concentrated alloys without losing ductility. This strategy relies on the rapid cooling of 3D printing to achieve a supersaturation state that is beyond conventional casting. Then, spinodal decomposition via aging is exploited to introduce high-density coherent nanoparticles for strengthening. The proposed strategy is demonstrated in a 3D printed Cu-based complex concentrated alloy. The rapid solidification during printing strongly inhibits elemental diffusion, leading to a high supersaturation state. High-density nanoparticles with coherent interface and size of ∼7 nm are introduced into the 3D printed samples through spinodal decomposition via simple aging treatment. The strength of the 3D printed alloy is increased by 30 % after aging with no ductility loss, leading to a strength-ductility combination superior to other Cu alloys. This strategy is readily applicable to other spinodal alloys fabricated by 3D printing for circumventing the strength-ductility trade-off dilemma.

In-situ EBSD investigation of excellent strength-ductility synergy mechanism achieved by thermal-mechanical processing of Al0·25FeCoNiV high-entropy alloy

The development of high-entropy alloys (HEAs) is plagued by a trade-off between strength and ductility. In this study, we investigated the effects of different cold rolling ratios and subsequent heat treatments on the microstructures and mechanical properties of the Al0·25FeCoNiV HEA. An efficient and economical thermal-mechanical processing route was successfully explored to prepare multiscale, multiphase heterogeneous structures consisting of heterogeneous grain structures and duplex structures. Yield strength, ultimate tensile strength, and fracture elongation of 1277.3 MPa, 1633.6 MPa, and 26.4 %, respectively, could be achieved. The strength-ductility trade-off was avoided. Detailed information on the evolution of grain boundary distribution, grain orientation changes, and grain rotation under stress and strain fields were obtained with in-situ electron backscatter diffraction (EBSD) tensile tests. The mechanism of coordinated plastic deformation and the intrinsic mechanism of excellent strength and ductility were investigated by quantitatively calculating the activation of the slip system, slip transfer at the phase boundaries, and grain rotation paths. It was found that the excellent mechanical properties can be attributed to multistage hetero-deformation-induced (HDI) strengthening and strain-hardening coupling mechanisms. In this study, an effective method for designing novel HEAs with excellent strength-ductility combinations and an analytical method based on in-situ tests are proposed.

Achieving high strength-ductility combination of Al–Mn–Mg-Sc-Zr alloy subjected by laser powder bed fusion and suitable post heat treatment

Post heat treatment is crucial for optimizing the performance of additive manufacturing components, yet it is still challenging to synergistically improve the strength and ductility through simple heat treatment. The present work investigated the microstructure evolution and mechanical properties of laser powder bed fusion (L-PBF) additive manufactured Al–Mn–Mg-Sc-Zr alloy after aging at 300 °C for different durations. All the as-built and aged samples exhibited a typical bi-modal microstructure consisting of coarse columnar and fine equiaxed grain regions. The as-built samples contained a number of primary Al6Mn precipitates distributed along grain boundaries; meanwhile, Mg solutes segregation was visible at grain boundaries, forming “Mg walls”. After aging for 4 h, the Al6Mn and a small content of Al3Sc were precipitated. Two types of Mg-rich precipitates evolved along grain boundaries simultaneously, increasing ultimate tensile strength from 401 MPa to 501 MPa, while the ductility was significantly reduced from 19.6 % to 4.6 %. When increased to 6 h, the Mg-rich precipitates disappeared, and the density of Al6Mn and Al3Sc precipitates reached its peak concurrently, providing a dual-precipitate strengthening effect. As a result, an excellent strength-ductility balance with an ultimate tensile strength of 528 MPa and ductility of 14.5 % was obtained. The Al6Mn precipitates became coarsening with increasing to 7 h, resulting in lower ductility. The strategy of modulating elemental distribution and precipitated phases by simple heat treatment is expected to be further developed for high strength and ductility Al alloys prepared by L-PBF.

Unveiling the impact of heat treatment on powder-metallurgy α+β titanium alloy for achieving a superior strength-ductility combination

The present study investigated the effects of heat treatment (solid solution and aging treatments) on the microstructure and mechanical properties of a hot-rolled α+β Ti–3Al–5Mo–4.5V (TC16) produced through blended elemental powder metallurgy (BEPM) process using master alloy hydrogenation (MAH) approach. After solid solution treatment, metastable martensite α'' phase and athermal ω phase were formed. Aging treatment facilitated decomposition of metastable phases and precipitation of αs particles. Dispersion strengthening, resulting from the formation of a significant amount of α/β interfaces between fine secondary αs particles and β matrix, played a decisive role in enhancing the strength of the TC16 alloy. However, elevated aging temperatures promoted the coarsening of αs particles, leading to a reduction in the strengthening effect. A systematic investigation of the effects of the solid solution and aging treatments enabled the evolution of a bimodal microstructure with the precipitation of fine αs particles, contributing to an outstanding combination of strength and ductility. The ultimate tensile strength reached 1424.6 MPa with an elongation at break of 7.7%, while the hardness reached 380.2HV.

A novel cast Co68Al18.2Fe6.5V4.75Cr2.55 dual-phase medium entropy alloy with superior high-temperature performance

A newly-designed Co68Al18.2Fe6.5V4.75Cr2.55 medium entropy alloy (MEA) with FCC + B2 structure exhibits not only an excellent strength-ductility combination at room temperature, but also an outstanding high-temperature mechanical properties in particular. The phase formation, microstructure and deformation mechanism of this designed dual-phase MEA at room and high temperatures were investigated and discussed in detail. Based on experimental results (XRD, SEM and TEM) and theoretical analysis (calculated diagram and empirical thermodynamic parameters), it is demonstrated that this newly-designed MEA consists of a dual-phase structure of a face-centered-cubic (FCC) phase (70.3 vol %) plus chemically ordered body-centered cubic phase (B2) (29.7 vol %). Meanwhile, the FCC phase is enriched with Co, Fe and Cr elements, while the B2 phase is enriched with Al and Co elements. Furthermore, compared with some HEAs/MEAs and commercial alloys, this newly-designed dual-phase MEA exhibits excellent strength-ductility combination with a high compressive yield strength of ∼1254 MPa, a fracture strength of ∼2657 MPa and a large compressive plasticity of ∼34.6 % at room temperature. In particular, the studied MEA also shows an outstanding high-temperature performance with an ultrahigh yield strength of ∼1213 MPa at 600 °C and ∼626 MPa at 800 °C, respectively. The excellent comprehensive mechanical properties of this designed dual-phase MEA at both room and temperatures mainly originate from synergy effect of alternate FCC + B2 structure and strengthening contributions of B2 phase. These important findings not only reveal the deformation and strengthening mechanisms of the newly-designed dual-phase MEA, but also provides an effective alloy design strategy for developing advanced structural materials serving as a wide range of temperature.

A bimodal microstructure to improve the strength-ductility properties of Ti-Cu alloys by using eutectoid reactive laser depositing

A novel bimodal structure was achieved in Ti-6wt%Cu by laser metal deposition. The microstructures and tensile behaviors of this heterostructure, comparing with typical colony structure, were examined to understand the mechanisms of strengthening and deformation. The bimodal structure consisting of nano-scale eutectoid lamellae and fine Widmanstätten-α exhibits the strength-ductility combination and excellent strain hardening ability. The shear deformation mechanism in ultrafine lamellae and hetero-deformation induced hardening in bimodal structure contribute to the high strain hardening. In contrast, the colony structure with coarse laths and Ti2Cu precipitates show inferior dislocation strengthening leading to a lower tensile strength, while the formation of deformation twins ensures a higher ductility.

Synergic strength-ductility enhancement in bimodal titanium alloy via a heterogeneous structure induced by the 〈c + a〉 slip

In this work, a heterogeneous structure (HS) with an alternating distribution of coarse and fine α lamella is fabricated in bimodal Ti6242 alloy via insufficient diffusion of alloying elements induced by fast heating treatment. Instead of a distinct interface between the primary α phase (αp) and β transformation microstructure (βt) in the equiaxed microstructure (EM), all αp/βt interfaces are eliminated in the HS, and the large αp phases are replaced by coarse α lamella. Compared to the EM alloy, the heterostructured alloy exhibits a superior strength-ductility combination. The enhanced strength is predominantly attributed to the increased interfaces of α/β plates and hetero-deformation induced (HDI) strengthening caused by back stress. Meanwhile, good ductility is ascribed to its uniform distribution of coarse and fine α lamella, which effectively inhibits strain localization and generates an extra HDI hardening. This can be evidenced by the accumulated geometrically necessary dislocations (GNDs) induced by strain partitioning of the heterostructure. Significantly, the HDI causes extra 〈c+a〉 dislocations piling up in the coarse α lamella, which generates an extra strain hardening to further improve the ductility. Such hetero-interface coordinated deformation mechanism sheds light on a new perspective for tailoring bimodal titanium alloys with excellent mechanical properties.

Enhanced strength-ductility combination in the aluminum-gold system by heterogeneous distribution of nanoparticles via ultra-severe plastic deformation and reactive interdiffusion

Ultrafine-grained aluminum alloys are of interest due to their high strength-to-weight ratio, but they usually suffer from poor uniform ductility. In this study, an Al-Au alloy with a good combination of strength and ductility is produced by the heterogeneous distribution of Al2Au nanoparticles in an aluminum matrix. To generate such heterogeneity, the alloy is synthesized by ultra-severe plastic deformation of aluminum and gold powders via the high-pressure torsion (HPT) method. Reactive interdiffusion occurs during the process leading to the heterogeneous formation of intermetallic particles and a good strength-ductility synergy (200 MPa yield stress and 15% uniform elongation). Nanoparticles gradually distribute within the matrix and once a uniform nanoparticle distribution is achieved, the alloy shows no further increase in strength, but it completely loses its ductility. It is concluded that not only the presence of nanoparticles but more importantly the heterogeneity of their distribution can positively influence the strength-ductility combination in ultrafine-grained aluminum alloys. The findings of this study suggest that future studies on heterogeneous precipitation hardening can be a solution to achieve ductile precipitation-hardened alloys.

Developing strong-yet-ductile light-weight medium-entropy alloy via the unusual oxide doping effect

High or medium entropy alloys (M/HEAs) possess outstanding mechanical and thermal properties, but their high mass density severely restricts their practical applications. In this study, we designed a novel light-weight MEA based on the TiAlCrNb-x(ZrO2) system. These ZrO2 particles doped can completely dissolve in the β-phase structure of the TiAlCrNb MEA, which contrasts with conventional ceramic strengthening introduced through powder metallurgy. Consequently, it promotes the formation of Ti3Al nanoparticles and simultaneously substantially reduces grain size, thereby achieving a remarkable strength-ductility combination with a high specific yield strength of 225/250 MPa/(g·cm−3) while maintaining an excellent tensile ductility of 19.5/10.1% in the newly designed TiAlCrNb-(1.2/1.5)(ZrO2) MEAs. This performance outperforms most other light-weight M/HEAs. Strengthened mechanisms analysis indicates that the strength increment is attributed to grain-refinement-induced Hall-Petch strengthening and solid-solution strengthening from the dissolved oxygen. The results offer insight into the innovative design of ultrastrong and ductile light-weight MEAs for advanced structural applications.

Grain refinement induced unusually large shape memory effect in lightweight titanium alloy

High-performance and lightweight shape memory alloys (SMAs) play a pivotal role in a variety of cutting-edge fields like aerospace, which however, are still quite difficult to be obtained until now. In the present work, we meticulously develop a novel lightweight Ti-12.5V-3.5Al-1Hf-0.8Fe-0.01B (at.%) SMA, which has a low mass density of ∼4.73 g/cm3. Prominently, distinct from the conventional wisdom in previous studies that is mainly based on eliminating grain boundaries or developing desired texture, we utilize the grain refinement strategy introduced by elaborate thermal-mechanical training, which effectively enables us to achieve exceptional shape memory response and strength-ductility combination simultaneously. It was revealed that a dominant β phase structure is achieved in this newly designed fine-grain SMA, accompanied by some α″ variants penetrating the grain boundaries. Compared to the coarse-grained counterpart, the deformation process becomes more intricate, involving stress-induced martensitic transformation and additional reorientations, which ultimately results in an unusually large recoverable strain up to 9.3 %. Furthermore, the fine-grain structure also contributes to an enhanced specific yield strength up to ∼159 MPa·m³/kg, while maintaining a large tensile ductility of ∼30 %. In light of its impressive structure-function integration and cost efficiency, this new-type SMA would emerge as a promising candidate for various advanced lightweight applications in a broad spectrum of fields.

Strength-Ductility Mechanism of CoCrFeMnNi High-Entropy Alloys with Inverse Gradient-Grained Structures.

The microstructures and mechanical properties of equiatomic CoCrFeMnNi high-entropy alloys (HEAs) treated with various processing parameters of laser surface heat treatment are studied in this paper. The typical inverse gradient-grained structure, which is composed of a hard central layer and a soft surface layer, can be obtained by laser surface heat treatment. A much narrower gradient layer leads to the highest yield strength by sacrificing ductility when the surface temperature of the laser-irradiated region remains at ~850 °C, whereas the fully recrystallized microstructure, which exists from the top surface layer to the ~1.05 mm depth layer, increases the ductility but decreases the yield strength as the maximum heating temperature rises to ~1050 °C. Significantly, the superior strength-ductility combination can be acquired by controlling the surface temperature of a laser-irradiated surface at ~1000 °C with a scanning speed of ~4 mm/s due to the effect of hetero-deformation-induced strengthening and hardening, as well as the enhanced interaction between dislocation and nanotwins by the hierarchical nanotwins. Therefore, retaining the partial recrystallized microstructure with a relatively high microhardness in the central layer, promoting the generation of hierarchical nanotwins, and increasing the volume proportion of gradient layer can effectively facilitate the inverse gradient-grained CoCrFeMnNi HEAs to exhibit a desirable strength-ductility synergy.

Achieving ultra-strong and ductile CoNi-based FCC multi-principal element alloys via alloying with refractory Mo and W

FCC single-phase multi-principal element alloys (MPEAs) often suffer from insufficient strength, which limits their wide applications. Here a CoNi-based FCC MPEA (Co44Ni44Mo9W3, at. %) was designed by alloying with refractory Mo and W. The Co44Ni44Mo9W3 alloy possesses an ultrahigh yield strength of ∼1050 MPa and good ductility of ∼30 % at ambient temperature. The excellent mechanical properties were correlated to its superior lattice friction stress and grain boundary strengthening compared to most reported FCC MPEAs and conventional alloys. By density-functional theory calculations, such extraordinary mechanical properties were further attributed to its high lattice distortion, low intrinsic stacking fault energy and high unstable stacking fault energy due to the addition of Mo and W. Transmission electron microscopy investigations revealed that the high strength-ductility combination arises from the synergistic effect of planar slip bands, stacking faults and deformation nano-twins.

Microstructures and precipitation behaviors of a hypoeutectic high-entropy alloy prepared by laser powder bed fusion

The trade-off between strength and ductility remains a significant challenge for high-entropy alloys (HEAs). The layer-by-layer printing manner inherent in the laser powder bed fusion (L-PBF) leads to distinctive microstructures in alloys, in turn yielding unexpected performances that are promising for resolving this contradiction. At present, the influence of L-PBF technology on the microstructures and mechanical properties of near-eutectic HEAs has yet to be explored. In the present work, a metastable AlCo0.4CrFeNi2.7 hypoeutectic HEA with an extraordinary strength-ductility combination was successfully fabricated using L-PBF. The precipitation behaviors and mechanical properties of the alloy were further modulated through annealing. The results reveal that the high cooling rates inherent in L-PBF promote the formation of a “pseudoeutectic” structure featuring the face-centered-cubic (FCC) + body-centered-cubic (BCC) dual-phase nanolamellae. Furthermore, after annealing at 600–750 °C, the L12 and B2 precipitates are generated in the FCC lamellae, accompanied by a coarsening of the original Cr-rich precipitates (BCC) in the BCC lamellae. The as-printed sample exhibits remarkable mechanical properties, posing a yield strength of 1007 MPa, an ultimate strength of 1249 MPa, and a total elongation of ∼25.1 %. After annealing at 750 °C, the tensile yield strength and ultimate strength are further increased to 1201 MPa and 1512 MPa, respectively, while the elongation is maintained at ∼15.1 %. The analysis results demonstrate that the high strength originates from the hard BCC lamellae and nanolamellar interfaces, while the good ductility stems from the soft FCC lamellae and coordinated deformation of the dual-phase nanolamellae. Furthermore, the strength increase is further substantiated through the introduction of annealing-induced nanoprecipitates, thereby contributing to additional precipitation strengthening.

A novel D022-strengthened medium entropy alloy with outstanding strength-ductility synergies over ambient and intermediate temperatures

Precipitation-strengthened medium/high-entropy alloys (MEAs/HEAs) have great potential for high-temperature applications. In this study, we designed a novel Ni45.9Fe23Cr23V4Nb3Mo1B0.1 (at.%) MEA alloy, hardened by the D022 (Ni, Fe, Cr)3(Nb, V)-type nanoprecipitates, with an excellent strength-ductility combination from room to elevated temperatures. Specifically, the tensile strengths, at 700 and 800 °C, could be maintained as high as 845 and 589 MPa, respectively; meanwhile, elongations at all testing temperatures exceeded 25 % without any intermediate-temperature embrittlement. The temperature-dependent deformation mechanisms were unraveled using multi-scale characterizations, which involved profound slip planarities, such as stacking fault (SF) networks and deformation twins (DTs). Furthermore, the critical resolved shear stress (CRSS) to initiate SFs in both face-centered cubic (FCC) and D022 phases was evaluated, and the possible reasons for the origin of anomalous DTs at 800 °C were discussed in detail. The main findings demonstrate that the shearable D022 nanoparticles can provide the FCC matrix with considerable dislocation storage capacity, reinforcing strain hardening at ambient and intermediate temperatures. This work provides fundamental insights into the controllable design and deformation mechanisms of high-performance D022-strengthened MEAs/HEAs.

Dual heterogeneous structure enabled ultrahigh strength and ductility across a broad temperature range in CrCoNi-based medium-entropy alloy

Developing alloys with exceptional strength-ductility combinations across a broad temperature range is crucial for advanced structural applications. The emerging face-centered cubic medium-entropy alloys (MEAs) demonstrate outstanding mechanical properties at both ambient and cryogenic temperatures. They are anticipated to extend their applicability to elevated temperatures, owing to their inherent advantages in leveraging multiple strengthening and deformation mechanisms. Here, a dual heterostructure, comprising of heterogeneous grain structure with heterogeneous distribution of the micro-scale Nb-rich Laves phases, is introduced in a CrCoNi-based MEA through thermo-mechanical processing. Additionally, a high-density nano-coherent γ' phase is introduced within the grains through isothermal aging treatments. The superior thermal stability of the heterogeneously distributed precipitates enables the dual heterostructure to persist at temperatures up to 1073 K, allowing the MEA to maintain excellent mechanical properties across a wide temperature range. The yield strength of the dual-heterogeneous-structured MEA reaches up to 1.2 GPa, 1.1 GPa, 0.8 GPa, and 0.6 GPa, coupled with total elongation values of 28.6 %, 28.4 %, 12.6 %, and 6.1 % at 93 K, 298 K, 873 K, and 1073 K, respectively. The high yield strength primarily stems from precipitation strengthening and hetero-deformation-induced strengthening. The high flow stress and low stacking fault energy of the dual-heterogeneous-structured MEA promote the formation of high-density stacking faults and nanotwins during deformation from 93 K to 1073 K, and their density increase with decreasing deformation temperature. This greatly contributes to the enhanced strain-hardening capability and ductility across a wide temperature range. This study offers a practical solution for designing dual-heterogeneous-structured MEAs with both high yield strength and large ductility across a wide temperature range.