Face-centered Cubic High-entropy Alloys Research Articles

Strengthening Al0.1CoCrFeNi high-entropy alloy via multiaxial cryogenic forging and low temperature annealing

Face-centered cubic (FCC) high-entropy alloys (HEAs) exhibit excellent fracture toughness and fatigue properties at both ambient and cryogenic temperatures. However, the relatively low strength of FCC HEAs at room temperature limits their widespread application. Herein, we present a novel approach to tailoring the microstructure of the Al0.1CoCrFeNi HEA and improving its room temperature tensile strength through multiaxial cryogenic forging (MACF) and low temperature annealing (LTA). After the treatments, the tensile strength increases from 687 MPa for the as-prepared fine-grained HEA to 1487 MPa for the MACF specimens annealed at 673 K for 48 h. Our analysis indicates that the strengthening mechanisms are twofold: the tensile strength increased by about 455 MPa after the MACF treatment, which is due to the synergistic effect of dislocations and nanotwins; after LTA, the tensile strength abnormally increased by about 345 MPa, which can be attributed to stacking faults, hexagonal close-packed, and 9R structures formed by thermal relaxation of nanotwin boundaries.

Unexpected sluggish martensitic transformation in a strong and super-ductile high-entropy alloy of ultralow stacking fault energy

Displacive martensitic transformation usually prevails in face-centered cubic (FCC) steels and high-entropy alloys (HEAs) of low stacking fault energy (SFE; e.g., <15 mJ/m2) upon deformation. Here, we present and rationalize an unexpectedly sluggish and suppressed martensitic transformation behavior in a newly developed non-equiatomic HEA (Fe30Ni20Co20Cr20Si10, at.%) with an ultralow SFE, i.e., ∼7 mJ/m2 measured by weak-beam dark-field scanning transmission electron microscopy (STEM) at room temperature. The recrystallized FCC HEA with an average grain size of ∼14.9 µm shows an ultimate tensile strength of 903 MPa and a fracture elongation of ∼83.1 %. Plastic deformation of this alloy with ultralow SFE is characterized by the intensive formation of stacking faults (SFs), whereas the volume fraction of hexagonal close-packed (HCP) martensite is very low even at cold-rolled state with an equivalent strain of 1.85, i.e., < 8.3 % according to neutron diffraction analysis. Atomic-scale TEM observation demonstrates that chemical short-range order (CSRO) domains are present in the FCC solid solution. Apart from contributing to the high strength, the CSRO domains and atomic size misfit (2.36 %) promoted by the high content of Si are regarded to enhance the local barrier for the successive accumulation of SFs, hindering the formation of HCP martensite and mechanical twin. The uniform formation of SFs in the FCC matrix upon deformation effectively alleviates strain localization, contributing to the sustained strain-hardening ability and super-ductility. These insights suggest a strategy to design low-SFE alloys with excellent mechanical properties via manipulating the rate of martensitic transformation by CSROs and atomic size misfit.

Connecting the mechanical properties of magnesium and high-entropy alloys

The deformation mechanisms in individual crystal structures have been extensively studied, such as the yielding phenomenon in face-centered-cubic (FCC) high-entropy alloys (HEAs) and the ductility in hexagonal close-packed (HCP) magnesium alloys. However, the physical connection of these mechanisms between the different crystal structures has rarely been investigated. For example, high-strength HEAs and ductile magnesium alloys both require low stacking fault energy (SFE), and the physical origin of this common feature is unclear. Here we provide a physical model for the SFE trend in FCC HEAs that connects the two phenomena. The physical model predicts the change of SFE based on elastic constants and geometrical parameters. This study paves the pathway to unifying the mechanics of close-packed materials.

Effect of silicon addition on the corrosion resistance of Al0.2CoCrFe1.5Ni high-entropy alloy in saline solution

In the paper, the effect of Si content on the microstructure evolution and corrosion properties of Al0.2CoCrFe1.5NiSix (x = 0, 0.1, 0.2, 0.3) high entropy alloys (HEAs) was systematically investigated. With the addition of Si element, the microstructure changed from single face-centered cubic (FCC) phase in the Al0.2CoCrFe1.5Ni and Al0.2CoCrFe1.5NiSi0.1 HEAs to a dual-phase structure composed of FCC and body-centered cubic (BCC) phases in the Al0.2CoCrFe1.5NiSi0.2 and Al0.2CoCrFe1.5NiSi0.3 HEAs. In addition, the corrosion mechanism has been analyzed by elemental analysis and corrosion morphology. The electrochemical measurement results indicated that the Al0.2CoCrFe1.5NiSi0.1 HEA possessed the best corrosion resistance (Ecorr = − 215 mVAg/AgCl, Icorr = 256 nA/cm2) because the small amount of Si element improves the corrosion resistance of the single-phase FCC HEA. When the Si content increased to 4.08 at% (x = 0.2) and 6.00 at% (x = 0.3), the galvanic corrosion was caused by BCC, resulting in the selective dissolution of the BCC phase and the decrease of corrosion resistance. This research results will help to understand the potential applications of doped trace Si element as a matrix element in single-phase FCC HEAs with good corrosion resistance.

Re alloying-driven stacking fault energy decrease enabled strength-ductility synergy in NiCoCrFe high-entropy alloys

Metallic materials have long been enslaved in the dilemma of strength-ductility trade-off, which limits their potential applications. Here, we report on a new class of non-equiatomic Re-added Ni-Co-Cr-Fe face-centered cubic (FCC) high-entropy alloys (HEAs) with superior yield strength to those of Mo-added and other FCC HEAs without compromising ductility. The strengthening mechanisms of these HEAs were discussed in detail by experimental observation of the recrystallized microstructure, dislocation configuration and deformation substructure combined with theoretical evaluation of yield stress and stacking fault energy (SFE). It has been clarified that the unique work-hardening capacity and deformability of these HEAs is attributed to solid solution strengthening and the grain boundary strengthening caused by refined crystalline on the one hand. And what's more important, the appropriate addition of Re greatly reduces SFE so as to modulate the dislocation slip and dissociation behavior and promote phase transformation. It has been confirmed that the plastic deformation process was controlled by abundant microbands, multiple nanoscale deformation twins and phase transformation in the Re-added HEAs.

Achieving High Strength and Good Ductility in a Nb-Containing CoCrNi-Based High-Entropy Alloy by Grain Boundary and Precipitates Strengthening

Face-centered cubic (FCC) high-entropy alloys (HEAs) have attracted considerable attention due to their excellent mechanical properties; however, an insufficient yield strength (YS) limits their widespread engineering applications. To improve the strength of FCC HEAs, the present work aims to develop fine-grained Nb-containing CoCrNi-based HEAs with precipitates. In the present work, a single-phase FCC CoCrNi1.5Nb0.2 HEA was processed by cold rolling followed by annealing at a higher temperature and aging at a lower temperature, yielding fine- and ultra-fine-grained FCC matrices and two types of precipitates: ultra-fine granular C15 Laves phase CoCrNb-based precipitates and ultra-fine lath-shaped D019-structured ε-Ni3Nb-based precipitates. The resultant alloy exhibits a combination of high strength (approximately 1409 MPa of yield strength) and good ductility (10.1% of uniform elongation). The contributions of grain boundary and precipitation strengthening to YS were analyzed and calculated. The mechanisms underlying good ductility were discussed.

Realizing excellent tribological properties of BCC/FCC gradient high -entropy alloy coating via an in-situ interface reaction

The simultaneous improvement of strength and toughness has been a long-standing problem in the pursuit of high-performance materials. Using a gradient-structure design that exploits the migration and recrystallization processes of elements in the molten pool, this study generates an FeCoCrNiAl intermediate interfacial layer from FeCoCrNi and FeCoCrAl powders in situ. Further, a strong and tough integrated composite high–entropy alloy (HEA) coating with an outer layer of high-strength body centered–cubic (BCC) HEA, an interface of BCC and face centered–cubic (FCC) dual-phase HEA, and an inner layer of high-toughness FCC HEA is innovatively designed and prepared. The Vickers hardness of the composite exhibited a stepped gradient of 594.4Hv0.5, 311.8Hv0.5, and 182.9Hv0.5 from top to bottom. Subsequently, the surface and cross-sectional tribological behaviors of the gradient coating are studied in various air environments. The composite HEA coating overcomes the brittle fracture problem of single FeCoCrAl coatings and the adhesion problem of single FeCoCrNi coatings. The composite HEA coating exhibits a low friction coefficient, a small width/depth, and a low wear rate (0.374 × 10−5 mm3/(N·m)). The cross-section wear is a dynamic friction process coupled with multiple wear mechanisms, and the wear marks are characterized by ladders, width changes, and debris accumulation. This new approach can guide the future development and design of HEA wear-resistant gradient coatings and HEA composite coatings.

Dislocation mediated dynamic tension-compression asymmetry of a Ni2CoFeV0.5Mo0.2 medium entropy alloy

Although tension-compression (T-C) asymmetry in yield strength was rarely documented in coarse-grained face centered cubic (FCC) metals as critical resolved shear stress (CRSS) for dislocation slip differs little between tension and compression, the T-C asymmetry in strength, i.e., higher strength when loaded in compression than in tension, was reported in some FCC high entropy alloys (HEAs) due to twinning and phase transitions activated at high strain regimes in compression. In this paper, we demonstrate a reversed and atypical tension-compression asymmetry (tensile strength markedly exceeds compressive strength) in a non-equiatomic FCC Ni2CoFeV0.5Mo0.2 medium entropy alloy (MEA) under dynamic loading, wherein dislocation slip governs dynamic deformation without twins or phase transitions. The asymmetry can be primarily interpreted as higher CRSS and more hard slip modes (lower average Schmid factor) activated in grains under dynamic tension than compression. Besides, larger strain rate sensitivity in dynamic tension overwhelmingly contributes to the higher flow stress, thanks to the occurrence of more immobile Lomer-locks, narrower spacing of planar slip bands and higher dislocation density. This finding may provide some insights into designing MEAs/HEAs with desired properties under extreme conditions such as blast, impact and crash.

Effects of grain size and Al addition on the activation volume and strain-rate sensitivity of CoCrFeMnNi high-entropy alloy

The activation volume (V*) and strain-rate sensitivity exponent (m) of CoCrFeMnNi and Al0.5CoCrFeMnNi high entropy alloys (HEAs) with various grain sizes (ranging between 2.4 and 356 μm) were measured at different strain rates and strain levels at room temperature. As the strain rate decreased, the plastic strain decreased, but the grain size and V* increased. The enhanced solid-solution strengthening by addition of aluminum decreased V*. The Hassen plot was modified to capture the grain-size dependence of V* by considering the grain-size dependence of the dislocation density. As the plastic strain increased, the strain rate, grain size and m decreased. The behavior of m could be quantitatively predicted by using equations derived for the grain-size-dependent V*and flow stress. The difference in the grain-size dependence of m between conventional face centered cubic (FCC) metals and FCC HEAs over the nanograin size range could not be explained in terms of a relatively large Hall-Petch slope in the latter compared with the former but was explainable in terms of a substantially higher probability of activation of grain-boundary diffusion-controlled grain-boundary-sliding mechanism at nanograin sizes in the former than in the latter.

Prediction of elastic properties of face-centered cubic high-entropy alloys by machine learning

The machine learning (ML) models were proposed for predicting elastic properties of face-centered-cubic (FCC) high-entropy alloys (HEAs). The data set was from the first-principles calculation, which contained 186 samples. The goodness-of-fit (R2) values of predicted bulk modulus (B) and shear modulus (G) in the test set were 0.81 and 0.84, respectively. According to the results of ML, CoNiCuMoW HEAs have the largest B, G, elastic modulus (Y) and good ductility (G/B≤0.57) among the FCC HEAs with equal components. The first-principles calculation results show that the elastic anisotropy of (CoNiCuMo)1−xWx HEAs increases and ductility decreases with increasing W content. According to the analysis of charge density difference, there is obvious charge accumulation at W—W and W—Mo bonds, indicating the directional covalent bonds formed between W atoms and their neighboring atoms.

Adiabatic shear localization induced by dynamic recrystallization in an FCC high entropy alloy

Due to their strong strain hardening, face-centered cubic (FCC) high/medium entropy alloys (H/MEAs) have remarkable resistance to shear localization even under forced dynamic shear. In this work we investigated the dynamic mechanical behavior of an FCC Al0.1CoCrFeNi HEA via a series of split Hopkinson pressure bar (SHPB) experiments at 77 K and room temperature. This HEA exhibited very strong strain hardening capacity, remarkable strain rate sensitivity and moderate temperature dependence. Such a combination of mechanical properties should be strongly unfavorable for adiabatic shear bands (ASBs). However, its dynamic compressive failure at 77 K was found to be caused by crack propagation within the primary ASBs along the directions of maximum shear stress. Detailed examination of the specimens using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed convincing microstructural evidences for massive dynamic recrystallization (DRX) prior to ASB. We suggested that the expansion and coalescence of these DRX softened regions served as the root cause for the occurrence of ASBs and the final catastrophic fracture of specimens. The severe local shear flow, substantial adiabatic temperature rise and extremely fast quenching within the ASB resulted in ultrafine grains therein. Our results and analyses provided a plausible understanding of the formation mechanisms of ASBs in the FCC HEA under impact loading.

Investigation on microstructure, superior tensile property and its mechanism in Al0.3CoCrFeNi high-entropy alloy via thermo-mechanical processing

In this study, the fully annealed Al0.3CoCrFeNi high entropy alloy (HEA) with single-phase face-centered cubic (FCC) was cryo-rolled and isochronal annealed at different temperatures from 1123 K to 1473 K for 1 h. The effects of annealing temperature on microstructure, grain boundary evolution, and mechanical properties, especially the deformation mechanism of cryo-rolled Al0.3CoCrFeNi alloys were systematically investigated using XRD, EBSD, TEM, microhardness, and tensile tests. A partial recrystallization was achieved after annealing in the 1123 K and 1223 K temperature ranges, which was mainly attributed to that the precipitation of B2 phase can effectively improve the nucleation rate of phase boundary and play a barrier role in growth of grain. Additionally, The remarkable combination of strength (yield strength of ∼911 MPa and ultimate tensile strength of ∼1146 MPa) and ductility (fracture elongation of ∼23%) achieved in cryo-rolled Al0.3CoCrFeNi alloy annealed at 1123 K with heterogeneous microstructure was mainly due to the contributions of back stress and grain size. It was noticeable that the FCC → 9R phase → twin transformation was found in cryo-rolled Al0.3CoCrFeNi HEA annealed at 1473 K during the tensile deformation, which indicated that twin-induced plasticity and phase transformation induced plasticity were reason for the outstanding ductility and strain hardening ability of this sample. This research are important not only for understanding the strengthening mechanisms of HEAs with various microstructure in general, but also for guiding thermo-mechanical processing of single-phase FCC HEAs.

Enhanced strengthening effect via nano-twinning in cryo-rolled FeCoCrNiMo0.2 high-entropy alloys

High-entropy alloys (HEAs) with face-centered cubic (FCC) structure generally exhibit extraordinary ductility but relatively low strength, which limits its widespread applications as engineering structural materials. Enhancing the strength of FCC HEAs is crucial to broaden its application. In present work, by performing rolling at cryogenic temperature (77 K), we obtained a significantly enhanced ultimate strength as high as 1557.7 MPa in a typical single-phase FCC FeCoCrNiMo0.2 HEA. A high intensity of Brass texture and Goss texture are formed in the cryo-rolled specimens. With the increasing of cryo-rolling reduction, twins are generated inside grains with the Copper texture, forming the Copper-twin texture. Due to the low stacking fault energy of FeCoCrNiMo0.2 HEA, abundant nanoscale deformation twins and stacking faults are generated inside the cryo-rolled specimens. Besides, a high density of dislocation networks and Lomer-Cottrell dislocation locks are also observed. Hence, the interactions among the dislocations, nano-twins, stacking faults, and Lomer-Cottrell dislocation locks are speculated to effectively contribute to the enhanced ultimate strength and work hardening. Our results are forward to improve the strength of single-phase FCC HEAs as structural materials.

Effects of stacking fault energy and temperature on grain boundary strengthening, intrinsic lattice strength and deformation mechanisms in CrMnFeCoNi high-entropy alloys with different Cr/Ni ratios

Tensile properties and deformation mechanisms at 293 and 77 K of single-phase face-centered-cubic (FCC) CrxMn20Fe20Co20Ni40-x (0 ≤ x ≤ 26 at.%) high-entropy alloys (HEAs) with grain sizes (d) in the range [6–283 µm] were studied. A specificity of these alloys is that their stacking fault energy (SFE) strongly decreases with increasing the Cr/Ni ratio while all the other characteristics affecting strength (elastic moduli and solute misfit volumes) remain approximately constant. Therefore, these alloys allow to investigate how the SFE affects the tensile behavior and deformation mechanisms of FCC HEAs. Grain boundary strengthening is found to be nearly independent of SFE, composition, and temperature. While the intrinsic lattice strength (σ0 ≈ 136 MPa) remains approximately constant at 293 K, it strongly increases with increasing Cr/Ni ratio and thus decreasing SFE at 77 K, presumably due to a reduction of dislocation line tension. Deformation twinning occurs at 293 K when x ≥ 20 at.% (SFE ≤ 35 mJ/m2) and in all alloys at 77 K. The strength-ductility combination is found to increase with decreasing SFE only when 14 ≤ x ≤ 24 at.% at 77 K because twinning must be triggered at relatively low plastic strains to yield a significant dynamic Hall-Petch effect, allowing to sustain a high work hardening rate and postpone necking instability. In addition to twinning, an FCC-to-HCP (hexagonal close-packed) martensitic transformation was observed at 77 K in the Cr26Mn20Fe20Co20Ni14 HEA, which induced early rupture when d = 212 µm.

Dynamic tensile behavior of Al0.1CoCrFeNi high entropy alloy: Experiments, microstructure and modeling over a wide range of strain rates and temperatures

High entropy alloys (HEAs) are considered as new metallic materials with great application prospects due to their excellent mechanical properties. Massive experimental studies have revealed simultaneous strength-ductility enhancements in face-centered-cubic (FCC) HEAs resulting from the plasticity mechanisms transition under much harsher conditions. However, it still lacks quantitative understanding of such extraordinary mechanical behaviors. Aiming at precisely comprehending and describing the mechanical behaviors of FCC HEAs, a physical mechanism-based constitutive model is quite desirable to be established. As such, we first systematically investigated the uniaxial tension behavior of Al0.1CoCrFeNi HEA over wide strain rate (10−3-7 × 103s−1) and temperature (77–298K) ranges. Subsequent microstructural analyses indicated that its plasticity was dominated by dislocation slip at 298K or during small strain, while deformation twins acted as an additional plasticity mechanism with an increase of flow stress at higher strain rates or cryogenic temperatures. Moreover, combining theoretical calculations and experimental verification we further determined the critical twinning stress for this HEA. On basis of these experimental analyses, some specific models were employed to quantify the evolution of dislocation density or twin volume fraction during the plastic deformation. Finally, a physical-based constitutive model was developed to accurately describe the strain rate and/or temperature-dependent behaviors of this typical FCC HEA. This work not only helps quantitatively understand effects of various micro-mechanisms on the mechanical behaviors, but also provides the numerical simulation foundation for their future applications.

Abnormal hardening and amorphization in an FCC high entropy alloy under extreme uniaxial tension

Ever more extreme environments in advancing industrial applications motivate efforts to design new generation of alloys with excellent mechanical properties. High entropy alloys (HEAs) are considered as promising candidates for bearing extreme loadings. However, the reported results of correspondence between extremely mechanical behaviors and micromechanisms are still in infancy. Here we showed mechanical responses over wide strain rate and temperature ranges and microscopic observations recovered from dynamic uniaxial tension of face-centered cubic (FCC) HEA to reveal plasticity mechanisms transition under various loading conditions. In particular, with an increasing strain during the extreme dynamic tension a sequence of mechanisms, including twinning, detwinning-induced localization, nanoscale body-centered cubic (BCC) phase transformation and symbiotic amorphization, were progressively activated for continued plasticity. At strains over 35%, twin boundaries hindrance to localized band propagation and more dislocations emitting from BCC phases even collectively caused a second sharp rise in hardening. Additionally, the plasticity kept increasing during crystal to amorphization transition, which should be promoted by severe lattice distortion in super-dense dislocations areas. By combining large scale molecular dynamic simulations, the causes of phase transition to BCC lattice structure or even to amorphous bands were explained in depth. The thorough uncovering of these brand-new mechanisms can help understand the plasticity and amorphization of HEAs under the extreme loadings.

Tuning deformation mechanisms of face-centered-cubic high-entropy alloys via boron doping

Face-centered-cubic (FCC) high-entropy alloys (HEAs) can be strengthened through boron doping. However, the existing form of boron and its effect on the deformation mechanism of FCC HEAs have been rarely reported. The present work demonstrates for the first time the effect of the interstitial boron and boron-rich precipitate on the deformation mechanisms of the FCC (CoCrFeNi) HEAs. The dominant deformation mechanism of CoCrFeNi HEAs is dislocation slip, whereas the interstitial boron atoms dissolved in the matrix, thereby reducing the stacking fault energy (SFE) and transforming the dominant deformation mechanism to deformation twins. The loss of Cr from the matrix after boride formation increased the SFE, which led to transforming the dominant deformation mechanism from deformation twins to dislocation slip. Based on the results, we firstly propose to tune the deformation mechanism of FCC HEAs via boron doping, which provide a new strategy for designing and optimizing the HEAs. • Adding boron to CoCrFeNi HEA can significantly improve the yield strength. • The different existing form of boron will change the SFE of CoCrFeNi HEA, thereby affecting its deformation mechanism. • We firstly propose to tune the deformation mechanism of FCC HEAs via boron doping.

Size effects and plastic deformation mechanisms in single-crystalline CoCrFeNi micro/nanopillars

As emerging and revolutionary alloy materials, high entropy alloys (HEAs) have been extensively studied because of their unique composition, microstructures and excellent mechanical properties and performances. However, there have been limited studies on the deformation behaviors and mechanisms of HEA single crystals at the micro/nanoscale. Here, we fabricated single-crystalline CoCrFeNi HEA pillars with typical orientations of <100>, <110> and <111> and diameters ranging from 272 nm to 1,253 nm and then conducted in situ uniaxial compression tests on these fabricated micro/nanopillars inside a scanning electron microscope. The in situ compression tests showed pronounced size effects on yield and flow stresses in the <100>-, <110>- and <111>-oriented micro/nanopillars, i.e., both yield and flow stresses follow a scaling law with pillar diameter. The scaling exponents of <110>- and <111>-oriented micro/nanopillars are approximately -0.60, which is close to those of face-centered cubic (FCC) pure metallic micro/nanopillars. However, the <100>-oriented pillars, having a scaling exponent of -0.32∼-0.17, exhibit weaker size effects on yield and flow stresses compared with the <110>- and <111>-oriented micro/nanopillars. Combining observations and analyses using transmission electron microscopy, and large-scale atomistic simulations, we revealed that the nucleation and slip of full dislocations dominate the plastic deformation in the <110>- and <111>-oriented micro/nanopillars, while deformation twinning is a governing mechanism in the <100>-oriented micro/nanopillars. These underlying deformation mechanisms are responsible for the size effects of single-crystalline HEA micro/nanopillars. Large-scale atomistic simulations further reveal that twin nucleation in <100>-oriented pillars is initiated by the slip of pre-existing partial dislocations from an internal source. By considering the free energy change during partial slip, we used a theoretical model to predict the scaling law for the size effects of <100>-oriented micro/nanopillars. The predicted scaling exponent for the size effect of <100>-oriented micro/nanopillars is consistent with our experimental results. Our current study sheds light on the underlying deformation mechanisms in FCC HEA single crystals at the micro/nanoscale, which provides a guide for the design and fabrication of HEAs with high strength and remarkable plasticity.

An investigation on the microstructure and mechanical properties of Al0.3CoCrFeNi high entropy alloy with a heterogeneous microstructure

In this study, the Al0.3CoCrFeNi high entropy alloy (HEA) with various heterogeneous microstructures was designed and fabricated through different process histories, such as severely cold-rolled plus annealing at low temperature for a long time (S1), cryo-rolled plus annealing at intermediate temperature (S2), cold-rolled plus annealing at intermediate temperature (S3). The microstructures were analyzed by SEM, EBSD, and TEM. The results showed that the heterogeneous grain structure of S1 alloy was composed of stretched original grains and fine recrystallized grains. Besides, the L12-(Ni, Co)3Al type phase was also observed within the face-centered cubic (FCC) matrix of the S1 alloy. However, the S2 and S3 alloys consisted of stretched original grains, survived original grains, fully recrystallized equiaxed grains, and B2 precipitates. Moreover, a mount of nano-deformation twins was also incorporated in the matrix of S2 alloy. Furthermore, the S2 alloy exhibited a superior balance of high ultimate tensile strength of ∼1019 MPa and high elongation of ∼28% as compared to S1 and S3 alloys due to its complex heterogeneous microstructure. This research would provide guidance for designing the heterogeneous structure in the FCC HEA and facilitate establishing the relationships between the heterogeneous structure and mechanical properties.

High temperature tensile and creep properties of CrMnFeCoNi and CrFeCoNi high-entropy alloys

High temperature tensile and creep properties of face-centered cubic (FCC) high-entropy alloys (HEAs), CrMnFeCoNi and CrFeCoNi, were evaluated at 500–700 and 650–725 °C, respectively, to evaluate high temperature structural integrity of the most well-known FCC HEAs with special attention on (i) sigma phase formation in CrMnFeCoNi and (ii) difference in solid solution strengthening between the two alloys in the temperature range where commercial heat-resistant wrought alloys including austenitic heat-resistant steels and Ni-based superalloys are used. No remarkable difference was detected in tensile behavior measured both at room and high temperatures between the two alloys. However, creep rupture life turned out to be significantly longer for the CrFeCoNi quaternary alloy. On top of the longer creep rupture life, CrFeCoNi alloy is characterized by lower minimum creep rate and larger creep activation barrier energy which could be attributed to higher level of lattice distortion of CrFeCoNi than CrMnFeCoNi resulting in higher solid solution strengthening. Also, grain boundary strength of CrMnFeCoNi was compromised by the formation of sigma phase during creep deformation which led to lower elongation especially for long-term creep conditions. Therefore, the longer creep life of CrFeCoNi is mainly attributed to the combination of enhanced solid solution strengthening and stronger grain boundary which is free from the deleterious sigma phase due to the reduced thermodynamic driving force for formation of intermetallic phase.