CoCrFeNi High-entropy Alloy Research Articles

Research on the Thermal Stability of MoCu/CoSb3 Thermoelectric Junction with CoCrFeNi High-Entropy Alloy Barrier Layer

Research on the Thermal Stability of MoCu/CoSb3 Thermoelectric Junction with CoCrFeNi High-Entropy Alloy Barrier Layer

Investigation on effect of L12 and B2 phases on tensile properties and mechanisms of Al0.3CoCrFeNi high entropy alloy through aging treatment

Al0.3CoCrFeNi high entropy alloys (HEAs) have excellent mechanical properties due to the large tunability of microstructures, which makes them one of the most promising candidates for engineering application. In the aspect of enhancing tensile properties, precipitation strengthening was verified effective in this HEA and more studies are needed to deepen relative understanding. In this work, Al0.3CoCrFeNi HEAs with coarse-grained single FCC (A) and fine-grained triplex FCC + B2 + σ (B) structures were aged at different temperatures and times to precipitate second phases, and the tensile properties and mechanisms of aged specimens were studied. The results showed that aging of A resulted in the main precipitation of L12 phases throughout FCC matrix and increasing aging temperature obviously increased L12 size, whereas increasing annealing time had minor effect on phase coarsening. At small L12 size of ~2.0nm, stacking fault shearing mechanism was activated during tension and planar dislocation configurations were formed, leading to simultaneous increase of strength and ductility. With increasing L12 size to ~10.0nm, the deformation mechanism was transformed into anti-phase boundary shearing and Orowan bypassing and wavy dislocation configurations predominated, which resulted in the significant increase of yield strength from ~263MPa to ~433MPa without sacrificing notable ductility. On the other hand, aging of B induced the formation of fine B2 phases at residual dislocations and enhancement of tensile yield strength from ~705MPa to ~759MPa. The ductility decreased slightly from ~32% to ~28%, which was related to the plastic deformation of B2 phases. However, it was found that the tensile strengths could not be continuously increased by further increasing aging temperature or time. These findings provide insights into the development of precipitation strengthened HEAs with good mechanical properties.

Investigation on tensile property and mechanism of partially recrystallized Al0.1CoCrFeNi high-entropy alloy after cold rolling and annealing treatment

Investigation on tensile property and mechanism of partially recrystallized Al0.1CoCrFeNi high-entropy alloy after cold rolling and annealing treatment

Enhancing strength and ductility synergy through heterogeneous laminated structure design in high-entropy alloys

Heterogeneous laminated structure (HLS) design offers new opportunities to enhance the mechanical performance of high-entropy alloys (HEAs) through synergistic effects from heterogeneity. However, it remains challenging to introduce the HLS into HEAs via severe plastic deformation due to their strong work-hardening capacity. In this study, a specially designed multi-level HLS, characterized by alternatively stacked micro-grained soft CoCrFeNi layers and nanostructured ultra-hard Al0.3CoCrFeNi layers containing a three-phase microstructure (composed of nanograined face-centered cubic matrix, (Al, Ni)-rich B2 precipitates, and Cr-rich σ precipitates), is controllably introduced into FCC HEAs via a conventional thermo–mechanical processing involving hot-pressing, cold-rolling, and annealing. Meanwhile, thermo–mechanical processing induces Al element diffusion across the layer interface, resulting in the formation of an interfacial transition layer and the establishment of a strong interface bonding between the neighboring CoCrFeNi and Al0.3CoCrFeNi layers. As a result, the multi-level HLSed CoCrFeNi/Al0.3CoCrFeNi composite exhibits a yield strength as high as 1127±25.4 MPa while maintaining a large fracture elongation (up to (26.3±2.4)%). Such an excellent strength–ductility synergy surpasses that of most previously reported high-performance monolithic bulk CoCrFeNi and Al0.3CoCrFeNi HEAs prepared through careful chemical composition optimization and/or thermo–mechanical processing. Strong hetero-deformation induced strengthening benefited from the apparent microstructural/microhardness difference and the strong interface bonding between the neighbouring CoCrFeNi and Al0.3CoCrFeNi layers, together with simultaneous activation of multiple strain hardening mechanisms containing mechanical twinning, stacking faults and precipitation strengthening, is responsible for the excellent strength–ductility combination. This multi-level HLS and its fabrication strategy provide an enlightening way to develop strong and ductile HEAs and can also be applied to high-performance designs of other metallic materials.

Deformation behavior of Gd alloyed CoCrFeNi high entropy alloy at high temperature under the influence of local nano-ordered structure and misorientation

The thermomechanical processing (TMP), which is an industrial process that includes deformation and heat treatment, represents a novel approach to enhancing the mechanical properties of metallic materials. However, it requires extensive research on the high-temperature deformation behavior of alloys during its application process. The present study conducts hot compression tests at 700 °C and 800 °C, as well as detailed microstructural characterization of the deformed material after hot deformation, to comprehensively discuss the mechanical properties and deformation behavior at high temperature of the (CoCrFeNi)100-xGdx (x = 0, 0.5, 1, 2, 3 at.%) HEAs. The (CoCrFeNi)100-xGdx HEAs exhibit an excellent combination of strength and plasticity in high temperature. The original dendritic microstructure in the (CoCrFeNi)100-xGdx HEAs was completely fractured and transformed into a mass of deformed structures during thermal deformation. The mechanical properties at high temperature of the alloy system originate from the misorientation in the microstructure. Dislocation slip is the main deformation mechanism of the alloy system at high temperature. The coordinated deformation of the soft orientation for FCC phase and the hard orientation for HS phase affects the dislocation morphology in the microstructure. Dynamic recovery (DRV) plays a dominant role in the flow behavior of the alloy at high temperature, and work hardening is the main strengthening mechanism during plastic deformation. The correlation between microstructure evolution and mechanical properties of Gd alloyed CoCrFeNi HEAs during hot deformation has been established through comprehensive studies on deformation behavior.

Microstructure and mechanical properties of the C/C composites and Nb joints brazed with CoCrFeNi HEAs

A robust brazing of C/C composites and Nb was achieved using CoCrFeNi high-entropy alloys (HEAs) with complete FCC structure. The effects of brazing temperature and holding time on microstructural evolution and mechanical properties of joints were evaluated. The filler infiltrated into the pores of the C/C composites and formed a mixture of FCC and carbides, expanding the bonding areas and interlocking the interface. Moreover, a lamellar eutectic structure composed of FCC and Laves formed in situ near Nb side, providing unique high-temperature performance. The optimal average shear strength of 29.1 MPa at room temperature and 23.6 MPa at 800 °C were obtained at 1260 °C for 10 min. The joining and fracture mechanism of brazed joints were also revealed.

Performance study of explosively formed projectile using CoCrFeNi high-entropy alloy as a liner

Based on mechanical tensile experiments on the CoCrFeNi high-entropy alloy (HEA), this study explores the forming patterns of the eccentrically shaped sub-hemispherical lined explosively formed projectile (EFP) made of the HEA material. The CoCrFeNi HEA material is initially prepared, and mechanical tensile tests are conducted at various temperatures and strain rates. The Johnson–Cook (J–C) constitutive equation for this material is derived by fitting the experimental data. Scanning electron microscopy and the energy dispersive spectrometer characterize the fracture surface of the tensile specimens, providing insights into the mechanical ductility and fracture mechanism of CoCrFeNi HEA. The EFP forming process under various charge configurations is simulated using AUTODYN software, leading to the identification of the optimal charge configuration. In addition, the damage performance is evaluated. This study provides a theoretical basis for applying HEA materials in the field of shaped charges and offers new ideas and methods for designing more efficient shaped charge warheads.

Minor tungsten addition enhances corrosive resistance in CoCrFeNi high-entropy alloy at elevated seawater temperature

Most research on the corrosive CoCrFeNi high-entropy alloy (HEA) has mainly modulated element species and atomic concentration recently. However, the corrosive resistance of the passive film in the elevated temperatures environment inevitably need to be extensively explored for application. In this work, the microstructure and corrosion behavior in the elevated temperatures of face-centered cubic (FCC) CoCrFeNiWx (x = 0 and 0.2) HEAs are investigated. The mechanism of passivation layers on the corrosion behavior of HEAs is further discussed for comparison to the alloy of addition tungsten. Through potentiodynamic polarization curves in 3.5 wt% NaCl solution from 25 °C to 70 °C, the addition of tungsten can raise the critical pitting temperature, while maintaining a lower passivation current density and a larger passivation range at the same time. The high resolution X-ray photoelectron spectrometer (HRXPS), electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are all conducted to analyze the passivation composition and chemical reaction of the present alloys detailly. The CoCrFeNiW0.2 HEA exhibits the uniform Cr2O3 layer on the FCC matrix; the proportion of dispersive WO3 layer on the μ phase precipitates form a double-layer passive film under elevated temperature in 3.5 wt% NaCl. It shows that the synergistic effect of the double-layer passive film significantly enhances the pitting and corrosive resistance in this HEA.

Influence of hot forging on grain formation in Al0.35CoCrFeNi high-entropy alloy: numerical simulation, microstructure and mechanical properties

One-step (F100) and three-step (F30-60-40) hot forging of Al0.35CoCrFeNi alloy was investigated to achieve a uniform equiaxed grain structure. In the as-cast and forged state, only a single-phase face-centered cubic structure was observed. The formation of twins, recrystallized and partially recrystallized grains in the volume of the samples was observed depending on used forging process. To predict uniform grain-size formation numerical simulation of the hot-forging process was used. The numerical model was calibrated and validated by means of measured compression experimental data of as-cast Al0.35CoCrFeNi alloy before forging. Thermal analysis using finite element analysis was used to simulate cooling of sample during the relocation from the furnace on the lower die. Simulations were run under different thermo-mechanical conditions and the regions for the formation of dynamically recrystallized grains were predicted. Room temperature mechanical properties were evaluated after F100 and F30-60-40 hot-forging process. The F30-60-40 hot forging optimized the grain size, which was evident in the very small dispersion of the room temperature mechanical properties in tension. Elongation after F30-60-40 hot forging increased by 17%. The correlation between temperature, equivalent stress, equivalent plastic strain, microstructure, tensile properties, and strain-hardening behavior is discussed.

Tensile properties and work hardening in Al0.3CoCrFeNi: The role of L12 precipitates and grain size

In this study, we investigate the FCC-Al0.3CoCrFeNi high entropy alloy fabricated via spark plasma sintering of atomized powders, focusing on its mechanical and work-hardening properties across three distinct microstructures: coarse-grained, fine-grained, and fine-grained with L12 nano-precipitates. Using a dislocation density-based model, we analyze the effects of grain size and L12 precipitates on these properties, achieving quantitative agreement between model predictions and experimental tensile and work-hardening behaviors. This exploration highlights the underlying deformation mechanisms at room temperature and their contributions to the strength/ductility trade-off. Significantly, our analysis reveals that twinning in HEAs manifests differently from that observed in steels. Furthermore, the incorporation of L12 precipitates emerges as a critical factor enhancing the alloy's mechanical attributes. Our findings underscore the essential roles of microstructural parameters in tailoring the mechanical properties of HEAs, offering insights that could guide the design of advanced alloys with optimized performance.

Wear behavior of CoCrFeNi high-entropy alloy prepared by mechanical alloying

Equiatomic quaternary CoCrFeNi powder was prepared from the individual metals by mechanical alloying of the mixed powders at room temperature. The as-milled powder contained particles of irregular shape with an average particle size of 0.62 μm, and exhibited a dual-phase composition of the face-centered cubic and the body-centered cubic structures. The as-milled powder was consolidated by hydraulic pressing at room temperature and sintered at 950 °C in argon and vacuum. The compacted material exhibited a single-phase composition of the face-centered cubic structure alone and a chemical composition close to equiatomic with uniform distribution of all constituents, indicating that the desired CoCrFeNi high-entropy alloy (HEA) had been formed. The microhardness of the vacuum-sintered CoCrFeNi HEA was measured to be 118±7 HV, exceeding that calculated by the rule of mixture. The coefficient of friction was determined by pin-on-disk dry sliding studies at room temperature and found to be 0.65 and 0.44 for the applied loads of 25 for 35N, respectively, and the corresponding wear rates were 4.6×10-4 and 9.1×10-4 mm3/N·m. XPS studies on the worn alloy surfaces showed the formation of oxidized tribo-layers, composed of predominantly Cr2O3. Overall, this study has demonstrated that mechanical alloying followed by simple furnace sintering at a moderate temperature is able to yield an abrasion-resistant CoCrFeNi HEA suitable for structural applications.

Effect of nitriding treatment on microstructure, mechanical property and wear mechanisms of Al CoCrFeNi high entropy alloy coatings prepared by cold spraying

Effect of nitriding treatment on microstructure, mechanical property and wear mechanisms of Al CoCrFeNi high entropy alloy coatings prepared by cold spraying

Effect of Ti, Mn and Mo on the microstructure and properties of CoCrFeNi high entropy alloy coatings prepared by laser cladding

CoCrFeNi-based High Entropy Alloys (HEAs) coatings, with additions of Ti, Mn, and Mo, were synthesized using laser cladding (LC). The pure CoCrFeNi HEA had a uniform FCC structure, exhibiting the lowest hardness and wear resistance but good corrosion resistance. The Ti-added HEA had an FCC+Laves+η phase structure, showing increased hardness and wear resistance due to abrasive and mild surface fatigue wear, though with the poorest corrosion resistance. The Mn-added HEA maintained a single FCC structure with coarser grain boundaries, exhibiting lower hardness and reduced wear resistance due to adhesive, abrasive wear, and minor surface fatigue wear, but with the best corrosion resistance.The Mo-added HEA developed a eutectic structure of FCC and σ phases, achieving the highest hardness and wear resistance due to oxidative wear, but showed moderately decreased corrosion resistance. The varying microstructures significantly impacted the coatings' mechanical properties and corrosion behavior, highlighting the critical role of alloying elements in customizing LCed-HEA coatings for specific applications.

SUPERIOR ANTIBACTERIAL PROPERTIES OF COPPER-BEARING HIGH ENTROPY ALLOY COATED STAINLESS STEEL SURFACE FABRICATED USING LASER CLADDING

A high-entropy alloy (HEA) coating is applied to a stainless-steel surface using the laser cladding (LC) process. This coating offers a broad-spectrum antibacterial ability and favorable mechanical properties. The release of copper ions from the HEA effectively inhibits the growth of Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus), enhancing the alloy’s suitability for various applications requiring antibacterial properties. The elemental compositions, sizes, and morphologies of the constructed HEAs are revealed through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). This study validates the feasibility of applying the antibacterial CoCrFeCuNi HEA alloy coating to stainless steel via LC. The antibacterial evaluation showcases the enhanced efficacy of the CoCrCuFeNi HEA coating, demonstrating an antibacterial effectiveness of approximately 89% against E. coli and 98% against S. aureus. In contrast, the CoCrFeNi HEA coating exhibits an antibacterial effectiveness of about 40% against E. coli and 93% against S. aureus. The Vickers hardness of the stainless steel coated with CoCrCuFeNi HEA has significantly increased to 352 HV, compared to the CoCrFeNi coating with a hardness of 300.6 HV and the uncoated stainless steel with a hardness of 256.6 HV. This HEA alloy demonstrates considerable potential for use in medical applications or other settings that require antibacterial features.

Ultra-High Strength in FCC+BCC High-Entropy Alloy via Different Gradual Morphology.

In this study, high-pressure torsion (HPT) processing is applied to the as-cast Al0.5CoCrFeNi high-entropy alloy (HEA) for 1, 3, and 5 turns. Microstructural observations reveal a significant refinement of the second phase after HPT processing. This refinement effect is influenced by the number of processing turns and the distance of the processing position from the center. As the number of processing turns or the distance of the processing position from the center increases, the fragmentation effect on the second phase becomes more pronounced. The hardness of the alloy is greatly enhanced after HPT processing, but there is an upper limit to this enhancement. After increasing the number of processing turns to 5, the increase in hardness at the edge becomes less significant, while the overall hardness becomes more uniform. Additionally, the strength of the processed alloy is significantly enhanced, while its ductility undergoes a noticeable decrease. With an increase in the number of processing turns, the second phase is further refined, resulting in improvement of strength and ductility.

Microstructure and tensile properties of Y2O3-dispersion strengthened CoCrFeNi high entropy alloys prepared via mechanical alloying using pre-alloyed powder

Nano-Y2O3-dispersion strengthened CoCrFeNi high entropy alloys are fabricated via mechanical alloying and spark plasma sintering using high purity elemental powders or pre-alloyed CoCrFeNi powder. All the alloys show an FCC matrix incorporated by small amount of BCC Cr-rich segregations, whose brittle nature is detrimental to the mechanical properties of the alloys. It has been found that using pre-alloyed powder significantly suppresses the formation of the Cr-rich phase, and its size and volume fraction can be further reduced by increasing the rotation speed of ball-milling during mechanical alloying. Besides, the grain refinement is also achieved under a higher rotation speed. Y2O3 nanoparticles with a number density of 1.8 × 1022 m−3 and an average diameter of 11.0 ± 7.3 nm are uniformly distributed in the alloy that produced from pre-alloyed powder under the rotation speed of 350 rpm during ball-milling. These Y2O3 nanoparticles share coherent interface with the FCC matrix, indicating the in-situ precipitation mechanism. Due to a good combination of grain boundary strengthening, dislocation strengthening and precipitation strengthening, this ODS high entropy alloy possesses a yield strength of 1281 MPa at room temperature.

Structural Changes in High-Entropy Alloys CoCrFeNi and CoCrFeMnNi, Irradiated by He Ions at a Temperature of 700 °C.

High-entropy alloys (HEA) are promising structural materials that will successfully resist high-temperature irradiation with helium ions and radiation-induced swelling in new generations of nuclear reactors. In this paper, changes in the elemental and phase composition, surface morphology, and structure of CoCrFeNi and CoCrFeMnNi HEAs irradiated with He2+ ions at a temperature of 700 °C were studied. Structural studies were mainly conducted using the X-ray diffraction method. The formation of a porous surface structure with many microchannels (open blisters) was observed. The average diameter of the blisters in CoCrFeMnNi is around 1.3 times smaller than in CoCrFeNi. It was shown that HEAs' elemental and phase compositions are stable under high-temperature irradiation. It was revealed that, in the region of the peak of implanted helium, high-temperature irradiation leads to the growth of tensile macrostresses in CoCrFeNi by 3.6 times and the formation of compressive macrostresses (-143 MPa) in CoCrFeMnNi; microstresses in the HEAs increase by 2.4 times; and the dislocation density value increases by 4.3 and 7.5 times for CoCrFeNi and CoCrFeMnNi, respectively. The formation of compressive macrostresses and a higher value of dislocation density indicate that the CoCrFeMnNi HEA tends to have greater radiation resistance compared to CoCrFeNi.

Effect of crystallographic orientation on mechanical properties of Al0.25CoCrFeNi high entropy alloy under tension

The deformation behavior of single-crystal materials is closely related to the crystallographic orientation. However, there are few simulations and experimental studies on the effect of crystallographic orientation on the tensile properties of single-crystal AlxCoCrFeNi high entropy alloys (HEAs). Molecular dynamics simulations were used to analyze the mechanical properties and structural variations of Al0.25CoCrFeNi HEAs with different crystallographic orientations under tension. Prior to yielding, elastic softening occurred in [111], [110] and [11̅0]-oriented HEAs, except for [100]-oriented HEA. During plastic deformation, [100]-oriented HEA had high strength and flow stress due to the low Schmid factors of leading partial dislocations and multiple activatable slip systems. [111]-oriented HEA also showed high strength and flow stress, which were attributed to the low Schmid factors of the activatable slip systems and the existence of incomplete stacking fault tetrahedrons. Both [110]- and [11̅0]-oriented HEAs showed low strength, but the presence of the secondary increased stress suggested they had good plasticity. The difference between them was that [110]-oriented HEA was easier to form hexagonal close-packed lamellae and experienced secondary yielding at larger strain. This suggested that different crystallographic orientations in the non-tensile direction also affected the mechanical properties.

Superior strength-ductility synergy in additively manufactured CoCrFeNi high-entropy alloys with multi-scale hierarchical microstructure

Additive manufacturing offers a feasible route for fabricating net-shape metallic materials with advanced physicochemical and mechanical properties, which is unattainable through conventional routes. This study focuses on the processing parameters, microstructural, and mechanical properties of the CoCrFeNi high-entropy alloys (HEAs) fabricated via laser powder bed fusion (LPBF). A near-dense LPBF CoCrFeNi HEA was manufactured by fine-tuning the parameters. Compared with conventionally processed CoCrFeNi HEAs, the as-built samples exhibit excellent strength-ductility synergy with 200 % higher yield strength and unsacrificed ductility. Microstructural analyses of the LPBF samples demonstrate a hierarchical structure including melt pools, columnar grains, dislocation cells, and elemental segregations. This excellent strength is attributed to the high-density dislocation cellular structure and Cr2O3 nanoparticles that impede the dislocation motion. The excellent ductility correlates to the steady work-hardening regulated by the interactions between cellular structure, dislocations, deformation twins, and nanoprecipitation. This work provides promising outlook towards fabricating high-performance alloys with tailored hierarchical microstructure.

Analysis and Prediction of Morphology and Properties of Laser-Directed Energy Deposition CoCrFeNi High-Entropy Alloy Using Response Surface Methodology and Non-dominated Sorting Genetic Algorithm II

Analysis and Prediction of Morphology and Properties of Laser-Directed Energy Deposition CoCrFeNi High-Entropy Alloy Using Response Surface Methodology and Non-dominated Sorting Genetic Algorithm II