CoCrFeMnNi High-entropy Alloy Research Articles

Effects of strain rate and low temperature on dynamic behaviors of additively manufactured CoCrFeMnNi high-entropy alloys

With the increasing demand for high-performance alloys in extreme environments, there has been an active development of new materials. High-entropy alloys (HEAs) are considered a promising choice due to their exceptional properties. Even though HEAs exhibit remarkable mechanical properties, only a limited number of studies have explored the mechanical properties and microstructures of additively manufactured HEAs under extreme conditions. In this work, the dynamic mechanical properties at high strain rates and different temperatures (i.e., 293 K and 77 K), as well as quasi-static mechanical properties for the CoCrFeMnNi HEAs fabricated by selective laser melting (SLM) are investigated. Microstructure observations have demonstrated that the as-printed sample displays a single face-centered cubic (FCC) phase with a columnar dendrite feature. Within dendrites, a large number of dislocations and substructures appear due to the applied high cooling rates during the SLM process. The as-printed sample exhibits a significant strain rate dependence from quasi-static to dynamic loading during deformation. Compared to the quasi-static mechanical properties, the as-printed HEA reveals higher dynamic yield strength (e.g., ∼1081 MPa and ∼1020 MPa along the build and scanning directions under 2800 s−1, respectively), more pronounced strain-hardening behavior, and larger remarkable plasticity under dynamic loading at cryogenic temperatures. When subjected to increasing strain rates ranging from 0.001 s−1 to 3500 s−1 at 293 K, the yield strength of the as-printed HEAs exhibits a notable enhancement, increasing from ∼517 MPa to ∼723 MPa along the BD direction, and from ∼545 MPa to ∼667 MPa along the SD direction. The corresponding variation in the strength of the BD samples is more pronounced than that of the SD samples. The enhancement in strength should be attributed to the differences in the features of grain orientation and dislocation distribution along different build directions, as well as the strain rate dependence. During plastic deformation, the dominant mechanisms of dislocation slips and deformation twins (DTs) are transformed into multi-stage-twin-controlled ones at cryogenic temperatures, which is beneficial to improve the strain hardening rate and plasticity. The present studies provide a deep understanding of the deformation mechanism of HEAs under extreme conditions and help promote the application of HEAs in the future.

Study on precipitation in (CoCrFeMnNi)90Al6Ti4 high entropy alloy

(CoCrFeMnNi)90Al6Ti4 high entropy alloy (HEA) was prepared by adding Al and Ti elements into CoCrFeMnNi HEA. The alloy underwent processes including copper mold suction casting, 1473 K homogenization, 30 % cold rolling + 1273 K annealing + 1073 K aging, and water quenching and heating treatment. Microstructure and phase composition of both as-cast and heat-treated (CoCrFeMnNi)90Al6Ti4 HEA were studied using metallographic microscope, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffractometer (XRD), and transmission electron microscope (TEM). The effect of heat treatment processes on the precipitation of second phases in (CoCrFeMnNi)90Al6Ti4 HEA was investigated, and mechanical properties of as-cast and heat-treated (CoCrFeMnNi)90Al6Ti4 HEA were studied via hardness tests. The results show that in as-cast (CoCrFeMnNi)90Al6Ti4 HEA, the second phase precipitated is primarily L21 structured AlTiNi2 phase. After homogenization, cold rolling, annealing, and aging processes, both the quantity and size of the second phase increase, with a small amount of nanoscale L12 phase appearing after aging treatment. With the progression of heat treatment, the hardness of (CoCrFeMnNi)90Al6Ti4 HEA increases, and mechanical properties are improved.

Research on hot cracking in laser welding of Al-contaminated CoCrFeMnNi high-entropy alloys

To study the thermal cracking susceptibility of laser-welded CoCrFeMnNi high-entropy alloys, stainless steel and aluminum alloy plates were each used as backing for welding. The microstructure of the weld and morphology of the fracture were examined. In addition, the chemical compositions of the fractures, the interfacial tension between the CoCrFeMnNi high-entropy alloy and liquid aluminum alloy, and the linear expansion coefficient of the CoCrFeMnNi high-entropy alloy were determined. The results show that when stainless steel is used as the base plate, no cracking is apparent in the weld, and the microstructure is made up of dendrites and equiaxed crystals. Conversely, when an aluminum alloy plate is adopted, solidification cracks are seen at the center of the weld, and the microstructure consists of bright polygonal dendrites scattered in a dark gray matrix. In the later stage of solidification, the contact angle between the Al-dominated low-melt liquid metal and the CoCrFeMnNi high-entropy alloy is about 14.6°, which is distributed in the form of a liquid film between the dendrite of the CoCrFeMnNi high-entropy alloy weld, and the linear expansion coefficient of the high-entropy alloy is 23×10-6-25×10-6 K-1 in the temperature range of 900-1100 K, which is higher than the thermal expansion coefficient of austenitic stainless steel in this range, and the solidification temperature range is 1000-1400K. Therefore, thermal cracks tend to occur during the solidification process.

Tensile behavior of CoCrFeMnNi high-entropy alloy with intermediate strain rate included

High-entropy alloys (HEAs) have garnered considerable attention for their exceptional mechanical properties, positioning them as promising candidates for diverse industrial applications. This study investigates the mechanical properties of CoCrFeMnNi HEAs over a wide range of strain rates. Based on a novel electromagnetic loading device equipped with high-speed photography, intermediate strain rate tests of the HEAs are conducted. The intermediate strain rate test results show the effectiveness of the experiments. Experimental results exhibit a remarkable strain rate sensitivity of the HEA during tension tests. Microstructural analysis reveals the collaborative interplay between deformation twins and dislocations, enhancing strength-plasticity relationships under tension loading. Finally, a constitutive model (M-JCK) integrated by Johnson-cook model and KHL model is proposed to describe the mechanical behavior of HEAs. The results demonstrate that the M-JCK model outperforms traditional models, providing enhanced accuracy in capturing the complex responses of HEAs across varying strain rates.

Bending Forming Characteristics of CoCrFeMnNi High-Entropy Alloy Sheets Induced by Nanosecond Pulse Laser Irradiation.

Laser bending forming, as a flexible and die-less forming approach, facilitates the three-dimensional shaping of sheets through the generation of thermal stress via laser-material interaction. In this study, the bending forming characteristics of CoCrFeMnNi high-entropy alloy sheets induced by nanosecond pulse laser irradiation were systematically investigated. The effects of parameters including laser power, scanning speed, number of scans, scanning interval, and sheet size on the bending angle, cross-sectional morphology, and hardness were studied in detail under both the laser single-line and multi-line scanning modes. The experimental results confirmed the effectiveness of nanosecond pulse laser irradiation for achieving accurate formation of CoCrFeMnNi sheets, with the successful fabrication of J, L, and U-shaped metal components. Apart from the forming ability, the cross-sectional hardness was significantly increased due to the grain refinement effect of nanosecond pulse laser irradiation. Furthermore, employing the laser single-line scanning mode enabled the effective rectification of overbending parts, showcasing complete recovery for small-angle overbending, and a remarkable 91% recovery for larger-angle overbending. This study provides an important basis for the bendability of CoCrFeMnNi sheets by laser forming and elucidates the evolution of the microstructure and mechanical properties in the bending region.

TEM Characterization of Radiation-Induced Segregation at Irradiation-Induced Dislocation loops in Al0.3CoCrFeNi and CoCrFeMnNi High Entropy Alloys

TEM Characterization of Radiation-Induced Segregation at Irradiation-Induced Dislocation loops in Al0.3CoCrFeNi and CoCrFeMnNi High Entropy Alloys

Study on Mechanical Properties of Nanopores in CoCrFeMnNi High-Entropy Alloy Used as Drug-Eluting Stent.

The CoCrFeMnNi high-entropy alloy is commonly used for vascular stents due to its excellent mechanical support and ductility. However, as high-entropy alloy stents can cause inflammation in the blood vessels, leading to their re-narrowing, drug-eluting stents have been developed. These stents have nanopores on their surfaces that can carry drug particles to inhibit inflammation and effectively prevent re-narrowing of the blood vessels. To optimize the mechanical properties and drug-carrying capacity of high-entropy alloy stents, a high-entropy alloy system with different wide and deep square-shaped nanopore distributions is created using molecular dynamics. The mechanical characteristics and dislocation evolution mechanism of different nanopore high-entropy alloy systems under tensile stress were studied. The results showed that the CoCrFeMnNi high-entropy alloy with a rational nanopore distribution can effectively maintain the mechanical support required for a vascular stent. This research provides a new direction for the manufacturing process of nanopores on the surfaces of high-entropy alloy stents.

Effects of annealing time on microstructural characteristics and mechanical properties of cryo-rolled CoCrFeMnNi high entropy alloys

High-entropy CoCrFeMnNi alloy was subjected to cryo-rolling with a reduction ratio of 25 % and was then annealed at 1050 °C for 5, 25, and 45 min. The results of the recrystallization textures of the various annealed samples showed the presence of similar texture components, indicating that cryo-rolling had only a limited effect on the formation of the subsequent annealing texture. The volume fractions of the different texture components similarly showed no obvious dependence on the annealing temperature. The uniformity of the recrystallization texture suggests the absence of strong preferential nucleation and growth during annealing. The cryo-rolled samples processed with different holding times exhibited a remarkable ambient temperature strength-ductility combination.

Characterization of the Manufacturing Process and Mechanical Properties of CoCrFeMnNi High-Entropy Alloys via Metal Injection Molding and Hot Isostatic Pressing

Characterization of the Manufacturing Process and Mechanical Properties of CoCrFeMnNi High-Entropy Alloys via Metal Injection Molding and Hot Isostatic Pressing

Effect of sputtering power and substrate bias on microstructure, mechanical properties and corrosion behavior of CoCrFeMnNi high entropy alloy thin films deposited by magnetron sputtering method

The CoCrFeMnNi high entropy alloy thin films with crystalline (solid solution) structure or amorphous state were fabricated by magnetron sputtering process at different target powers and different substrate bias. Effects of the target power and substrate bias on the characteristics of the CoCrFeMnNi thin films such as microstructures, morphology and surface topography, as well as mechanical properties and corrosion behavior were studied. Phase prediction was calculated by using thermodynamic and kinetic criteria under various deposition conditions. The enthalpy, entropy, δ and Ω parameters, and electronegativity differences influenced the solid solution stability of thin films. These parameters led to the formation of solid solutions with crystalline or amorphous structures under specific limits, which corresponded to the practical XRD and TEM analysis results. Using a substrate bias in the deposition of CoCrFeMnNi thin film changed the amorphous structure to a crystalline (solid solution (BCC + FCC)) at higher target powers (200 and 300 W). On the other hand, the film structure was entirely amorphous at all sputtering target powers without the substrate bias. The mechanical properties, such as hardness, Young's modulus, film strength, and fracture toughness of CoCrFeMnNi thin film, can be enhanced by using the substrate bias and increasing the sputtering power. The potentiodynamic polarization test in 3.5 wt% NaCl aqueous solution indicated that the deposition of CoCrFeMnNi thin films at different sputtering powers and without substrate bias could increase the corrosion resistance of 304 stainless steel substrate. Using the substrate bias of −100 V in the fabrication of CoCrFeMnNi thin film decreased the corrosion current density and increased its polarization resistance.

Ion nitriding of a face-centered cubic high-entropy alloy: Nitriding kinetics, the effect of temperature and contact stress on tribological behavior

Face-centered cubic (FCC) high-entropy alloys (HEAs) have good mechanical properties, but their low hardness makes it difficult to meet the wear resistance required by actual working conditions. Therefore, the effect of ion nitriding on the microstructure, mechanical properties and tribological properties of Fe40Mn20Cr20Ni20 FCC HEA was investigated in this work. It was found that the activation energy of ionic nitriding of Fe40Mn20Cr20Ni20 FCC HEA was 41.388 kJ/mol, which was lower than that of CoCrFeMnNi HEA (64.034 kJ/mol). When nitriding at 400 °C, the nitrided layer was mainly expanded FCC phase; above 400 °C, the expanded FCC phase began to decompose into CrN and γ phase, the generation of γ phase and the growth of CrN caused the hardness of the nitrided layer to decrease with the increase of temperature. After evaluating the hardness, brittleness and nitriding growth kinetics of the nitrided layer obtained from different nitriding processes, the nitrided layer obtained from the nitriding process with the nitriding temperature of 550 °C and the nitriding time of 5 h was selected for the friction and wear test under different normal forces and temperatures. It was demonstrated that at 20–400 °C, the wear resistance of the nitrided alloy was relatively stable over a wide range of normal forces and was substantially improved over the matrix alloy. At 500–600 °C, due to a certain degree of softening of the surface of the nitrided layer, the wear rate increased slightly, but it was still much lower than that of the matrix alloy.

Evolution of microstructure, mechanical properties and phase stability of CoCrFeMnNi high entropy alloys

In the present study, the microstructure and mechanical properties of various CoCrFeMnNi high-entropy alloys were investigated. Analytical studies were conducted to determine the optimal chemical composition, mixing entropy, mixing enthalpy, atomic radii, valence electron concentration (VEC), dimensionless parameter, and melting point on the Co-Cr-Fe-Mn-Ni system. The microstructure of the alloys was analyzed using FESEM, XRD, and TEM. The results showed that the microstructure of all HEAs had dendritic and interdendritic regions, with secondary precipitations detected along the grain boundaries of the alloy, mainly composed of Mn and Ni. High-density dislocation structures and nano-precipitates were predominantly present in the alloy. The mechanical characteristics such as microhardness and tensile properties are conducted at room temperature. The HEA Co25Cr25Fe10Mn30Ni10 exhibited the highest average microhardness, while the Co20Cr20Fe30Mn10Ni20 HEA had the lowest mean hardness value. This significant difference of 7.2 % may be attributed to the hard phases composed of Mn and Ni. The results of the tensile experiments indicate that the Co20Cr20Fe20Mn20Ni20 alloy has the most favorable overall properties, with an ultimate tensile strength of 441 MPa. This represents a significant increase of 37.8 % compared to 20Cr20Fe20Mn20Ni20. Furthermore, the study examines the instability of the solid-solution state caused by differences in the valence electron concentrations of the constituent elements and phase stability.

Higher entropy-induced strengthening in mechanical property of Cantor alloys/Zr-3 joints by laser in-situ eutectic high-entropy transformation

To effectively regulate the grain boundary infiltration in CoCrFeMnNi high-entropy alloy (Cantor alloys, HEA) caused by the violent atomic interdiffusion, the higher configuration entropy on Cantor alloys surface was designed and realized via eutectic high-entropy (EHEA) transformation. Meanwhile, to effectively alleviate the residual stress caused by the notable difference in the thermal expansion coefficient (CTE) between Cantor alloys and Zr-3 alloys, a cladding layer was applied to the HEA surface using laser cladding technology of Nb, followed by brazing to Zr-3 alloys with Zr63.2Cu filler. The cladding layer's microstructure comprised Nbss and FCC+(Co,Ni)2Nb eutectic structure, resulting from an in-situ reaction between Cantor alloys and Nb. The Nbss and FCC demonstrated good plasticity, and the (Co,Ni)2Nb Laves phase provided increased strength, endowing both good plastic deformation ability and strength of the cladding layer. Notably, the existence of EHEA in the laser cladding layer made the Cantor alloy entropy from 1.61 R to 1.77 R, greatly enhancing its thermal stability and suppressing the grave grain boundary infiltration. Joints produced via laser cladding with Nb-assisted brazing exhibited a complex microstructure (HEA/Nbss + FCC + (Co,Ni)2Nb/(Zr,Nb)(Cr,Mn)2 + (Zr,Nb)ss/(Zr,Nb)2(Cu,Ni,Co,Fe) + (Zr,Nb)(Cr,Mn)2 + (Zr,Nb)ss/Zr-3 and a significantly improved shear strength of 242.8 MPa at 1010 °C for 10 min, 42.4 % higher than that of directly brazed joints. This improvement was attributed to reduced grain boundary infiltration, alleviated residual stress due to CTE disparity, and eliminated micro-cracks in the brazing seam. This study presents an effective solution for reducing residual stresses and achieving reliable bonding between Cantor alloys and Zr-3 alloys, with potential applications in brazing CoCrFeNi-based HEA and Zr-3 due to the beneficial eutectic reaction between CoCrFeNi and Nb.

Revealing the Microstructure Evolution and Mechanical Properties of Al2O3-Reinforced FCC-CoCrFeMnNi Matrix Composites Fabricated via Gas Atomization and Spark Plasma Sintering

In the present work, novel Al2O3 particles were used to reinforce heterogeneous CoCrFeMnNi high-entropy alloy (HEA) matrix composites with nano- (5.0 wt.%) and nano- + micro- (5.0 wt.% + 10.0 wt.%) specimens. Al2O3 particles were fabricated via gas atomization and spark plasma sintering. The microstructure evolution and properties, i.e., density, hardness, and room temperature compression, were systematically investigated. The results indicate that the concentration of the Cr element in the pure CoCrFeMnNi HEA and the HEA matrix composite can be effectively reduced by using a gas-atomized HEA powder as the matrix. The formation of an impurity phase can also be inhibited, while the distribution uniformity of matrix elements can be improved. The composites prepared via gas-atomized powders formed a network microstructure composed of continuous Al2O3-rich regions and isolated Al2O3-poor regions, exhibiting good plasticity and improved density. The relative densities of the pure HEA, nano- (5.0 wt.%), and nano- + micro- (5.0 wt.% + 10.0 wt.%) composites were 98.9%, 97%, and 94.1%, respectively. The results demonstrate a significant improvement in the relative densities compared to the values (97.2%, 95.7%, and 93.8%) of the composites prepared via mechanical alloying. In addition, compared to the compressive fracture strains of nano- (5.0 wt.%) and nano- + micro- (5.0 wt.% + 10.0 wt.%) composites based on the mechanically alloyed HEA powder, the values of the nano- (5.0 wt.%) and nano- + micro- (5.0 wt.% + 10.0 wt.%) specimens prepared via gas atomization and spark plasma sintering increased by 80% and 67%, respectively.

In-situ microstructural evolution during tensile loading of CoCrFeMnNi high entropy alloy welded joint probed by high energy synchrotron X-ray diffraction

The research regarding high entropy alloys (HEAs) has proved them to be suitable for engineering applications. Nevertheless, assessing their processability is key for industrial deployment. Of special relevance in terms of processing techniques arised welding. In this work, Gas Tungsten Arc Welding (GTAW) was chosen as a processing method to attest for the suitability of the equiatomic CoCrFeMnNi HEA for one of the most common real-world mechanical solicitations, tensile loading. We delve into an in-situ synchrotron X-ray diffraction analysis of the mechanical behavior of a high-performing GTAW CoCrFeMnNi HEA joint. Local analysis of the microstructure evolution, considering the base material, heat affected zone and fusion zone, was performed by tracking changes in the diffracted intensity and lattice strain. Orientation-dependent evolution is highlighted by considering partial azimuthal integration detailing texture impact across the joint. Evidence of strain concentration at specific locations is correlated with the microstructure and overall macroscopic mechanical behavior.

Molecular dynamics simulation of CoCrFeMnNi with twin boundaries under high-speed impact

Based on the hybrid modeling framework of atomic scale and continuum, molecular dynamics (MD) was the method to study the shock wave, elastic-plastic dual-wave, and microstructure of CoCrFeMnNi high entropy alloy (HEA) with twin boundary under high-speed impact. The interaction between dislocation and twin interfaces is significant to the plastic deformation of HEA, but research on them as initial defects is still limited. Therefore, this article simulates and analyzes the impact velocity and twin spacing and proves that HEA with twin boundaries can synergistically improve strength and flexibility by comparing the wave simulation process and variation of spalling strength between HEA with TB and crystal Ni. The simulation results show that TB affects the propagation of shock waves in HEA and provides deformation sites for the generation of dislocations. Unlike single-crystal HEA, there is a smoother impact response process, and the maximum tensile stress during the fracture stage can reach 41 GPa, which is about 34.12% higher than that of single-crystal HEA. The research results may contribute to a better understanding of HEA with TB and help apply complex load scenarios such as explosive loads.

Suppression of helium migration in arc-melted and 3D-printed CoCrFeMnNi high entropy alloy

We compare the evolution and migration of helium cavities in 3D-printed and arc-melted CoCrFeMnNi high entropy alloys (HEAs), and 304 stainless steel (SS), using transmission electron microscopy (TEM). The materials are subjected to Ni and He sequential implantation as well as subsequent thermal annealing. After irradiation at room temperature, all samples initially display similar He cavity sizes. Annealing up to 673 K has a minimal impact on cavity size. However, annealing at 873 K leads to significant cavity growth. Both HEA materials exhibit larger average cavity sizes compared to 304 SS, while a higher proportion of small cavities is found in the 304 SS. This indicates that Ostwald Ripening is more prevalent in 304 SS, confirmed by in-situ thermal annealing in TEM. Depth distribution analysis reveals distinct cavity belts after annealing: from the narrowest in 3D printed HEA to the broadest belts in 304 SS. These results indicate that the He concentration at the peak of irradiation damage decreases in the same order. We suggest that the long-range migration of He atoms is limited in both the Cantor and 3D-printed Cantor HEAs, potentially due to the complex diffusion paths of the He atoms.

Modulated heating rate effect on optimizing sintered density and microstructure in CoCrFeMnNi high-entropy alloy fabricated through metal injection molding

The fabrication of high-entropy alloys (HEAs) using metal injection molding (MIM) represents a significant leap forward in the field of materials engineering, enabling the production of components with complex shapes while maintaining the unique mechanical properties of HEAs. This study investigates the effect of heating rate variations on the densification behavior in the equiatomic CoCrFeMnNi HEA, focused on microstructural analysis and densification behavior for each thermal schedule. By optimizing the sintering behavior with single and double-step heating rates, significant findings reveal that sintering under double-step heating rates leads to marked improvements in densification and microstructural uniformity. Microstructural analyses indicate that the modulated heating rates during sintering crucially affect the segregation of Cr and secondary phase formation. This research highlights the importance of sintering profiles in optimizing the microstructure and properties of HEAs processed through MIM, contributing valuable insights to a refined strategy for the improved microstructure and mechanical strength of HEAs in powder metallurgy.

Spatially varied stacking fault energy induced low twinning ability in high entropy alloys

Nanostructured high-entropy alloys (HEAs) are promising candidates for extreme load-bearing applications due to their superior performance. In this work, we investigate the deformation behaviors of CoCrFeMnNi HEA under high-speed impact by molecular dynamics simulations. Compared with Al, Ni, and Cu representing pure metals with low to high stacking fault energies, it is found that the CoCrFeMnNi HEA exhibits remarkably low twinning density under shock, despite its extremely low stacking fault energy. Shear loading is then applied to stacking-faulted HEAs and these pure metals to study the evolution of stacking faults under shear stress. The results further show a low tendency for stacking faults to transform into deformation twinning in HEAs, regardless of the initial density of stacking faults. The energy path for deformation twins and stacking faults was calculated, and a direct comparison of fault energies could not explain the deformation mechanism of HEA. We reveal that the inhomogeneous energy profile of dislocation slip caused by the inherent heterogeneity of HEA leads to dispersed stacking fault propagation, which suppresses twinning formation. These results address the spatially tunable defects and further urgent need for the synergistic design of components and microstructures in HEAs.

Effects of Ti addition and annealing on microstructure and mechanical properties of CoCrFeMnNi high-entropy alloy

Effects of Ti addition and annealing on microstructure and mechanical properties of CoCrFeMnNi high-entropy alloy