CoCrCuFeNi High-entropy Alloy Research Articles

AlxCoCrCuFeNi (X = 0, 0.8) high entropy alloys fabricated by laser powder bed fusion

ABSTRACT The AlxCoCrCuFeNi (x = 0, 0.8) high entropy alloys (HEAs) were prepared by laser powder bed fusion (LPBF), and the effects of volumetric energy density (VED) and heat treatment on microstructures and mechanical properties of HEAs were investigated. The results show that the CoCrCuFeNi HEA has a single FCC phase structure and exhibits good ductility. Its yield strength is relatively low, being 520 MPa. After the alloy composition was adjusted from CoCrCuFeNi to Al0.8CoCrCuFeNi, the HEA changed from FCC single-phase to FCC + BCC dual-phase structure and the hardness and compressive properties were significantly improved. Among them, the Al0.8CoCrCuFeNi HEA prepared with the VED of 167 J/mm3 has the best mechanical properties, with a compressive yield strength of 652 MPa and good plasticity. After annealing at 600°C and 900°C for 1 h, the mechanical properties of HEAs are significantly improved. In particular, the microhardness and compressive yield strength of the Al0.8CoCrCuFeNi HEA annealed at 600°C for 1 h are 378 HV and 810 MPa, respectively. The coordinated deformation between FCC and BCC domains, back stress strengthening, solid solution strengthening and precipitation strengthening after heat treatment make the Al0.8CoCrCuFeNi HEA achieve a balance of high strength and good ductility.

Atomistic study of CoCrCuFeNi high entropy alloy nanoparticles: Role of chemical complexity

High entropy alloy nanoparticles are envisaged as one of the most interesting materials compared to monoatomic materials due to their modulated properties in terms of their convenient surface-to-volume ratio. However, studies are still missing to unveil how composition or nanoparticle size can influence nanoparticle morphology. Based on molecular dynamics simulations, we perform a structural characterization as a function of nanoparticle size and the chemical composition of high entropy alloy nanoparticles subject to multiple annealing cycles. After the multiple thermal loads, we observe a substantial migration of copper atoms towards the np surface, consistent with the experimental results of Cu-based high entropy alloys. The resulting high entropy alloy nanoparticle behaves as a core–shell nanostructure with a rich fcc phase on the surface (50% of Cu) and 5% fcc phase in the nanoparticle core. Inspecting the nanoparticle surface, it is observed that high entropy alloy nanoparticles have a lack of surface facets, leading to a more spherical shape, quite different from mono-metallic nanoparticles with a high number of facets. Performing an average atoms simulation, it showed that nanoparticles are prone to form 111 surface facets independent of the nanoparticle size, suggesting that for high entropy alloy nanoparticles, the chemical complexity avoids the formation of surface facets. The latter can be explained in terms of the lattice distortion inducing tensile/compressive stress that drives the surface reconstruction. All in all our results match extremely well with experimental evidence of FeNiCrCoCu nanocrystalline materials, explaining the Cu segregation in terms of surface energy and mixing enthalpy criteria. We believe that our results provide a detailed characterization of high entropy nanoparticles focusing on how chemical complexity induces morphological changes compared to mono-crystalline nanoparticles. Besides, our findings are valuable for experimental works aimed at designing the shape and composition of multicomponent nanoparticles.

Effect of Silver Addition on Microstructure, Phase Composition, Microhardness and Surface Oxide Layer Formation of Al0.5CoCrCu0.5FeNi and Al0.5CoCrCuFeNi High-Entropy Alloys

Effect of Silver Addition on Microstructure, Phase Composition, Microhardness and Surface Oxide Layer Formation of Al0.5CoCrCu0.5FeNi and Al0.5CoCrCuFeNi High-Entropy Alloys

The Effect of La on the Surface Properties of Plasma Nitrided CoCrCuFeNi High-Entropy Alloys at 440 Degrees Celsius

This paper investigates the effect of the element La on plasma nitriding of the CoCrCuFeNi high-entropy alloy (HEA) at 440 °C for 8, 16, and 24 h. The phase composition, morphology, and hardness distribution of the nitrided layer are characterized using XRD, optical microscopy, and a microhardness tester. Furthermore, the corrosion resistance is tested using an electrochemical workstation. The study evaluated the friction and wear performance using a testing machine and scanning electron microscope. The thickness of the effective hardening layer after 16 h of treatment with La was similar to that after 24 h of treatment without La. The addition of La significantly increased the growth rate constant of the effective hardening layer from 0.53 × 10−14 m2/s to 0.72 × 10−14 m2/s. In addition, an expanded FCC phase with greater interplanar spacing can be formed on the surface of the sample by introducing La into the plasma nitriding process. This indicates that the expanded FCC phase, with a higher concentration of interstitial nitrogen atoms, can effectively improve the corrosion resistance of the specimen surface. The corrosion rate of the specimen surface was reduced by 27.5% and the wear rate was reduced by 41.7% after 16 h of treatment with the addition of La compared to 24 h of nitriding without the addition of La. It has been shown that the addition of La to the plasma nitriding process results in a higher quality nitrided layer in a shorter time and also demonstrates that La has the potential to optimize the surface properties of plasma nitrided HEAs.

Metastable phase separation and crystalline orientation feature of electromagnetic levitation processed CoCrCuFeNi high entropy alloy

The metastable liquid phase separation and rapid solidification kinetics of CoCrCuFeNi high entropy alloy (HEA) has been investigated by electromagnetic levitation (EML) technique. The maximum liquid undercooling attained 452 K (0.27TL), and the critical undercooling to initiate phase separation was determined as 172 K. By modulating the degree of alloy undercooling, the overall macrosegragation pattern achieved a transition from homogeneous dendrites into dispersed structures and finally into core-shell structures. Both microstructure and electron backscatter diffraction (EBSD) characterizations revealed that liquid undercooling played a crucial role in the modulation of dendritic anisotropy correlated with crystalline orientations. Coarse high entropy face-centered cubic (HEF) dendrites were produced with 〈100〉 preferred orientation at small undercoolings. Once undercooling exceeded the threshold of 172 K, the previously uniform liquid alloy was separated into Cu-depleted and Cu-rich zones. Numerous equiaxed HEF grains of relatively random orientation together with some twin crystals were displayed in Cu-depleted zone, whereas in Cu-rich zone HEF phase solidified into many slender dendrites. Theoretical analyses indicated that equiaxed grains were produced in the regime of thermal diffusion-controlled dendritic growth with actual solidifying velocity up to 42.7 m s−1. Furthermore, physical properties examinations demonstrated that large liquid undercoolings enhanced the alloy microhardness and low temperature saturated magnetization but reduced its electrical resistivities.

Impact of edge dislocation and grain boundaries on mechanical properties in CoCrCuFeNi high entropy alloy

This study uses molecular dynamics simulations to explore the mechanical behavior of a CoCrCuFeNi high-entropy alloy (HEA) with Σ5 and Σ13 grain boundaries (GBs) as well as without GBs and dislocation. The analysis focused on understanding the influence mechanisms of these grain boundaries on the mechanical behavior of the HEA. Our findings reveal that the atomic size disparity among the constituent elements induces lattice distortion, leading to deformation in HEAs. The determined elastic constants met Born stability requirements, ensuring mechanical stability across both the examined GBs. Higher elastic moduli were associated with increased strength and stiffness, particularly evident in HEAs with Σ5 GB, surpassing those of non-GB structures. Notably, GB Σ5 demonstrated enhanced strength and hardness, indicated by larger elastic moduli compared with those of non-GB structures. Conversely, GB Σ13 exhibited increased Cauchy pressure and Poisson and Pugh's ratios. The ductility of face-centered cubic HEAs was found to be significantly influenced by the GBs, affecting mechanical properties. The Kleinman parameter highlighted a bending-type bonding with reduced strength at the GBs. Machinability indices indicated high machinability of the CoCrCuFeNi alloy, further enhanced by the presence of the GBs. Direction-dependent parameters underscored the anisotropic nature of the HEA, mitigated by the GBs. Overall, this study elucidates the nuanced influence of different GBs on the mechanical properties of HEAs, offering valuable insights for materials design and applications. The results of this investigation shed light on HEAs with improved mechanical properties via GB engineering.

Modeling of the Flow Field and Clad Geometry of a Molten Pool during Laser Cladding of CoCrCuFeNi High-Entropy Alloys

A flow field analysis was performed in this research using the ANSYS Fluent module, and a dynamic heat source employing UDF was constructed using the DEFINE_PROFILE macro. A VOF model was developed to track the volume fraction of each fluid throughout the computational domain as well as the steady-state or transient condition of the liquid–gas interface in the free liquid surface area. To determine the distribution state and regularity of the molten pool flow field, the flow field velocity was calculated iteratively by linking the Simple algorithm with the horizontal set method. The molten pool was concave, indicating that the key hole was distributed narrowly. Inserting cross-sections at different depths yielded the vector distribution of the molten pool flow velocity along the depth direction. We set up monitoring sites along the molten pool’s depth direction and watched the flow change over time. We investigated the effects of the process parameters on the flow field’s vector distribution.

Elucidating the warm compression of CoCrCuFeNi high entropy alloy: Modeling and microstructural evolution

The warm compression tests were carried out at various temperatures of 400, 550, and 700 °C and strain rates of 0.01, 0.001, and 0.0001 s−1 to unravel the warm deformation behavior of CoCrCuFeNi high entropy alloy (HEA). The microstructural evolutions were characterized using x-ray diffraction (XRD), differential scanning calorimetry (DSC), a field emission gun scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS), and electron backscattered diffraction (EBSD) detector. These microstructure studies show a dendritic structure containing a bright phase rich in Cu and a darker phase rich in Fe, Cr, and Co. The solidification and melting points of the bright phase are 1136 °C and 904 °C, respectively and the dark phase nucleates at 1365 °C based on DSC results. A Zener-Hollomon parameter was employed to model and predict the warm compression behavior by calculating the activation energy (∼451 kJ/mol), developing a precise constitutive equation (correlation coefficient: R = 0.99), and constructing a processing map for determining the stable temperature and strain rate (strain rates lower than 10−2.75 s−1 and temperatures between 400 and 700 °C) of the plastic flow behavior of the alloy. The FESEM results confirmed the stability and instability domains predicted by the processing map. Moreover, the EBSD analysis indicated that the continuous dynamic recrystallization (CDRX) grains nucleated in the strain rate of 0.0001 s−1 at temperatures higher than 400 °C. Larger grains were formed at 550 and 700 °C due to lower Z parameters in higher temperatures.

High-Temperature Oxidation Behaviour of High Entropy Alloys

High-temperature operation, such as in gas turbine engines, is typically limited by the ability of materials to withstand extreme operating conditions. The need for a combination of high-temperature strength and oxidation resistance, as well as microstructural stability, has led to the continued use of nickel-based superalloys as materials for turbine blades, the most critical component in aero gas turbine engines. Due to their limited melting points, materials beyond superalloys are needed to meet increasing fuel efficiency requirements. High entropy alloys have recently been considered as candidates for materials to replace nickel-based superalloys. This paper discusses the microstructures and high-temperature oxidation behavior of Al0.75CoCrCuFeNi high entropy alloy at 900, 1000, and 1100°C. It was found that the oxidation rate constants for Al0.75CoCrCuFeNi oxidized at 900, 1000, and 1100°C are 3.84 x 10-10, 5.99 x 10-10, and 8.97 x 10-10 (mg/cm2.s), respectively, with an activation energy for oxidation of 66.58 kJ/mol.

Microstructure and Mechanical Properties of CoCrCuFeNi High-Entropy Alloys Synthesized by Powder Metallurgy and Spark Plasma Sintering

Microstructure and Mechanical Properties of CoCrCuFeNi High-Entropy Alloys Synthesized by Powder Metallurgy and Spark Plasma Sintering

Molecular dynamics simulation of tension and compression deformation behavior in CoCrCuFeNi high-entropy alloy: Effects of temperature and orientation

This study utilizes molecular dynamics (MD) simulations to investigate the uniaxial tensile and compressive deformation behavior of the face-centered cubic (FCC) CoCrCuFeNi high-entropy alloy (HEA) with varying orientations at different temperatures. The results indicate that the material experiences temperature-induced softening in all simulation cases. In addition, it has been observed that the mechanical properties of the CoCrCuFeNi HEA exhibit orientation dependence and tension-compression asymmetry. Analysis of the Schmid factor of the leading and trailing partial dislocations suggests that the yield asymmetry is contingent upon the leading Schmid factor asymmetry. Microstructure evolution analysis and quantitative statistics of deformation defects indicate that the plastic deformation mechanisms of uniaxial tensile deformation along the [001], [111], and [112] orientations and compressive deformation along the [110], [111], and [112] orientations can be characterized by the formation and annihilation of intrinsic stacking fault (ISF), extrinsic stacking fault (ESF), and a few twins resulting from the gliding of Shockley partial dislocations. Twins play a crucial role in compression along the [001] direction and tension along the [110] direction due to the proliferation of twin boundaries (TBs)-assisted dislocations. The interactions between stacking faults (SF) and abundant TBs cause an increase in dislocation density in the later stage of plastic deformation in the aforementioned models. A comparison with low-entropy materials and theoretical calculations reveals that alloying reduces the stacking fault energy (SFE) of the CoCrCuFeNi crystal and enhances its twinnability.

Friction stir welding of AA5010 aluminum alloy to St-12 carbon steel using CoCrCuFeNi high entropy alloy interlayer

Dissimilar friction stir welding (FSW) of AA5010 aluminum alloy to St-12 carbon steel using an equiatomic CoCrCuFeNi high-entropy alloy (HEA) interlayer was investigated. Mechanical properties of the joints fabricated under different plunge depths of the FSW tool were examined and correlated to microstructural evolutions. The incorporation of the HEA interlayer successfully prohibited the formation of AlFe intermetallic compounds (IMCs). Also, the interlayer was metallurgically consistent with both base metals, although quite thin complex IMC layer was formed at the aluminum/HEA interface as a result of sluggish diffusion of constituent elements of HEA towards the aluminum. The application of suitable plunge depth in the steel part, i.e. 0.2 mm, resulted in the perturbation of the HEA/steel interface and then the formation of interfacial mechanical interlocks that were beneficial to the strength of the joints. However, excessive FSW tool plunge depth caused large voids to form near hooks, acting as stress concentration areas during shear-tensile test. An appropriate plunge depth of the tool achieved an ideal joint that exhibited fracture from the aluminum side rather than the joint interfaces. It also provided a strength value as high as 80 % of the nominal strength of aluminum base metal.

The effect of grain boundary on irradiation resistance of CoCrCuFeNi high entropy alloy

Defect traps have been well known to influence the irradiation resistance of traditional structural materials, but to what extent is not fully clear for high entropy alloys. Herein, the effect of grain boundary on irradiation resistance of CoCrCuFeNi is investigated through defects capture, strength variation, and void annihilation via molecular dynamics simulations and compared to that of pure Ni, taking ∑5(210) as a representative grain boundary. The grain boundaries in both CoCrCuFeNi and Ni are effective in capturing point defects. The number of captured defects is related to the distance between primary knock-on atoms and grain boundary. The change in tensile stress of CoCrCuFeNi after irradiation is smaller than that of Ni. The grain boundary of CoCrCuFeNi can annihilate voids at room temperature. However, that of Ni is hindered in the migration process and cannot annihilate voids completely. Our results provide atomistic insights into the mechanism of radiation resistance via grain boundaries as recombination centers.

Real-time atomic deformation behavior of nano CoCrCuFeNi high-entropy alloy

Tensile tests were performed by molecular dynamics at various strain rates ranging from 2×108 to 1×1010 s−1 and various temperatures ranging from 10 to 1200 K, and the real-time deformation behavior of a nanocrystalline CoCrCuFeNi high-entropy alloy was investigated. The results indicate that the main deformation mechanism is grain boundary slip at high temperatures and low strain rates. Dislocation slip replaces grain boundary slip to control plastic deformation with decreasing temperature and increasing strain rate, correspondingly enhancing the strength of the alloy. Furthermore, the alloys with different grain sizes were simulated to evaluate the effects of grain boundaries on the mechanical behavior. It is found that the strength of the nanocrystalline high-entropy alloys increases with increasing grain sizes when the grain size is too small, exhibiting an inverse Hall—Petch relationship.

Effect of Segregation on Deformation Behaviour of Nanoscale CoCrCuFeNi High-Entropy Alloy

A molecular dynamics (MD) simulation method is used to investigate the effect of grain boundary (GB) segregation on the deformation behavior of bicrystals of equiatomic nanoscale CoCrCuFeNi high-entropy alloy (HEA). The deformation mechanisms during shear and tensile deformation at 300 K and 100 K are analyzed. It is revealed that upon tensile deformation, the stacking fault formation, and twinning are the main deformation mechanisms, while for the shear deformation, the main contribution to the plastic flow is realized through the GB migration. The presence of the segregation at GBs leads to the stabilization of GBs, while during the shear deformation of the nanoscale CoCrCuFeNi HEA without the segregation at GBs, GBs are subject to migration. It is found that the GB segregation can differently influence the plasticity of the nanoscale CoCrCuFeNi HEA, depending on the elemental composition of the segregation layer. In the case of copper and nickel segregations, an increase in the segregation layer size enhances the plasticity of the nanoscale CoCrCuFeNi HEA. However, an increase in the thickness of chromium segregations deteriorates the plasticity while enhancing maximum shear stress. The results obtained in this study shed light on the development of HEAs with enhanced mechanical properties via GB engineering.

The Effect of Copper on the Microstructure, Wear and Corrosion Resistance of CoCrCuFeNi High-Entropy Alloys Manufactured by Powder Metallurgy.

This paper focuses on the microstructure, phase composition, mechanical, tribological and corrosion properties of high-entropy alloys (HEAs) in the CoCrCuFeNi system depending on copper content, which was varied from 0 to 20 at. % with an increment of 5%. CoCrCuFeNi alloys were manufactured by powder metallurgy methods: mechanical alloying and hot pressing of element mixtures. The solubility limit of copper in CoCrFeNi solid solution was found to be 9 at. %. Segregation of irregularly shaped copper grains sized 1-30 μm is observed at concentrations above this solubility limit. As copper concentration increases, the phase composition of CoCrCuFeNi alloys changes from the single phase based on FCC1 solid solution (Cu = 0-5 at. %) to the dual-phase FCC1 + FCC2 alloy (Cu = 10-20 at. %), where FCC1 is the main phase and FCC2 is the secondary copper-rich phase. Tribological tests have shown that doping the CoCrFeNi alloy with copper increased wear resistance by 23% due to solid solution hardening. As copper content rises above 20%, the content of the secondary FCC2 phase increases, while wear resistance and alloy hardness decline. An analysis of wear tracks and wear products has shown that abrasion of CoCrCuFeNi alloys occurs via the abrasive-oxidative wear mechanism. The corrosion tests of CoCrCuFeNi HEAs in 3.5% NaCl solution had demonstrated that doping the alloy with copper at low concentrations (5-10%) leads to decreasing of corrosion resistance, possibly due to the formation of undesirable oxide Cu2O along with protective Cr2O3. At high copper concentrations (15-20%) galvanic corrosion is suppressed due to coarsening of FCC2 grains and thus decreasing the specific contact surface area between the cathode (FCC2) and the anode (FCC1).

Enhanced corrosion resistance and hardness of CoCrCuFeNi alloy under high-pressure solidification

This study adopts a high-pressure method to prepare a CoCrCuFeNi high-entropy alloy (HEA). The effects of high pressure on the microstructure, phase composition, hardness, and corrosion resistance of the alloy are studied. The Vickers hardness of the CoCrCuFeNi HEA solidified under ambient pressure and 4 GPa is determined to be 203.4 HV and 822.4 HV, respectively. This presents an approximately three-fold increase in hardness. The nanohardness of the matrix is also increased three-fold, and the value of the intergranular phase is increased by 0.2 times, which is attributed to the precipitation of a finer nano Cu-rich phase and nano Fe–Cr phase under high pressure. A more complete, dense, and uniform CoCrCuFeNi HEA passive film is obtained under high pressure, and the passivation film resistance (Rpass) is increased by 1.3 times. From a polarization curve test, the corrosion rates of the CoCrCuFeNi HEA solidified under ambient pressure and 4 GPa are 0.0209 mm/a and 0.0076 mm/a, respectively. Thus, the corrosion rate is reduced by 64%.

Thermal stress-assisted formation of submicron pillars from a thin film of CoCrCuFeNi high entropy alloy: experiments and simulations.

In this work, for the first time, the thermal stress-assisted formation of submicron pillars (SPs) from a high entropy alloy (HEA) thin film is made possible, and novel molecular dynamics (MD) simulations are proposed to assess the underlying mechanisms. In a series of experiments, the growth of quasi-equiatomic HEA SPs from CoCrCuFeNi HEA thin films was demonstrated under different heating and cooling conditions. Atomistic simulations are performed to probe possible formation mechanisms in two ways. One is to first obtain surface elastic constants and then conduct surface stability analysis with the consideration of size-dependent surface stress. The other is to effectively apply large compressive stress while simplifying the molecular dynamics (MD) model by using the Stoney equation to perform long-term MD simulations. From the former, it is suggested that surface diffusion is likely not the dominant cause for the observed pillar formation. From the latter, it is revealed that the level of compressive stress plays a much greater role than the crystalline structure of the film sample. Light has been shed on the stress-assisted formation of submicron pillars from CoCrCuFeNi HEA films by both experimental and simulation approaches.

Study on Microstructure of Fiber Laser Welding of CoCrCuFeNi High Entropy Alloy.

A CoCrCuFeNi high-entropy alloy was successfully welded in this study using fiber laser welding. The effects of the welding parameters on the microstructure and mechanical properties were studied. Three zones were formed: the fusion zone, partial melting zone, and base metal. The base metal exhibited a typical dendrite structure, and the Cu element segregated in the interdendrite. The fusion zone consisted of fine equiaxed crystals and columnar crystals with the same crystalline structure as the base metal. The fusion zone exhibited minimal compositional microsegregation after laser welding. Electron backscatter diffraction results showed that the low-angle grain boundary fraction in the fusion zone increased. Furthermore, some dislocations and dislocation pile-ups were present in the fusion zone, and the densities of the dislocations and dislocation pile-ups were higher than those of the base metal. The hardness of the fusion zone was considerably higher than that of the base metal, while the ultimate tensile strength and elongation values were lower than those of the base metal for all conditions. The ultimate tensile strength and the elongation increased gradually and then decreased with increasing laser power. The maximum ultimate tensile strength exceeded that of the base metal by 90%.

Microstructure, thermodynamics and compressive properties of Al0.5CoCrCuFeNi-X (X=V,Si,VSi) high-entropy alloys

ABSTRACT The article investigated the effect of V and Si on the microstructure, thermodynamics and compressive properties of Al0.5CoCrCuFeNi high-entropy alloy (HEA). Four Al0.5CoCrCuFeNi-X(X = V, Si, VSi) HEAs were prepared by vacuum arc melting. With the addition of V and Si, the microstructure of the Al0.5CoCrCuFeNi HEA changes from FCC to FCC + BCC. The (Ω, δ)-values of the four HEAs meet the formation condition of solid solution. Their VEC-values in the range of [6.87–8] explain the reason for FCC and FCC + BCC formation in the four HEAs. The V and Si elements greatly improve the hardness of the three HEAs. The fracture mechanisms of X = V, Si, VSi HEAs are the composite fracture, quasi-cleavage fracture and purely brittle cleavage fracture, respectively.