CrMnFeCoNi High-entropy Alloy Research Articles

Structure and properties of the CrMnFeCoNi high-entropy alloy irradiated with a pulsed electron beam

According to the technology of wire-arc additive manufacturing a nonequiatomic composition CoCrFeMnNi high-entropy alloy (HEA) was obtained. Deformation curves of samples in tension are plotted and analyzed after the HEA fabrication by the methods of wire-arc additive manufacturing (initial state) and after the electron-beam processing (EBP). The EBP results in a decrease in the HEA's strength and plastic properties. Along with a pit character of the fracture a presence of micropores and microlayerings are identified. A study of the HEA's fracture surface after the EBP except for regions with a ductile fracture mechanism revealed regions with a band (lamellar) structure. The area with a band structure increases with a growth in the beam electron density from 25% at 10 J/cm2 to 65% at 30 J/cm2. A diameter of separation pits in fracture bands varies in the limits (0.1–0.2) μm, which is considerably less than those of the remaining part of the HEA sample. An average size of crystallization cells formed in the EBP depends on the energy density of electron beam and increases from 310 nm at 15 J/cm2 to 800 nm at 30 J/cm2. A non-monotonous change in the scalar dislocation density, reaching a maximum value of ∼5.5 · 1010 cm-2 at a distance of 25 μm from the irradiation surface is revealed. It is suggested that defects being formed in surface layers in the EBP may be one of the reasons for decreasing the values of HEA strength and plasticity.

Fractography of fracture surface of CrMnFeCoNi high-entropy alloy after electron-beam processing

In the past decade the attention of scientists in the field of physical materials science is attracted to studying the high-entropy alloys. By the technology of wire-arc additive manufacturing (WAAM) a high-entropy alloy (HEA) of a nonequiatomic composition was obtained. Deformation curves obtained under uniaxial tension at a rate of 1.2 mm/min at room temperature using Instron 3369 unit were analyzed in two states: initial/after fabrication and after electron-beam treatment (EBT). EBT was conducted to detect its influence on structural-phase states and mechanical properties. The EBP leads to a decrease in strength and plastic properties of the HEA. By means of scanning electron microscope LEO EVO 50, analysis of structure of fracture surface and the near-surface zone was performed. Dependences of the ultimate strength and relative elongation to failure on EBT parameters were revealed, and it was shown that values of strength and plasticity decrease nonmonotonically with an increase in electron beam energy density in the range ES = 10 – 30 J/cm2 at constant values of duration, frequency, and number of pulses. Along with a pit character of the fracture a presence of micropores and microlayering was detected. Investigation of the HEA’s fracture surface after EBP except for areas with a ductile fracture mechanism revealed the regions with a band (lamellar) structure. At ES = 10 J/cm2, the area of such structure is 25 %; it increases nonmonotonically to 65 % at ES = 30 J/cm2. The diameter of pits of detachment in fracture bands varies in the limits of 0.1 – 0.2 μm, which is considerably less than that in the remainder of the HEA samples. After EBP the thickness of the molten layer varies in the limits of 0.8 – 5.0 μm and grows with an increase in the energy density of electron beam. EBT leads to generation of crystallization cells, the sizes of which change within the range 310 – 800 nm as ES increases from 15 to 30 J/cm2. It is suggested that the defects being formed in surface layers in ЕВР may be the reason for decreasing the HEA’s maximum values of strength and plasticity.

Epitaxial growth of high-entropy alloy thin film with spontaneous exchange bias

High-entropy alloys (HEAs) have attracted extensive research interest for their outstanding mechanical, electrochemical, and magnetic properties due to the highly random occupation of near equimolar multi-principal elements. The functionalities are severely influenced by the highly disordered lattice structure at grain boundaries and random orientation of grains in polycrystalline samples. Here, epitaxial CrMnFeCoNi HEA thin films of (001) orientation are prepared by laser-assisted molecular beam epitaxy on MgO substrates using a Cu buffer layer. A spontaneous exchange bias (SEB) effect is observed in a 98 nm thick CrMnFeCoNi HEA thin film. The SEB effect is related to a newly formed ferromagnetic (FM)/spin glass (SG) interface generated by field-induced irreversible growth of FM regions. The interfacial spins of SG domains are gradually consumed during the initial magnetizing process, leading to the consequent reservation of unidirectional anisotropy. The observation of SEB in HEA films expands their functions, which may be applied in novel magnetic devices.

Chloride corrosion property evolution of CrMnFeCoNi high-entropy alloy coating

CrMnFeCoNi coating was produced on the X65 steel substrate by the arc cladding method. The microstructure and chemical composition of the coating were explored. The results reveal that the CrMnFeCoNi coating exhibited a single-phase structure. The corrosion behaviour of the coating was investigated by the electrochemical test in NaCl solution (3.5 wt-%). The results show that the CrMnFeCoNi coating possessed lower corrosion current density and higher charge transfer resistance than the X65 steel. The superior corrosion resistance of the CrMnFeCoNi coating compared to the X65 steel substrate was due to the formation of the protective passive film. The coating effectively improved the corrosion resistance of the X65 steel substrate.

The effect of bond coat on the high-temperature behavior of HVOF-sprayed (Co,Ni)O coating on Cu-Ni-Fe anodes

The high-temperature behavior of Cu-Ni-Fe-coated materials is evaluated as inert anodes for zero‑carbon Al production. Co0.5Ni0.5O top coat is deposited on CrMnFeCoNi high entropy alloy (HEA) and Ni–20Cr bond coats with Cu-rich and Ni-rich CuNiFe as the substrate materials. The bond coats as well as the top coat are deposited by high velocity oxy fuel (HVOF) process. The effect of the bond coat layers on the oxidation resistance of coated samples is investigated at 1000 °C in air. The formation of a dense thin layer of Cr2O3 on both bond coats inhibits oxygen diffusion to the substrates. No diffusion of elements is observed from NiCr to (Co,Ni)O, while Mn atoms diffuse from HEA into the top coat layer during oxidation process.

Diffusion kinetics and boronizing of high entropy alloy produced by TIG melting reverse suction method

ABSTRACT In this study, CrMnFeCoNi high entropy alloy was produced using TIG melting reverse vacuum method using a shielding gas atmosphere. Produced samples were boronized with pack boriding method at 1173, 1223, 1273 K temperatures and for 2, 4 and 6 h. As a result of the experiments, the morphology of the boride layers formed on the high entropy alloy surfaces was investigated by SEM microscope. The hardness values were measured with a microhardness device and the phases formed on the boride layer were determined by the XRD method. In addition, the thickness and growth kinetics of the boride layer on the high entropy alloy were investigated. The boride coating thickness ranges from 38 µm to 268 µm. FCC crystal structure is present in the HEAs produced in XRD examinations and the hardness value was determined as 242 HV0,05 (± 15). B-dependent phases of Cr, Mn, Fe, Co, Ni were obtained in the coating layer and the hardness value was measured as 2250 HV0,05 maximum. The boride layer activation energy value of the coated HEA was calculated as 376,465 kJ/mol.

Revealing the deformation mechanisms of the heterogeneous structured CrMnFeCoNi high entropy alloy with ameliorated mechanical and corrosion resistance properties

Developing high entropy alloy (HEA) with both high strength and ductility is a longstanding challenge as potential structural materials. The strength and ductility trade-off can achieve via tailoring heterogeneous structures. Herein, a gradient structure with grain size variation from surface to center was successfully introduced into the CrMnFeCoNi HEA by ultrasonic nanocrystal surface modification (UNSM) process. More importantly, the deformation mechanisms and strengthening mechanisms of the heterogeneous structured CrMnFeCoNi HEA during tension were also investigated. Experimental results demonstrated that the heterogeneous structured CrMnFeCoNi HEA shows excellent strength and ductility combination properties ascribed to the grain refinement strengthening, dislocation density strengthening, and twinning strengthening. Besides, the corrosion resistance properties are also simultaneously improved, resulting from the low surface roughness, small grain size, and large compressive residual stress value. During tensile deformation process, the deformation mode is the dislocation slip at low strain and the slip system of the alloy is mainly {111}〈110〉. With the increase of deformation amount, twin crossing occurs, and grain size gradually decreases. The deformation mode changes from dislocation slip to twinning at high strain. In conclusion, the UNSM process ameliorates the comprehensive mechanical and corrosion resistance properties of CrMnFeCoNi HEA.

Enhancement mechanism of fracture-resistance of CrMnFeCoNi high-entropy alloys: A molecular dynamics study

It has been found that amorphization in high-entropy alloys (HEAs) can happen under the condition of severe plastic deformation. Therefore, understanding the interaction of amorphization with other kinds of deformations has become an important issue for the design of the mechanical properties of HEAs. However, the underlying mechanism of such interaction in HEAs remains unclear. Here we found in our molecular dynamics simulations that the synergistic effects of amorphization and dislocations could significantly improve the fracture-resistance of CrMnFeCoNi HEAs. It shows that amorphous structures and dislocations appeared simultaneously in HEAs, where the amorphous structures are confined by the surrounding dislocations. Once a crack propagates, some dislocation zones would become amorphous, and the local stress concentration could be relaxed during such kind of transformation instead of crack propagation, resulting in the improvement of the fracture-resistance of CrMnFeCoNi HEAs. In contrast, in nickel single crystals, only dislocations appear and fracture is more likely to occur when dislocations are emitted. It was also found that the length of cracks could affect the evolution of the microstructures in the HEAs. These findings provide new insights into the synergistic effects of the interaction between amorphization and dislocations on the enhancement of the fracture-resistance of the HEAs, which may help to improve the mechanical properties of high-performance HEAs.

Atomic Stress State Inside fcc and bcc Random Alloys: A First-Principles Approach

The stress state at an atomic level and its governing physics inside a random alloy are essential elements in developing a model for solid solution strengthening in random alloys, which is one of the primary strengthening mechanisms of high-entropy alloys (HEAs). Through first-principles calculation, we investigated the atomic stress in fcc and bcc random alloys that were subsets of CrMnFeCoNi and VNbMoTaW HEAs, respectively. The results showed a correlation between the atomic pressure dispersion and the experimental yield stress for the bcc random alloys, as observed in a previous study on fcc alloys. By focusing on the charge transfer and volume change with respect to a bulk crystal, we examined whether the internal stress fields in the fcc and bcc alloys could be interpreted from a unified viewpoint in terms of these physical quantities. Regression analyses using the random forest method revealed that the charge transfer and volume change simultaneously govern the stress state inside an alloy, albeit with varying degrees of intensity.

Microstructure and defect effects on strength and hydrogen embrittlement of high-entropy alloy CrMnFeCoNi processed by high-pressure torsion

High-entropy alloys (HEAs) are considered as new hydrogen compatible materials, but enhancing their yield strength without deteriorating their hydrogen embrittlement resistance is challenging. In this study, various kinds of defects are introduced into a CrMnFeCoNi Cantor alloy by plastic straining via the high-pressure torsion method, and the correlations of applied strain, microstructural features, strength, and hydrogen embrittlement are studied. The unstrained coarse-grained alloy shows elongations over 80% under hydrogen, but its yield strength is only 220 MPa. Twinning is a major deformation mechanism at the early stages of straining, resulting in over 1 GPa yield strength and 9% elongation in the presence of hydrogen. With a further increase in strain, dislocation-based defects including Lomer-Cottrell locks and D-Frank partial dislocations with low mobility are formed, enhancing the strength further. At large strains, nanograins with high-angle boundaries are generated, resulting in over 1900 MPa strength with poor hydrogen embrittlement resistance due to large hydrogen diffusion and hydrogen-enhanced decohesion. These results suggest that twins and defects with low mobility such as Lomer-Cottrell locks and D-Frank partial dislocations are effective to achieve a combination of high yield strength and good hydrogen embrittlement resistance by suppressing the hydrogen-enhanced localized plasticity in HEAs.

Dynamically reversible shear transformations in a CrMnFeCoNi high-entropy alloy at cryogenic temperature

Shear transformation, such as twinning and martensitic phase transformation, is generally unidirectional under monotonic thermal or mechanical loading. Here, we report the dynamically reversible shear transformations in a CrMnFeCoNi high-entropy alloy (HEA) under uniaxial tension at the extremely low temperature of 4.2 K. Since stacking fault energy (SFE) of CrMnFeCoNi HEA with a face-centered cubic (fcc) structure is low and decreases with decreasing temperature, plastic deformation is accommodated by dislocation slips and shear transformation bands, such as {111} stacking faults (SFs), {111} nano-twins and fcc → hcp (hexagonal close-packed structure) shear transformation bands. When deformed at 4.2 K, the lower SFE promotes fcc → hcp shear transformation, forming hcp grains. Besides basal and non-basal dislocation slips in hcp grains, high-density {0001} SFs and {101¯1} nano-twinning are activated to accommodate plastic deformation. More intriguingly, reverse hcp → fcc shear transformations are stimulated by deformation-induced local dissipative heating. The reversible fcc↔hcp shear transformations and both {101¯1} and {111} nano-twinning lead to dynamic nano-laminated dual-phase (NL-DP) structures, which advances the monotonic “dynamic Hall-Petch” effect in enhancing strength, strain-hardening ability, and ductility by dynamically tailoring the type and width of shear transformation bands. Our work provides a promising strategy for evading the strength-ductility dilemma via dynamically developing NL-DP structures through activating reversible shear transformations.

Deformation and damage of heterogeneous-structured high-entropy alloy CrMnFeCoNi under plate impact

Plate impact experiments are conducted on a hot-rolled, heterogeneous-structured, high-entropy alloy CrMnFeCoNi consisting of two types of domains: the UR domains containing unrecrystallized stretched grains and the FR domains containing fine recrystallized grains. Free surface velocity histories are obtained along with microstructure characterizations. Shock-induced deformation twinning is activated firstly in the UR domains, and the dislocation density of the FR domains increases more significantly during shock compression. Under similar shock stress, spall strength is the highest for loading along the rolling direction as a result of the texture-induced highest longitudinal sound velocity. Voids prefer to nucleate at the triple junctions of high angle grain boundaries in the FR domains or around the UR–FR domain boundaries. Compared to the homogeneous (annealed) structure, the heterogeneous structure leads to a negligible increase in spall strengths because of grain refinement in the FR domains and relatively severe plastic deformation around the UR–FR domain boundaries.

Theoretical modeling of toughening mechanisms in the CrMnFeCoNi high-entropy alloy at room temperature

While multiple plastic deformation mechanisms at room temperature in the CrMnFeCoNi high-entropy alloy (HEA) have been revealed experimentally, the qualitative and/or quantitative effects of the microstructural evolution on crack growth are studied rarely. In this work, the effects of crack bridging, crack-tip dislocation emission, and crack deflection on crack growth are modeled and analyzed. The results show that crack bridging via plastically deformed nanobridges has a significant inhibiting effect on crack growth, and the fracture toughness increases with an increase of the bridging-zone length. The crack growth resistance is also influenced by the structural transformation of the near-crack-tip dislocation pileup, which is sensitive to the grain-boundary (GB) misorientation angle Δθ. The pileup dislocations can penetrate across the GB at a small GB misorientation angle (at Δθ≤θc, where θc=33.8∘∼35.5∘), while at large GB misorientation angles (at Δθ>θc), the dislocation pileup initiates nanocrack generation at the GB. Crack deflection has little effect on stress field near the crack tip but has a great shielding effect on the driving force for crack growth. The driving force is decreased by ∼60% when the crack deflection angle reaches 70∘. This paper provides a comprehensive understanding of toughening mechanisms and suggests some strategies of microstructure design to improve the toughness of the HEA.

High-temperature oxidation and wear properties of TiC-reinforced CrMnFeCoNi high entropy alloy composite coatings produced by laser cladding

Extensive studies have been conducted on high entropy alloy composite coatings reinforced with micro-sized ceramic particles at the room-temperature. However, studies on their high temperatures properties are rare. The oxidation and tribological properties of CrMnFeCoNi/xTiC composite coatings produced by laser cladding were studied at 600 °C. The addition of TiC to the coating promoted the oxidation process due to “short-circuit diffusion”. During the high temperature wear process, brittle cracking did not occur on the worn surface, and a good wear-resistant intermediate layer was formed. The coatings exhibited low hardness, low wear rate, and high coefficient of friction due to the formation of composite oxide film formed on the worn surface with good adhesion to the matrix.

Gradient-structured high-entropy alloy with improved combination of strength and hydrogen embrittlement resistance

High-strength materials usually exhibit poor hydrogen embrittlement resistance, and thus, there are demands for materials with high strength and good ductility under hydrogen. Here, gradient structures containing surface nanotwins are introduced in a CrMnFeCoNi high-entropy alloy by surface mechanical attrition treatment, and hydrogen embrittlement resistance is compared with coarse- and nano-structured alloys produced by high-temperature homogenization and high-pressure torsion, respectively. The coarse-grained alloy shows high ductility, but its yield stress is low. The nanostructured alloy shows ultrahigh yield stress, but with poor hydrogen embrittlement resistance. The gradient-structured alloys have both high yield stress (500–700 MPa) and good ductility (15–33%) under hydrogen.

Microstructural and Tribological Behavior of Thermal Spray CrMnFeCoNi High Entropy Alloy Coatings

Microstructural and Tribological Behavior of Thermal Spray CrMnFeCoNi High Entropy Alloy Coatings

Texture evolution in a CrMnFeCoNi high-entropy alloy manufactured by laser powder bed fusion

Additive manufacturing (AM) techniques including laser powder bed fusion have been widely used to produce metallic components with microstructures and mechanical properties distinctly different from the conventionally manufactured counterparts. Understanding how AM parameters affect the evolution of microstructure, including texture, of these AM metallic components is critical for appropriate manipulation of their processing and therefore their mechanical properties. Here we conducted a systematic investigation of texture evolution of a face-centred cubic CrMnFeCoNi high-entropy alloy cuboid fabricated using laser powder bed fusion. Our results showed that the texture evolutions along the build direction were different between the corner and central parts of the sample. Detailed analysis suggested that the texture evolution is closely related to local thermal gradient, which is a property that can be manipulated through changing AM parameters. The different textures lead to the significant variations of mechanical properties within the sample.

Thermally activated dependence of fatigue behaviour of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion

The CrMnFeCoNi high-entropy alloy demonstrates a promising potential for applications over a range of temperature. The alloy also shows excellent printability to be fabricated by additive manufacturing for complex structures. Nevertheless, there are limited studies on the thermo-mechanical behaviour of the alloy, in particular when fabricated by laser powder-bed fusion. This study provides an in-depth understanding of the relationship between as-built cellular microstructures and fatigue behaviour at a range of temperatures (22–600 °C) in particular concerning the stability of dislocation cells and thermo-mechanical dependence of the fatigue behaviour of the alloy. At all tested temperatures, the alloy exhibits a very short duration cyclic hardening with a low hardening rate followed by a cyclic softening. The high density of dislocations already existing in as-built condition were able to accommodate most of the prescribed strain. Hence, only a small number of mobile dislocations needs to be generated, causing a short cyclic hardening phase. Upon further loading, the back stress associated with the long-range stress field was dominant factor governing the cyclic softening behaviour. The similitude relationship provided insights into the stability of as-built cells, in particular it explains why the size of as-built cells did not change during cyclic loading at 22 °C. The significant reduction in dislocation density due to the increased annihilation rate and untanglement of dislocation substructures thanks mainly to thermal assistance at elevated temperatures led to a decrease in cyclic strength and related properties (yield stress, friction and back stress, hysteresis loop shape parameter and energy per cycle). The LPBF HEA shows an insignificant strain rate dependence of the primary cyclic hardening and softening in the range of 10 −3 s −1 and 10 −2 s −1 . However, the dynamic strain ageing results in a secondary cyclic hardening at 400 °C and the reversed strain sensitivity at temperatures from 200° to 400 °C. The fracture mode was transgranular at 22–400 °C but changed to more intergranular-like at 600 °C due to the decohesion of grain boundaries, resulting in a reduction in fatigue life.

Mictomagnetism and suppressed thermal conduction of the prototype high-entropy alloy CrMnFeCoNi

• CrMnFeCoNi is found to be mictomagnetic. • There is weak lattice anharmonicity in CrMnFeCoNi. • The thermal conductivity is reduced down to about 10% of nickel due to disorder scattering. • Lattice of CrMnFeCoNi is remarkably softened compared to nickel. High-entropy alloys are characteristic of extensive atomic occupational disorder on high-symmetric lattices, differing from traditional alloys. Here, we investigate the magnetic and thermal transport properties of the prototype face-centered-cubic high-entropy alloy CrMnFeCoNi by combining physical properties measurements and neutron scattering. Direct-current and alternating-current magnetizations measurements indicate a mictomagnetic behavior with coexisting antiferromagnetic and ferromagnetic interactions below room temperature and three anomalies are found at about 80, 40, and 20 K, which are related to the paramagnetic to antiferromagnetic transition, the antiferromagnetic to ferromagnetic transition, and the spin freezing, respectively. The electrical and thermal conductivities are significantly reduced compared to Ni, and the temperature dependence of lattice thermal conductivity exhibits a glass-like plateau. Inelastic neutron scattering measurements suggest weak anharmonicity so that the thermal transport is expected to be dominated by the defect scattering.

Prediction of short-range order in CrMnFeCoNi high-entropy alloy

To theoretically examine the short-range order (SRO) in CrMnFeCoNi high-entropy alloy, we performed first-principles-based Monte Carlo simulations. A significant change in the number of the first and second nearest neighbor (NN) pairs occurred around the Cr atoms: Cr–Cr and Cr–Mn first NN pairs decreased and the other pairs increased. The second NN pair showed the opposite trend. These changes led to the formation of L12-type ordering. This chemical SRO originates from the instability of the parallel-spin first NN pairs around the Cr atoms in the face-centered cubic structure. The Cr and Mn atoms tend to occupy the corner sites of the L12 structure to reduce the number of first NN Cr–Cr and Cr–Mn pairs. The degree of the L12-type ordering is approximately 0.3 in the range of the 2nd NN distance. The elastic modulus is predicted to increase by forming the chemical SRO, whereas the influence of the chemical SRO on the averaged structural properties is small.