High-entropy Alloys For Applications Research Articles

High Entropy Alloy Coatings and Technology

Recently, the materials research community has seen a great increase in the development of multicomponent alloys, known as high entropy alloys (HEAs) with extraordinary properties and applications. In surface protection and engineering, diverse applications of HEAs are also being counted to benefit from their attractive performances in various environments. Thermally sprayed HEA coatings have outperformed conventional coating materials and have accelerated further advancement in this field. Therefore, this review article overviews the initial developments and outcomes in the field of HEA coatings. The authors have also categorized these HEA coatings in metallic, ceramic, and composite HEA coatings and discussed various developments in each of the categories in detail. Various fabrication strategies, properties, and important applications of these HEAs are highlighted. Further, various issues and future possibilities in this area for coatings development are recommended.

MnO2-decorated metallic framework supercapacitors fabricated from duplex-phase FeCrCoMnNiAl0.75 Cantor high entropy alloy precursors through selective phase dissolution

To exploit functional applications of high entropy alloys (HEAs), HEA metallic frameworks (MFs) have been fabricated through selective phase dissolution of fcc and bcc duplex-phase FeCrCoMnNiAl0.75 HEAs precursors in H2SO4. Preferential dissolution of less-noble cubic fcc phases leads to the formation of three dimensional continuously-distributed residual bcc-phase MFs with wall thickness of 31–45 nm and the regular cubic voids with an average size of 121–150 nm similar with cubic fcc phases in size. Microstructural inheritance of Cantor HEAs governs the formation of MFs. MnO2-decorated HEA metallic framework (MnO2/MF) have been fabricated by electrodepositing MnO2 thin layers on MFs with average thickness of about 9 nm. The MnO2 layers-decorated MnO2/MF composite electrodes can fulfill the high specific capacitance of 961 F g−1, close to the theoretical value, due to the large specific surface area and superior electronic conductivity. Enhanced capacitive performances may result from the fast ion/electron transports between the active MnO2 and the electrolytes. The results suggest that the uniform MF composite electrodes are capable for improving capacitance performance of MnO2 in supercapacitors.

Comparative irradiation response of an austenitic stainless steel with its high-entropy alloy counterpart

Two metallic alloys in the quaternary system Fe–Cr–Mn–Ni were irradiated in situ within a transmission electron microscope (TEM) using Xe+ heavy ions in the temperature range of 293–873 K and in the regime of low- (30 keV) and medium-energies (300 keV) with respective maximum doses of around 40 and 140 dpa. The first alloy is the FeCrMnNi high-entropy alloy (HEA) synthesised with the alloying elements close to equimolar composition. The second alloy is a commercial austenitic stainless steel AISI-348 (70.5Fe-17.5Cr-1.8Mn-9.5Ni wt.%), selected as the “low-entropy” counterpart of the FeCrMnNi HEA. Microstructural characterisation was carried out in the TEM with in situ heavy ion irradiation to investigate the role of entropy on radiation induced segregation and precipitation (RIS and RIP). The results demonstrated that among all the irradiation cases investigated, the FeCrMnNi HEA had its random solid solution matrix phase preserved in 80% of the experiments whilst the austenite matrix of the AISI-348 steel underwent RIP in 80% of the cases. It is therefore demonstrated that small differences between two alloys can lead to different radiation responses, confirming the trend that, by tuning the elemental composition superior radiation resistance can be achieved in metallic alloy systems, but emphasising that some of the constitutive core-effects of HEAs are still in need of further confirmation especially when the application of HEAs in energetic particle irradiation environments is considered.

Manufacturing of FeCoCrNi + FeCoCrNiAl laminated high-entropy alloy by laser melting deposition (LMD)

The ductility and strength of Fe-Co-Cr-Ni-Al high-entropy alloys (HEAs) present contradictory trends with the increasing Al element content. To broaden the applications of HEAs, new methods must be explored to effectively improve the combination of their mechanical properties. In this study, FeCoCrNi + FeCoCrNiAl-laminated HEA was manufactured by laser melting deposition (LMD), a highly efficient and flexible technology. After manufacturing, the microstructure and mechanical properties of the composite HEA were evaluated. FeCoCrNi LMD-HEA contains only the soft face-centered cubic (FCC) phase, which has excellent dislocation storage capacity, ductility, and toughness. FeCoCrNiAl LMD-HEA has high strength and low toughness due to the hard and brittle body-centered cubic (BCC) phase. The FeCoCrNi + FeCoCrNiAl-laminated HEA presented excellent ductility and strength and superior crack resistance.

Microwave sintered porous CoCrFeNiMo high entropy alloy as an efficient electrocatalyst for alkaline oxygen evolution reaction

Developing efficient, stable and economical electrocatalyst for oxygen evolution reaction (OER) is a significant pathway to produce clean energy from water splitting. Herein, the promising and highly efficient porous CoCrFeNiMo high entropy alloys (HEAs) were prepared by microwave sintering using Mg space holder. Owing to unique properties of high entropy alloys and abundant active surface area, the porous CoCrFeNiMo-20 Mg exhibits excellent catalytic performance and prominent electrochemical stability with a low overpotential of 220 mV to reach current density of 10 mA cm−2, a small Tafel slope of 59.0 mV dec-1 and long-term durability for 24 h in 1.0 M KOH. The results of microstructure and element states indicate that crystal defects, porous structure and villous hydroxides are the reasons for high OER performance of the CoCrFeNiMo. This work not only provides a new way to prepare electrocatalysts, but also proves the important application of high entropy alloys in functional materials.

Yield strength prediction of high-entropy alloys using machine learning

Yield strength at high temperature is an important parameter in the design and application of high entropy alloys (HEAs). However, the experimental measurement of yield strength at high temperature is quite costly, complicated, and time-consuming. Therefore, it is essential to identify and apply a robust method for the accurate prediction of yield strength at high temperature from the available experimental and simulation data. In this study, for the first time, a machine learning (ML) method based on the regression technique of random forest (RF) regressor is used to predict the yield strength of HEAs at the desired temperature. The yield strengths of MoNbTaTiW and HfMoNbTaTiZr at 800 °C and 1200 °C, are predicted using the RF regressor model. We find that the results are consistent with the experimental reports, showing that the RF regressor model predicts the yield strength of HEAs at the desired temperatures with high accuracy.

Investigation on microstructure and properties of laser cladded AlCoCrCuFeNi high entropy alloy coating by ultrasonic impact treatment

The application of high entropy alloy (HEA) as coating materials has become a research hotspot. The combination of laser cladding technology and ultrasonic impact treatment (UIT) may provide an opportunity for better performance of HEA and it is possible to enhance the application prospect of HEA. To research the effects of UIT on microstructure and properties of laser cladded HEA coating, AlCoCrCuFeNi HEA coating was prepared by means of laser cladding and UIT was applied to strengthen the surface properties of AlCoCrCuFeNi HEA coating. The phase structures of laser cladded AlCoCrCuFeNi HEA coating were composed of FCC and BCC phases. There were no new phases formed in the AlCoCrCuFeNi HEA coating after UIT, while the BCC phase volume fraction was increased. Additionally, a hardening layer with a depth of ~15 μm was formed at the top surface and the average values of grain sizes were refined from 34.4 μm to 18.6 μm. As expected, the hardness of AlCoCrCuFeNi HEA coating with UIT was significantly improved to 715 HV0.2 benefitted from the nano crystal structure and enhanced plastic deformation and then decreases gradually along the depth to approximately the same hardness value as that of without UIT (530 HV0.2). More impressively, the impact hardening AlCoCrCuFeNi HEA coating exhibited higher corrosion resistance in 3.5% NaCl solution resulting from the lower surface roughness and fewer microstructure defects.

Revisiting the role of prestrain history in the mechanical properties of ultrafine-grained CoCrFeMnNi high-entropy alloy

The yield strength of face-centered cubic (FCC) alloy is always insufficient for applications. In this work, different prestrain histories were imposed to improve the yield strength and strain-hardening capability of an ultrafine-grained (UFG) CoCrFeMnNi high-entropy alloy (HEA). In contrast to the specimens prestrained at 293 K, the specimens prestrained at 77 K possess higher yield strength, elongation, and strain-hardening capability, which were intensely related to the formation of deformation twins. The cryogenic strengthening magnitude is found to be strongly associated with the grain size, but slightly affected by dislocations. By modulating the prestrain history, an ultrahigh yield strength of 1.84 GPa and a considerable uniform elongation of 13% were achieved at 77 K in the CoCrFeMnNi HEA. Hence, imposing prestrain on HEAs at 77 K could be an efficient strategy to harmonize the mechanical properties of the FCC HEAs, which would enrich the application of HEAs in cryogenic fields.

Precipitation Hardening of the HVOF Sprayed Single-Phase High-Entropy Alloy CrFeCoNi

The application of high-entropy alloys (HEA) in surface technology has great potential due to the high corrosion and wear resistance. A further improvement can be achieved by applying thermochemical treatments. Powder-pack boriding enables the formation of a protective precipitation layer. This process has already been applied for cast HEAs causing the formation of a diffusion-enriched surface layer and a distinct increase in wear resistance. In the current investigations, the alloy CrFeCoNi with a single-phase face-centred cubic (fcc) structure is considered. An efficient application can be achieved by limiting the material usage of HEAs to the surface. Therefore, the high-velocity-oxygen-fuel (HVOF) thermal spray process is applied. Boriding was conducted with an adapted powder-pack routine. Furthermore, borided bulk HEAs were considered as a reference. The influence of the production route and boriding treatment on the microstructure, phase formation, and properties was investigated in detail. For the coating and the cast HEA, a precipitation layer is formed. Hence, the hardness and wear resistance are significantly increased. The current study proves the suitability of the investigated process combination.

Functional properties and promising applications of high entropy alloys

High-entropy alloys are a novel class of complex materials discovered near the center of phase diagram, and show exceptional performances over the traditional alloys. The ability of high-entropy alloys to be designed unique combinations of mechanical and functional properties in unlimited space of alloy compositions is encouraging. The unconventional chemical structures hold promise for achieving unprecedented functional performances, making it a potential functional material in the field of soft-magnetic, anti-radiation, catalyst, photothermal conversion materials, etc. Here, the recent progress in understanding the salient features of high-entropy alloys is reviewed. The functional performances and underlying mechanisms are carefully discussed.

Irradiation effects on Al0.3CoCrFeNi and CoCrMnFeNi high-entropy alloys, and 316H stainless steel at 500 °C

To evaluate the potential of high entropy alloys (HEAs) for nuclear applications, two HEAs, Al0.3CoCrFeNi and CoCrFeMnNi, and a conventional Type 316H stainless steel (SS) were irradiated with 1 MeV krypton ions at 500 °C up to 1 dpa, and examined in-situ with a transmission electron microscope (TEM). After irradiation, a high density of ordered L12 nanoparticles was observed in Al0.3CoCrFeNi. In contrast, no phase transformation was observed in CoCrFeMnNi and 316H SS. In the thin foil regions of TEM samples, stacking-fault tetrahedra were observed in the HEAs. In the thick foil regions, interstitial dislocation loops were observed for all alloys, and the areal loop density increased linearly with foil thickness. The Al0.3CoCrFeNi had the largest loop size (the lowest density), followed by the CoCrFeMnNi and then the 316H SS. The higher loop nucleation rate in the 316H SS was attributed to carbon content. The degree of irradiation hardening was slightly lower for the HEAs than for the 316H SS, which is a promising sign for the nuclear application of HEAs at high temperatures.

State-of-the-Art Diffusion Studies in the High Entropy Alloys

The development of the high entropy alloys (HEAs) is amongst the most important topics in the field of materials science during the last two decades. The concept of multicomponent, near-equimolar systems has been already applied to the number of other systems, including oxides, carbides, diborides, silicides, and it can be expected that other groups of materials will follow. One of the main driving forces for the development of HEAs is the so-called “four core effects”: high entropy effects, severe lattice distortion, cocktail effect, and sluggish diffusion effect. Their existence and extent has been a subject of heated discussion. Probably the least studied of them is the sluggish diffusion effect, which is of the, especially, high importance from the point of view of the most possible applications of HEAs—as high-temperature materials. Its alleged existence carries a promise of obtaining materials with superior mechanical properties, higher creep resistance, and less susceptibility to high-temperature corrosion. In the current review, the state-of-the-art of diffusion studies in HEAs was presented, as well as the resulting conclusions concerning the existence of the sluggish diffusion effect. Based on the literature analysis, it can be stated that there is no experimental evidence, which would support the existence of the sluggish diffusion in HEAs on the level of tracer and self-diffusivities. Nevertheless, it can be pointed out that our current state of knowledge on the diffusion in HEAs is still far from complete; therefore, further directions of studies are proposed.

Effects of annealing on the structure and mechanical properties of FeCoCrNi high-entropy alloy fabricated via selective laser melting

To widen the applications of FeCoCrNi high-entropy alloys (HEAs) fabricated via selective laser melting, their mechanical properties must be improved, and annealing plays an important role in this regard. In this study, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773–1573 K for 2 h were compared. As the annealing temperature increased, the specimen structure recrystallized from all columnar grains to equiaxial grains containing numerous annealing twins. The dislocation network, which formed during the solidification process under considerable shrinkage strain, decomposed into dislocations. The residual stress, yield strength, and hardness decreased, while the plasticity and impact toughness increased. During the deformation of as-printed and low-temperature-annealed specimens, the dislocation network remained unchanged and provided resistance to the dislocations moving within it, thus strengthening the specimen. The tensile strength remained largely unchanged owing to the reduction in the residual stress during low-temperature annealing, as well as the formation of the twinning network and dislocation wall under large deformation upon high-temperature annealing. Meanwhile, the ductility greatly increased, thus increasing the potential for industrial application of HEAs.

The low-temperature mechanical properties of the Fe40Mn40Co10Cr10 high-entropy alloy, the ductility of which is induced by twinning

In this work, we studied the low-temperature plasticity, elastic and dissipative characteristics, as well as the microstructural evolution of the Fe40Mn40Co10Cr10 high-entropy alloy in a wide temperature range of 300 - 0.5 K. The temperature dependences of yield strength, strain hardening, strength and ductility, as well as acoustic absorption and dynamic Young's modulus are obtained. It was found that the structure of Fe40Mn40Co10Cr10 alloy in the initial state is single-phase with fcc lattice, and in the deformed state at low temperatures it becomes two-phase due to the deformation-induced phase transition. In addition, EBSD analysis of the alloy structure revealed a change in grain morphology and the appearance of twin dislocations after plastic deformation at all investigatedtemperatures. Acoustic studies showed that the transition from the initial to the deformed state changes the character of the temperature dependence of the dynamic Young's modulus from almost linear to exponential, and reduces the absolute values. The Fe40Mn40Co10Cr10 alloy has excellent strength and ductility at a high strain hardening rate, which is explained by the significant contribution of the twinning process. With a decrease in temperature from 300 to 4.2 K, a strong temperature dependence of the yield strength is observed, which indicates the thermal activation of the nature of the plastic deformation of the alloys in this temperature range. In the temperature range of 0.5-4.2 K, an anomaly of the yield strength was observed, namely a decrease in the value of the yield strength with decreasing temperature from 4.2 K to 0.5 K. The anomalous dependence of the yield strength is due to a change in the mechanism of overcoming local barriers from thermoactivated to inertial one, when part of the local obstacles is overcome by dislocations without activation. This leads to a decrease in yield strength with decreasing temperature. At a temperature of 4.2 K and below, the smooth nature of the plastic deformation changes from smooth to serrated. The jumps begin immediately after the yield strength and gradually increase from 40 MPa to ~ 160 MPa. The results obtained are important for practical applications of high-entropy alloys at low temperatures.

Friction and wear behavior of single-phase high-entropy alloy FeCoNiCrMn under MoS2-oil lubrication

PurposeThe face-centered cubic structured single-phase FeCoNiCrMn high-entropy alloys (HEAs) were prepared to study the friction and wear behavior of HEAs under MoS2-oil lubrication.Design/methodology/approachFeCoNiCrMn alloys were subjected to ball-on-disc reciprocating sliding against the GCr15 ball. L25(56) orthogonal wear tests were designed for velocity Vrel (4.167-20.833 mm/s), load FN (10-50 N), temperature T (RT-140 °C) and time t (5-20 min). Based on orthogonal test results, multivariate repeated measures ANOVA was performed, and further comparative experiments were conducted for Vrel, FN and T. Energy dispersive spectrometer and scanning electron microscope were applied to characterize the surface morphology of wear scar and its element distribution.FindingsVrel, FN and t exerted the most significant influence (p < 0.01) on the average friction coefficient f. Vrel and FN were identified as the momentous effect (p < 0.01) on wear volume ΔV. T (≥50 °C) had positive correlation with f and ΔV, and both Vrel and FN correlated negatively with f. The dominant abrasive wear was attributed to the large hardness difference of the friction pair. Fatigue wear and delamination wear were experienced at higher speeds (Vrel ≥ 12.5 mm/s) and loading levels (FN ≥ 40 N). Elevated temperature weakens the lubrication effect of MoS2-oil and the mechanical properties of FeCoNiCrMn matrix, intensifying abrasive wear.Originality/valueThis study is expected to provide references for exploration on the wear behavior of single-phase HEAs under complex working conditions with lubrication and hence will help develop the application of HEAs in practical engineering.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2019-0303

Swamps of hydrogen in equiatomic FeCuCrMnMo alloys: First-principles calculations

High-entropy alloys (HEAs) merit promising applications in nuclear reactors. A FeCuCrMnMo HEA with low activation under neutron irradiation was studied, which was first focused on the search of an equilibrium state by a hybrid method combining the Metropolis Monte Carlo method and density functional theory (DFT). As a transmutation byproduct in fission reactions or fuel for fusion reactions, the evolution of hydrogen, in the HEA was then investigated through a systematic analysis of H solution and diffusion using DFT and molecular dynamics simulations. Rooted in the unique distortion of HEA lattices, i.e., the destroyed translational symmetry of the energy landscape, tetrahedral and octahedral interstitial positions have no significant difference in the priority of H residing. Diffusion of H, as a guest atom, also presents a sluggish effect. The dramatic increases and decreases in potential energy generate a great number of insurmountable barriers pervading the matrix and largely suppressing the mobility of H. However, this effect originates not only from the difference between the potential energies of interstitial positions as observed with host atoms, which increases the fluctuation of migration barriers and decreases the effective atomic jump frequency, but also from the destabilization of interstitial positions for H residing. This blocks the diffusion channels of H and further decreases the atomic jump probability in Einstein's equation. The present investigation provides fundamental insight into H behavior in HEAs and clues for the application of HEAs as materials of tritium permeation barriers or resistance to hydrogen irradiation.

A strategy of designing high-entropy alloys with high-temperature shape memory effect

Shape memory effect, the ability to recover a pre-deformed shape on heating, results from a reversible martensitic transformation between austenite and martensite phases. Here, we demonstrate a strategy of designing high-entropy alloys (HEAs) with high-temperature shape memory effect in the CrMnFeCoNi alloy system. First, we calculate the difference in Gibbs free energy between face-centered-cubic (FCC) and hexagonal-close-packed (HCP) phases, and find a substantial increase in thermodynamic equilibrium temperature between the FCC and HCP phases through composition tuning, leading to thermally- and stress-induced martensitic transformations. As a consequence, the shape recovery temperature in non-equiatomic CrMnFeCoNi alloys can be increased to 698 K, which is much higher than that of conventional shape memory alloys (SMAs) and comparable to that of B2-based multi-component SMAs containing noble metals (Pd, Pt, etc.) or refractory metals (Zr, Hf, etc.). This result opens a vast field of applications of HEAs as a novel class of cost-effective high-temperature SMAs.

Development of a large size FCC high-entropy alloy with excellent mechanical properties

A large size 10 kg Fe40Mn40Co10Cr10 high-entropy alloy (HEA) with 3.3 at. % C-doped was prepared by electromagnetic levitation melting. The hot deformation behavior of the as-cast ingot was investigated to guide the hot forging process. Annealing was further conducted on the as-forged samples to obtain more optimized mechanical properties. The as-forged HEA had yield strength (YS) of 792 MPa, ultimate tensile strength of 1055 MPa and a satisfactory fracture elongation (EL) of 22.6%. The YS of the HEA annealed at 1050 °C increased from 393 MPa to 512 MPa and EL from 26.5% to 55.3% in comparison with the as-cast HEA. It is suggested that the enhanced yield strength of the as annealed HEA was attributed to carbon interstitial solid solution strengthening, grain refining and precipitation of tiny carbides, and the improvement of ductility was due to the elimination of cast defects and twinning-induced plasticity. This work paved the way for the application of HEA as high performance structure materials.

Welding of High Entropy Alloys-A Review.

High-entropy alloy (HEA) offers great flexibility in materials design with 3–5 principal elements and a range of unique advantages such as good microstructure stability, mechanical strength over a broad range of temperatures and corrosion resistance, etc. Welding of high entropy alloy, as a key joining method, is an important emerging area with significant potential impact to future application-oriented research and technological developments in HEAs. The selection of feasible welding processes with optimized parameters is essential to enhance the applications of HEAs. However, the structure of the welded joints varies with material systems, welding methods and parameters. A systemic understanding of the structures and properties of the weldment is directly relevant to the application of HEAs as well as managing the effect of welding on situations such as corrosion that are known to be a service life limiting factor of welded structures in conditions such as marine environments. In this paper, key recent work on welding of HEAs is reviewed in detail focusing on the research of main HEA systems when applying different welding techniques. The experimental details including sample preparation, sample size (thickness) and welding conditions reflecting energy input are summarized and key issues are highlighted. The microstructures and properties of different welding zones, in particular the fusion zone (FZ) and the heat affected zones (HAZ), formed with different welding methods are compared and presented in details and the structure-property relationships are discussed. The work shows that the weldability of HEAs varies with the HEA composition groups and the welding method employed. Arc and laser welding of AlCoCrFeNi HEAs results in lower hardness in the FZ and HAZ and reduced overall strength. Friction stir welding results in higher hardness in the FZ and achieves comparable/higher strength of the welded joints in tensile tests. The welded HEAs are capable of maintaining a reasonable proportion of the ductility. The key structure changes including element distribution, the volume fraction of face centered cubic (FCC) and body centered cubic (BCC) phase as well as reported changes in the lattice constants are summarized and analyzed. Detailed mechanisms governing the mechanical properties including the grain size-property/hardness relationship in the form of Hall–Petch (H–P) effect for both bulk and welded structure of HEAs are compared. Finally, future challenges and main areas to research are highlighted.

Effect of the addition of Mg, Ti, Ni on the decoloration performance of AlCrFeMn high entropy alloy

Due to the excellent mechanical properties of high entropy alloys (HEAs), they have attracted wide attention of materials researchers, but their functional properties have rarely been reported. In order to study the functional properties of HEAs, the decoloration of azo dye Direct Blue 6 (DB6) using equiatomic AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) HEAs synthesized by mechanical alloying was reported in this work. The decoloration rate of DB6 by ball-milled (BM) AlCrFeMn was about 3 times faster than that of by BM MgZn-based amorphous alloy, which was the best one reported in the metallic glasses so far. In order to further improve the decoloration efficiency, we considered adding the fifth elements (Mg or Ti or Ni) to AlCrFeMn. Both of Mg and Ti could improve the decoloration performance of AlCrFeMn, but Ni played a negative role. The reaction activity of AlCrFeMnMg and AlCrFeMnTi was more than 2 and 1.2 times faster than that of AlCrFeMn. The effects of initial pH, temperatures and dye concentration on the decoloration efficiency of AlCrFeMnMg during reactions were systematically investigated. The reaction activity of AlCrFeMnMg in alkaline and acidic azo dye solution was about 37.5 and 16.6 times faster than that of neutral solution, respectively. This work had implications in reaching an attractive, low cost and efficient method for functional applications of HEAs.