Development Of High-entropy Alloys Research Articles

In-situ EBSD investigation of excellent strength-ductility synergy mechanism achieved by thermal-mechanical processing of Al0·25FeCoNiV high-entropy alloy

The development of high-entropy alloys (HEAs) is plagued by a trade-off between strength and ductility. In this study, we investigated the effects of different cold rolling ratios and subsequent heat treatments on the microstructures and mechanical properties of the Al0·25FeCoNiV HEA. An efficient and economical thermal-mechanical processing route was successfully explored to prepare multiscale, multiphase heterogeneous structures consisting of heterogeneous grain structures and duplex structures. Yield strength, ultimate tensile strength, and fracture elongation of 1277.3 MPa, 1633.6 MPa, and 26.4 %, respectively, could be achieved. The strength-ductility trade-off was avoided. Detailed information on the evolution of grain boundary distribution, grain orientation changes, and grain rotation under stress and strain fields were obtained with in-situ electron backscatter diffraction (EBSD) tensile tests. The mechanism of coordinated plastic deformation and the intrinsic mechanism of excellent strength and ductility were investigated by quantitatively calculating the activation of the slip system, slip transfer at the phase boundaries, and grain rotation paths. It was found that the excellent mechanical properties can be attributed to multistage hetero-deformation-induced (HDI) strengthening and strain-hardening coupling mechanisms. In this study, an effective method for designing novel HEAs with excellent strength-ductility combinations and an analytical method based on in-situ tests are proposed.

Sigma Phase Stabilization by Nb Doping in a New High-Entropy Alloy in the FeCrMnNiCu System: A Study of Phase Prediction and Nanomechanical Response

The development of high-entropy alloys has been hampered by the challenge of effectively and verifiably predicting phases using predictive methods for functional design. This study validates remarkable phase prediction capability in complex multicomponent alloys by microstructurally predicting two novel high-entropy alloys in the FCC + BCC and FCC + BCC + IM systems using a novel analytical method based on valence electron concentration (VEC). The results are compared with machine learning, CALPHAD, and experimental data. The key findings highlight the high predictive accuracy of the analytical method and its strong correlation with more intricate prediction methods such as random forest machine learning and CALPHAD. Furthermore, the experimental results validate the predictions with a range of techniques, including SEM-BSE, EDS, elemental mapping, XRD, microhardness, and nanohardness measurements. This study reveals that the addition of Nb enhances the formation of the sigma (σ) intermetallic phase, resulting in increased alloy strength, as demonstrated by microhardness and nanohardness measurements. Lastly, the overlapping VEC ranges in high-entropy alloys are identified as potential indicators of phase transitions at elevated temperatures.

Role of the structure order in the transport and magnetic properties of high-entropy alloy films

The fabrication and development of high-entropy alloys (HEAs) with exceptional functionalities is a rapidly expanding field with numerous applications. When the role of entropy in HEAs is considered, the extrinsic factors, such as the existence of grains and different phases, need to be separated from the intrinsic configurations of the atomic lattice. Here, we fabricated the CoCrFeNi2Al0.5 HEA/muscovite heterostructures, and some were prepared as epitaxial bilayers and others were prepared as an amorphous system. These two systems are classified into atomic-site disordered (ASD) and structurally disordered (SD) states, respectively, without the extrinsic effects for the determination of the crystal lattice role in high-entropy states. In this study, we determined the role of the structure order in correlation with the structural, electronic, and magnetic properties of HEAs using a combination of energy-dispersive X-ray spectrometry, X-ray diffraction, transmission electron microscopy, magneto-transport, ac magnetometry, and X-ray absorption spectroscopy with magnetic circular dichroism. The ASD state showed fully metallic behavior. In contrast, the SD state showed a metallic behavior with intense magnetic saturation, which was called Kondo-like behavior, under 50 K with a low-temperature coefficient of resistivity of ~64 ppm/°C. The difference between the saturation magnetic moment and the electron relaxation behavior in the ASD and SD states resulted from the existence of the structural order affecting the atomic distance and periodicity to modify the exchange interaction and tune the electron-phonon interaction for scattering. The ferromagnetic behavior contributed by Co, Fe, and Ni atoms was probed by X-ray absorption and magnetic circular dichroism to understand the magnetic interactions in the ASD and SD states.

High-Temperature Oxidation and Microstructural Changes of Al0.75CoCrFeNi High-Entropy Alloy at 900 and 1100 °C

The development of high-entropy alloys (HEAs) for high-temperature applications has been driven by the limitation of nickel-based superalloys in achieving optimal efficiency at higher temperatures for higher efficiency in power generation engines. The alloys must have high oxidation resistance and microstructural stability at high temperatures. Relatively equimolar multi elements involved in HEAs produce microstructure containing a single solid solution or multiphase that improves the mechanical properties and oxidation resistance resulting from sluggish diffusion and core effects. In this study, the oxidation behavior and microstructural changes of Al0.75CoCrFeNi HEA at 900, 1000, and 1100 °C in air atmosphere were investigated. Based on the XRD and SEM-EDS analysis, the mechanism of oxide scale formation and microstructural changes of the substrate are proposed. The results show that the oxidation behavior of the alloy follows a logarithmic rate law. Different oxide compounds of CoO, NiO, Cr2O3, and CrO3, θ-Al2O3, α-Al2O3, and Ni(Cr,Al)2O4 with semicontinuous oxides of Al2O3 with Cr2O3 subscale and an oxide mixture consisting of spinel of Ni(Cr,Al)2O4 and Co(Cr,Al)2O4 were found. During oxidation, Widmanstätten of FCC-A1 and BCC-B2/A2 phases in the substrate have changed. Spheroidization of B2 and a reduction in volume fraction decrease the hardness of the substrates.

Machine learning – informed development of high entropy alloys with enhanced corrosion resistance

This study demonstrates the use of machine learning as a potential tool to efficiently develop new biomedical alloys with improved corrosion resistance by exploring the whole compositional space in the HfNbTaTiZr system. Owing to the small volume and inherited uncertainty of available corrosion data in the literature, k-fold cross-validation and bootstrapping were used to quantify the uncertainty of models and select a robust one. Potentiodynamic polarization experiments were performed on the predicted composition in simulated body fluid at 37 ± 1 °C for validation, demonstrating the new alloy's superior corrosion properties with a homogeneous microstructure as opposed to the dendritic structure.

Development of High-Entropy Alloys as Bond Coats: A Thermodynamic and Kinetic Perspective

Development of High-Entropy Alloys as Bond Coats: A Thermodynamic and Kinetic Perspective

Fundamental design strategies for advancing the development of high entropy alloys for thermo-mechanical application: A critical review

Nearly three decades since the discovery of high entropy alloys (HEAs), it has greeted a broad interest in the field of materials research as a better alternative to conventional alloy materials due to the exceptional combinatorial properties they offer in terms of lightweight, high-specific strength in elevated temperatures, excellent oxidation and corrosion resistance properties (among others). Leveraging on the “four core effects”: high-entropy effect, sluggish/hysteretic diffusion effect, severe-lattice-distortion effect and cocktail effect which define the special features responsible for their outstanding properties, HEAs have been successfully employed for high-temperature applications in automobile and in the aerospace. An emerging sub-field of HEAs is the incorporation of a secondary strengthening phase that can be provided by the precipitation of intermetallic (IM) compounds to enhance the microstructure which will concomitantly affect the properties of a material (thermal, chemical and mechanical properties) for broader engineering applications. In this article, design concepts brewed from thermo-physical parameters calculation and computational thermodynamics using a CALPHAD-based tool were reviewed as fundamental design strategies in the development of IM-containing HEAs.

Development of High-Entropy Alloy (HEA) Anode for Carbon-Free and Electrochemically-Stable Operation of SOFC on Hydrocarbon Fuels

State of the art SOFCs, consisting of Ni base anode, experience excessive localized endothermic cooling and carbon deposition during internal reforming of hydrocarbon fuels. To mitigate the above issues, we developed high-entropy alloy (HEA) anode consisting of optimized mixture of Cu, Ni, Co, Fe and Mn transition metals. Alloys were synthesized using co-precipitation technique and experimentally evaluated for internal reforming and electrochemical activity under SOFC operating conditions. As-synthesized HEA catalysts were also characterized by techniques including N2 adsorption/desorption analysis, SEM, XRD, TPR, TPO and TPD. Compared with standard Ni/YSZ and Ni/GDC anodes, HEA anode containing gadolinium-doped ceria (GDC) showed controlled reforming of methane. Raman spectroscopy and SEM microscopy confirmed the absence of carbon on the HEA/GDC anode.

Effect of heat treatment on the microstructure and mechanical properties of a dual phase Al14Co41Cr15Fe10Ni20 high entropy alloy

The microstructure and mechanical properties of Al14Co41Cr15Fe10Ni20 high entropy alloy is investigated in this work. Both the as-cast and homogenized (1200 °C/20 h) alloys show a dual phase microstructure consisting from a dendritic solid solution FCC phase and an interdendritic B2 (BCC) phase. Aging treatments in the temperature range of 750 °C–1000 °C were applied on samples for 20 h. Uniform distribution of precipitates in both dendritic and interdendritic regions were observed after performing the aging treatments. It is observed that with increasing the aging temperature from 750 °C to 1000 °C, the size of precipitates increases and the hardness of alloys decreases due to over-aging. The aged alloy with optimized hardness was selected for further room temperature and high temperature tensile tests. The results of mechanical tests indicate that the room temperature mechanical properties of the investigated alloy are comparable to the mechanical properties of commercial Co-based alloys, although the high temperature ductility of the alloy is very poor in comparison with commercial alloys. The obtained results in this work could be used for the further development of high entropy alloys with dual phase FCC/B2 microstructures.

Enhanced mechanical properties and in vitro biocompatibility of TiMOVWCr high-entropy alloy synthesized by magnetron sputtering

This study proposes the synthesis of a high-entropy alloy (HEA) composed of TiMoVWCr and a new synthesis route specialized for high-entropy alloys (HEAs) using radio-frequency (RF) magnetron sputtering. The development of HEAs with diversification for various elements is paramount for bridging some limitations in the usage of materials in harsh environments. In developing HEA-preparation techniques, the emergence of a novel sputtering target system is promising for preparing a wide range of HEAs. Thus, a reproducible single target sputtering system is proposed for HEAs and is evaluated systematically. This new target-preparation technique aids in tailoring the elemental compositions to optimize the alloy for important mechanical and biomedical applications, e.g., titanium implants. Films are analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and field emission scanning electron microscopy cross-sectional thickness. XRD and valence electron concentration (VEC) data showed a body-centered cubic structure with a major peak orientation of (110). Additionally, this alloy exhibited a very high hardness (29.2 GPa (±1.5)), elastic modulus (321 ± 8.9 GPa), smooth surface, and good cell viability compared to CP-Ti. The remarkably high hardness and elastic modulus are compared with the literature. Further, the crystalline phase is obtained at room temperature (RT) without undergoing post-treatment.

Design and Development of High-Entropy Alloys with a Tailored Composition and Phase Structure Based on Thermodynamic Parameters and Film Thickness Using a Novel Combinatorial Target.

This study presents a novel synthesis route for high-entropy alloys (HEAs) and high-entropy metallic glass (HEMG) using radio frequency (RF) magnetron sputtering and controlling the HEA phase selection according to atomic size difference (δ) and film thickness. The preparation of HEAs using sputtering requires either multitargets or the preparation of a target containing at least five distinct elements. In developing HEA-preparation techniques, the emergence of a novel sputtering target system is promising to prepare a wide range of HEAs. A new HEA-preparation technique is developed to avoid multitargets and configure the target elements with the required components in a single target system. Because of a customizable target facility, initially, a TiZrNbMoTaCr target emerged with an amorphous phase owing to a high δ value of 7.6, which was followed by a solid solution (SS) by lowering the δ value to 5 (≤6.6). Thus, this system was tested for the first time to prepare TiZrNbMoTa HEA and TiZrNbMoTa HEMG via RF magnetron sputtering. Both films were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, field emission scanning electron microscopy cross-sectional thickness, and atomic force microscopy (AFM). Furthermore, HEMG showed higher hardness 10.3 (±0.17) GPa, modulus 186 (±7) GPa, elastic deformation (0.055) and plastic deformation (0.032 GPa), smooth surface, lower corrosion current density (Icorr), and robust cell viability compared to CP-Ti and HEA. XRD analysis of the film showed SS with a body-centered cubic (BCC) structure with (110) as the preferred orientation. The valence electron concentration [VEC = 4.8 (<6.87)] also confirmed the BCC structure. Furthermore, the morphology of the thin film was analyzed through AFM, revealing a smooth surface for HEMG. Inclusively, the concept of configurational entropy (ΔSmix) is applied and the crystalline phase is achieved at room temperature, optimizing the processing by avoiding further furnace usage.

An overview of microstructure, mechanical properties and processing of high entropy alloys and its future perspectives in aeroengine applications

Modern engineering applications continually strive to develop greater performance mechanical components with good microstructural stability, improved mechanical properties, corrosion resistance and decreased cost of repairing and maintenance. This necessitates the broad use of advanced high performance materials like high entropy alloys (HEAs). These alloys are created by combining five or more alloying elements in equal or substantial amount. About 5 to 35 at. % of the alloying element is present in HEAs. It is characterized primarily by greater entropy, slow diffusion, severe lattice distortion, and cocktail effects. Due to its advanced microstructural stability throughout a larger temperature span and for longer length of time, it demonstrates improved mechanical characteristics at ambient temperature, cryogenic temperature, and elevated temperature. The diversity of elemental contents and significantly higher mixing entropy of HEAs make them mechanically superior to classic metals and alloys. It also shows better strength to weight ratio. Hence, it qualifies as a possible structural and functional material for aeroengine applications. In this work, the studies on the HEAs are briefly reviewed. A basic explanation of the four core effects of HEAs is given. Discussion is held on microstructure and mechanical properties of HEAs. The processing routes for manufacturing of HEAs (arc melting, bridgman solidification, mechanical alloying and vapour deposition) are presented briefly. The influence of heat treatment on mechanical behavior and microstructure of HEAs is presented. The simulation approach of CALPHAD modeling for designing of HEAs is discussed briefly. The future scope for research and development of HEAs in aeroengine applications is briefed.

Development of High-Entropy Alloy (HEA) Anode for Carbon-Free and Electrochemically-Stable Operation of SOFC on Hydrocarbon Fuels

High-entropy alloy (HEA) anode, consisting of multicomponent alloy and gadolinium-doped ceria (GDC) powders, were synthesized and evaluated for ethanol steam reforming under SOFC operating conditions using a fixed-bed reactor as well as electrochemical performance using a button cell test configuration. Both bench-top and cell tests were conducted at 700 °C with a steam-to-carbon (S/C) ratio of 2. While Ni anode showed filamentary carbon formation, and performance degradation with time, HEA/GDC catalyst showed stable performance without carbon deposition. As an anode, HEA/GDC demonstrated high electrochemical stability and comparable current density compared to the Ni electrode.

Construction of FeCrVTiMox high-entropy alloys with enhanced mechanical properties based on electronegativity difference regulation strategy

The development of high-entropy alloys (HEAs) as coating materials for protecting cutting tools remains a big challenge due to the diversity of their compositions and structures. In this work, we demonstrated an electronegativity regulation strategy to improve the mechanical properties of FeCrVTiMox HEAs coatings by introducing the high-electronegativity Mo element, which has been confirmed to be effective both in theoretical calculations and experimental studies. Thermodynamic analysis and the first-principles calculations predicted that FeCrVTiMox could readily form a single solid solution with a body-centered cubic (BCC) phase, and the electronegativity difference with increased Mo content showed a linear correlation for the improved mechanical properties of HEAs. The FeCrVTiMox HEA coatings synthesized using the laser cladding technique exhibited the increased Young's modulus from 227.8 GPa to 251.6 GPa, the increased Vickers hardness from 2.15 GPa to 8.19 GPa, and in the decreased frictional wear weight loss from 1.14 mg to 0.18 mg, supporting the predictions. The microscopic mechanism of solid solution strengthening of Mo elements was analyzed by Density of states (DOS) and electron localization function (ELF). The d‐orbital electron of each metal atom hybridizes with one another, leading to that Mo element with high electronegativity can attract and localize electrons to form covalent bonds. The larger electronegativity difference is, the more covalent bonds can be formed, in turn enhancing the mechanical properties of HEAs.

Characterisation and property evaluation of High Entropy Alloy coating on 316L steel via thermal spray synthesis

The advancement in the alloys led the development of High Entropy Alloys (HEA) that possess improved performance and characteristics. In the current study, AlCoCrFeNi HEA powder was synthesised using gas atomisation and coated onto SS316L stainless steel substrate using the atmospheric plasma spraying process. The microstructural characterisation of the synthesised HEA powder exhibited spherical morphology with a mean particle size of 20 µm. The crystal structure is observed to be BCC phase. The coated samples displayed uniform coating with homogenous distribution of HEA over the coating thickness of 150 µm. The microhardness of the coated samples had an increment of 2.46 times against the uncoated samples due to the excellent bonding and fine dispersion of HEA. The subsequent wear test performed by varying the process parameters revealed improved wear resistance on the coated samples than the uncoated samples. The lower range of load, sliding velocity and sliding distance of coated samples offered 83.92 %, 76.56 % and 74.59 % increments in wear resistance respectively and the characterisation of the worn surface revealed the mechanism to be plastic delamination and adhesive wear. The corrosion test conducted in 3.5 wt% NaCl electrolytic solution also exhibited better corrosion resistance for the coated samples due to the formation of an oxide layer along the coating acting as a protective layer for the enhanced resistance to corrosion.

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.

Evaluation of Equiatomic CrMnFeCoNiCu System and Subsequent Derivation of a Non-Equiatomic MnFeCoNiCu Alloy.

Investigation into non-equiatomic high-entropy alloys has grown in recent years due to questions about the role of entropy stabilization in forming single-phase solid solutions. Non-equiatomic alloys have been shown to retain the outstanding mechanical properties exhibited by their equiatomic counterparts and even improve electrical, thermal, and magnetic properties, albeit with relaxed composition bounds. However, much remains to understand the processing-structure-property relationships in all classes of so-called high-entropy alloys (HEAs). Here, we are motivated by the natural phenomena of crystal growth and equilibrium conditions to introduce a method of HEA development where controlled processing conditions determine the most probable and stable composition. This is demonstrated by cooling an equiatomic CrMnFeCoNiCu alloy from the melt steadily over 3 days (cooling rate ~4 °C/h). The result is an alloy containing large Cr-rich precipitates and an almost Cr-free matrix exhibiting compositions within the MnFeCoNiCu system (with trace amounts of Cr). From this juncture, it is argued that the most stable composition is within the CrMnFeCoNiCu system rather than the CrMnFeCoNi system. With further optimization and evaluation, a unique non-equiatomic alloy, Mn17Fe21Co24Ni24Cu14, is derived. The alloy solidifies and recrystallizes into a single-phase face-centered cubic (FCC) polycrystal. In addition to possible applications where Invar is currently utilized, this alloy can be used in fundamental studies that contrast its behavior with its equiatomic counterpart and shed light on the development of HEAs.

Dry abrasive wear and solid particle erosion assessments of high entropy alloy coatings fabricated by cold spraying

Recent advances in advanced metallic alloys have enabled the development of high entropy alloys (HEAs) for extreme engineering environments. HEAs are characterized by their mixing of elements with equimolar concentrations or near to equimolar concentrations, which enables the formation of solid-solution structures due to inherent high configurational entropy. With the advancement of HEAs, the attractiveness of these alloys as cold-sprayed coatings for wear resistant surfaces has been increasing. In the present study, a low-pressure cold spraying technique was employed to fabricate two new mechanically alloyed AlCoCrFeMoW and AlCoCrFeMoV HEA coatings. The phase formations, microstructure evolutions, and average microhardness of the fabricated HEA coatings were systematically investigated, in order to assess their resistance to dry abrasive wear and solid-particle erosion. The results showed that the cold-sprayed HEA coatings exhibited refined microstructure and low porosity, with the presence of body-centered cubic (BCC1 and BCC2) structures and CrMo intermetallic phases. Microhardness results revealed that AlCoCrFeMoV HEA coatings achieved the highest average hardness of around 561±63HV, such that it was 35% and 52% higher than that of AlCoCrFeMoW and AlCoCrFeMo HEA coatings, respectively. Dry abrasive wear results showed lower wear rates were achieved for AlCoCrFeMoV HEA coatings (590×10−6mm3/Nm), followed by AlCoCrFeMoW (660×10−6mm3/Nm) and AlCoCrFeMo HEA coatings (720×10−6mm3/Nm). However, damage during solid particle erosion was found to be higher in case of AlCoCrFeMoV HEA coatings compared to the other two coatings, suggesting no apparent correlation with microhardness values. These results suggest that the existence of high weight fractions of solid-solution phases in conjunction with refined microstructure and low porosity resulted in improved resistance to abrasive loading for the AlCoCrFeMoV HEA coatings but undermine performance and resistance to solid particle erosion damage.

A novel high-entropy alloy with exceptional strength and elongation via bimodal grains and lamellar nano-precipitates

This study presents a new effective approach for enhanced mechanical properties of the high-entropy alloy (HEA) Al7.3Co21.3Cr10.7Ti4.9Fe21.3Ni32.0Cu2.5 obtained by cold-rolling of columnar grains and subsequent annealing. This processing strategy aims to construct a microstructure of bimodal grains and coherent lamellar nano-precipitates. An ultrahigh strength of ∼1.68 GPa and a high elongation of ∼13.5% after annealing at 700 °C for 4 h were obtained, which are superior to those of previously reported similar HEAs. The ultrahigh strength originates mainly from the lamellar nano-precipitates strengthening and the bimodal grain size; however, the large elongation correlates to a progressive work-hardening mechanism regulated by the collaborative deformation of the ultrafine grains and the coarse grains and the unique nano-lamellar precipitates. This study provides a new strategy to utilize bimodal grains and coherent nano-precipitates in the development of HEAs or other similar alloys.

Exploring the relationship between lattice distortion and phase stability in a multi-principal element alloy system based on machine learning method

Lattice distortion is a basic characteristic of multi-principal element alloys (MPEAs), or high entropy alloys (HEAs). The severe lattice distortion strategy is an effective way to improve the solid solution strengthening of HEAs, but the phase stability will decrease with the increase in lattice distortion. The trade-off between lattice distortion and phase stability is yet to be investigated. Herein, the complex relationship between lattice distortion and phase stability in the Co-Ni-Cr-Mo-V system has been established by using ab initio calculations, thermodynamic calculations and machine learning methods. The results show that electronegativity difference and valence electron concentration are the most important descriptors in the prediction models of lattice distortion and phase stability, respectively, and the support vector machine algorithm has the highest prediction accuracy in the above two groups of models. The establishment of the relationship between lattice distortion and phase stability in the whole composition space has two meanings. One is to point out that the lattice distortion strategy must consider the temperature factor, and the other is to guide the manufacturing and heat treatment process of HEAs with severe lattice distortion. In addition, the correlations between lattice distortion and constituent elements were discussed. It is pointed out that the V element plays an important role in increasing lattice distortion degree in the Co-Ni-Cr-Mo-V system. This study is helpful to understand the relationship between lattice distortion and phase stability and to promote the development of high entropy alloys with severe lattice distortion.