Development Of High-entropy Alloys Research Articles

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.

Multimaterial direct energy deposition: From three-dimensionally graded components to rapid alloy development for advanced materials

Laser-based direct energy deposition (L-DED) with blown powder enables the simultaneous or sequential processing of different powder materials within one component and, thus, offers the possibility of additive multimaterial manufacturing. Therefore, the process allows a spatially resolved material allocation and fabrication of sharp or even graded material transitions. Within this contribution, the latest results from two major research fields in multimaterial L-DED—(I) automation and (II) rapid alloy development of high entropy alloys (HEAs) by in situ synthesis—shall be presented. First, an automated multimaterial deposition process was developed, which enables the automated manufacturing of three-dimensionally graded specimens. For this, a characterization of the deposition system regarding powder feeding dynamics and resulting powder mixtures in the process zone was conducted. The obtained system characteristics were used to achieve a three-dimensional deposition of specified powder mixtures. The fabricated specimens were analyzed by energy-dispersive x-ray spectroscopy, scanning electron microscopy, and micro hardness measurement. The research demonstrates the increasing readiness of L-DED for the fabrication of multimaterial components. Second, the latest results from rapid alloy development for HEAs by DED are presented. By the simultaneous usage of up to four powder feeders, a vast range of alloy compositions within the Al–Ti–Co–Cr–Fe–Ni HEA system was investigated. For this, tailored measurement systems such as an in-house developed powder sensor were beneficially used. The study shows the influence of a variation of Al on the phase formation and resulting mechanical properties and demonstrates the potential of L-DED for reducing development times for new alloys.

Molecular dynamics perspective of the effects of laser thermal configurations on the dislocation and mechanical characteristics of FeNiCrCoCu HEA through powder bed fusion process

The concept of alloying multiple principal elements at high concentrations has led to the development of High Entropy Alloys (HEAs) with exceptional mechanical properties, making them the focus of major recent scientific endeavors. Geometrically complex HEAs with tailored microstructural characteristics can be produced using additive manufacturing technologies such as powder bed fusion (PBF). However, an in-depth study on the effect of process thermal conditions during PBF is required to effectively fabricate HEAs with desirable mechanical characteristics. Thus, in our present molecular dynamic (MD) study we have explored the implication of PBF process thermal conditions on the mechanical characteristics of FeNiCrCoCu HEA by systematically varying laser scan speed from 0.4 {\AA}/ps to 0.1 {\AA}/ps, unidirectional and reversing laser passes from 1 to 4, and laser power from 100 microwatts to 220 microwatts. Our investigation suggests that reducing the laser scanning speed up to a critical velocity of 0.2 {\AA}/ps considerably improves mechanical strengths, with further reduction creating severe surface defects. Decreased ultimate tensile strength (UTS) is associated with the annihilation of the bulk sessile dislocations during tensile straining marking an early yield failure. Alternately, the material's strength could be improved by annealing with several unidirectional laser passes over the same target region, resulting in enhanced UTS due to subtler yield points. Increasing laser power aids in ameliorating material density ultimately leading to higher UTS even in non-dislocation-free structures. These findings will assist researchers to understand the underlying effects and optimize process thermal parameters to fabricate superior HEAs utilizing additive manufacturing.

Strain hardening and strengthening mechanism of laser melting deposition (LMD) additively manufactured FeCoCrNiAl0.5 high-entropy alloy

In order to develop the high-entropy alloy (HEA) with low cost and excellent mechanical properties for structural applications, the FeCoCrNiAl 0.5 HEA has been fabricated by laser melting deposition, one of the advanced additive manufacturing methods. Strain hardening behaviour has been analysed and discussed using the combination of characterisation techniques. The LMD-ed FeCoCrNiAl 0.5 had a true yield strength and strain of 463 MPa and 2.94%. Also, the true tensile strength of the LMD-ed FeCoCrNiAl 0.5 reached 876 MPa, together with the ductility of ~24.97% (engineering strain). The LMD-ed FeCoCrNiAl 0.5 HEA exhibited a dual-phase structure of 93% face-centred cubic (FCC) phase and 6.9% ordered B2 phase. The phase boundary between the disordered FCC and ordered B2 phases played a key role in the barrier, which can block the movement of dislocations because of the lattice distortion, very large angle, and mismatch of the lattice. Dislocation pile-up and tangle caused the dislocation density near the phase boundaries to be higher than that in other areas, meanwhile, they further prevented the movement of dislocation under stress as they generated back stress, therefore LMD-ed FeCoCrNiAl 0.5 HEA had a good strain hardening behaviour with a strain hardening exponent of 0.92. This study provided an innovative insight into the development of HEAs with ordered phase by laser additive manufacturing for structural applications in extreme environments. • The LMD-ed FeCoCrNiAl0.5 HEA displayed a good strain hardening behaviour. • The boundary between the disordered FCC and ordered B2 phases blocks the movement of dislocations under stress. • Dislocation pile-up and tangle nearby the phase boundary improve the strain hardening property.

Microstructure and Hydrogen Permeability of Nb-Ni-Ti-Zr-Co High Entropy Alloys.

Hydrogen separation membranes are one of the most promising technologies for hydrogen purification. The development of high-entropy alloys (HEAs) for hydrogen separation membranes is driven by a "cocktail effect" of elements with different hydrogen affinities to prevent hydride formation and retain high permeability due to the single-phase BCC structure. In this paper, equimolar and non-equimolar Nb-Ni-Ti-Zr-Co high entropy alloys were fabricated by arc melting. The microstructure and phase composition of the alloys were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The hydrogen permeation experiments were performed at 300-500 °C and a hydrogen pressure of 4 bar. In order to estimate the effect of composition and lattice structure on hydrogen location and diffusivity in Nb-Ni-Ti-Zr-Co alloy, ab initio calculations of hydrogen binding energy were performed using virtual crystal approximation. It was found that Nb-enriched and near equimolar BCC phases were formed in Nb20Ni20Ti20Zr20Co20 HEA while Nb-enriched BCC and B2-Ni(Ti, Zr) were formed in Nb40Ni25Ti18Zr12Co5 alloy. Hydrogen permeability tests showed that Nb20Ni20Ti20Zr20Co20 HEA shows lower activation energy and higher permeability at lower temperatures as well as higher resistance to hydrogen embrittlement compared to Nb40Ni25Ti18Zr12Co5 alloy. The effect of composition, microstructure and hydrogen binding energies on permeability of the fabricated alloys was discussed.

Design and characterization of a novel MgAlZnCuMn low melting point light weight high entropy alloy (LMLW-HEA)

Development of high-entropy alloys (HEAs) is one of the remarkable advancements in the progress of new materials. In recent years, continuous efforts have been made to design light weight high-entropy alloys (LWHEAs) with suitable microstructure and superior properties. This paper reports the preliminary results of a study aimed at design and development of a new low melting point LWHEA (LMLW-HEA) based on AlMgZnCuMn alloy system without excessive intermetallic compounds formation. In this research, first, the initial chemical composition of the alloy was designed according to thermodynamic parameters, and then the alloy was melted in a resistance electric furnace. Since the thermodynamic indicators have been defined based on alloys containing 3d transition elements, formation of binary intermetallic compounds was predicted with the help of Midema model. To investigate the effects of cooling rate and heat treatment on the structure of the designed alloy, the molten alloy was poured into two different molds made of silica sand and steel, and both as cast samples were also heat treated. Microstructural features, mechanical properties and thermal stability of the samples were then examined. Density of the samples was measured to be less than 3.4 gr/cm3 which was very close to the theoretical density of the alloy. Structures of all the samples were complex and multi-phase and contained intermetallic (ordered solid solution) binary and ternary compounds including Mg7Zn3, MgZn2, Cu2Mg, Al2Cu, Mg32 (Al, Zn)49, Mg32Cu7Al47, Al25Mg37.5Zn37.5, and CuMgZn phases. The designed alloy was brittle under all the processing conditions. Compressive strength of the alloy reached to about 95 MPa, and its hardness was in the range of 110–120 HV. Oxidation resistance of the alloy was evaluated at a temperature about 30 °C below its solidus temperature and no weight change was observed in the sample after 14 h. It is suggested that the designed alloy is a suitable candidate for high temperature applications not involving impact or cyclic loading.

Development of High Entropy Alloy as Catalyst for Azo Dye Degradation in Fenton Process

Development of High Entropy Alloy as Catalyst for Azo Dye Degradation in Fenton Process

Recent progress on the development of high entropy alloys (HEAs) for solid hydrogen storage: A review

High entropy alloys (HEA) represent a kind of materials with unique structural and functional properties, and have attracted wide attentions in many fields including hydrogen storage. Due to the huge diversity in the composition of HEAs, novel hydrogen storage materials with superior comprehensive performance are expected to be developed following the concept, with some notable progress made in the past decade. In this study, the present research status in HEAs for hydrogen storage is summarized from the aspects of theoretical guide, composition and preparation, microstructure and hydrogen storage properties. Moreover, the key issues in future development and possible application scenarios are analyzed. • A rigorous literature survey on the status and trend of high entropy alloys for hydrogen storage was presented. • Pros and cons of hydrogen storage HEAs were shown regarding capacity, thermal stability, kinetics, activation and cycling. • The promising scenarios for hydrogen storage HEAs were summarized and problems to solve pointed out.

Microstructure of and mechanical properties of an as-cast fine-grain dual-phase Fe-based high entropy alloy formed via solid-state phase transformation

In this paper, a novel Fe49.85Cr10.03Mn10.03Co10.03Ni10.03Al10.03 high entropy alloy (HEA) with dual-phase fine-grain structure was designed and fabricated by arc-melting. Its microstructure, as well as mechanical properties at room temperature and cryogenic temperature, have been systematically investigated. It is found that this as-cast Fe-based HEA is composed of face-centered cubic (FCC) and body-centered cubic (BCC) phases, and the stick-like FCC fine grains are derived from solid-state phase transformation of BCC phase during cooling. Moreover, it shows similar tensile strength and good plasticity at room temperature and cryogenic temperature, indicating high and stable mechanical properties at a wide range of temperatures. This work provides a novel route for designing high-performance fine-grain HEAs without mechanical/heat treatment, which is beneficial for further development of HEAs.

High Entropy Alloy Towards Functional Materials Application: A Review

The development of high entropy alloy (HEA) is considered a new bloom area as the ideas open up the exploration of a various multi-disciplinary elements in many applications. It is a novel class of complex materials identified in the phase diagram’s core, and they have been proven to outperform typical alloys. This HEA involves mixing unique combinations of mechanical and functional qualities across an infinite space of alloy composition. It is also possessing a great understanding of the thermodynamic behaviour of the materials which influence greatly their physical and mechanical properties. This review paper aims to open up the possibilities of developing HEA on different types of approaches based on the core effects which is very useful in functional materials applications especially in semiconductor, refractory and many more. However, to obtain this material, the most crucial thing is to understand and develop their synthesising routes of HEA production, which has become the main priority. Therefore, this paper focuses on discussing different metal elements that are commonly used in HEA and few fabrication routes on HEA. Some of recent functional materials based HEA is introduced by their enhance properties compare to conventional alloys.

Low neutron cross-section FeCrVTiNi based high-entropy alloys: design, additive manufacturing and characterization

The development of high-entropy alloys (HEAs) based on the novel alloying concept of multi-principal components presents opportunities for achieving new materials with desired properties for increasingly demanding applications. In this study, a low neutron cross-section FeCrVTiNi-based HEA was developed for potential nuclear applications. A face-centred cubic (FCC) HEA with the nominal composition of FeCr0.4V0.3Ti0.2Ni1.3 is proposed based on the empirical thermodynamic models and the CALculation of PHAse diagrams (CALPHAD) calculation. Verifications of the predictions were performed, including the additive manufacturing of the proposal material and a range of microstructural characterizations and mechanical property tests. Consistent with the prediction, the as-fabricated HEA consists of a dominant FCC phase and minor Ni3Ti precipitates. Moreover, significant chemical segregation in the alloy, as predicted by the CALPHAD modelling, was observed experimentally in the produced dendritic microstructure showing the enrichment of Ni and Ti elements in the interdendritic regions and the segregation of Cr and V elements in the dendritic cores. Heterogenous mechanical properties, including microhardness and tensile strengths, were observed along the building direction of the additively manufactured HEA. The various solid solution strengthening effects, due to the chemical segregation (in particular Cr and V elements) during solidification, are identified as significant contributing factors to the observed mechanical heterogeneity. Our study provides useful knowledge for the design and additive manufacturing of compositionally complex HEAs and their composition-microstructure-mechanical property correlation.

Development of High Entropy Alloy (HEA) as Catalyst for Azo Dye Degradation in Fenton Process

Azo dye is widely used in the textile industry since it is cost effective and simple to use. However, it becomes a continuous source of environmental pollution due to its carcinogenicity and toxicity. Various methods had been used to remove the azo dye in solution. One of the famous and repeatedly used is Fenton process. The Fenton’s process is one of the advanced oxidation process where iron catalysed hydrogen peroxide to generate hydroxyl radical. Treating azo dyes in solution requires a catalyst to enhance the process of degradation. Herein, high entropy alloy (HEA) has been proposed as a catalytic material to enhance the performance of Fenton process for azo dye degradation. HEA has been reported as a promising catalyst due to its high surface area. The higher the number of active sites, the higher the rate of azo dye degradation as more active sites are available for adsorption of azo dyes. The results have shown that HEA can be used as a catalyst to fasten the Fenton’s reaction since the degradation time is proven to be shorter in the presence of HEA. The method derived from the result of this study will contribute in treating azo dyes for wastewater management in Fenton process.

Development of high entropy alloys in Australia: a review

ABSTRACT First reported in the early 2000s, a new generation of metals called ‘high-entropy alloys’ (HEAs) have the potential to outperform conventional alloys that rely on a primary base metal. The plethora of five or more metallic elements when alloyed in equiatomic or near equiatomic creates a vast HEA material hyperspace that can lead to exceptional technical properties. This paper provides insights into HEAdevelopment in Australia; in particular, the additive manufacturing of HEA deposits through thermal spray and laser-based methods.

Friction Stir Welding/Processing of High Entropy Alloys (HEAs)

Design and development of high entropy alloys is very important to overcome the shortcomings of conventionally used alloys in applications where operating conditions of temperature and loading are extreme. In this recent paper, the application of Friction Stir Welding for joining the high entropy alloys and also using Friction Stir Processing for improving the mechanical and microstructure properties of high entropy alloys are discussed.

Development of high entropy alloys: A review

Abstract High-entropy alloys (HEAs) are alloys that have at least five or more alloying elements of principal type. Because of the concept based on a different design, these materials provide a requiring remarkable property. Thus, interest has been developed of a significant amount in these types of materials. It is emerging as well as an exciting new field in the area of material science and engineering. This paper briefly reviews some critical aspects of HEAs, including various effects of core, phases and crystal structures, stabilities in structural applications, mechanical properties.

Promising Ballistic Behavior of CoCrFeMnNi High Entropy Alloy

The development of high entropy alloys led to a great scientific interest owing to the fact that these alloys are responsible for presenting previously unexplored possibilities. Not only the breakdown of the classical paradigm of metal alloying development, but also the infinite possibilities on compositions and the unique properties exhibited, attracted the attention of a number of researchers since the "discovery" of high entropy alloys in 2004. Recently, it was suggested that some of these alloys could be suitable for ballistic application. This works aims to study the behavior of a CoCrFeMnNi high entropy alloy against a 271 m/s lean projectile. It is shown for the first time that high entropy alloys might provide a substantial protection against this ammunition threat. Furthermore, the results suggested that this specific alloy could be strengthen by shock-wave propagation and that the main ballistic energy absorption mechanism was the spalling of surfaces.

Awakening the metastability of an interstitial high entropy alloy via severe deformation

The metastability of a typical non-equiatomic FeMnCoCr high-entropy alloy (HEA) system was demonstrated to be dormant upon co-doping with C and N interstitials, i.e., displacive γ-ε phase transformation disappears during tensile deformation. Interestingly, we found in the present study that the displacive phase transformation prevails again in the C-N co-doped HEA upon severe deformation via cold-rolling. The generation of ε-phase lamellae is found to be motivated by strain localization which facilitates the formation of shear bands. Such highly concentrated strains significantly intensify the local elastic fields to neighboring γ matrix and finally eke out the driving force to overcome the interstitials-enhanced energy barrier of ε-martensite transformation. The results suggest that the suppressed phase metastability is awakened, which inspires efforts to tune the phase metastability via modifying the strain state during alloy processing and gives new insights into the development of HEAs with desirable phase stability and mechanical properties.