Development Of High-entropy Alloys Research Articles

Cyclic plasticity of an interstitial high-entropy alloy: experiments, crystal plasticity modeling, and simulations

The development of high-entropy alloys (HEAs) comprising multiple principal components is an innovative design strategy for metallic materials from the perspective of thermodynamic entropy. However, despite their potential candidacy for engineering applications, the lack of research on the cyclic loading responses as well as constitutive modeling of the HEAs is a major constraint. Therefore, the present work focuses on the cyclic plasticity of a typical carbon-doped interstitial HEA (iHEA) with nominal composition Fe49.5Mn30Co10Cr10C0.5 (at.%). The results of stress-controlled cyclic tests with nonzero mean stress showed that the iHEA exhibits significant cyclic hardening and stress level–dependent ratcheting. Owing to its improved cyclic hardening, the saturated ratcheting strain rate of the iHEA is lower than that of conventional steels such as the 316L stainless steel. Furthermore, microscopic characterizations revealed that the cyclic deformations caused massive martensitic phase transformation and hierarchical structures in the iHEA. The experimental results were used to develop a physical mechanism-based crystal plasticity constitutive model that is capable of describing the cyclic plasticity of the iHEA, which was implemented into a finite element framework. The simulation results showed that the loading stress significantly affected the microstructural evolutions, leading to a stress level–dependent cyclic plasticity. Thus, this investigation provides a fundamental basis for fatigue tests and service life prediction/optimization of the iHEA in the future, which can promote its engineering applications.

Synthesis and Mechanical Characterization of a CuMoTaWV High-Entropy Film by Magnetron Sputtering.

Development of high-entropy alloy (HEA) films is a promising and cost-effective way to incorporate these materials of superior properties in harsh environments. In this work, a refractory high-entropy alloy (RHEA) film of equimolar CuMoTaWV was deposited on silicon and 304 stainless-steel substrates using DC-magnetron sputtering. A sputtering target was developed by partial sintering of an equimolar powder mixture of Cu, Mo, Ta, W, and V using spark plasma sintering. The target was used to sputter a nanocrystalline RHEA film with a thickness of ∼900 nm and an average grain size of 18 nm. X-ray diffraction of the film revealed a body-centered cubic solid solution with preferred orientation in the (110) directional plane. The nanocrystalline nature of the RHEA film resulted in a hardness of 19 ± 2.3 GPa and an elastic modulus of 259 ± 19.2 GPa. A high compressive strength of 10 ± 0.8 GPa was obtained in nanopillar compression due to solid solution hardening and grain boundary strengthening. The adhesion between the RHEA film and 304 stainless-steel substrates was increased on annealing. For the wear test against the E52100 alloy steel (Grade 25, 700–880 HV) at 1 N load, the RHEA film showed an average coefficient of friction (COF) and wear rate of 0.25 (RT) and 1.5 (300 °C), and 6.4 × 10–6 mm3/N m (RT) and 2.5 × 10–5 mm3/N m (300 °C), respectively. The COF was found to be 2 times lower at RT and wear rate 102 times lower at RT and 300 °C than those of 304 stainless steel. This study may lead to the processing of high-entropy alloy films for large-scale industrial applications.

Improvement of corrosion resistance and magnetic properties of FeCoNiAl0.2Si0.2 high entropy alloy via rapid-solidification

In this work, a rapid-solidification method was utilized on FeCoNiAl0.2Si0.2 high entropy alloy (HEA) and its effects on microstructure, corrosion resistance and magnetic properties were systematically investigated. Rapid-solidified and slowly-cooled FeCoNiAl0.2Si0.2 specimens were produced by melt-spinning and induction melting, respectively. It was found that, slowly-cooled FeCoNiAl0.2Si0.2 exhibits dendrite structure with compositional segregation of Al, Si and Ni, as well as the precipitation of L12-type ordered phase in the solid solution matrix, while rapid-solidification effectively promotes the structural and compositional homogeneity. Moreover, rapid-solidification leads to significant enhancement in the stability of the passive film, and thus results in remarkable increase in corrosion resistance compared to its slowly-cooled counterpart. Besides, the distribution of constituent elements plays an important role in the formation of passive film on the surface of this HEA. In addition, the Hc of the rapid-solidified specimen exhibits only nearly 1/5 of its slowly-cooled counterpart, indicating significant positive effect of rapid-solidification on soft magnetic properties of FeCoNiAl0.2Si0.2. This work provides a novel route for further development of HEAs with excellent corrosion resistance and soft magnetism.

Experimental and numerical study of laser surface alloying of Al<SUB align="right">xCu<SUB align="right">0.5FeNiTi high entropy alloy

High entropy alloy possesses superior properties such as high hardness, wear-resistance, thermally stable, high strength to weight ratio, etc. required in industries. The development of high entropy alloy is critical due to the high segregation possibilities of multi-component systems. Processing of high entropy alloy can be done using laser surface alloying. This work reports 3D simulation and, subsequently, an experimental comparison of laser surface alloying process. Such a comparison is crucial to understand the influence of process parameters on bead geometry formation for better process optimisation. This work deals with the processing of AlxCu0.5FeNiTi high entropy alloy over aluminium alloy (AA1050) substrate. The quality parameters (dilution percent, aspect ratio, and bead angle) obtained from experimental and numerical studies are compared. The calculated melt pool depth and width are in a good agreement with the experimental results. An error of 1.72%, 13.95%, and 6.09% in dilution percent, aspect ratio, and bead angle is reported.

Development of high entropy alloys for brazing applications

High entropy alloys are novel and innovative metallic materials, which have intensively moved into the focus of research over the last decade. The high entropy effect in those multi-component alloys promotes the formation of a characteristic crystal structure, the random solid solution, which features unique material properties, and reduces the number of possible brittle phases. In this publication, the influence of gallium as a melting point depressant on the melting range and the microstructure of the two-phased equimolar CoCrCoFeNi were determined. In order to integrate the vacuum brazing process into the solutioning heat treatment of the nickel-based super alloy Mar-M 247 between 1180 and 1270 °C, the liquidus temperature of CoCrCoFeNi was aimed to be below 1270 °C. The changes in the melting ranges due to the modified compositions were predicted by CALPHAD simulations and verified by differential thermal analysis measurements. The promising multi-component filler alloy CoCrCuFeNiGa was determined for further investigations. For this purpose, the microstructures of the filler metal itself and the brazement were conducted. A shear strength of 388 ± 73 MPa was achieved for a brazing gap of 200 μm. The crack, which led to joint failure propagated through high-entropic, fcc-structured phases in the brazing seam.

Microstructure, Mechanical Properties, and Corrosion Behavior of MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi High-Entropy Alloys

The development of high-entropy alloys (HEAs) has stimulated an ever-increasing interest from both academia and industries. In this work, three novel MoNbFeCrV, MoNbFeCrTi, and MoNbFeVTi HEAs containing low thermal neutron absorption cross section elements were prepared by vacuum arc melting. The microstructure, mechanical properties, and corrosion behaviors were investigated. A dominant body-centered cubic (BCC) phase was present in all these three HEAs. In addition, an ordered Laves phase was found to be another major phase in both MoNbFeCrV and MoNbFeCrTi alloys, whereas an ordered BCC (B2) phase was observed in the MoNbFeVTi alloy. The phase formation in these three alloys was discussed. It is found that the formation of the secondary phase in these alloys is mainly ascribed to the large atomic size difference and electronegativity difference. All the three HEAs show high hardness, high yield strength but limited plasticity. Moreover, the MoNbFeCrV, MoNbFeCrTi and MoNbFeVTi alloys exhibit excellent corrosion resistance in both deaerated 1 mol/L NaCl and 0.5 mol/L H2SO4 solutions at room temperature. However, further composition adjustment and/or thermomechanical processing is required to enhance the mechanical properties of the three alloys.

Intermetallic Phases in High-Entropy Alloys: Statistical Analysis of their Prevalence and Structural Inheritance

Strengthening high entropy alloys (HEAs) via second phases is a very effective approach. However, the design of intermetallic (IM) phases in HEAs is challenging, mainly because our understanding of IM phases in HEAs is still very limited. Here, a statistical approach is used to enhance our understanding towards IM phases in HEAs. A database consisting of 142 IM-containing HEAs was constructed. Our aim is twofold. The first is to reveal the most common IM phase types in published HEAs. The second is to understand whether HEAs inherit their IM structures from their binary/ternary subsystems, or whether they tend to form new structures irrelevant to their subsystems. The results show that the five most prevalent IM structures in the HEAs surveyed here are Laves, σ, B2, L12, and L21. This trend is evidently different from the overall trend among known binary/ternary IMs. As for structural inheritance, all the IM phases contained in the alloys are existing structures in the binary/ternary subsystems of the respective alloys. This suggests that the compositional complexity in HEAs does trigger additional complexity in IM structure formation. These findings have important implications in the future design and development of HEAs.

Ultrahigh cryogenic strength and exceptional ductility in ultrafine-grained CoCrFeMnNi high-entropy alloy with fully recrystallized structure

Similar to the conventional alloys, attaining excellent combination of high strength and ductility is still challenging in the high-entropy alloys (HEAs). In this work, a fully recrystallized ultrafine-grained (UFG) microstructure was introduced into the single-phase CoCrFeMnNi HEA via cold rolling and annealing process. The UFG HEA displays ultrahigh yield and ultimate tensile strengths of 1.24 GPa and 1.46 GPa, respectively, and excellent ductility with a uniform elongation of 41% at 77 K. The exceptional strength-ductility combination in the UFG CoCrFeMnNi HEA surpasses those of all existing HEAs and other advanced engineering alloys. Such unparalleled mechanical properties stem from the fully recrystallized ultrafine grains that significantly strengthen the material and simultaneously provide high strain-hardening capability and the activations of multiple deformation mechanisms consisting of prominent dislocation activities, profuse stacking faults, and deformation twins. The fully recrystallized UFG microstructures do not only shed light on the design and development of HEAs with superior mechanical properties, but also enrich the potential applications of HEAs at the cryogenic environment.

An approach for screening single phase high-entropy alloys using an in-house thermodynamic database

A new screening methodology is proposed to aid in the development of high-entropy alloys (HEAs). This approach takes into account three commonly used criteria and methods to guide the design of HEAs: empirical parameters, binary phase diagram inspection and the Calculation of Phase Diagrams (CALPHAD) method. In addition, two novel concepts are introduced: a criterion to evaluate the likeliness of single phase solid solution in an alloy system's non-equiatomic compositional space, and a binary priority list, which allows us to make the employed in-house thermodynamic database a more reliable tool for solid solution screening in a time-effective manner.

Current and emerging practices of CALPHAD toward the development of high entropy alloys and complex concentrated alloys

Abstract

Phase-Transformation Ductilization of Brittle High-Entropy Alloys via Metastability Engineering.

High-entropy alloys (HEAs) in which interesting physical, chemical, and structural properties are being continuously revealed have recently attracted extensive attention. Body-centered cubic (bcc) HEAs, particularly those based on refractory elements are promising for high-temperature application but generally fail by early cracking with limited plasticity at room temperature, which limits their malleability and widespread uses. Here, the "metastability-engineering" strategy is exploited in brittle bcc HEAs via tailoring the stability of the constituent phases, and transformation-induced ductility and work-hardening capability are successfully achieved. This not only sheds new insights on the development of HEAs with excellent combination of strength and ductility, but also has great implications on overcoming the long-standing strength-ductility tradeoff of metallic materials in general.

Rapid Identification of Highly Corrosion Resistant Alloy Compositions

Typically, corrosion studies are conducted over large areas (mm2 – cm2) which make them inadequate to investigate corrosion behavior of localized areas of interest. To address the above shortcoming, a number of micro-electrochemical methods have been investigated over the past couple decades. This presentation will report the results from the microelectrochemical corrosion studies conducted on diffusion couples, which form the initial studies in the development of High Entropy Alloys (HEAs) or Multi-Principal Component Alloys. The study will discuss methods to identify compositions of lightweight alloys that have high corrosion resistance and exhibit reasonable mechanical properties compared to commercial alloys.

Three Strategies for the Design of Advanced High-Entropy Alloys

High-entropy alloys (HEAs) have recently become a vibrant field of study in the metallic materials area. In the early years, the design of HEAs was more of an exploratory nature. The selection of compositions was somewhat arbitrary, and there was typically no specific goal to be achieved in the design. Very recently, however, the development of HEAs has gradually entered a different stage. Unlike the early alloys, HEAs developed nowadays are usually designed to meet clear goals, and have carefully chosen components, deliberately introduced multiple phases, and tailored microstructures. These alloys are referred to as advanced HEAs. In this paper, the progress in advanced HEAs is briefly reviewed. The design strategies for these materials are examined and are classified into three categories. Representative works in each category are presented. Finally, important issues and future directions in the development of advanced HEAs are pointed out and discussed.

Design of high entropy alloys based on the experience from commercial superalloys

High entropy alloys (HEAs) have been drawing increasing attention recently and gratifying results have been obtained. However, the existing metallurgic rules of HEAs could not provide specific information of selecting candidate alloys for structural applications. Our brief survey reveals that many commercial superalloys have medium and even to high configurational entropies. The experience of commercial superalloys provides a clue for helping us in the development of HEAs for structural applications.

Exploration and Development of High Entropy Alloys for Structural Applications

We develop a strategy to design and evaluate high-entropy alloys (HEAs) for structural use in the transportation and energy industries. We give HEA goal properties for low (≤150 °C), medium (≤450 °C) and high (≥1,100 °C) use temperatures. A systematic design approach uses palettes of elements chosen to meet target properties of each HEA family and gives methods to build HEAs from these palettes. We show that intermetallic phases are consistent with HEA definitions, and the strategy developed here includes both single-phase, solid solution HEAs and HEAs with intentional addition of a 2nd phase for particulate hardening. A thermodynamic estimate of the effectiveness of configurational entropy to suppress or delay compound formation is given. A 3-stage approach is given to systematically screen and evaluate a vast number of HEAs by integrating high-throughput computations and experiments. CALPHAD methods are used to predict phase equilibria, and high-throughput experiments on materials libraries with controlled composition and microstructure gradients are suggested. Much of this evaluation can be done now, but key components (materials libraries with microstructure gradients and high-throughput tensile testing) are currently missing. Suggestions for future HEA efforts are given.

Computational Thermodynamics Aided High-Entropy Alloy Design

Thermodynamic calculation is used to shed light on the design and development of high-entropy alloys (HEAs) in this article. A thermodynamic database for the Al-Co-Cr-Fe-Ni was developed, and phase diagrams of this system were calculated. The calculated results, such as primary solidified phases, which are fractions of stable phases at a given alloy composition, explain the published experimental observations fairly well for both as-cast and homogenized alloys. These calculations also confirm the effect of each element on the face-centered cubic (fcc)/body-centered cubic (bcc) structure transition as published in the literature. The role of thermodynamic calculation in aiding effective design of HEAs is clearly demonstrated by this work.

Determination of the transition to the high entropy regime for alloys of refractory elements

The development of high entropy alloys is currently limited to experimental work aimed at the determination of specific compositions that exhibit particular properties. The main feature of these alloys is their particular phase structure, which tends to be a continuous solid solution in spite of the large number of constituents which would otherwise form a large number of intermetallic phases. While it is known that equimolar concentrations and large number of elements are two necessary conditions for achieving high entropy behavior, not much is known regarding the transition to this regime in the presence of specific elements. Such knowledge would be useful when determining alloy compositions, as it would set boundaries for the necessary concentrations of each element in experimental situations. In this work, results of a computational modeling effort are presented, where a recently developed 5-element high entropy alloy of refractory elements is used as the foundation needed to examine such transition and determine the necessary lower bounds for the concentration of each element. Details of the phase structure of the quaternary combinations of W, Nb, Mo, Ta and V as they evolve upon the addition of a fifth element are discussed. The results are compared to the experimental case for the case of V added to W–Nb–Mo–Ta. Using these examples as a reference, the concept of critical concentrations for each element, signaling the transition to the high entropy regime, is developed, based on a simple analysis of only bulk properties of such alloys (lattice parameter, bulk modulus, and cohesive energy).

Phase stability in high entropy alloys: Formation of solid-solution phase or amorphous phase

The alloy design for equiatomic multi-component alloys was rationalized by statistically analyzing the atomic size difference, mixing enthalpy, mixing entropy, electronegativity, valence electron concentration among constituent elements in solid solutions forming high entropy alloys and amorphous alloys. Solid solution phases form and only form when the requirements of the atomic size difference, mixing enthalpy and mixing entropy are all met. The most significant difference between the solid solution forming high entropy alloys and bulk metallic glasses lies in the atomic size difference. These rules provide valuable guidance for the future development of high entropy alloys and bulk metallic glasses.