Concept Of High-entropy Alloys Research Articles

Theoretical Prediction of Strengthening in Nanocrystalline Cu with Multi-Element Grain Boundary Segregation Decoration.

The composition of grain boundaries (GBs) determines their mechanical behavior, which in turn affects the mechanical properties of nanocrystalline materials. Inspired by GB segregation and the concept of high-entropy alloys (HEAs), we investigated, respectively, the mechanical responses of nanocrystalline Cu samples with and without multi-element GBs, as well as the grain size effects, aiming to explore the effects of GB composition decoration on mechanical properties. Our results show that introducing multi-element segregation GBs can significantly improve the mechanical properties of nanocrystalline Cu by effectively inhibiting GB migration and sliding. Additionally, we proposed an improved a theoretical model that can reasonably describe the strengths of the materials with multi-element or single-element segregation GBs. Notably, the introduction of multi-element segregation GBs inhibits both migration and sliding behavior, with migration being more effectively suppressed than sliding. These results present a novel approach for designing high-performance nanometallic materials and offer valuable insights into the role of GB composition decoration in enhancing mechanical properties.

Investigations on microstructure and properties of five TiZr-based refractory multi-principal element alloys for clinical metallic biomaterials

The development of new materials with high strength, ductility, low modulus, wear resistance, and good biocompatibility remains a requirement in the field of clinical biomaterials. Based on the concept of high-entropy alloys, five TiZr-based refractory multi-principal element alloys (RMPEAs), i.e. MoTiZr, NbTiZr, MoTiVZr, NbTiVZr, and NbMoTiZr were designed and their microstructure, mechanical properties, elastic properties, and biocompatibility were assessed in this study. Results indicate that NbTiZr and NbTiVZr RMPEAs are single-phase BCC alloys, while the other alloys exhibit multiphase structures. In terms of mechanical behaviors, NbMoTiZr RMPEA has the highest hardness (496±15 HV) among the five alloys and also combines a high yield strength and good ductility, with values of 1275 MPa and 21.66%, respectively. According to atomic calculations, the ideal shear strengths of the five alloys on the {110} plane are all lower than those on the {112} plane. Furthermore, NbMoTiZr RMPEA also exhibits a lower Young’s modulus than that of Ti6Al4V, which can reduce “stress shielding” effects. The in vitro biocompatibility evaluation revealed that NbMoTiZr RMPEA can support cell adhesion and proliferation of MC3T3-E1 cells and exhibits good biocompatibility comparable to Ti6Al4V. These results demonstrate the remarkable potential of NbMoTiZr RMPEA for orthopedic bioimplant applications.

High Entropy Alloy Deposition from an Aqueous Bath

A new alloy design approach based on the concept of high entropy alloy (HEA) has attracted attention during the last few years. HEA is defined as an alloy composed of five or more principal elements with a concentration between 5 and 35 at% of each element. Depending on the combinations of the elements, HEA could show high resistance to corrosion, fatigue and fracture toughness besides unique electrical and magnetic responses. The outstanding characteristic of HEA is attributed to the high mixing entropies and the severe lattice distortion related to the addition of multiple components. Due to this unique combination of properties and the potential for designing a wide variety of new materials, interests have been directed toward high entropy alloy films and coatings for applications where corrosion and oxidation protection are strongly required. This study attempted to deposit five elements (Ni-Co–Cu–Mo-W) from an aqueous bath on a low-carbon steel substrate by the direct current. The goal was to combine the properties of Ni-Co-Cu with Ni-W and Ni-Mo coatings. In this case, a HEA coating can be produced, which might maintain corrosion and wear resistance even at high temperatures and be suitable for application in aggressive environments. To understand better the role of entropy on the coatings properties, three electroplating systems were designed, including electroplating of five elements (Ni-Co-Cu-Mo-W), which was compared with electroplating of four elements (Ni-Co-Cu-W) and (Ni-Co-Mo-W). In this case, systems with different configuration entropy can be produced, and by characterizing them, the relationship between the entropy and the properties of the coating might be cleared.Electrodeposition was carried out in Hull-cell to find the alloy deposition's proper working window. The role of different parameters such as pH, current density, and complex agent on the coating's composition was studied. The morphology and composition of the Hull-cell coatings were checked by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy along with X-ray fluorescence spectroscopy. Finally, the hardness and corrosion resistance of the coatings was studied by the nanoindentation technique and polarization in NaCl. This study developed a single aqueous bath for the electrodeposition of Ni-Co-Cu-Mo-W, Ni-Co-Cu-W and No-Co-Mo-W, and suitable deposition conditions were determined. It was shown that besides the metal concentration ratio, pH was the most influential parameter in the codeposition of elements with different deposition mechanisms. Ni and Co were the main inducing elements in the deposition of Mo and W than Cu. In addition, Ni, Co, Mo and W deposited uniformly, while Cu showed a different behavior. A competition between Mo and W deposition was seen, and Mo deposited more preferentially than W. The coatings showed weak passivity behavior and comparison between coating’s corrosion resistance was a challenge since their corrosion behavior does not only depend on coating composition but also on coatings compactness and integrity. By comparing the nano-hardness of the coatings, it was seen that the Ni-Co-Cu-W system with even the lowest configurational entropy has the highest hardness value. Keywords: Electrodeposition; High entropy alloy; Alloy deposition; hardness; Corrosion resistance

Effect of Al content on the microstructure and mechanical alloying behavior of AlCoCuMnNi and Al0.25CoCuMnNi high entropy powders

The concept of high entropy alloys (HEAs) is an emerging subject in materials science and engineering. The presence of at least four elements in equiatomic or near equiatomic composition in HEAs has led to superior properties and vast unexplored alloying space. In this study, we investigated two new HEA powders, AlCoCuMnNi and Al0.25CoCuMnNi, by subjecting mechanically milled (MM) powders to a series of analyses over a 60 h ball milling process. It was found that alloying starts after 20 h of MM and after 45 h, full elemental integration into the host lattice was detected. Meanwhile, further milling to 60 h can cause powder contamination. The High-Resolution Transmission Electron Microscopy (HRTEM) results validated the XRD findings and provided further insights into the substantial influence of Al content on the microstructure and the extent of amorphization within the milled powders. Differential Thermal Analysis (DTA) analysis on the MM powders showed one exothermic peak for AlCoCuMnNi and one for Al0.25CoCuMnNi at high temperatures, which may suggest the formation of the BCC or second FCC phases for these alloys. The results confirmed the significant influence of the increased Al content on increasing both the lattice constant and lattice strain while decreasing thermal stability of the investigated MM powders. The present results provided insights needed for consolidation of such HEA powders.

Research and Application Progress of High-Entropy Alloys

With the continuous improvement of global technological levels and the increasing demand for high-performance alloy materials in national economic construction, the traditional single principal component alloy is increasingly unable to meet people’s increasing service needs. High-entropy alloys play an important role in aerospace, mechanical manufacturing, biomedicine, energy development and other engineering fields because of their unique physical, chemical and mechanical properties. Based on the concept of high-entropy alloys, the high-entropy effect, lattice distortion effect, sluggish diffusion effect and cocktail effect of high-entropy alloys are represented in this paper. The common preparation methods of high-entropy alloys are summarized according to the classification of melting-casting method, mechanical alloying method and coating method. The strength and toughness, wear resistance, corrosion resistance, high temperature resistance, fatigue resistance, radiation resistance and magnetic properties of the high-entropy alloys are discussed. The application prospect of high-entropy alloys is summarized, and the future research and development direction of high-entropy alloys are prospected on this basis.

Development of a magnesium based multicomponent alloy with low density and high ductility

Driven by the concept of high entropy alloy, Mg62.9Li30Al3Y1.3Gd1.5Sc1.3 (at. %), a new multicomponent alloy is designed to solve the brittleness of lightweight high entropy alloys (LWHEAs). The microstructure and mechanical properties are also studied. As cast, the alloy consists of bcc solid solution phase with a large number of second phases and stands out for the low density (1.62 g/cm3) and the high compression strain (>45 %). Then two typical deformation processes, cold rolling and equal channel angular pressing (ECAP), were applied to improve the strength. After cold rolling, the ultimate tensile strength improved from 107 MPa to 145 MPa and the ductility increased by two times; After ECAP processing, the ultimate tensile strength considerably increased to 211 MPa without ductility loss.

High-performance magnesium alloy with multi-element synergistic strengthening: Design, microstructure, and tensile properties

Given the limitations of the phase diagram method and the blindness of the trial-and-error method in the composition design of multi-element Mg alloys, the design of multi-element synergistic strengthening Mg alloys was conducted in this work inspired by the design concept of high entropy alloys. Out of the requirement of low-cost and high-performance wrought Mg alloys, inexpensive Al, Ca, Mn, Zn and Sn were added in different ratios. The experimental alloys were prepared by casting and one-step hot extrusion. The optimal composition was Mg-1.4Sn-0.93Zn-0.83Ca-0.67Mn-0.39Al (named as AXMZT-5) showing the best combination of strength and elongation. The microstructure of extruded pure Mg and the AXMZT-5 alloy was studied in detail to reveal the reason for the excellent yield strength (YS) of AXMZT-5 up to 280 MPa, 197% higher than that of pure Mg. Fine-grained structure, residual dislocations, and relatively uniformly distributed nanoparticles of CaMgSn, Ca2(Mg, Al)6Zn3, and Al-Mn phases are attributed to the high YS. In addition, the sharp (0001) basal fiber texture from the un-recrystallized grains in the bimodal microstructure also contribute to the increase of strength. A large number of fine recrystallized grains make the AXMZT-5 alloy exhibit a ductile-dominated fracture mode, which results in its excellent elongation.

Phase composition and its effect on the mechanical performance of an AlCoCrFeNiTi high-entropy alloy

The concept of High-Entropy Alloy (HEA) expanded the research field of advanced metallic materials for various applications, like the development of ultra-hardness ballistic protection materials for national security. Although there can be hundreds of compositions, carefully selecting the constituent is mandatory to improve their mechanical behavior, keeping in mind their microstructural array based on chemical composition. In the present study, the AlCoCrCuFeNi HEA was modified, replacing copper with titanium, looking for the formation of a Ti-rich BCC phase for hardening effects. Samples were prepared following the powder metallurgy route, including mechanical alloying favoring the generation of a nanocrystalline microstructure. Studies based on structure, microstructure and nanoindentation testing on each phase of the alloy were performed to determine the correlation between their composition, crystalline structure and mechanical properties. Evidence showed the formation of three main micrometric phases (two BCC and one tetragonal) coexisting with a nanometric dispersion of rounded precipitates. Nanoindentation testing shows that the main hardening effect was related to the tetragonal phase formation by a solid-solution strengthening mechanism. This phase reached the highest hardness (14.9 GPa); meanwhile, the richest Ti phase showed the lowest elastic modulus, titanium favors the material ductility.

Enhanced antibacterial behavior of a novel Cu-bearing high-entropy alloy.

Contact infection of bacteria and viruses has been a critical threat to human health. The worldwide outbreak of COVID-19 put forward urgent requirements for the research and development of the self-antibacterial materials, especially the antibacterial alloys. Based on the concept of high-entropy alloys, the present work designed and prepared a novel Co0.4FeCr0.9Cu0.3 antibacterial high-entropy alloy with superior antibacterial properties without intricate or rigorous annealing processes, which outperform the antibacterial stainless steels. The antibacterial tests presented a 99.97% antibacterial rate against Escherichia coli and a 99.96% antibacterial rate against Staphylococcus aureus after 24 h. In contrast, the classic antibacterial copper-bearing stainless steel only performed the 71.50% and 80.84% antibacterial rate, respectively. The results of the reactive oxygen species analysis indicated that the copper ion release and the immediate contact with copper-rich phase had a synergistic effect in enhancing antibacterial properties. Moreover, this alloy exhibited excellent corrosion resistance when compared with the classic antibacterial stainless steels, and the compression test indicated the yield strength of the alloy was 1015 MPa. These findings generate fresh insights into guiding the designs of structure-function-integrated antibacterial alloys.

Engineering defect energy landscape of CoCrFeNi high-entropy alloys by the introduction of additional dopants

The concept of high-entropy alloys (HEAs) focusing on tuning the overall chemical complexity represents a novel alloy design strategy. In contrast, alloying of a metallic matrix with minor doping elements with limited and localized tunability has been a common practice to improve material performance. Combining the idea of globally engineering defect energy landscape in HEAs and the localized doping strategy in dilute alloys, in this work, we explore doping effects of minor elements in a HEA matrix to further enhance the overall and localized chemical tunability, aiming to improve its irradiation resistance. Specifically, we study the influence of minor Al, Cu, Ti, and Pd substitutional doping elements on defect energetics in a CoCrFeNi model HEA based on density-functional theory (DFT) calculations. The DFT results indicate that the formation and migration energies of vacancies can be strongly influenced when a dopant is introduced at the first nearest neighbor shells around a vacancy. On the other hand, interstitial energetics are only slightly affected. Among the four elements, Ti and Pd generally decrease vacancy formation energies and increase vacancy migration energies more significantly than Al and Cu. The doping effects become more pronounced when the concentration of the substitutional dopants increases. Based on the energy distributions obtained from DFT, we build a kinetic Monte Carlo (kMC) model to assess the impact of dopants on vacancy-mediated diffusivity in the doped HEAs. Our results suggest that Ti and Pd can lower the tracer diffusivity in the considered HEAs and act as trapping sites, whereas Cu may enhance the atomic transport. This work indicates that substitutional doping in HEAs is an effective strategy in metallurgy to further tune the defect and transport properties of complex alloys.

New trends in additive manufacturing of high-entropy alloys and alloy design by machine learning: from single-phase to multiphase systems

Alloys with excellent properties are always in significant demand for meeting the severe conditions of industrial applications. However, the design strategies of traditional alloys based on a single principal element have reached their limits in terms of property optimization. The concept of high-entropy alloys (HEAs) provides a new design strategy based on multicomponent elements, which may overcome the bottleneck problems that exist in traditional alloys. To further maximize the capability of HEAs, a novel additive manufacturing (AM) technique has been utilized to produce HEA components with the desired structures and properties. This review considers a new trend in the AM of HEAs, i.e., from the AM of single-phase HEAs to multiphase HEAs. Although most as-printed single-phase HEAs show superior tensile properties to as-cast ones, their strength is still not satisfactory, especially at elevated temperatures. Thus, multiphase HEAs are developed by introducing hard second phases, such as L12, BCC, carbides, oxides, nitrides, and so on. These phases can be introduced to the matrix using in situ alloying during AM or the subsequent heat treatment. Dislocation strengthening is considered as the main reason for improving the tensile properties of as-printed single-phase HEAs. In contrast, multiple strengthening and toughening mechanisms occur in as-printed multiphase HEAs, which can synergistically enhance their mechanical properties. Furthermore, machine learning provides an effective method to design new alloys with the desired properties and predict the optimal AM parameters for the designed alloys without tedious experiments. The synergistic combination of machine learning and AM will significantly speed up scientific advances and promote industrial applications.

High-throughput design of high-performance lightweight high-entropy alloys

Developing affordable and light high-temperature materials alternative to Ni-base superalloys has significantly increased the efforts in designing advanced ferritic superalloys. However, currently developed ferritic superalloys still exhibit low high-temperature strengths, which limits their usage. Here we use a CALPHAD-based high-throughput computational method to design light, strong, and low-cost high-entropy alloys for elevated-temperature applications. Through the high-throughput screening, precipitation-strengthened lightweight high-entropy alloys are discovered from thousands of initial compositions, which exhibit enhanced strengths compared to other counterparts at room and elevated temperatures. The experimental and theoretical understanding of both successful and failed cases in their strengthening mechanisms and order-disorder transitions further improves the accuracy of the thermodynamic database of the discovered alloy system. This study shows that integrating high-throughput screening, multiscale modeling, and experimental validation proves to be efficient and useful in accelerating the discovery of advanced precipitation-strengthened structural materials tuned by the high-entropy alloy concept.

An Overview of High-Entropy Alloys as Biomaterials

High-entropy alloys (HEAs) have been around since 2004. The breakthroughs in this field led to several potential applications of these alloys as refractory, structural, functional, and biomedical materials. In this work, a short overview on the concept of high-entropy alloys is provided, as well as the theoretical design approach. The special focus of this review concerns one novel class of these alloys: biomedical high-entropy alloys. Here, a literature review on the potential high-entropy alloys for biomedical applications is presented. The characteristics that are required for these alloys to be used in biomedical-oriented applications, namely their mechanical and biocompatibility properties, are discussed and compared to commercially available Ti6Al4V. Different processing routes are also discussed.

HIGH-ENTROPY ALLOYS AS A PROSPECTIVE CLASS OF NEW RADIATION-TOLERANT MATERIALS RESEARCH DEVELOPMENT ANALYSIS BASED ON THE INFORMATION DATABASES

A new class of metallic materials, so-called “high-entropy alloys” (HEAs), was under review. Various definitions of these alloys are given, their main differences from the conventional alloys are indicated and the dynamics of publications in the period from the first publications in 2004 to the end of 2020 are presented. It is noted the almost exponential growth of the article numbers concerning these alloys, and the main reasons of such high interest are discussed. Experimental results of development the radiation-tolerant materials based on the concept of high-entropy alloys and study of the radiation damage mechanisms are summarised.

Research progress of the preparation of high entropy alloy coatings by spraying

In the 1990s, Ye, a scholar from China, broke the routine with a creative idea of the concept of high entropy alloys (HEAs). Since then, new alloys, which breached the traditional concept of “primary component”, have caught great attention of scientists at home and abroad. Due to their excellent properties such as high hardness, strength, wear resistance, corrosion resistance, thermal resistance, and irradiation resistance, HEAs have been considered as a new generation of thermal spray materials with immense potential for industrial applications. Previous studies show similar or even better properties of HEA coatings compared to those of the HEA block. The preparation of the coatings has become the key point since then. Spraying is one of the common methods on preparing HEA coatings, including conventional methods such as plasma spraying, supersonic flame spraying, high-velocity arc spraying, and cold spraying. With their own advantages and disadvantages, appropriate spraying methods and parameters should be selected according to different matrix and spraying materials, which will be discussed in this paper. First, the theoretical basis of the HEAs were introduced. Next, starting from different thermal spraying processes, the research and development status of plasma spraying, supersonic flame spraying, high-velocity arc spraying, and cold spraying on the preparation of HEA coatings were reviewed. Raw material selection, preparation process optimization, performance research, coating post-treatment, and other aspects of the above four thermal spraying techniques to prepare HEA coatings were systematically summarized. Finally, the three problems of limited existing thermal spraying techniques for preparing HEA coatings, limited thermal spraying materials, and aimless design of HEA coatings were proposed. Moreover, three future development issues of new thermal spraying techniques and optimization of existing techniques were discussed along with the development of high entropy ceramics, high entropy amorphous alloys, high entropy composite materials, and other new thermal spray materials. Finally, an HEA database was established to prepare HEA coatings using the concept of material genome to solve the three problems.

Analysis of Carbides in Multi-component Cast Iron Design Based on High Entropy Alloys Concepts

Nowadays, the concept of high entropy alloys (HEAs) has expanded the limits of metallurgy and high-quality metallurgical HEAs ingots have already been successfully obtained on an industrial scale. High Chromium Cast Irons (HCCI) and Multi-Component Cast Irons (MCCI) are known to be a useful material for applications when abrasion resistance is required. Inspired by the concepts of high-entropy alloys, a base HCCI was modified, by adding other carbide-forming elements, at concentrations higher than what is currently used, in order to develop a new class of white cast iron, High Entropy White Cast Iron (HEWCI). The characterization of carbides precipitated during the HEWCI solidification was performed by scanning electron microscopy, energy dispersive spectroscopy, electron backscattered diffraction (SEM/EDS/EBSD) and X-ray diffraction (XRD). With the characterization techniques employed, the as-cast microstructure obtained of HEWCI is composed of approximately 50% of austenite matrix and 50% of different types of carbides, MC, M7C3 and M2C carbides.

Novel Multicomponent B2-Ordered Aluminides: Compositional Design, Synthesis, Characterization, and Thermal Stability

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi followed to produce Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi), and Al(CoCuFeMnNi) multicomponent alloys. CALculation of PHAse Diagrams (CALPHAD) approach was used to predict the phases in these alloys. X-ray diffraction and transmission electron microscopy were used to confirm the B2 ordering in the alloys. Thermal stability of the B2 phase in these alloys was demonstrated by prolonged heat treatments at 1373 K and 1073 K up to 200 h.

Cutting Edge of High-Entropy Alloy Superconductors from the Perspective of Materials Research

High-entropy alloys (HEAs) are a new class of materials which are being energetically studied around the world. HEAs are characterized by a multicomponent alloy in which five or more elements randomly occupy a crystallographic site. The conventional HEA concept has developed into simple crystal structures such as face-centered-cubic (fcc), body-centered-cubic (bcc) and hexagonal-closed packing (hcp) structures. The highly atomic-disordered state produces many superior mechanical or thermal properties. Superconductivity has been one of the topics of focus in the field of HEAs since the discovery of the bcc HEA superconductor in 2014. A characteristic of superconductivity is robustness against atomic disorder or extremely high pressure. The materials research on HEA superconductors has just begun, and there are open possibilities for unexpectedly finding new phenomena. The present review updates the research status of HEA superconductors. We survey bcc and hcp HEA superconductors and discuss the simple material design. The concept of HEA is extended to materials possessing multiple crystallographic sites; thus, we also introduce multisite HEA superconductors with the CsCl-type, α-Mn-type, A15, NaCl-type, σ-phase and layered structures and discuss the materials research on multisite HEA superconductors. Finally, we present the new perspectives of eutectic HEA superconductors and gum metal HEA superconductors.

Tailoring nanoprecipitates for ultra-strong high-entropy alloys via machine learning and prestrain aging

The multi-principal-component concept of high-entropy alloys (HEAs) generates numerous new alloys. Among them, nanoscale precipitated HEAs have achieved superior mechanical properties and shown the potentials for structural applications. However, it is still a great challenge to find the optimal alloy within the numerous candidates. Up to now, the reported nanoprecipitated HEAs are mainly designed by a trial-and-error approach with the aid of phase diagram calculations, limiting the development of structural HEAs. In the current work, a novel method is proposed to accelerate the development of ultra-strong nanoprecipitated HEAs. With the guidance of physical metallurgy, the volume fraction of the required nanoprecipitates is designed from a machine learning of big data with thermodynamic foundation while the morphology of precipitates is kinetically tailored by prestrain aging. As a proof-of-principle study, an HEA with superior strength and ductility has been designed and systematically investigated. The newly developed γ′-strengthened HEA exhibits 1.31 GPa yield strength, 1.65 GPa ultimate tensile strength, and 15% tensile elongation. Atom probe tomography and transmission electron microscope characterizations reveal the well-controlled high γ′ volume fraction (52%) and refined precipitate size (19 nm). The refinement of nanoprecipitates originates from the accelerated nucleation of the γ′ phase by prestrain aging. A deeper understanding of the excellent mechanical properties is illustrated from the aspect of strengthening mechanisms. Finally, the versatility of the current design strategy to other precipitation-hardened alloys is discussed.

Snoek-type damping performance in strong and ductile high-entropy alloys.

Noise and mechanical vibrations not only cause damage to devices, but also present major public health hazards. High-damping alloys that eliminate noise and mechanical vibrations are therefore required. Yet, low operating temperatures and insufficient strength/ductility ratios in currently available high-damping alloys limit their applicability. Using the concept of high-entropy alloy (HEA), we present a class of high-damping materials. The design is based on refractory HEAs, solid-solutions doped with either 2.0 atomic % oxygen or nitrogen, (Ta0.5Nb0.5HfZrTi)98O2 and (Ta0.5Nb0.5HfZrTi)98N2. Via Snoek relaxation and ordered interstitial complexes mediated strain hardening, the damping capacity of these HEAs is as high as 0.030, and the damping peak reaches up to 800 K. The model HEAs also exhibit a high tensile yield strength of ~1400 MPa combined with a large ductility of ~20%. The high-temperature damping properties, together with superb mechanical properties make these HEAs attractive for applications where noise and vibrations must be reduced.