High-entropy Composites Research Articles

Atomic-Scale High-Entropy Design for Superior Capacitive Energy Storage Performance in Lead-Free Ceramics.

Dielectric ceramics with high energy storage performance are crucial for the development of advanced high-power capacitors. However, achieving ultrahigh recoverable energy storage density and efficiency remains challenging, limiting the progress of leading-edge energy storage applications. In this study, (Bi1/2Na1/2)TiO3 (BNT) is selected as the matrix, and the effects of different A-site elements on domain morphology, lattice polarization, and dielectric and ferroelectric properties are systematically investigated. Mg, La, Ca, and Sr are shown to enhance relaxation behavior by different magnitudes; hence, a high-entropy strategy for designing local polymorphic distortions is proposed. Based on atomic-scale investigations, a series of BNT-based high-entropy compositions are designed by introducing trace amounts of Mg and La to improve the electric breakdown strength and further disrupt the polar nanoscale regions (PNRs). A disordered polarization distribution and ultrasmall PNRs with a minimum size of ≈1 nm are detected in the high-entropy ceramics. Ultimately, a high recoverable energy density of 10.1 Jcm-3 and an efficiency of 90% are achieved for (Ca0.2Sr0.2Ba0.2Mg0.05La0.05Bi0.15Na0.15)TiO3. Furthermore, it displays a high-power density of 584 MWcm-3 and an ultrashort discharge time of 27 ns. This work presents an effective approach for designing dielectric energy storage materials with superior comprehensive performance via a high-entropy strategy.

Excellent cryogenic temperature strength-ductility synergy in laser-powder-bed-fused TiB2p/CrMnFeCoNi high-entropy composite

Excellent cryogenic temperature strength-ductility synergy in laser-powder-bed-fused TiB2p/CrMnFeCoNi high-entropy composite

Excellent strength-ductility synergy in oxide dispersion strengthened AlCrFeNi high-entropy composites by heterostructure strategy

Oxide dispersion strengthened (ODS) AlCrFeNi high-entropy composites were produced by different heterostructure strategies to achieve strength and ductility synergy. The effects of different reinforcement types, reinforcement contents and oxide-forming elements in the matrix on the microstructure and mechanical properties of ODS-AlCrFeNi composites were investigated. The results showed that in the composites with different reinforcement types (ternary ODS-CrFeNi, quaternary ODS-CoCrFeNi and quinary ODS-CoCrFeNiMn), spinodal decomposition is observed in the all reinforcements, resulting in the formation of ellipsoidal/cuboidal B2-structured NiAl-rich phase and BCC-structured FeCr phases. As the number of the principal elements in the reinforcing phase decreases, the spinodal decomposition size gradually decreases. In the composites with varying ODS-CrFeNi reinforcement contents (5%, 10%, 15%, 20%), the occurrence of spinodal decomposition is also observed in the reinforcement. The spinodal decomposition size in the composite with 15% and 20% reinforcement content is smaller than that with 5% and 10% content. It is noteworthy that the incorporation of oxide-forming elements of Zr and Ti or only Zr together with Y2O3 in to the matrix result in different reinforcement structures. The former is typical spinodal decomposition, whereas latter displays a gradient network structure comprising a FCC-structured FeNi phase and a BCC-structured Cr-rich phase. The superior strength-ductility synergy, a compressive strength and strain of 7,690 MPa and 15.5%, which are 2.7 and 2 times higher than those of the unreinforced reference alloy, respectively, is achieved. This is mainly contributed by the novel gradient network structure in the reinforcement.

MoSe2 nanosheets implanting on 3D hollow carbon sphere for high-efficiency electromagnetic wave absorption

Carbon-based electromagnetic wave absorption (EMA) materials are extensively studied due to their unique advantages. Nevertheless, impedance matching issues was still constraining their further research. Consequently, we successfully synthesized MoSe2 nanosheets implanting on 3D hollow carbon sphere (HCS@MoSe2) composites with core–shell structure. The complex core–shell structure high-entropy composites provided multiple dissipation mechanisms for efficient dissipation of incident electromagnetic wave energy. The implanting MoSe2 nanosheets optimized the dielectric constant of the composites, thus ensuring optimal impedance matching. The HCS@MoSe2-1 composites exhibit excellent EMA properties and a significant minimum reflection loss (RLmin) of −56.19 dB. And HCS@MoSe2-1 had an ultra-wide effective absorption bandwidth (EAB) of 7.43 GHz. The significant EMA capability attributed to the synergistic effects of conductive loss, polarization loss, and optimized impedance matching. This work paved a new path for synthesizing transition metal selenide-modified carbon-based composite materials, offering great potential as high-performance materials for EMA applications.

Two-dimensional single-crystalline mesoporous high-entropy oxide nanoplates for efficient electrochemical biomass upgrading.

Mesoporous single crystals have received more attention than ever in catalysis-related applications due to their unique structural functions. Despite great efforts, their progress in engineering crystallinity and composition has been remarkably slower than expected. In this manuscript, a template-free strategy is developed to prepare two-dimensional high-entropy oxide (HEO) nanoplates with single-crystallinity and penetrated mesoporosity, which further ensures precise control over high-entropy compositions and crystalline phases. Single-crystalline mesoporous HEOs (SC-MHEOs) disclose high electrocatalytic performance in 5-hydroxymethylfurfural oxidation reaction (HMFOR) for efficient biomass upgrading, with remarkable HMF conversion of 99.3% and superior 2,5-furandicarboxylic acid (FDCA) selectivity of 97.7%. Moreover, with nitrate reduction as coupling cathode reaction, SC-MHEO realizes concurrent electrosynthesis of value-added FDCA and ammonia in the two-electrode cell. Our study provides a powerful paradigm for producing a library of novel mesoporous single crystals for important catalysis-related applications, especially in the two-electrode cell.

Achieving superior compressive strength-ductility synergy in a novel ODS-CrFeNi/AlCrFeNi heterostructured high-entropy composite

A novel high-entropy heterostructured composite (ODS-CrFeNi/AlCrFeNi) was fabricated by powder metallurgy method. The microstructure and mechanical properties of the composites with various sintering temperatures (950 °C to 1100 °C) and various addition amounts (0% to 20%) of FCC-structured reinforcing phase (ODS-CrFeNi) were investigated. The results revealed a reduction in porosity and an increase in relative density, reaching up to 99% with increasing sintering temperature. The optimal sintering temperature is 1100 °C. In the composites with the addition of 5% and 10% ODS-CrFeNi reinforced phases, spinodal decomposition (SD), forming BCC and B2 phases, takes place both in the matrix and in the reinforcing phase. As the reinforcing phase content is increased to 15% and 20%, SD occurs exclusively in the matrix while is suppressed in the reinforcing phase. The reinforcing phase consists of hard BCC structured Cr rich phases and soft FCC-structured FeNi phases, forming a gradient network structure. The composite with 15% reinforcing phase exhibits high hardness of 841 HV and compressive yield strength, ultimate strength and strain of 6960 MPa, 7690 MPa and 15.5%, respectively, which are 2.5, 2.7 and 1.9 times higher than those of the reference alloys (without reinforcing phase). The excellent strength-ductility synergy originates from the unique gradient network structure formed in the reinforcing phase, i.e. complex heterogeneous structure, which results in hetero-deformation induced (HDI) strengthening for higher strength and HDI strain hardening for higher ductility.

Preparation, microstructure and mechanical properties of novel WC-(Ti,Ta,Mo,V,Cr)(C,N)-Co high-entropy composites

In this work, novel WC-(Ti,Ta,Mo,V,Cr)(C,N)-Co (WC-HECN-Co) high-entropy composite powders were prepared by carbothermal reduction nitriding at 1400 °C and subsequently sintered by spark plasma sintering (SPS) to obtain cemented carbide. The preparation, microstructure and mechanical properties were investigated. The results show that, during the synthesis of WC-HECN-Co, phase evolution follows: (oxide raw material) → (WO3, CoWO4, WO2, V2O5, Ta2O5, Cr2O2.4, CoO, V6O11, TiMoO5, Co3W3C) → (W2C, Co3W3C, WC, WO3, Ta2O5, VO2, Ti6O11) → (WC, Co3W3C, TiTaC2, Ti6O11, Co) → (WC, Co3W3C, HECN, Co) → (WC, HECN, Co). The TEM elemental mapping results show that Ta, Ti, Mo, V and Cr are uniformly distributed with the WC grains to form a composite hard phase that is well bonded with Co. With the temperature increase, the Vickers hardness of SPS sintered WC-HECN-Co bulks gradually increased and the fracture toughness showed a trend of increasing and then decreasing. The Vickers hardness and fracture toughness of the WC-HECN-Co composite reached 1439 Kg·mm−2 and 12.19 MPa·m1/2, respectively. The fracture mechanism of sintered samples is jointly influenced by intergranular fracture and transgranular fracture. This study provides a novel strategy to develop high-performance cemented carbides that contain high-entropy carbonitrides.

Ultrathin High-Entropy Fe-Based Spinel Oxide Nanosheets with Metalloid Band Structures for Efficient Nitrate Reduction toward Ammonia.

Spinel oxides with tunable chemical compositions have emerged as versatile electrocatalysts, however their performance is greatly limited by small surface area and low electron conductivity. Here, ultrathin high-entropy Fe-based spinel oxides nanosheets are rationally designed (i.e., (Co0.2Ni0.2Zn0.2Mg0.2Cu0.2)Fe2O4; denotes A5Fe2O4) in thickness of ≈4.3nm with large surface area and highly exposed active sites via a modified sol-gel method. Theoretic and experimental results confirm that the bandgap of A5Fe2O4 nanosheets is significantly smaller than that of ordinary Fe-based spinel oxides, realizing the transformation of binary spinel oxide from semiconductors to metalloids. As a result, such A5Fe2O4 nanosheets manifest excellent performance for the nitrate reduction reaction (NO3 -RR) to ammonia (NH3), with a NH3 yield rate of ≈2.1mmol h-1 cm-2 at -0.5V versus Reversible hydrogen electrode, outperforming other spinel-based electrocatalysts. Systematic mechanism investigations reveal that the NO3 -RR is mainly occurred on Fe sites, and introducing high-entropy compositions in tetrahedral sites regulates the adsorption strength of N and O-related intermediates on Fe for boosting the NO3 -RR. The above findings offer a high-entropy platform to regulate the bandgap and enhance the electrocatalytic performance of spinel oxides.

Considering sustainability when searching for new high entropy alloys

Alloying elements bring with them a level of undesirable societal impact. The present paper examines this impact, in terms of primary production, for 340 newly proposed high entropy compositions. Three areas of sustainability are considered: economic viability, environmental impact and human well-being. The high entropy alloys are considered in two classes: body centred cubic and related alloys with potential for high temperature application and face centred cubic and related Cantor-type alloys. The former are compared to Ni-based superalloys and the latter to advanced steels. It is seen that with improved performance of Ni-based superalloys there has been an increase in undesirable impact across many of the indicators we employ. However, some recently proposed high temperature high entropy alloys – especially Yeh-type High Entropy Superalloys - display notably lower levels of undesirable impact than Ni-based superalloy compositions. Cantor-type face centred cubic alloys, however, do not compare so favourably when stacked up against the best steels, for room temperature strength and ductility. We show that, in general, there is good scope for high entropy alloys to be designed that target lower levels of undesirable impact.

Cellular structure engineering of additive manufactured CoCrFeMnNi high-entropy composite: The role of hard ceramic reinforcements in elemental segregation of constitutive elements

Cellular structure engineering of additive manufactured CoCrFeMnNi high-entropy composite: The role of hard ceramic reinforcements in elemental segregation of constitutive elements

Low-, medium-, and high-entropy pyrochlores in (Bi,Li,Na,La,Eu)1.9(Mg0.5Nb1.5)O7-δ compositions, their optical and dielectric properties

The pyrochlore phase formation in the (Bi,Li,Na,La,Eu)1.9(Mg0.5Nb1.5)O7-δ compositions with different element fractions in the A sites of the A2B2O6O' structure was studied. The low- and medium-entropy pyrochlores were formed after calcination of the batch at 1000–1150 °C. The high-entropy pyrochlore composition was detected during the investigations. Its stability by ab initio calculation was proven. The main impurities (Eu0.55La0.35Bi0.04)NbO4-δ and Na0.65MxNbO3-δ were identified in the initial medium- and high-entropy compositions as more stable phases. The samples belong to wide band gap semiconductors (Egdir = 3.2–3.6 eV, Egindir = 2.9–3.0 eV). The dielectric parameters calculated from impedance spectra depend on polarizability created by different types of cations in the A2O' sublattice. The dielectric permittivity (ε' = 67–162, at 25 °C and 1 MHz) decreases with decreasing polarizability of the system, while the tangent loss remains equally low (tanδ = 0.002–0.003 up to 240 °C). The presence of impurity phases in the initial high-entropy pyrochlores adversely affected the dielectric properties. Two mechanisms of conductivity were distinguished.

Boost in mechanical strength of additive manufactured CoCrFeMnNi HEA by reinforcement inclusion of B4C nano-particles

As a way to improve the strength of high-entropy alloys (HEAs), reinforcement by composite fabrication of HEAs has recently been investigated in various ways. In this study, CoCrFeMnNi+B4C (2 wt%) high-entropy composite (HEC) parts were fabricated using the direct energy deposition (DED) process of pre-ball milled powders. The grain growth is severely hindered by B4C nano-particles during the layer-by-layer printing of CoCrFeMnNi+B4C HEC compared to CoCrFeMnNi HEA. In addition, as a collateral effect, the sequence of elemental segregation in the dislocation cellular boundary differs in CoCrFeMnNi HEA and CoCrFeMnNi+B4C HEC. Additional segregation of Cr, Mn, and C elements into the cell boundary has been observed. In fact, unlike the CoCrFeMnNi HEA, the segregation of Cr, Mn, and C provides a presence of carbide-rich regions because of the partial decomposition of B4C nano-particles. Furthermore, the B4C nano-particle promotes the pinning effect of moving dislocation leading to the high dislocation density in the matrix, which contributes the biggest portion of strength reinforcing of CoCrFeMnNi+B4C HEC. Besides, the well-distributed B4C nano-particles boost the final strength of the HEC component by providing an additional dispersion hardening effect. Therefore, by co-activation of various strengthening mechanisms, the CoCrFeMnNi+B4C HEC showed magnificent yield and ultimate tensile strengths of 1024 ± 20 MPa and 1264 ± 16 MPa, respectively, with more than 7% of total elongation, which is much higher mechanical strength than the ones reported in the literature.

Microstructure and properties of TiC/Fe24Ni24Co24Mn18 high-entropy composite with exceptionally low coercivity

It is challenging to achieve good combinations of excellent mechanical properties and low coercivity (≤200 A/m) in typical soft magnetic alloys. In this work, TiC/Fe24Ni24Co24Mn18 high-entropy composite (HEC) prepared by mechanical alloying (MA) and spark plasma sintering (SPS) was produced to achieve good combinations of strength-ductility synergy and low coercivity. After MA, the TiC/Fe24Ni24Co24Mn18 HEC powders were composed of a main face-centered cubic (fcc) phase and some body-centered cubic (bcc) phase. Following SPS, the bulk TiC/Fe24Ni24Co24Mn18 HEC consisted of a fcc phase with nanosized TiC. In detail, the HEC showed multilevel heterogeneous microstructure, i.e., ultra-fine fcc grains (∼0.38 µm), micron-sized fcc grains (∼1.49 µm), and uniform dispersion of the in-situ formed TiC nanoparticles (∼98 nm) in ultra-fine fcc grains. The bulk TiC/Fe24Ni24Co24Mn18 HEC exhibits a high tensile yield strength of ∼1167 MPa and an exceptionally low coercivity of ∼100.5 A/m, suggesting an outstanding combinations of mechanical and magnetic properties. Compared with previously reported Fe-Co alloys or high entropy alloys (HEAs), the TiC/Fe24Ni24Co24Mn18 HEC shows a significantly enhanced yield strength while maintaining a relatively low coercivity, primarily resulting from the heterogeneous microstructure caused by the in-situ formed TiC.

ДОПИРОВАНИЕ НИОБАТОВ ВИСМУТА КАК ПОДХОД К ПОЛУЧЕНИЮ ВЫСОКОЭНТРОПИЙНЫХ ПИРОХЛОРОВ С ДИЭЛЕКТРИЧЕСКИМИ СВОЙСТВАМИ

An approach to the obtaining of high-entropy compositions with the pyrochlore structure with dielectric properties based on substituted bismuth niobates is shown. A series of compositions has been synthesized, and the regions for the formation of compounds with the pyrochlore structure have been established. The optical and dielectric properties of the resulting compositions have been studied. It is shown that the samples are wide-gap semiconductors and the dielectric indices depend on the polarizability of the doped cations and the size of the agglomerates.

Influence of Oxide Dispersions (Al2O3, TiO2, and Y2O3) in CrFeCuMnNi High-Entropy Alloy on Microstructural Changes and Corrosion Resistance

This study investigates the influence of 3 vol.% Al2O3, 3 vol.% TiO2, and 3 vol.% Y2O3 in the CrFeCuMnNi equimolar high-entropy alloy on its microstructural changes and corrosion resistance. These oxide-dispersed high-entropy composites (ODS-HECs) were synthesized via high-energy ball milling (50 h) followed by uniaxial hot-compaction (550 MPa, 45 min), medium-frequency sintering (1100 °C, 20 min), and hot forging (50 MPa). The microstructures of the developed composites produced a stable FCC phase, a small amount of ordered BCC-B2 structure, Fe2O3, and corresponding dispersed oxide phases. The corrosion of the developed high-entropy composites was tested in 3.5% NaCl solution using several electrochemical techniques. The results revealed that the corrosion rate (RCorr) decreased with the incorporation of oxide particles. Among the investigated samples and based on the electrochemical impedance spectroscopy results, CrFeCuMnNi-3 vol.% TiO2 ODS-HECs were seen to possess the highest value of corrosion resistance (RP). The change in the chronoamperometric current with time indicated that the CrFeCuMnNi alloy suffered pitting corrosion which decreased when Al2O3 was added, forming a CrFeCuMnNi-3 vol.% Al2O3 sample. In contrast, the incorporation of a 3 vol.% Y2O3, and 3 vol. TiO2, prevents pitting.

Ceramic-reinforced HEA matrix composites exhibiting an excellent combination of mechanical properties

CoCrFeNi is a well-studied face centered cubic (fcc) high entropy alloy (HEA) that exhibits excellent ductility but only limited strength. The present study focusses on improving the strength-ductility balance of this HEA by addition of varying amounts of SiC using an arc melting route. Chromium present in the base HEA is found to result in decomposition of SiC during melting. Consequently, interaction of free carbon with chromium results in the in-situ formation of chromium carbide, while free silicon remains in solution in the base HEA and/or interacts with the constituent elements of the base HEA to form silicides. The changes in microstructural phases with increasing amount of SiC are found to follow the sequence: fcc → fcc + eutectic → fcc + chromium carbide platelets → fcc + chromium carbide platelets + silicides → fcc + chromium carbide platelets + silicides + graphite globules/flakes. In comparison to both conventional and high entropy alloys, the resulting composites were found to exhibit a very wide range of mechanical properties (yield strength from 277 MPa with more than 60% elongation to 2522 MPa with 6% elongation). Some of the developed high entropy composites showed an outstanding combination of mechanical properties (yield strength 1200 MPa with 37% elongation) and occupied previously unattainable regions in a yield strength versus elongation map. In addition to their significant elongation, the hardness and yield strength of the HEA composites are found to lie in the same range as those of bulk metallic glasses. It is therefore believed that development of high entropy composites can help in obtaining outstanding combinations of mechanical properties for advanced structural applications.

Short-range order and origin of the low thermal conductivity in compositionally complex rare-earth niobates and tantalates

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Of these, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. Furthermore, doping 2% of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite-type phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite-type order, which is indeed revealed by diffuse scattering in X-ray diffraction and neutron total scattering. Specifically, neutron total scattering is used to characterize five selected specimens. A better fit to a weberite-type structure is found at the nanoscale. The characteristic length (domain size) is appears to be larger in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and “diffusons” give rise to the observed amorphous-like thermal conductivity.

In-situ synthesized age-hardenable high-entropy composites with superior wear resistance

This work reports a new approach to in-situ synthesize high-entropy composite (HEC) with high age-hardening ability and superior wear resistance. The as-cast FeNiCrMoTiC composite consists of a face-centred cubic (FCC) solid solution matrix with in-situ formed randomly-distributed carbides and FCC/intermetallics eutectic structures. Aging at 800 °C for 96 h effectively increases the hardness and compressive yield strength of the composite due to the precipitation strengthening of intermetallics. The peak-aged composite thus shows significantly enhanced wear resistance, even higher than the high-chromium cast iron (HCCI) with much higher hardness. In contrast to the severe delamination in the HCCI, the peak-aged HEC shows moderate abrasive wear and minor delamination under sliding friction due to its unique microstructure. The in-situ formed carbides, eutectic structures and precipitates effectively hardens the soft FCC matrix of the HEC and thus decreases the material loss caused by abrasive wear. The relatively lower cracking susceptibility of the finer reinforced particles in HEC can also prevent severe brittle delamination. In addition, propagation of micro-cracks from the brittle particles is inhibited by the ductile FCC matrix, which suppresses the delamination behaviour. During the wear test, the spalled carbides or intermetallics were welded onto the surface of the FCC phase, which further strengthened the matrix and thus decreased the abrasive wear. The developed composite also exhibited superior oxidation and corrosion resistance, indicating a strong application potential in extreme environments associated with abrasion, corrosion and oxidation.

A theoretical and experimental investigation on the SHS synthesis of (HfTiCN)-TiB2 high-entropy composite

In this work, a fundamental possibility of obtaining a high-entropy ceramic (HfTiCN)-TiB2 composite material by the coupled self-propagating high-temperature synthesis is shown. To search for a stable fixed composition of the HfTiCN compound, the USPEX code was used with the CASTEP interface at 0K. According to the XRD analysis, the obtained SHS product is represented by HfTiCN phase (60 wt%) and TiB2 phase (40 wt%). Based on the results of XRD, elemental analysis, and the heat pattern of combustion of the Hf-Ti-C-N-B powder mixture, a probable mechanism for the formation of the (HfTiCN)-TiB2 composite material during the coupled self-propagating high-temperature synthesis was proposed.

Formation of High-Entropy Multilayer Compositions with a Hierarchical Structure

The authors carried out complex studies of a composite multilayer high-entropy coating obtained by HVOF in a protective atmosphere. They also investigated metallophysical properties of the coating (using electron microscopy and X-ray diffraction analysis) in order to obtain new information on its composition, mechanical properties, and phase composition. Functional properties of the high-entropy layer were also determined. The effect of mechanical activation of powders on the structural-phase state and quality of the layered coating has also been studied. On the basis of complex metallophysical studies, they investigated the formation of a structure in a composite multilayer high-entropy coating after HVOF in a protective atmosphere and subsequent thermomechanical treatment. Calorimetric tests of the functional high-entropy layer were carried out to reveal the exo effect corresponding to the manifestation of phase transformation. The mechanical properties of steel with a multilayer composite coating have been determined.