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Refractory metals have increasingly attracted the attention of researchers due to their excellent high-temperature strength, thermal conductivity, radiation resistance, and biocompatibility for applications in extreme environments such as aerospace and nuclear industries. However, beyond traditional manufacturing processes, the complex post-treatment process, high cost, and difficulty in manufacturing complex geometry components limit its further application in modern industry. Compared to conventional manufacturing processes, selective laser melting (SLM) technology, an emerging technology, can significantly simplify the production process and has the advantage of manufacturing parts with complex geometry. Therefore, there is increasing research on manufacturing refractory metals using SLM technology. This article describes the current research progress of pure refractory metals manufactured by SLM regarding the preparation process, microstructure, metallurgical defects, and mechanical properties of refractory metals (tungsten, molybdenum, niobium, tantalum, etc.). Generally speaking, there are some technical difficulties in the fabrication of refractory alloys by SLM, summarized in this paper. Finally, the article presented the prospect of developing SLM refractory metals. [ABSTRACT FROM AUTHOR]

High strength-ductility synergy is difficult to achieve in Nb alloys. Although high strength has been achieved through severe plastic deformation (SPD) technology, led to low ductility in alloys. In this work, FSP technology was applied to treat Nb–5W–2Mo–1Zr-0.1C (Nb521) alloys in preparation of fine-grained (FG)/ultrafine-grained (UFG) Nb521 with excellent strength and ductility. The microstructure evolution and mechanical property improvement mechanism were systematically studied for Nb521 alloy through various characterization pathways. The research results indicated that UFG Nb521 alloy with a grain size of 0.63 ± 0.41 μm can be prepared using low shoulder plunge depth FSP (LPD-FSP), which is the first report of UFG Nb521 alloy. The main reason for the formation of onion rings structure in SZ is the periodic wear of the stirring tool, and the onion rings structure does not cause mechanical damage. The texture formed by Nb521 alloy under different processing parameters is off-axis shear texture, which matches the ideal shear texture of D2 (112‾)[111] after rotation. In addition, this study also elaborated on the refinement mechanism of the second phase particles (Nb, Zr) C in Nb521 alloy during FSP. This study also indicated that the increase in yield strength of FSP samples at room temperature is mainly determined by grain refinement. These findings provided new ideas for the development of high-performance niobium alloys.

Abstract Our research focuses on enhancing the broadband absorption capability of organic solar cells (OSCs) by integrating plasmonic nanostructures made of Titanium nitride (TiN). Traditional OSCs face limitations in absorption efficiency due to their thickness, but incorporating plasmonic nanostructures can extend the path length of light within the active material, thereby improving optical efficiency. In our study, we explore the use of refractory plasmonics, a novel type of nanostructure, with TiN as an example of a refractory metal. TiN offers high-quality localized surface plasmon resonance in the visible spectrum and is cost-effective, readily available, and compatible with CMOS technology. We conducted detailed numerical simulations to optimize the design of nanostructured OSCs, considering various shapes and sizes of nanoparticles within the active layer (PM6Y6). Our investigation focused on different TiN plasmonic nanostructures such as nanospheres, nanocubes, and nanocylinders, analyzing their absorption spectra in a polymer environment. We assessed the impact of their incorporation on the absorbed power and short-circuit current (Jsc) of the organic solar cell.

Molten salt electrodeposition is the process of producing impressively dense deposits of refractory metals using the electrolysis of molten salts. However, predicting which electrochemical parameters and setup will best control different kinds of deposition (density, homogeneity, etc.) is an ongoing challenge, due to our limited understanding of the properties and mechanisms that drive molten salt electrodeposition. Because these advancements have been made rapidly and in different arenas, it is worth taking the time to stop and assess the progress of the field as a whole. These advancements have increasing relevance for the energy sector, the development of space materials and engineering applications. In this review, we assess four critical facets of this field: (1) how the current understanding of process variables enhances the electrodeposition of various molten salts and the quality of the resulting product; (2) how the electrochemical setup and the process parameters (e.g., cell reactions) are known to impact the electrodeposition of different metal coatings and refractory-metal coatings; (3) the benefits and drawbacks of non-aqueous molten salt electrodeposition, and (4) promising future avenues of research. The aim of this work is to enhance our understanding of the many procedures and variables that have been developed to date. The expectation is that this review will act as a stimulant, motivating scientists to delve further into the investigation of refractory-metal alloys by utilizing molten salt electrodeposition. [ABSTRACT FROM AUTHOR]

The isothermal and cyclic oxidation behavior of a multi-principal element (MPE) TiNbCr alloy at 800–1000 °C in air was studied and compared to Co-based alloy 188. The phase constitution of the MPE alloy consisted of a Nb-rich body-centered cubic (BCC) matrix and Cr-rich Laves precipitates. While isothermal tests conducted at 800 °C led to the formation of a complex mixture of Nb, Ti and Cr oxides, tests at 900 and 1000 °C resulted in the formation of an innermost Cr2O3-rich scale layer which provided improved oxidation resistance. However, for all exposure temperatures, the scaling kinetics of the alloy were linear and therefore deemed non-protective. In contrast, alloy 188 exhibited parabolic scaling kinetics and smaller mass gain per area than the MPE alloy. The similarity between isothermal and cyclic test results for the MPE alloy confirmed that the scale does not offer much protection. Additionally, for all tests, there was extensive internal oxidation and nitridation. [ABSTRACT FROM AUTHOR]

Additive friction stir deposition (AFSD) is a solid-state metal additive manufacturing technique, which utilizes frictional heating and plastic deformation to create large deposits and parts. Much like its cousin processes, friction stir welding and friction stir processing, AFSD has seen the most compatibility and use with lower-temperature metals, such as aluminum; however, there is growing interest in higher-temperature materials, such as titanium and steel alloys. In this work, we explore the deposition of an ultrahigh-temperature refractory material, specifically, a tantalum–tungsten (TaW) alloy. The solid-state nature of AFSD means refractory process temperatures are significantly lower than those for melt-based additive manufacturing techniques; however, they still pose difficult challenges, especially in regards to AFSD tooling. In this study, we perform initial deposition trials of TaW using twin-rod-style AFSD with a high-temperature tungsten–rhenium-based tool. Many challenges arise because of the high temperatures of the process and high mechanical demand on AFSD machine hardware to process the strong refractory alloy. Despite these challenges, successful deposits of the material were produced and characterized. Mechanical testing of the deposited material shows improved yield strength over that of the annealed reference material, and this strengthening is mostly attributed to the refined recrystallized microstructure typical of AFSD. These findings highlight the opportunities and challenges associated with ultrahigh-temperature AFSD, as well as provide some of the first published insights into twin-rod-style AFSD process behaviors. [ABSTRACT FROM AUTHOR]

This study focuses on synthesizing and characterizing mechanically alloyed refractory high entropy alloys (RHEA) containing TiMoNb, Cr, W, and Cr + W. Analytical techniques including XRD, SEM, EDX, DTA, and TGA were employed to explore the influence of Cr and W on the alloys' structure, morphology, and thermal stability towards oxidation. XRD analysis confirmed the formation of a single-phase bcc solid solution in TiMoNbCr, TiMoNbW, and TiMoNbCrW alloys. Crystallite size decreased as milling progressed, yielding average sizes of approximately 7.2 nm, 7.4 nm, and 9.7 nm, with lattice strains of 1.143%, 1.148%, and 1.15%, respectively. Both experimental and calculated values of lattice parameters converged to around 3.1685 ± 0.002 and 3.1791 ± 0.010 Å for all three alloys. The synergy of Cr and W in TiMoNb was observed for the first time, impacting dislocation density, hardness, and oxidation stability. TiMoNbCrW exhibited reduced dislocation density and the highest hardness (502 HV). TGA indicated enhanced oxidation resistance up to 400 °C for TiMoNbCrW compared to the most vulnerable TiMoNbCr alloy. [ABSTRACT FROM AUTHOR]

This work focuses on the parameter development and microstructural characterization of Nb-based alloy C103 for thin wall structures produced via laser powder – directed energy deposition (LP – DED). Laser power and scanning speeds were varied as part of a design of experiments to identify adequate print parameters. Combinations were evaluated for relative density, porosity, and geometrical accuracy. A combination of a laser power of 1420 W and scanning speed of 14 mm/s resulted in a relative density >99%, exhibited a consistent weld bead profile and was used for further microstructural evaluation. A mix of optical and scanning electron microscopy (SEM) revealed small, slightly elongated grains along the edges and large epitaxial grains in the central region along the Build – Transverse view. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis revealed Hf rich columnar cells in the center that transition into an evenly spaced cellular structure towards the edges of the built sample. Electron backscatter diffraction (EBSD) scans show a sharp [001] texture when looking at the cross section of the sample along the build direction. The Build – Scan view revealed a zig – zag pattern that follows the back – and – forth deposition strategy that was used. Finally, microhardness measurements were taken in the as – built (AB) and stress relieved (SR) conditions to baseline preliminary mechanical properties. The AB condition exhibited a large amount of scatter in the data and averages up to 11% larger than the SR condition. The reduction in scatter upon applying the SR cycle are indicative of a large concertation of dislocations present in the AB condition.

Due to their excellent high-temperature strength, good processing plasticity and corrosion resistance, refractory metals and their alloys are extensively utilized in the aviation, aerospace and nuclear industries, serving as important high-temperature structural materials. However, their susceptibility to oxidation often causes serious oxidation before reaching service temperatures, leading to rapid failure. High performance high-temperature protective coatings are essential for maintaining the performance of these refractory alloy materials at present. However, the actual service conditions of high-temperature protective coatings on surfaces of refractory metals and their alloy are very harsh, often accompanied by strong thermal shock, which is an important reason for coating failure. Therefore, a high-temperature protective coating on refractory metals must have excellent constant temperature oxidation resistance and good thermal shock resistance. In this paper, the thermal shock failure mechanism of high-temperature protective coatings on the surfaces of refractory metals was reviewed, and the key parameters affecting the coatings’ thermal shock resistance were discussed. The research status of the thermal shock resistance of three main coating systems, including silicides, metals and composite coatings on the surface of refractory metals, was expounded. Additionally, the modification methods and their improvement effects, such as optimizing the coating structure, adding ceramic particles and designing composite coatings to improve the thermal shock resistance of coatings, were reviewed. Finally, the future development direction of high-temperature protective coatings for refractory metals was prospected from three aspects: reducing the mismatch of thermal expansion coefficient between the coating and the substrate, improving the interface bonding performance between the substrate and coating and designing composite gradient coatings.

The present study reports on additive manufacturing of a Ti-30Ta (at.%) high-temperature shape memory alloy (HT-SMA) using electron beam powder bed fusion (PBF-EB/M) technique. Detailed microstructure analysis was conducted to reveal the microstructural evolution along the entire process chain, i.e. from gas-atomised powder to post-processed material. PBF-EB/M processed structures with near full density and an isotropic, β-phase stabilised microstructure, i.e. equiaxed β-grains of around 20 µm in diameter with no preferred crystallographic orientation, are reported. As revealed by differential scanning calorimetry, post-process heat-treated Ti-Ta demonstrates a reversible martensitic phase transformation well above 100°C. Although partly unmolten Ta-particles after both gas atomisation and PBF-EB/M remain a challenge towards robust processing, PBF-EB/M appears to show significant potential for fabrication of Ti-Ta HT-SMAs, especially when functional metal parts and components with complex shapes are required, which are difficult to fabricate conventionally.

The paper presents the results of creating protective coatings on niobium and tantalum samples. These coated samples are tested for heat resistance in air in the temperature range of 1700–1900°C. The coating is formed by the method of activated vacuum diffusion saturation at temperatures of 1150–1310°C. When forming a part of the protective coatings, a layer of slurry is applied to the surface of the samples, followed by chemical and thermal treatment in a vacuum in the presence of NaCl vapours. A thermodynamic analysis of possible chemical reactions taking place in the process of formation of protective coatings is carried out. The developed heat-resistant coatings provide protection of niobium samples from air oxidation at a temperature of 1700°C from 8 to 13.5 hours. Heat-resistant coatings created on samples with Nb and Ta protect them from exposure to an oxidizing environment from 3 to 6.3 hours at a temperature of 1800°C and for 0.5 hours at a temperature of 1900°C. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of refractory metal on the high temperature properties of phenolic resin matrix composites, modified quartz fiber reinforced ceramizable composites were prepared by a molding process with a refractory component, tungsten, as the functional component and boron carbide as the ceramic forming agent. The effects of the tungsten and the boron carbide on the heat resistance of the composite were investigated. The results showed that the introduced refractory metal tungsten and the boron carbide can react to form tungsten borides and tungsten carbides at high temperature, and form a ceramic layer on the surface of the composite, which can fill the defects caused by pyrolysis of matrix and improve the high temperature performance of the composite. When the content of boron carbide was 10 wt% and the content of tungsten powder was 30 wt%, the flexural strength of the composite before and after heat treatment at 1200 °C were increased by 54.6% and 30.2% respectively compared with that without filler. [ABSTRACT FROM AUTHOR]

The development of Refractory High Entropy Alloys (RHEAs) for Additive Manufacturing has been advancing rapidly in recent years, buoyed by the triumph of RHEAs generated using traditional techniques in various applications. RHEAs, a recent category of materials possessing extraordinary high-temperature attributes, have only lately emerged. However, owing to their intricate microstructure, the production of RHEAs employing conventional methods such as casting and forging poses a challenge. On the other hand, Additive Manufacturing (AM) has exhibited significant potential for creating RHEAs due to its ability to generate intricate microstructures and complex geometries with minimal material wastage. The fundamentals of AM and RHEAs are introduced at the outset, in conjunction with the unique attributes of 3D-printed RHEA products, such as their enhanced strength and unique microstructure. The techniques for producing RHEA powders, including atomization and mechanical alloying, as well as the characteristics of the powder, are then elucidated. In this review article, we will provide a comprehensive overview of RHEAs produced using Additive Manufacturing (AM). We will examine the most innovative AM methods presently available for producing RHEAs, their microstructure, attributes, and potential applications.

An impact mill has been developed to produce powders from shavings of refractory metals using the impact grinding method for reuse in electrometallurgy in devices with screw feed, for example, in 3D printers. The proposed device provides high uniformity of grinding with a minimum content of dust fraction and impurity content at low technical and economic costs. The result is achieved using a Laval nozzle, which operates in the supersonic jet formation mode. In the area of the first Mach disk, there are rod fenders arranged in a cascade, and the impact plate is located in the turbulence zone and is equipped with winglets with holes for separating crushed metal. [ABSTRACT FROM AUTHOR]

Outstanding inherent environmental resistance is a precondition for the use of superalloys in high-temperature applications. Besides high Al and Cr levels, also refractory metal concentrations (W and Ta) are reported to affect protective scale formation, as these elements are expected to affect the chemical activity and also the transport of protective scale formers within the alloy. In this study, we elucidate the high-temperature oxidation behavior of 3 Co-based (Co/Ni ratio: 1.4) and 3 Ni-based (Co/Ni ratio: 0.7) superalloys differing in W and Ta levels. Time-resolved thermogravimetric analysis (TGA) in synthetic air at 1050 °C and 1150 °C for 100 h, scanning electron microscopy analysis (SEM), thermodynamic calculations using the CALPHAD software Thermo-Calc, and diffusion couple experiments were conducted to assess the impact of the Co/Ni ratio and the refractory metal content on the oxidation performance. The results indicate that a low W content (3 vs. 5 at.%) and a high Ta content (2.1 vs. 1.5 at.%) beneficially affect the oxidation resistance, as alumina scale formation is facilitated. [ABSTRACT FROM AUTHOR]

Sintering of tungsten nominally requires several hours at ultrahigh temperatures. We show this refractory metal can be sintered quickly by direct injection of current into dog bone shaped specimens. The current rate was varied from 10 A s−1 (fast) to 0.1 A s−1 (slow), leading to sintering in 2–200 s, respectively. Sintering occurred at the same current density, regardless of the current rate. In all instances, the samples sintered when they reached 1000°C. The phenomenological behavior of flash sintering of metals is described by three stages: an incubation time followed by electroluminescence, and finally by abrupt sintering to full density. It is conjectured that rapid sintering is instigated by the formation of Frenkel pairs (vacancies and interstitials), as well as electrons and holes. The point defects accelerate mass transport, whereas electrons and holes recombine to form photons. Calorimetric measurements show an endothermic reaction attributed to the creation of defects. Estimates suggest an unusually large concentration of Frenkel pairs. PS: Flash sintering is different than electro‐discharge‐sintering where a capacitor is discharged in a few milliseconds to sinter a metal. Here, instead of dumping large amount of energy at once, a power supply is programed to control the rate of current injection. [ABSTRACT FROM AUTHOR]

The characteristics of spherical powders of (Mo,Nb) and (Nb,Mo) alloys prepared by the spark erosion technique are described. Two modes (U = 24 V, I = 10 A and U = 24 V, I = 20 A in distilled water) were chosen for powder preparation. The pyrolysis of the dielectric liquid during spark erosion leads to the formation of stable oxides NbO and NbO2 as thin crusts and spherical inclusions from 10 nm to 10 μm in powder particles. The largest deviations from electrode compositions (from 3 to 7 wt % Nb) are inherent to Mo alloys with 9 and 10 wt % of Nb. The number of NbO and, to a lesser extent, NbO2 inclusions in spherical powder increases with increasing Nb content in the alloy, but only in the series of (Mo,Nb) samples. Such regularity is not observed for (Nb,Mo) series. A decrease in current value results in a change in the granulometric compositions of spherical powders. The main fraction of powders at I = 20 A is 40–80 μm and I = 10 A is 20–50 μm. [ABSTRACT FROM AUTHOR]

The appropriate hetero interfacial structure between metal and ceramics is a challenging topic in materials science. In this paper, a sandwich hybrid cladding structure made of SiCf/SiC composite and refractory metal niobium (Nb) is designed for application in a lead-cooled fast reactor. A diffusion reactive interfacial layer composed of β-SiC | NbC | Nb5Si4C | α-Nb5Si3 | β-Nb2C | Nb between SiCf/SiC composite and Nb is experimentally validated after heat treatment in a simulating reactor environment. Moreover, Kirkendall voids are found at the interface between Nb5Si4C and Nb5Si3 sublayers when the annealing temperature is higher than 1500 °C. Lastly, the formation mechanisms of interfacial structures are revealed based on thermodynamics and kinetics, meanwhile the diffusion coefficients k at 1500 °C and activation energy are calculated to be 2.81 × 10−14 m2/s and 270 kJ/mol, respectively. This work provides a practical strategy for developing hybrid composites of metal/ceramic matrix composites.

Additive friction stir deposition (AFSD) is a solid-state metal additive manufacturing technique, which utilizes frictional heating and plastic deformation to create large deposits and parts. Much like its cousin processes, friction stir welding and friction stir processing, AFSD has seen the most compatibility and use with lower-temperature metals, such as aluminum; however, there is growing interest in higher-temperature materials, such as titanium and steel alloys. In this work, we explore the deposition of an ultrahigh-temperature refractory material, specifically, a tantalum–tungsten (TaW) alloy. The solid-state nature of AFSD means refractory process temperatures are significantly lower than those for melt-based additive manufacturing techniques; however, they still pose difficult challenges, especially in regards to AFSD tooling. In this study, we perform initial deposition trials of TaW using twin-rod-style AFSD with a high-temperature tungsten–rhenium-based tool. Many challenges arise because of the high temperatures of the process and high mechanical demand on AFSD machine hardware to process the strong refractory alloy. Despite these challenges, successful deposits of the material were produced and characterized. Mechanical testing of the deposited material shows improved yield strength over that of the annealed reference material, and this strengthening is mostly attributed to the refined recrystallized microstructure typical of AFSD. These findings highlight the opportunities and challenges associated with ultrahigh-temperature AFSD, as well as provide some of the first published insights into twin-rod-style AFSD process behaviors.

Molten salt electrodeposition is the process of producing impressively dense deposits of refractory metals using the electrolysis of molten salts. However, predicting which electrochemical parameters and setup will best control different kinds of deposition (density, homogeneity, etc.) is an ongoing challenge, due to our limited understanding of the properties and mechanisms that drive molten salt electrodeposition. Because these advancements have been made rapidly and in different arenas, it is worth taking the time to stop and assess the progress of the field as a whole. These advancements have increasing relevance for the energy sector, the development of space materials and engineering applications. In this review, we assess four critical facets of this field: (1) how the current understanding of process variables enhances the electrodeposition of various molten salts and the quality of the resulting product; (2) how the electrochemical setup and the process parameters (e.g., cell reactions) are known to impact the electrodeposition of different metal coatings and refractory-metal coatings; (3) the benefits and drawbacks of non-aqueous molten salt electrodeposition, and (4) promising future avenues of research. The aim of this work is to enhance our understanding of the many procedures and variables that have been developed to date. The expectation is that this review will act as a stimulant, motivating scientists to delve further into the investigation of refractory-metal alloys by utilizing molten salt electrodeposition.

Refractory metals such as molybdenum, tantalum, tungsten, niobium and their alloys have the characteristics of high melting point and good high temperature performance, which play a major role in the field of protection. However, due to their high cost, high melting point, low thermal conductivity and high brittleness, their processing is relatively difficult. By using specific processes to prepare refractory metal coatings on the substrate surface to achieve excellent mechanical properties and high-temperature performance of refractory metals without changing the substrate properties, and effectively reduce manufacturing costs, which has significant research value for equipment surface protection. Because the refractory metal coatings play an increasingly important role in the manufacturing industry, the plasma spraying, explosive spraying, laser cladding and cold spraying preparation technology of molybdenum, tantalum, tungsten, niobium and other refractory metal coatings were introduced firstly, and the characteristics of each process were summarized. On this basis, the microstructure and properties of refractory metal coatings prepared by different processes were analyzed and compared, and the improvement progress was summarized from three aspects of coating material composition and structure, process parameters and post-treatment. Finally, the existing problems and development directions of the refractory metal coating preparation technology were prospected.

The most promising directions of the development of heat-resistant alloys (HRAs) based on refractory metals are analyzed. The microstructures characteristic of HRAs, which it is advisable to form in promising alloys, are considered. The stability factors of the microstructure with respect to the diffusion coarsening of the hardening phases are discussed. Two groups of alloys are considered as the most promising HRAs based on refractory metals. First, the principles for design of HRAs based on (Pt, Ir)-Sc with heterophase γ-γ’ microstructure, where γ-matrix is a (Pt, Ir) solid solution with a FCC lattice, and γ’ is a strengthening phase with the structure L12 by analogy with Ni-base superalloys, are developed. The resistance of γ-γ’ microstructure in Ni, Pt and Ir alloys against the process of diffusion-limited coarsening is analyzed. It is shown that the diffusion permeability of Pt is several times less than that of Ni, so one should expect that Pt-based HRAs will not be inferior to Ni-based HRAs in terms of structural stability. The second group includes HRAs based on many not noble refractory metals. It is shown that solid solutions of the system (Ti, Zr, Hf, Ta, Nb) with a BCC lattice can be considered as a matrix of advanced refractory HRAs. The results of experimental studies of alloys based on (Ti, Zr, Hf, Ta, Nb) additionally alloyed with elements contributing to the formation of strengthening intermetallic and silicide phases are discussed. The issues of segregation of alloying elements at the grain boundaries of refractory alloys and the effect of segregation on the cohesive strength of the boundaries are considered.

A series of electric explosions were carried out on single and twisted conductors of various diameters made of titanium (Ti) and tungsten (W) in propane-butane. Analysis of the electro-physical characteristics of the explosion showed that the process of resistive heating of the conductors is characterized by two monotonically increasing sections on the voltage and current curves, separated by a flat segment (plateau), which corresponds to a relatively stable specific electrical resistance of refractory metals in a liquid state. The energy introduced into the conductor during the resistive heating stage, which can be higher or lower than the energy of sublimation of the conductor and can be regulated by changing the external parameters of the discharge circuit, is a key indicator that determines the structural-phase state of the destruction products and the chemical interaction of the conductor. Conditions were realized under which micro- and nanosized powder products of the electric explosion do not contain residual metals and consist entirely of carbide phases (TiC with an average microhardness of 29 580 MPa in the explosion of titanium conductors, and a mixture of W2C + WC1 – x dominated by stabilized high-temperature nonstoichiometric cubic carbide WC1 – x with an average microhardness of 16 770 MPa in the explosion of tungsten conductors). [ABSTRACT FROM AUTHOR]

Diboride‐based ultra‐high temperature ceramics (UHTCs) are a special class of ceramics with excellent comprehensive properties, which have extensive potential applications in extreme environments. However, their practical applications are limited, mainly due to the poor fracture toughness and thermal shock resistance. Refractory metals have high melting points, good ductility, and high toughness, which have huge potential to improve the properties of diboride‐based ceramics. As a special class of additives, they have been adopted to promote densification, improve microstructure, and properties. However, diboride‐based ceramics containing refractory metals have not received adequate attention due to relatively weak practical effects on property improvement. The present review highlights the progress and existing problems of transition metal diborides with refractory metal additives, including W, Ta, Mo, Nb, Hf, V, Cr, and Zr, focusing mainly on the microstructure change and property improvements, followed by challenges and possible future development strategies. [ABSTRACT FROM AUTHOR]

General information about the method of electrospark plasma sintering (ESPS) of powder materials is presented, and its advantages and disadvantages are given. A review of results and achievements of domestic and foreign researchers in the field of refractory metal ESPS and their compounds is provided. The ESPS method is especially effective in consolidating powders of materials with a high melting point that are difficult to deform. During electrospark plasma sintering special attention should be paid to raw powder quality and cleaning processes for interparticle surfaces in order to obtain optimum finished product properties. Methods of comparative temperature measurement should be used to obtain more reliable data for ESPS, for example, according to input power. [ABSTRACT FROM AUTHOR]

The photovoltaic cells have undergone a series of metamorphosis since the perovskite materials are being used as light absorber in it. Owing to its superior light absorbing ability, perovskite materials have offered a bit of hope for future photovoltaic application. However, the stability and toxicity of perovskite-based solar cells have always remained a major concern. In this context, electronic characteristics pertaining to compounds of refractory metals, i.e., molybdenum and tungsten; and non-toxic properties of tin halide-based perovskite material may be considered to vanquish the issues related to durability and toxicity. This paper comprehends SCAPS 1D simulation and study of tin-based perovskite solar cell structures consisting of oxides and dichalcogenides of refractory metals viz oxides of molybdenum (Mo O x) , tungsten di-selenide (WS e 2) , molybdenum di-telluride (MoT e 2) and molybdenum di-sulfide (Mo S 2) as hole transport materials (HTMs). Post simulation, the optimized efficiencies were observed to be 31.95%, 30.89%, 31.92% and 31.86% for Mo O x , WS e 2 , MoT e 2 and Mo S 2 , respectively. Among these, perovskite solar cell consisting Mo O x as hole transport layer (HTL) displayed conspicuous result exhibiting open circuit voltage ( V oc ) of 1.1093 V, short circuit current density ( J sc ) of 33.88 mA/ cm 2 , fill factor (FF) of 85.01% and power conversion efficiency (PCE) of 31.95%. These parameters indicate that oxides and dichalcogenides of refractory metals viz oxides of molybdenum (Mo O x) , tungsten di-selenide (WS e 2) and molybdenum di-telluride (MoT e 2) can be optimistic materials for future generation solar cells. A comparative analysis of the proposed perovskite solar cells is carried out using oxides and dichalcogenides of refractory metals as HTLs, i.e., MoOx, WSe2, MoTe2 and MoS2 using SCAPS 1D simulator. Power conversion efficiency, fill factor, short circuit current density and open circuit voltage of the proposed device are characterized by thickness, doping concentrations, defect density and temperature. The proposed solar cells with MoOx, MoTe2 and MoS2 as HTLs, yielded high performance as compared to existing one. [ABSTRACT FROM AUTHOR]

Electrodeposition of composite coatings based on cobalt, including refractory metals, allows obtaining coatings with a unique combination of physicochemical properties that cannot be achieved with other coating methods. The application of high-quality electrochemical composite coatings is only possible by establishing the characteristics of the electrolyte and electrolysis parameters. The characteristics of the scattering ability and specific electrical conductivity of Co-MoOx-WOx, Co-WOx-ZrO2, and Co-MoOx-ZrO2 electrolytes for depositing cobalt-based composite coatings have been established. The electrical conductivity values of the electrolytes for depositing cobalt-based composite coatings linearly increase with the temperature of the electrolyte, but their values at 25-30°C are sufficient for the deposition of high-quality coatings. It has been determined that within the current density range of 0.5--3.0 A/dm2 for Co-MoOx-WOx and up to 4.0 A/dm2 for Co-MoOx-ZrO2 and Co-WOx-ZrO2, the scattering ability remains above 85 %. The calculated activation energy of the electrical conductivity of complex electrolytes ranges from 22 to 29 kJ/mol, indicating the occurrence of mass transfer processes in the diffusion regime. Using insoluble inert stainless steel anodes in the technological process is justified. [ABSTRACT FROM AUTHOR]

The most promising directions of the development of heat-resistant alloys (HRAs) based on refractory metals are analyzed. The microstructures characteristic of HRAs, which it is advisable to form in promising alloys, are considered. The stability factors of the microstructure with respect to the diffusion coarsening of the hardening phases are discussed. Two groups of alloys are considered as the most promising HRAs based on refractory metals. First, the principles for design of HRAs based on (Pt, Ir)-Sc with heterophase γ-γ' microstructure, where γ-matrix is a (Pt, Ir) solid solution with a FCC lattice, and γ' is a strengthening phase with the structure L12 by analogy with Ni-base superalloys, are developed. The resistance of γ-γ' microstructure in Ni, Pt and Ir alloys against the process of diffusion-limited coarsening is analyzed. It is shown that the diffusion permeability of Pt is several times less than that of Ni, so one should expect that Pt-based HRAs will not be inferior to Ni-based HRAs in terms of structural stability. The second group includes HRAs based on many not noble refractory metals. It is shown that solid solutions of the system (Ti, Zr, Hf, Ta, Nb) with a BCC lattice can be considered as a matrix of advanced refractory HRAs. The results of experimental studies of alloys based on (Ti, Zr, Hf, Ta, Nb) additionally alloyed with elements contributing to the formation of strengthening intermetallic and silicide phases are discussed. The issues of segregation of alloying elements at the grain boundaries of refractory alloys and the effect of segregation on the cohesive strength of the boundaries are considered. [ABSTRACT FROM AUTHOR]

Interdiffusion of the elements in a diffusion pair consisting of Ti and an equiatomic high-entropy alloy (HEA) TiZrHfNbTaMo in the temperature range of 1473–1673 K has been studied. A calculated results phase diagram of the alloy by Thermo-Calc 2021-B software as used to determine the temperature stability range of the β-phase in the alloy. Ti–HEA diffusion pairs were obtained by low = temperature welding and then diffusion annealing was carried out at temperatures of 1473, 1573, and 1673 K during 12, 9, and 6 h, respectively. The interdiffusion zone was profiled using electron probe microanalysis (EPMA). The diffusion parameters of the HEA's elements were obtained using Hall's method. An experimental results discussion is given. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) have recently attracted widespread attention due to their outstanding mechanical properties at elevated temperatures, making them appealing for concentrating solar power and nuclear energy applications. Here, the corrosion behavior of equimolar HfTaTiVZr and TaTiVWZr RHEAs was investigated in molten FLiNaK eutectic salt (LiF-NaF-KF: 46.5−11.5−42 mol.%) at 650 °C. Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and immersion test measurements were carried out for these two RHEAs and compared with Inconel 718 (IN718) superalloy and SS316 stainless steel under identical test conditions. Both TaTiVWZr and HfTaTiVZr refractory high-entropy alloys exhibited an order of magnitude lower corrosion rate than SS316. IN718 and TaTiVWZr showed similar corrosion rates. Corrosion products enriched with noble alloying elements formed in the case of TaTiVWZr and IN718 were stable and protective on the substrate. SS316 showed the lowest corrosion resistance and void formation along the exposed surface due to the active dissolution of Cr and Fe, which provided diffusion paths for the corroded species. The surface analysis results showed that IN718 underwent pitting corrosion, while TaTiVWZr experienced selective dissolution in the inter-dendritic area. In contrast, HfTaTiVZr and SS316 experienced corrosion at the grain boundaries. [ABSTRACT FROM AUTHOR]

Binary Ti-Ta and ternary Ti-Ta-Al alloys attracted considerable attention as new potential biomaterials and/or high-temperature shape memory alloys. However, conventional forming and manufacturing technologies of refractory based titanium alloys are difficult and cost-intensive, especially when complex shapes are required. Recently, additive manufacturing (AM) emerged as a suitable alternative and several studies exploited elemental powder mixing approaches to obtain a desired alloy and subsequently use it for complex shape manufacture. However, this approach has one major limitation associated with material inhomogeneities after fabrication. In present work, novel pre-alloyed powder material of a Ti-Ta-Al alloy was additively manufactured. Hereto, electron beam powder bed fusion (PBF-EB/M) technique was used for the first time to process such Ti-Ta based alloy system. Detailed microstructural analysis revealed that additively manufactured structures had a near full density and high chemical homogeneity. Thus, AM of pre-alloyed feedstock material offers great potential to overcome major roadblocks, even when significant differences in the melting points and densities of the constituents are present as proven in the present case study. The homogeneous microstructure allows to apply short-term thermal post treatments. The highly efficient process chain detailed will open up novel application fields for Ti-Ta based alloys.

Lead-based fast reactor (LFR) has become one of the most promising reactors for Generation IV nuclear systems. A developing trend of LFR is high efficiency, along with operation temperatures up to 800 °C or even higher. One of key issues in the high-efficiency LFR is corrosion of cladding materials with lead at high temperatures. In this study, corrosion behavior of some refractory metals (Nb, Nb521, and Mo-0.5La) was investigated in static lead at 1000 °C for 1000 h. The results showed that Nb and Nb521 exhibited an intense dissolution corrosion with obvious lead penetration after corrosion, and lead penetration extended along the grain boundaries of the specimens. Furthermore, Nb521 showed a better corrosion resistance than that of Nb as a result of the elements of W and Mo included in Nb521. Mo-0.5La showed much better corrosion resistance than that of Nb and Nb521, and no lead penetration could be observed. However, an etched morphology appeared on the surface of Mo-0.5La, indicating the occurrence of corrosion to a certain degree. The results indicate that Mo-0.5La is compatible with lead up to 1000 °C. While Nb and Nb alloys might be not compatible with lead for high-efficiency LFR at such high temperatures.

The study of the viscosity of Ti-Zr-Hf-Si alloy is of great help in preparing Ultra-High Temperature Ceramic Matrix Composites. In this work, the viscosity of Ti 15 Zr 60 Hf 20 Si 5 is 11.8 m Pa·s by the drop oscillation method under electrostatic levitation conditions at 2273 K. For qualitative assessment of the samples, it was found that Ti 15 Zr 60 Hf 20 Si 5 does not contain solid phase at 2273 K using experimental analysis (SEM, EPMA, XRD, and TEM). Based on the experimental results, the viscosity database of the liquid phase for Ti-Zr-Hf-Si quaternary systems was constructed by the viscosity model in our previous work. • Viscosity model only requires less experimental values and basic thermodynamic data. • Successfully constructed the Ti-Zr-Hf-Si alloy liquid viscosity database. • Obtaining accurate viscosity prediction results for range of liquid components. [ABSTRACT FROM AUTHOR]

Energy efficiency demands finding new materials for applications at elevated temperatures. Refractory multicomponent alloys with a BCC/B2 microstructure promise to serve as alternatives to the current state-of-the-art, Ni-based superalloys for applications at higher temperatures. With the vast compositional space of these alloys, the first fundamental question is whether the B2 phase can form among refractory metals. In this study, we span through the periodic table to investigate the potential for creating ordered B2 intermetallics among the refractory metals, using first principles calculations, and establish the correlation between the nature of the atomic bonds and stability of the B2 structure. We use our newly developed Multi-Cell Monte Carlo method to explore the extensive compositional space of these alloys, refine the trends in ordering, and pinpoint the most promising combinations for detailed analysis. We discover that B2 phases form exclusively with the inclusion of elements from groups VII and VIII, such as Re, Ru, and Os, and we identify these phases among a total of eleven elements. Further analysis of the electronic structure and bonding characteristics of these B2 systems reveals a pseudogap in the electronic density of states at the Fermi level. This pseudogap is attributed to the hybridization of d orbitals, leading to the formation of strong covalent bonds in the most stable compounds. This finding explains why B2 formation is restricted to elements from groups IV-VIII and V-VII refractory metals. [Display omitted] [ABSTRACT FROM AUTHOR]

• Microwave heating was used for alkaline fusion of scheelite and columbite-tantalite ores. • The fusion temperature was lowered by using an eutectic NaOH:KOH mixture. • Subsequent water leaching gave efficient extraction of W after MW-fusion in NaOH:KOH. • Ta and Nb leaching in water was efficient after MW-fusion in KOH. • MW heating allowed for shorter alkaline fusion times. The refractory metals tungsten, tantalum and niobium are considered critical raw materials in Europe. Diversifying the supply of such materials, for instance by recovering refractory metals from local low-grade ore materials could reduce their supply risks. However, the extraction processes require to be efficient with a low energy and material consumption and a minimal environmental impact. In this work, microwave (MW) assisted alkaline fusion was explored to extract tungsten from scheelite containing ore materials with different grades (high to very low) and tantalum and niobium from a columbite-tantalite [(Fe,Mn)(Nb,Ta) 2 O 6 ] concentrate. During the fusion process scheelite was converted into soluble alkali tungstate salts [Na 2 WO 4 , K 2 WO 4 and/or K 3 Na(WO 4) 2 ] and the reaction temperature could be lowered to 150–200 °C by fusion with a low-melting eutectic alkali salt system of an 1:1 (m/m) NaOH:KOH mixture. The application of MW-assisted heating allowed for fast and volumetric heating, thereby lowering reaction times to 10 min – 30 min. After fusion, tungsten was extracted by a simple water leaching step leading to extraction efficiencies ranging from 77 % to 97 %. Noteworthily, also P, S and As, which all form soluble oxyanions were extracted from the studied materials. Extraction of niobium and tantalum was also achieved though MW-assisted alkaline fusion as an alternative to acidic fluorine-based hydrometallurgical processes. MW-assisted heating allowed for a fast conversion of the (Fe,Mn)(Nb,Ta) 2 O 6 into alkali tantalate and niobate salts. Under optimal conditions, MW-assisted fusion with KOH (salt:sample = 3:1 (m/m)) at 400 °C for 10 min leached ca. 74 %–88 % of tantalum and 77 %–86 % of niobium into water (L/S=10) at 40 °C for 120 min, while the main matrix elements Mn and Fe displayed limited dissolution (<4%). Due to the low solubility of sodium tantalate and niobate salts in water, MW-assisted alkaline fusion in an eutectic NaOH:KOH mixture followed by water leaching was not efficient. [ABSTRACT FROM AUTHOR]

Opportunities are described for developing coatings for refractory alloys beyond the current state-of-the-art silicide coatings, which are inadequate for long-term application in combustion environments due to silica volatility. It is proposed that multicomponent rare earth oxides provide an ideal material system to tailor all necessary properties for environmental/thermal barrier coatings in a single layer, differing from current trends to adopt multi-layer systems that are prone to thermochemical-mechanical interface failure mechanisms. The article discusses opportunities, proof-of-concept, and challenges to accomplish these single-layer rare earth oxide coatings. Properties of interest include isotropic phase stability, processability, thermal expansion, thermal conductivity, ability to perform as a radiation barrier, stability in combustion environments, CMAS resistance, and ability to act as an oxygen diffusion barrier. Both experimental and computational approaches for property optimization are described. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) has transformed the process of shaping refractory metals into complex form factors. This study was undertaken to investigate the AM processing parameters for pure tantalum (Ta) and to understand their influence on the attendant properties. Computational fluid dynamics simulations informed a subset of AM laser parameters that were manufactured and characterized for their porosity and mechanical response. Manufacturing with high energy densities (350-470 J/mm3) increased material density and mechanical strength, but dramatically reduced the ductility owing to the prolific formation of pre-existing edge cracks. While low porosity is typically a key metric for success in AM, the results presented here elucidate that a balance between maximizing bulk material density and minimizing steep thermal gradients during processing is needed to optimize the strength-ductility tradeoff.

When it is asked, "where can refractory metals be used?," the possible shortest answer is, "where cannot they be used?" The uses of refractory‐metal‐based compounds in research and industry are too many to be enumerated; nevertheless, some outstanding examples are briefly mentioned here. Essentially, chalcogenide forms of refractory metals are preferred in the fabrication of high‐performance structures. Therefore, expanding the current studies that usually focus on tungsten‐ and molybdenum‐based structures to other materials may open new opportunities. Moreover, research on ternary and quaternary structures can also be a keystone in creating high‐performance products. The rationale of the present review is to give a brief overview of the recent history of refractory‐metal‐based chalcogenides (RMCs). Initially, the framework is confined to the general design and approaches for the synthesis of refractory metal chalcogenides. The assay is continued by extending with characteristic features of materials from crystalline properties to thermoelectric attributes and examining device fabrication processes. Taken together, the device fabrication part where RMCs are mainly used is extensively focused upon. Finally, outlook and future perspectives are given on the design and construction of RMCs to enable future inspiration and innovation. [ABSTRACT FROM AUTHOR]

Molybdenum and a molybdenum alloy were fusion welded to ZrB 2 -based ceramics to determine if the electrical and thermal properties of the metals and ceramics affected their weldability. Commercial ceramic powders were hot pressed, machined into coupons, and preheated to 1600 °C before joining the ceramics to commercial metals using plasma arc welding. Weldability varied as indicated by the range of porosity observed within the fusion zones. Measured thermal and electrical properties appeared to have little to no effect on the weldability of metal-ceramic welds despite the large range of values measured across each property. Differences in melting temperatures between metal and ceramic coupons did affect weldability by changing the weld penetration depth into ceramic coupons. Future studies on metal-ceramic welds are suggested to investigate the effect that work function, melt viscosity, wetting, or other properties have on weldability. [ABSTRACT FROM AUTHOR]

Based on the analysis of theoretical and experimental studies, this article proposes that rolling (or flattening) while passing a high-density electric current through a deformation center is an effective method for obtaining precision tapes and micron cross-section profiles from difficult-to-form metals and alloys. Reliable experimental data on the deformation resistance of difficult-to-form metals and alloys at various deformation temperatures, degrees, and rates are required to develop a mathematical model of the electroplastic deformation process and to solve practical problems associated with the pressure treatment of these metals and alloys. Regarding the electroplastic deformation process, it is proposed that a multiplier that considers the influence of electric current be added to the dependence of deformation resistance on the temperature, degree, and rate of deformation. An analysis of the experimental data obtained during the electroplastic flattening of wires from several nonferrous and refractory metals and alloys revealed that when the effect of electroplastic deformation was registered, the deformation resistance could be considered to decrease linearly in the first approximation as the density of the electric current passing through the deformation focus increased. A mathematical model of the resistance of metals and alloys to electroplastic deformation was developed based on the mathematical processing of the experimental data; in this model, the deformation resistance decreased linearly as the density of the electric current passing through the deformation focus increased. [ABSTRACT FROM AUTHOR]

Measurement of the surface tension of refractory liquid metals up to date is a difficult task. Values of the surface tension of liquid metals, in general, in the undercooled region are lacking. Here we have calculated the surface tensions of undercooled liquids Aluminium (Al) and the three refractory metals Tantalum (Ta), Tungsten (W) and Niobium (Nb) using our previously reported statistical thermodynamic equation. The calculations have been carried out below and above the melting temperature (Tm) of the metal thus covering the undercooled region by about 300 K below Tm. For Al, there was a notable jump at the melting point upon cooling where the surface excess entropy changed by 30%. This stems from the increased ordering at the surface upon cooling. The observed abrupt change indicates a first order surface phase transition at Tm from a pure liquid surface to a solid like surface structure upon cooling. For the other metals studied, the change was less pronounced. From the calculated value of the fraction of surface broken bond, it is concluded that the surface structure of the studied metals resemble that of simple metals. The calculated surface tensions above the melting point of the metals show good agreement with measured ones. Below the melting point of the metals, measurements are lacking that emphasise the importance of our calculations. [ABSTRACT FROM AUTHOR]