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This review explores additive manufacturing (AM) for refractory tungsten (W) and its alloys, highlighting the primary challenges and determining factors in the AM of pure W, W alloys and composites. The challenges mainly arise from W's high melting point, low laser absorptivity, high thermal conductivity, high melt viscosity, high oxygen affinity, high ductile-to-brittle transition temperature, and inherent embrittlement, which lead to defects and anomalies in AM-produced parts. This review focuses on both processes and alloying strategies to address the issues related to densification, micro-cracking, and the resultant properties in W-based components. Cracking in additively manufactured W remains a persistent issue due to thermal stress, embrittlement, and oxide formation. Powder characteristics, process parameters, and thermal management strategies are crucial for W densification. Throughout the review, existing knowledge and insights are organized into comprehensive tables, serving as valuable resources for researchers delving deeper into this topic. Future research in W-AM should focus on understanding the interaction between AM process parameters and microstructural and material design. Advances in atomic-level understanding, thermodynamic modeling, and data analytics have the potential to significantly enhance the precision, sustainability, and applicability of W-AM. [ABSTRACT FROM AUTHOR]

This study evaluates the effectiveness of hot isostatic pressing (HIP) in mitigating porosity in TZM-NbZr1 bimetallic structures fabricated via wire-arc directed energy deposition (DED). Two distinct HIP conditions and a combined HIP and heat treatment (HT) were investigated. The pore area fraction decreased from 4 ± 0.05% in the as-built condition to 3.05 ± 0.02% and 1 ± 0.01% after treatment at 1200°C and 1500°C, respectively. Porosity was further reduced to 0.01 ± 0.005% with HIP at 1800°C followed by HT. Plastic deformation strongly influenced pore size and shape at elevated temperatures, particularly above the recrystallisation temperatures of both alloys. At lower treatment temperature, disparity in material flow around the pores resulted in interconnected micro-cracks or micro-void formation. However, the combined HIP and HT process effectively eliminated both cracks and porosities from the interface altering the pore recovery mechanism from micro-void formation to dynamic recrystallisation. Additionally, enhanced Mo diffusion from TZM to NbZr1 was observed at higher temperature.

The microstructure of high-entropy alloys with refractory elements and Al as constituents can be considered to be analogous to superalloys. These so-termed refractory high-entropy superalloys (RHSAs) can show remarkable compressive strength up to temperatures exceeding 1200 °C. Here, we examine the microstructure and properties - compressive, tensile, and fracture toughness - of a precipitation-hardened, body-centered cubic, RHSA, Al0.5Nb1.25Ta1.25TiZr, at ambient temperature (RT) to 1200 °C. Two dual-phase microstructures comprising ordered B2 (brittle) and disordered A2 (ductile) phases were produced in this alloy - one with B2 as the matrix, the other with A2 - for evaluation of the mechanical properties. Under compression, both microstructures display RT compressive strengths above 1.5 GPa and considerable ductility exceeding 40% at elevated temperatures; the alloy with the A2 matrix has ∼15% compressive ductility even at RT. However, properties are very different under tensile loading; at all temperatures, both microstructures fail predominately in an intergranular mode in the elastic regime at a fracture stress less than 200 MPa and ductility below 0.15%. The microstructure with the A2 matrix has a KIc fracture toughness of ∼15 MPa√m at RT, although at all temperatures above 800 °C, measured KIc values for both dual-phase microstructures are less than 5 MPa√m. In this study, we investigate the microstructural origin of these mechanical properties, and emphasize the importance of evaluating these alloys in tension.

This review explores additive manufacturing (AM) for refractory tungsten (W) and its alloys, highlighting the primary challenges and determining factors in the AM of pure W, W alloys and composites. The challenges mainly arise from W’s high melting point, low laser absorptivity, high thermal conductivity, high melt viscosity, high oxygen affinity, high ductile-to-brittle transition temperature, and inherent embrittlement, which lead to defects and anomalies in AM-produced parts. This review focuses on both processes and alloying strategies to address the issues related to densification, micro-cracking, and the resultant properties in W-based components. Cracking in additively manufactured W remains a persistent issue due to thermal stress, embrittlement, and oxide formation. Powder characteristics, process parameters, and thermal management strategies are crucial for W densification. Throughout the review, existing knowledge and insights are organized into comprehensive tables, serving as valuable resources for researchers delving deeper into this topic. Future research in W-AM should focus on understanding the interaction between AM process parameters and microstructural and material design. Advances in atomic-level understanding, thermodynamic modeling, and data analytics have the potential to significantly enhance the precision, sustainability, and applicability of W-AM.

Refractory alloys can be difficult to fabricate by laser-based manufacturing methods due to their high melting temperatures, high interstitial solubility, and propensity for low temperature brittleness. Laser-based processes, such as welding and additive manufacturing (AM), yield similar populations of defects, including microsegregation and solidification and solid-state cracking. Given the extreme challenges and cost associated with the production of refractory powders, this research aimed to develop a rapid screening methodology that combines predictive defect formation metrics with single track melting experiments. A flexible single laser track melting platform was designed to perform screening experiments on conventional and multi-principal element refractory alloys across a wide range of laser energy inputs. The platform was employed to investigate laser melting on solid substrates, or on a substrate with a single layer of powder feedstock, and is demonstrated with the highly fabricable Nb-base alloy C103. Preliminary investigations are performed on refractory multi-principal element alloys in the Hf-Mo-Nb-Ta-Ti family, and significant differences in cracking resistance and solidification morphology are observed. Implications for future alloy design and processing strategies for defect-resistant refractory alloys for AM are discussed.

Hot oscillating pressure (HOP) at different amplitudes (0, 5, and 10 MPa) is used to create a tungsten refractory alloy with narrow grain size distribution, higher hardness, and excellent corrosion resistance. The relative density and properties of sintered alloys are significantly improved and enhanced through the amplitude applied. The highest relative density about 99.2% with the narrowest average thickness bonding phase is presented in 10 MPa amplitude alloy. And its average grain size is only 3.67 µm, with a grain growth rate that is only 1.12 × 10-15 m²s-1 slower than that without the amplitude alloy (0 MPa amplitude alloy). Then the excellent hardness (454.7 HV0.5) is obtained. More importantly, the corrosion current of 10 MPa amplitude alloy is only just 5.09 × 10-6A cm-2 and reduces by nearly 9% compared with 0 MPa amplitude alloy. The hardness and corrosion resistance are better than those of other similar materials ever reported. The above results show that the HOP with amplitudes has a positive effect on improving the densification, hardness, corrosion resistance, and inhibiting the growth of grain size. [ABSTRACT FROM AUTHOR]

The mechanical properties of the bcc refractory high-entropy alloy (RHEA) NbMoTaW with columnar and equiaxed microstructures and a nanoscale metal oxide layer on the grain boundaries are investigated at ambient temperatures (RT) to 1200 °C. Under compression, the alloy shows a yield strength, σ y , of ⁓1390 ± 20 MPa at RT and retains a high yield strength, σ y , of ⁓301.5 MPa at 1200 °C. However, in tension, the fracture strengths, σ f , of both columnar and equiaxed microstructures are far lower - below 70 MPa - and the material fails in the elastic regime without showing any measurable ductility at all temperatures. The fracture mechanisms in tension transition from transgranular cleavage at RT to a mixture of transgranular and intergranular fracture at 800 °C to a complete intergranular fracture at 1200 °C due to selective weakening of the metal oxide layer on the grain boundaries driven by a structural change. The transition in the fracture mechanism in the equiaxed microstructure promotes extrinsic toughening at higher temperatures, resulting in a ⁓6.4 times higher fracture toughness (K Ic ⁓10.3 MPa√m) at 1200 °C compared to that at RT. NbMoTaW is a well-known RHEA, but the poor tensile strength and fracture toughness measured in this study highlight the critical importance of investigating the mechanical properties of these high-temperature RHEAs in tension. [Display omitted] [ABSTRACT FROM AUTHOR]

The correlations between tensile properties and some common parameters in BCC refractory multi-principal element alloys (RMPEAs) have been systematically investigated. The findings can be used as guidelines for the alloy design. We found that there exists a positive linear correlation between average atomic shear modulus mismatch and yield strength, and more negative enthalpy of mixing is peculiarly prone to brittle fracture in RMPEAs. Meanwhile, some BCC RMPEAs with first-class tensile properties were designed successfully. The analyses of alloys’ toughen mechanism revealed that their high strength, as well as good ductility, is attributed to lattice distortion and dislocation glide respectively.

Refractory tungsten-based medium- and high-entropy alloys form a promising class of strong, heat- and radiation-resistant materials for nuclear applications. Here, we systematically explore the radiation tolerance of Mo–Nb–Ta–V–W alloys using molecular dynamics simulations and a machine-learned interatomic potential. Going from pure W to W-based equiatomic binaries, ternaries, quaternary, and the quinary high-entropy alloy, we simulate the radiation damage accumulation up to 0.4–0.8 dpa through cumulative collision cascades. We find that the radiation damage and evolution are controlled by a combination of cascade-induced clustering, the balance in migration of vacancies and interstitials, and defect cluster binding energies. These mechanisms are strongly influenced by atom size, among which vanadium as the smallest atom is decisive. In pure W and alloys without V, the microstructure is characterised by large and growing interstitial dislocation loops. In stark contrast, in alloys containing V, vacancy dislocation loops are formed directly in collision cascades and the balanced migration rate of vacancies and interstitials promotes recombination and prevents growth of interstitial loops. The atom size differences also lead to clear segregation, with small atoms (V) in interstitial clusters and larger atoms (Ta, Nb) segregating around vacancy clusters. Furthermore, the small cluster sizes, formation of vacancy loops, segregation, and balanced migration in V-containing alloys lead to low swelling and an exceptionally efficient defect annealing compared to pure W and V-free alloys. Our results highlight WTaV as a promising low-activation material, where almost 90% of defects are annealed at 2000 K during 5 ns compared to < 20 % in pure W. [Display omitted] [ABSTRACT FROM AUTHOR]

The growing interest in refractory high-entropy alloys (HEAs) in the last decade is mainly due to their thermal stability, outstanding mechanical properties, and excellent corrosion resistance. However, currently HEAs are still not considered for use as common structural materials due to their inherent drawbacks in terms of processing and machining operations. The recent progress witnessed in additive manufacturing (AM) technologies has raised the option of producing complex components made of HEAs with minimal machining processes. So far, this could be achieved by using pre-alloyed powders of HEAs that were mainly produced by a conventional arc melting furnace (AMF) in the form of small compounds that were transformed into powder via a gas atomization process. To significantly reduce the production cost, the present study aims to analyze the ability to synthesize HEA WTaMoNbV via a laser powder bed fusion (LPBF) process using mixed elemental alloying powder as the raw material. For comparison, a counterpart alloy with the same chemical composition was analyzed and produced by an AMF process. The microstructures of the tested alloys were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The physical properties were evaluated in terms of density and mechanical strength, while the electrochemical behavior was assessed by potentiodynamic polarization analysis. The results disclosed similarities in microstructure, physical properties and electrochemical behavior between HEA WTaMoNbV manufactured by the proposed LPBF process and its counterpart alloy produced by an AMF process. [ABSTRACT FROM AUTHOR]

The correlations between tensile properties and some common parameters in BCC refractory multi-principal element alloys (RMPEAs) have been systematically investigated. The findings can be used as guidelines for the alloy design. We found that there exists a positive linear correlation between average atomic shear modulus mismatch and yield strength, and more negative enthalpy of mixing is peculiarly prone to brittle fracture in RMPEAs. Meanwhile, some BCC RMPEAs with first-class tensile properties were designed successfully. The analyses of alloys' toughen mechanism revealed that their high strength, as well as good ductility, is attributed to lattice distortion and dislocation glide respectively. This work provided some effective guidelines for the design of BCC refractory multi-principal element alloys with superior strength-ductility combinations via utilizing several common physical parameters. [ABSTRACT FROM AUTHOR]

In recent publications [Schliephake et al. in Intermetallics 104 (2019) 133–142 and Obert, Kauffmann & Heilmaier in Acta Materialia, 184 (2020) 132–142], an unexpected pesting-stability of fully eutectic and specific eutectic–eutectoid Mo–Si–Ti alloys was found. Several potential reasons were proposed: microstructural length scale being typically very fine due to the eutectic and eutectoid reactions, the phase distribution including the fraction of the eutectic and eutectoid regions, as well as the chemical composition of the individual phases. In the present study, we prove the Ti content to be decisive for the pesting-resistance in air at 800 °C by investigating the microstructure and oxidation behaviour of Mo–Si–Ti alloys with systematically varying nominal Ti content. A critical Ti content of 43 at% was identified. Due to the phase equilibrium, this corresponds to a local Ti content in Mo solid solution of 35 at%. Other microstructural properties such as (i) lamellar size within eutectic and eutectoid regions and (ii) volume fraction of the eutectic and eutectoid regions were demonstrated to have an insignificant influence on the pesting-resistance of the alloys. Rather, the assessment of the oxidation behaviour of the monolithic phase MoSS, which was identified to be crucial for the oxidation behaviour of the Mo–Si–Ti alloys, confirmed an improvement in oxidation behaviour with increasing Ti content.

The thermophysical properties of refractory MoNbReTaW high-entropy alloy in both supercooled liquid and high-temperature solid states were explored by an electrostatic levitation technique. The maximum supercooling attains 504 K, and the hypercooling limit is derived as 571 K. The liquid density at liquidus temperature is measured to be 13.3 g cm−3, which increases linearly with decreasing temperature at a slope of 6.83 × 10−4 g cm−3 K−1. The liquid alloy exhibits 5.3% relative volume shrinkage during crystallization. The thermal expansion coefficient of liquid and solid alloy at liquidus temperature are determined as 5.0 × 10−5 K−1 and 3.6 ×10−5 K−1, respectively. The liquid specific heat at liquidus temperature is found to be 38.2 J mol−1 K−1, and basically displays a linear decreasing tendency with temperature. According to the calculated enthalpy of fusion 24.7 kJ mol−1 and measured specific heats, the temperature-dependent entropy and Gibbs free energy difference between supercooled liquid and crystalline solid are obtained. [ABSTRACT FROM AUTHOR]

Use of soldering methods for preparing high-temperature permanent joints of silicon carbide ceramic matrix composites (CMC) with metals is analyzed. The main problems in joining CMC with metals are considered associated with poor wettability of CMC with molten metals, a significant difference in the LTEC of CMC and metals, and formation of substances with high hardness and brittleness within a connecting layer. Analysis of practical experience of using traditional and diffusion soldering methods for joining CMC of the Cf/SiC type with metal alloys based on Ni, Nb, Ti, Mo, and stainless steel is provided. The most typical solder compositions and soldering process production methods are considered. It is shown that these technologies make it possible to create reliable and strong joints for CMC components and various metals. [ABSTRACT FROM AUTHOR]

We study the phase stability of the B2 phase in a two-phase Al 0. 5 NbTa 0. 8 Ti 1. 5 V 0. 2 Zr refractory high entropy alloy. We find that the ordered phase of this alloy primarily contains Zr, Al, and Ti, where Zr and Al occupy opposite sublattices. Although the site occupancy of Ti is not conclusive from the experiments, first-principles calculations confirm the preference of Ti atoms to occupy the Zr sublattice. We also find that the B2 phase composed of Al, Zr, and Ti is unstable and that it transforms into Zr 5 Al 4 Omega, L 1 0 , or an ordered hexagonal phase depending on strain and the presence of antisite defects. [Display omitted] [ABSTRACT FROM AUTHOR]

The Peierls barrier represents the inherent lattice resistance to dislocation glide, controlling dislocation movement and dictating the resulting mechanical properties. The rise of multi-principal element alloys, with their vast compositional space and local chemical fluctuations, introduces notable challenges in efficiently computing the Peierls barriers. Here, we propose a neural network model that captures the chemistry and structure of screw dislocations. By incorporating two crucial inputs, the local atomic type and displacement, our model enables accurate and efficient prediction of the Peierls barriers in multi-component space. Using this neural network, we construct a barrier diagram across the entire ternary space of refractory Nb-Mo-Ta alloys, from which the compositions with high or low barriers can be quickly identified. We discover that the NbMo binary alloy can have a higher barrier than NbMoTa ternary system, suggesting chemical complexity may not be the predominant factor governing dislocation barrier. Subsequently, three screened compositions with different barriers are selected for studying their effects on dislocation motion. Atomistic simulations reveal that a higher mean barrier slows down dislocation mobility, while a broader distribution of barriers facilitates kink pair nucleation, altering the rate-limiting process from kink pair nucleation to kink glide and cross kinking. This study presents a general neural network model that enables rapid screening composition with the desired barrier spectrum and will impact the exploration and discovery of alloys with improved dislocation-controlled properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Post irradiation examination of two commercial tantalum alloys (based on Ta-8 %W) revealed significant irradiation hardening following neutron irradiation in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Tensile samples of commercial Ta alloys T-111 and ASTAR-811C were irradiated to a total neutron fluence (E > 0.1 MeV) in the range (2–4)x1021 n/cm2 (0.39–0.75 dpa) at an irradiation temperature near 900 °C. Tensile testing conducted at room temperature and at 800 °C revealed significant irradiation hardening under all test conditions. Samples that were tensile tested at room temperature exhibited brittle failure, whereas ductility was maintained for elevated temperature testing. The irradiation hardening and embrittlement noted for both alloys was attributed to irradiation-induced dislocation loop formation as confirmed through scanning transmission electron microscopy analysis. Corresponding energy dispersive x-ray spectroscopy revealed radiation-induced segregation of Hf in these loop populations. This Hf enrichment is shown to result in eventual co-precipitation of elongated (Hf,O)-rich precipitates along the dislocation loops. These results show that irradiation hardening is enhanced via irradiation-enhanced precipitation at elevated irradiation temperatures. Consequently, the minimum recommended operating temperature window for Ta alloys should be increased to account for observed hardening/embrittlement at irradiation temperatures as high as 900 °C (1,173 K). [Display omitted] [ABSTRACT FROM AUTHOR]

We employ first-principles calculations to examine the role of oxidation on the thermal expansion behaviour of a refractory multicomponent alloy. Our results reveal that the linear expansion and the coefficient of volumetric expansion over a range of temperatures have higher values for the oxidised alloy than the pure material. The enhanced expansion for the oxidised alloy is attributed to significant changes in the lattice parameter of the alloy upon surface oxidation. During oxidation, the diffusion of oxygen atoms to the subsurface layer of the alloy after complete surface coverage produces a displacement of the constituent elements that form metal oxides. This depletion creates vacancy sites in the lattice for enabling enhanced expansivity in the oxidised structure relative to the pure refractory alloy. [ABSTRACT FROM AUTHOR]

The freezing shrinkage and dendritic growth are of great importance for various alloys solidified from high-temperature liquids to solids since they dominate microstructure patterns and follow-up processing. However, the microgravity freezing shrinkage dynamics is scarcely explored on the ground as it is hard to suppress the strong natural convection inside liquid alloys. Here, a series of in-orbit solidification experiments is conducted aboard the China Space Station with a long-term stable 10 -5 g 0 microgravity condition. The highest temperature up to 2265 K together with substantial liquid undercoolings far from a thermodynamically stable state are attained for both Nb 82.7 Si 17.3 and Zr 64 V 36 refractory alloys. Furthermore, the solidification under microgravity of a droplet is simulated to reveal the liquid-solid interface migration, temperature gradient, and flow field. The microgravity solidification process leads to freezing shrinkage cavities and distinctive surface dendritic microstructure patterns. The combined effects of shrinkage dynamics and liquid surface flow in outer space result in the dendrites growing not only along the tangential direction but also along the normal direction to the droplet surface. These space experimental results contribute to a further understanding of the solidification behavior of liquid alloys under a weaker convection condition, which is often masked by gravity on the ground., (© 2024 Wiley‐VCH GmbH.)

Refractory high-entropy alloys (RHEAs) are widely studied because of their promising potential for ultrahigh-temperature applications. One key challenge towards the development of RHEAs as high-temperature structural materials is to concurrently achieve optimum oxidation resistance and mechanical properties. Here in this work, the effect of alloying on the oxidation behavior of ductile RHEAs was studied. Specifically, a ductile RHEA, Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, was alloyed with Al and Zr aiming to improve its oxidation resistance. The two modified RHEAs, Al0.75Cr0.25Nb0.5Ta0.5Ti1.5 and Al0.5Cr0.25Nb0.5Ta0.5Ti1.5Zr0.01, indeed show enhanced oxidation resistance at 800 °C and 1,100 °C, compared with Al0.5Cr0.25Nb0.5Ta0.5Ti1.5. In addition, all three RHEAs studied here show an excellent oxidation resistance at 800 °C compared with other RHEAs, although there is still a large space to further improve their performance at 1,100 °C. Internal oxidation and even nitridation are still present after oxidation exposure, indicating further efforts are required to form protective oxide scales on the surface of ductile RHEAs. Nevertheless, the work is expected to shed some light on future directions of improving the oxidation of ductile RHEAs, via the alloying route.

The structures and phase compositions of a series of refractory alloys of the Co – Al – Mo – Nb system (the cobalt angle) are studied by differential scanning calorimetry (DSC) and scanning microscopy. The effect of the contents of niobium and molybdenum on the phase composition and the temperature boundaries of the range of existence of the Co3(Al, Mo, Nb) intermetallic phase are determined. [ABSTRACT FROM AUTHOR]

Nanostructured materials exhibit remarkable properties significantly different from their bulk counterparts. Metal alloys at the nanoscale show an impressive potential to produce new systems having well-designed functionalities. By using a nanothermodynamic approach, here, we present the effects of the size and shape of the nanoparticles (NPs) on the phase diagrams (PDs) of the Mo-M (M = Nb, Ta, and W) alloys. A well-known group of morphologies at 50, 20, and 10 nm in diameter was considered, which are as follows: tetrahedron, cube, octahedron, decahedron, dodecahedron, cuboctahedron, rhombic dodecahedron, sphere, icosahedron, and truncated octahedron. From an examination of the liquidus and solidus curves, we calculated the expansion or contraction of the coexistence solid-liquid region of the PDs and how these changes are related to the size and shape of the NPs. Through a detailed Gibbs free energy (GFE) analysis, we also determined the thermal stability of the three Mo-based nanoalloys as a function of the size (10–50 nm) of the mentioned polyhedra, by fixing the temperature and the chemical composition. Finally, the surface segregated element was predicted in each bimetallic system. Graphical abstract [ABSTRACT FROM AUTHOR]

• Successful fabrication of TZM-NbZr1 bimetallic structure using wire-based DED. • Fabricated structure did not form any cracks and intermetallic compounds. • Non-uniformly distributed pores in NbZr1 matrix near to TZM-NbZr1 interface. • Ultimate tensile strength of 249 ± 126 MPa with elongation less than 6 % was achieved. The outcome of this study highlights the feasibility of additively manufactured bimetallic structures from refractory alloys for high-temperature applications. Using a wire-based directed energy deposition (DED) process, a bimetallic structure composed of molybdenum alloy (TZM) and niobium alloy (NbZr1) was successfully fabricated. The TZM-NbZr1 interface displayed no significant defects, such as cracks or delamination, and showed no presence of intermetallic compounds. A few pores were observed on the deposited NbZr1 side near the interface. During uniaxial tensile testing, the specimen's failure occurred on the NbZr1 side in close proximity to the interface, resulting in an ultimate tensile strength of 249 ± 126 MPa and an elongation of less than 6 %. [ABSTRACT FROM AUTHOR]

Refractory alloys can be difficult to fabricate by laser-based manufacturing methods due to their high melting temperatures, high interstitial solubility, and propensity for low temperature brittleness. Laser-based processes, such as welding and additive manufacturing (AM), yield similar populations of defects, including microsegregation and solidification and solid-state cracking. Given the extreme challenges and cost associated with the production of refractory powders, this research aimed to develop a rapid screening methodology that combines predictive defect formation metrics with single track melting experiments. A flexible single laser track melting platform was designed to perform screening experiments on conventional and multi-principal element refractory alloys across a wide range of laser energy inputs. The platform was employed to investigate laser melting on solid substrates, or on a substrate with a single layer of powder feedstock, and is demonstrated with the highly fabricable Nb-base alloy C103. Preliminary investigations are performed on refractory multi-principal element alloys in the Hf-Mo-Nb-Ta-Ti family, and significant differences in cracking resistance and solidification morphology are observed. Implications for future alloy design and processing strategies for defect-resistant refractory alloys for AM are discussed. • A single track laser set-up was developed for rapid screening of refractory multi-principal element alloys. • Similarities in solidification morphology and defects observed in laser melted C103 wrought and powder feedstock substrates. • Significant changes in crack resistance, solidification morphology observed with Ta substitution in the Hf-Nb-Mo-Ti family. • Predictive ductility and cracking criteria in tandem with single track experiments can assist refractory alloy design for AM. [ABSTRACT FROM AUTHOR]

• Subsolidus Ni-activated sintering of MoNbTaW is discovered. • The first observation of activated sintering in a high-entropy alloy is made. • Grain boundary (GB) wetting is confirmed, implying subsolidus GB prewetting. • A GB diagram is computed to predict GB disordering and explain activated sintering. • Weak ni segregation in furnace-cooled specimens suggests GB drying during cooling. Activated sintering of refractory metals represents a classical phenomenon in powder metallurgy. This study discovers that Ni addition can enhance sintering of MoNbTaW both above and below the bulk solidus composition at 1800 °C, thereby demonstrating the first example of activated sintering of a high-entropy alloy. To probe the underlying mechanism, experiments reveal complete grain boundary (GB) wetting above the bulk solidus composition, which logically infers the stabilization of a liquid-like interfacial phase (GB complexion) in a prewetting region below the bulk solidus composition. Furthermore, bulk CALPHAD (calculation of phase diagram) methods have been extended to model GBs in Ni-doped MoNbTaW to compute a GB "phase" diagram to forecast high-temperature GB disordering in the prewetting region and rationalize the observed Ni-activated sintering of MoNbTaW. Only weak GB segregation of Ni is found in furnace-cooled specimens, which suggests GB "drying" during cooling, consistent with the observed nanoscale precipitates along GBs. [Display omitted] [ABSTRACT FROM AUTHOR]

Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

In order to reduce the environmental impact of air travel, it is desirable that the efficiencies of gas turbine engines are increased. One way to achieve this goal is to increase the operating temperatures of the engine cores. Unfortunately for aero-engine manufacturers, the temperature capability limits of the Ni-base superalloys used currently have been reached. Hence, new alloys need to be developed that are capable of operating at significantly higher temperatures. In this article, the potential of tantalum-base superalloys is discussed and explored. A suite of alloys based on the Ta-Al-Co system was investigated. It was found that an array of fine carbide precipitates was formed in the Ta-rich matrix in a subset of the alloys, which is promising in terms of developing a strong and damage-tolerant microstructure, but that the elemental partitioning of Al out of the matrix accompanying precipitation is likely to degrade environmental resistance. Nevertheless, it is believed that the design principles described have the potential to facilitate the development of the next generation of high-temperature alloys based on systems of this type. • A suite of Ta-Al-Co alloys are designed and investigated. • Aim to produce BCC (A2) matrix + A2-superlattice precipitates. • Fine Ta3Al2CoC precipitates in Ta-rich BCC matrix found after ageing. • precipitates had a consistent orientation relationship with the matrix [ABSTRACT FROM AUTHOR]

Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, T m) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000 °C and 1200 °C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 T m , so that the alloys with higher T m retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000 °C is probably due to dissolution of secondary phases. Image 1 [ABSTRACT FROM AUTHOR]

Abstract Equilibrium phase diagrams for ten Me X (NbTiZr) 100-X alloy systems, where Me is Al, Cr, Fe, Hf, Mo, Re, Si, Ta, V or W and X ranges from 0 to 25 at% were calculated using the PanNb2018a database recently developed by CompuTherm, LLC. By the type of phases and sequence of formation, these alloy systems can be divided into two groups. The first group of quaternary alloys, containing Al, Hf, Mo, Ta, or V has a single-phase BCC region below the solidus line over the entire X range and can be considered as potential candidates for the development of single-phase high entropy alloys. The second group consists of the quaternary alloy systems in which the fourth element (Cr, Fe, Re, Si or W) has limited solubility in BCC NbTiZr, which leads to the formation of an additional phase below the solidus and above the solubility limit. These quaternary alloy systems can be used for the development of precipitation or dispersoid strengthened complex concentrated alloys. To verify CALPHAD calculations three representative alloys, Cr 10 Nb 30 Ti 30 Zr 30 , Ta 10 Nb 30 Ti 30 Zr 30 and Re 10 Nb 30 Ti 30 Zr 30 , were prepared by arc melting. The alloy densities were 6.56, 7.81 and 7.85 g/cm3, respectively. The phase compositions of the produced alloys agreed satisfactorily with CALPHAD calculations. Mechanical properties were also studied and compared with those of NbTiZr. At room temperature (RT) all the alloys showed high hardness exceeding 350 Hv and high compression yield stress exceeding 1000 MPa. RT compression ductility of Re and Ta containing alloys was above 50%, but Cr-containing alloy showed low ductility of 5%. With an increase in temperature ≥800 °C, compression strength decreased more rapidly for the Cr-containing alloy, which at 1200 °C became softer than NbTiZr. The least temperature dependence of the strength was observed for the Ta-containing alloy, which became the strongest at 1200 °C. Highlights • Mo, Ta, V, Hf and Al have unlimited solubility range in BCC NbTiZr. • Cr, Fe, Re, Si and W have limited solubility in BCC NbTiZr resulting in secondary phases. • Alloying of NbTiZr with Ta and Re improve high-temperature mechanical properties. • Alloying of NbTiZr with Cr increases RT strength but softens the alloy at T ≥ 1000 °C. [ABSTRACT FROM AUTHOR]

Abstract Twelve different compositions of the ternary Nb–Ni–Si system were synthesized and heat-treated at 1073 K for up to 1000 h. The microstructure of the samples were analyzed using Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and X-ray Diffraction. The results from this work solve inconsistencies between the existing experimental information and the new computational predictions, in favor of the latter, due to the difficulty of reaching thermal equilibrium at this temperature. Graphical Abstract [ABSTRACT FROM AUTHOR]

Abstract The Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr refractory high-entropy alloy with excellent corrosion resistance in the 3.5 wt% NaCl solution is identified in this work. This refractory high-entropy alloy exhibits much better general corrosion resistance than that of the 316L stainless steel, due to its corrosion current density being about one fifth of that in the latter. Meanwhile, the pitting potential of Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr reaches an unusually high value of +8.36 V, much higher than that of reported high-entropy alloys. The superior passivity of Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr is accredited to the formation of a single-phase solid solution containing high amount of homogenously distributed passivity-promoting elements, and also the existence of metallic Ta and OH− species in the passive film, which contribute to the high immunity to passive film breakdown. Highlights • Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr contains finely distributed passivity-promoting elements. • Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr exhibits better general corrosion resistance than 316L SS. • E pit of Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr reaches +8.36 V, beating all other reported RHEAs. [ABSTRACT FROM AUTHOR]

Ceramic-metallic composite (cermet) fuel forms are promising options for use in nuclear thermal propulsion (NTP) due to their high melting point, inherent hydrogen compatibility, retained strength at high temperatures, and high thermal conductivity. Spark plasma sintering was used to fabricate cermet materials, resulting in improved microstructural characteristics. An alloy of molybdenum with 30 wt% tungsten was used as the matrix material. Spherical yttria stabilized zirconia (YSZ) and both spherical and angular hafnium nitride (HfN) were used as surrogate fuel particles for the cermets. Utilizing a new lab-scale particle coating method, high density composites with excellent particle-matrix interfaces were fabricated. The effects of volume loading on the microstructure of the matrix were also examined, testing particle loadings up to 90 vol%. Initial stage sintering models were applied to understand the effects of the added particles on the sintering of the Mo30W matrix. Recommendations for fabrication of uranium bearing cermets using Mo30W are also included. • Mo30W cermets with nitride and oxide particles were sintered using SPS. • Effect of volume loading on matrix density was examined up to 90%. • Formation of Mo,W-Hf,N Laves phase characterized. • Effect of particles on initial stage sintering quantified. • Presented recommendations for sintering conditions for UO 2 and UN cermets. [ABSTRACT FROM AUTHOR]

Refractory alloys with their brilliant high-temperature tolerance earn a place for applications in extremely harsh environments. Recent development in additive manufacturing technologies speeds up these alloys' design and forming processes before entering the market. However, the desire for better microstructures and superior high-temperature mechanical behaviors has never been fed up. As was wished, in this study, an additively manufactured WTaRe alloy with compressive strength, ultimate compressive strength, and strain of 356 ± 15 MPa, 731 ± 15 MPa, and > 40% at 1600 °C is herein developed. Composition characterization after deformation of the as-deposited WTaRe alloy suggests the super high thermal stability of this alloy. Considering the excellent high-temperature mechanical behaviors, the printable WTaRe refractory alloy may serve as an alternative in a high-temperature environment. • The as-deposited WTaRe alloy has a superb thermal resistance up to 1600 °C. • Thecomposition of WTaRe alloy remains high-level homogeneity after deformed at 1200 °C, 1400 °C, and 1600 °C. • At 1600 °C, WTaRe alloy has the ultimate compressive strength and ductility of 731 ± 15 MPa and > 40%, respectively. [ABSTRACT FROM AUTHOR]

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Phase composition, microstructure and mechanical properties of six complex concentrated alloys, NbTiZr, NbTiZrV, NbTiZrVMo, NbTiZrVTa, NbTiZrVCr, and NbTiZrVAl 0.24 , are reported. The alloy density was in the range from 6.34 g/cm 3 to 8.43 g/cm 3 . After homogenization annealing at 1400 °C or 1200 °C, only the ternary NbTiZr alloy had a single-phase BCC crystal structure. NbTiZrV and NbTiZrVTa produced predominantly single-phase BCC structure with clusters of fine V-rich precipitates inside NbTiZrV grains and fine, Zr-rich grain boundary precipitates in NbTiZrVTa. The other three alloys contained BCC and Laves phases, and NbTiZrVMo also contained a second BCC phase. After compression deformation at 1000 °C, noticeable changes in the phase composition were found in NbTiZrV, NbTiZrVTa and NbTiZrVAl 0.24 , while the phase composition of other alloys retained unchanged. NbTiZr, NbTiZrV and NbTiZrVTa showed good ductility and noticeable work hardening rate at room temperature, while the ductility of other alloys rapidly decreased with increasing the amount of Laves phase. The RT yield stress varied from 975 MPa for NbTiZ to 1510 MPa for NbTiZrVMo. At 1000 °C, all the alloys showed excellent compression ductility; however only NbTiZrVCr and NbTiZrVMo were stronger than NbTiZr. The results indicate that increasing the composition complexity of refractory near-equiatomic alloys by increasing the number of the alloying elements does not necessary result in a simpler phase structure and/or better mechanical properties. The type of the alloying elements seems to be more important than their number. Additions of V, Al or Ta decrease while additions of Cr or Mo increase the 1000 °C strength of NbTiZr. [ABSTRACT FROM AUTHOR]

A Mo-Si-B based coating has been applied on a refractory multi-principal element alloy (RMPEA) using a two-step coating strategy and has endured more than 750 h of thermal cycling oxidation exposure between room temperature to 1300 °C with a minimal weight change. The formation of the Mo 5 SiB 2 diffusion barrier in the coating prevents the inward diffusion of the coating components. Then, boron is captured by the RMPEA, resulting in the formation of a solid solution X 5 SiB 2 (X = W, Mo, Ta, Nb, V) layer and an RMPEA-B boride layer underneath it that acts as the second diffusion barrier. • A Mo-Si-B based coating was deverloped for a WMoTaNbV alloy by using a two-step coating strategy. • The diffusion pathway was modified by Mo and Si-B surface enrichment, resulting in a sequenced multilayered structure. • Boron is captured by the RMPEA substrate at the interface, enabling the formation of the second diffusion barrier. • The coating protects the WMoTaNbV from thermal cyclic oxidation for 750 × 1 h exposures at a 1300 °C. [ABSTRACT FROM AUTHOR]

High-resistivity Ru–Mo–W-alloy wires, exhibiting excellent strength and ductility, were fabricated using the micro-pulling-down melt-growth method under vacuum for application as resistance heating wires. Furthermore, their mechanical and electrical properties were investigated. The fabricated Ru 0.6 Mo 0.15 W 0.25 wires (⌀0.8 mm × 10.8 m long) showed a room-temperature tensile strength and nominal rupture strain of 1287 MPa and 21.2%, respectively. Transmission electron microscopy of Ru 0.6 Mo 0.15 W 0.25 revealed the presence of active 11 2 ¯ 2 < 11 2 ¯ 3 > pyramidal slip systems. Ru 0.6 Mo 0.15 W 0.25 demonstrated an electrical resistivity of ∼0.936 μΩ∙m at 1873 K, 1.77 and 1.24 times higher than those of W and Ta, respectively. The dependence of the hemispherical total emissivity on temperature was also measured. The temperature dependence of the electrical resistivity of Ru 0.6 Mo 0.15 W 0.25 was lower than those of Ta and W. Furthermore, the hemispherical total emissivities of Ru 0.6 Mo 0.15 W 0.25 and Ta were comparable at ∼1800 K. When tested at 1873 K for approximately 3000 h, the durability of a Ru 0.6 Mo 0.2 W 0.2 wire was equal or superior to that of a Ta wire. The current (power) consumption required to deposit LiF and tris (8-hydroxyquinolinato)aluminum(III) (Alq 3) at the same deposition rate using Ru 0.6 Mo 0.2 W 0.2 was 21% (11%) and 34% (12%) lower, respectively, than that when using conventional Ta heating wires. The results reveal that the Ru 0.6 Mo 0.15 W 0.25 heat-stabilization time was reduced by approximately 44%. Moreover, its deposition rate recovered more rapidly from errors such as a sudden boiling during heating. • The dewetting μ-PD method is used to grow single-crystal Ru–Mo–W alloy wires. • The alloy exhibits high tensile strength, ductility, and temperature durability. • The temperature dependence of its electrical properties is small. • It requires less current and power to deposit LiF and Alq 3 than a Ta wire. [ABSTRACT FROM AUTHOR]

To meet the growing demand for transportation and energy consumption around the world, more efficient turbine engines and power generators with higher operating temperatures are needed, which require alloys with retained strength levels at elevated temperatures. In the past decade, refractory complex concentrated alloys (RCCAs) have gained prominence through numerous reports of superior strengths at higher homologous temperatures compared to those of conventional refractory and super alloys. However, these RCCAs, comprised of transition metals from subgroups IV, V, and VI, tend to be brittle at room temperature, hindering their broad applicability. Recent findings reveal that interstitial constituents may significantly contribute to, and convolute observations of, the ductility and strength of RCCAs at room temperature. This review of the literature examines and discusses the field's current understanding of the role of interstitial constituents, specifically oxygen and nitrogen, in the microstructure and mechanical behavior of RCCAs. Moreover, we provide context derived from the binary interactions of interstitial constituents with refractory metals and their contribution to the development and processing of conventional refractory alloys as a framework to gain insight into interstitial constituent mechanisms in RCCAs. In some cases, the mechanisms of interstitial constituents in RCCAs are similar to those of unalloyed subgroup VI transition metals and their dilute alloys, segregating to and embrittling grain boundaries. In other cases, interstitial constituents can be accommodated in solid solution, strengthening the RCCA, similar to interstitial constituents soluble in unalloyed subgroup IV and V metals and their dilute alloys. With the understanding of interstitial constituent element interactions with RCCA constituents, more holistic approaches to the design of RCCAs are suggested to engineer the mechanisms of intended and unintended interstitial constituents through alloy design and processing. For the development of strong and ductile RCCAs, the interactions between interstitial constituents and RCCA constituents and their resulting mechanisms must be understood and controlled. [ABSTRACT FROM AUTHOR]

Methods of optical and electron microscopies were used to study the structure of particles of niobium carbide in a cast refractory Fe-Cr-Ni-C alloy modified by Nb and Ti. Particles of niobium carbide in the structure of the cast alloy are predominantly multiphase polycrystalline clusters that are inhomogeneous in the chemical composition and crystal structure. The misorientation angle between individual crystals that compose the carbide particles is 30°-60°. The polycrystalline character of carbides is probably associated with significant thermal stresses that arise at the interphase boundaries in the structure of the alloy upon the primary cooling of the ingot. To explain the polymorphism of the cluster of niobium carbide, a further analysis of the structural and geometrical crystallography is required. [ABSTRACT FROM AUTHOR]

Amechanism of local formation of crystals of random orientation in growth of single-crystal billets of refractory nickel alloys for blades of gas turbine engines is suggested. Aturn of the lattice is initiated at the place of contact between the initial melt and the surface of hard particles of NbC carbide and/or TiN nitride. [ABSTRACT FROM AUTHOR]