Search

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]

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]

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.)

• 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]

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]

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]

Microstructure and mechanical properties at 25–1200 °C of new refractory alloys, Nb-18Ti-12W (NTW), Nb-18Ti-12W-5Re (NTW-R) and Nb-18Ti-12W-5Re-1Al (NTW-RA), all compositions are in atomic percent, are reported. The alloys are developed as an alternative to a denser, more expensive and difficult-to-process refractory alloy WC-3009. After production by arc melting and hot isostatic pressing at 1400 °C, 207 MPa for 3 h, the alloys have a BCC crystal structure with the average grain size of ∼150–200 μm and contain low volume fraction of (Ti,N)-rich precipitates near grain boundaries. NTW shows excellent malleability at all studied temperatures. Its yield stress at 25 °C, 1000 °C and 1200 °C is 874 MPa, 353 MPa and 202 MPa, respectively. The NTW-R and NTW-RA alloys are stronger at all the studied temperatures; however, they lack in compression ductility at 25 °C. Extensive deformation twinning and formation of intergranular voids precede the fracture of these two alloys at 25 °C. • Nb-18Ti-12W (NTW), Nb-18Ti-12W-5Re (NTW-R) and Nb-18Ti-12W-5Re-1Al (NTW-RA) have a single-phase BCC crystal structure. • NTW shows excellent malleability and medium strength at 25–1200 °C. • NTW-R and NTW-RA are stronger than NTW at all studied temperatures but they lack in ductility at 25 °C. • Extensive deformation twinning and formation of intergranular voids precede the fracture of NTW-R and NTW-RA at 25 °C. • NTW-R and NTW-RA have excellent combination of strength and ductility at 600-1200°C. [ABSTRACT FROM AUTHOR]

MoNbTi, MoNbTiZr, CrMoNbTiZr, and MoNbTiVZr multi-principal element alloys (MPEAs) were fabricated via spark plasma sintering (SPS) and investigated for use in high-strength applications. The fabrication method by SPS and powder metallurgy differs from those presented in the prior literature, where most MPEAs are fabricated using arc melting (AM) methods. Cryogenic milling was used to maximize potential defect sinks (grain boundaries) for radiation resistance and to increase ultimate tensile strength through the Hall-Petch effect. SPS was chosen for consolidation in order to maintain a fine-grained structure during densification. Each alloy was characterized using x-ray diffraction and scanning electron microscopy for phase identification and compositional homogeneity. The base ternary alloy MoNbTi presented a predominantly single BCC system with minor cubic phases. The introduction of additional alloying elements—Zr, V, and Cr—heightened the phase complexity and increased the fractions of a secondary BCC phase and an HCP phase from Zr. The addition of Cr induced a larger fraction of the Laves phase to form. The addition of V caused the precipitation of small Mo inclusions. Thermodynamic analysis was performed to understand the separation of phases in each alloy. Discrepancies among the phase predictions generated by thermodynamic models, phases previously presented in the literature, and the characterization results suggest that MPEA fabrication methods, especially solid-state methods, require significant investigation to ensure that alloys can remain stable throughout their anticipated service lifetimes. [Display omitted] [ABSTRACT FROM AUTHOR]

Hf–Ta based alloys have recently been investigated as potential candidates for high-temperature structural applications. While most attentions have been given to the properties above ~1200 °C where the alloys are mainly single-phase BCC structures and have excellent oxidation performance due to formation of super-oxides, structural properties at lower temperatures are equally important for applications in which an alloy may experience a range of temperatures. In the present work, microstructure, phase composition, mechanical properties and oxidation behavior of six Hf-25Ta-5Me alloys (Me is Mo (HTM alloy), W (HTW), 0.5Mo + 0.5 W (HTMW), Cr (HTC) or Zr (HTZ), the compositions are in at.%) are reported at temperatures below the eutectoid transformation. The alloys were prepared by arc melting followed by hot isostatic pressing for 3 h at 1400 °C and 207 MPa. All the alloys display coarse grains of partially or fully transformed (by a eutectoid reaction) high-temperature BCC phase. The eutectoid regions consist of fine lamellae of Hf-rich HCP and Ta-rich BCC phases. The ternary alloys containing W or Cr also contained small amounts of a cubic Laves phase. At 25 °C, the HTW alloy was the strongest (yield stress σ y = 1966 MPa) but brittle and HTZ was the weakest (σ y = 1120 MPa) but ductile among the studied alloys. Other alloys showed intermediate behaviors. In general, the room temperature ductility of the alloys increased with decreasing σ y. All alloys maintained high strength up to 800 °C, but displayed a noticeable strength decrease at 1000 °C. At 1000 °C, HTMW was the strongest alloy (σ y = 468 MPa) and HTC was the weakest alloy (σ y = 368 MPa). All alloys had excellent deformability at 1000 °C. Oxidation behavior of the alloys was studied at 800 °C and 1000 °C and compared with that of Hf–27Ta binary alloy. Although the ternary additions improved oxidation resistance, overall oxidation performance of the studied alloys at 800 °C and 1000 °C was poor. • Hf-25Ta-5Me (Me = Mo, W, Mo+W, Nb, Cr or Zr) alloys have a complex microstructure caused by eutectoid transformation. • The alloys have high strength, exceeding 1000 MPa, at temperatures up to 800 °C. • The strength of the alloys decreases rapidly between 800 °C and 1000 °C. • Interfaces in the eutectoid structures provide a noticeable contribution to strengthening at 25 °C - 800 °C, not at 1000 °C. • Ternary additions to Hf-27Ta improve oxidation resistance at 800 °C and 1000 °C in air, although it is still poor. [ABSTRACT FROM AUTHOR]

The oxidation of refractory compositionally complex alloys (RCCAs) is one of the most critical subjects for their high temperature applications. In this work, several RCCAs were systematically designed and isothermally oxidised at 1000 °C for up to 24 h; the elemental effects of Al, Cr, Mo, Nb Ta, Ti, V, and Zr on the oxidation of the RCCAs were studied. It was found that Al, Cr, Ti could enhance the oxidation resistance of the RCCAs by promoting the formation of rutile-type AlTaO 4 , CrTaO 4 , and their related oxides, while Zr and V formed non-protective oxides such as Nb 2 Zr 6 O 17 , Ta 2 Zr 6 O 17 , ZrTiO 4 , and VTa 9 O 25 , resulting in severe internal oxidation. Mo appeared to be effective in suppressing internal oxidation, but it could form non-protective MoTiTa 8 O 25. Ta was slightly more beneficial than Nb for reducing the oxidation mass gain. This study provides insights on the elemental effects on the oxidation and guidelines for improving the oxidation resistance of RCCAs. [Display omitted] • The oxidation of refractory compositionally complex alloys is systematically studied at 1000 °C. • Multiple refractory complex oxides are observed and analysed. • The effects of the selected elements on the oxidation behaviours are discussed. • Complex oxide formation is explained with thermodynamic simulation. [ABSTRACT FROM AUTHOR]

The emergent multi-principal element alloys (MPEAs) provide a vast compositional space to search for novel materials for technological advances. How to screen promising compositions from such an ample design space for targeted properties is a grand challenge. Here, we demonstrate the state-of-the-art deep learning technology—convolutional neural network (CNN)—in predicting path-dependent vacancy migration energy barrier spectra, which are critical to diffusion behavior and many high-temperature properties, in the hyperdimensional composition space of MPEAs. The developed CNN model, fully capturing local chemical features surrounding each vacancy, accurately and efficiently predicts migration energy barrier of MPEAs with different degrees of chemical short-range order and at any unseen compositions. By varying the size of the local region encapsulating vacancy in the CNN model, we reveal that the length scale influencing vacancy migration is surprisingly extensive, up to its six nearest neighboring shells. The efforts of the CNN model make it promising for developing a database of diffusion barriers for various MPEA systems, which would have profound implications for accelerating alloy screening and discovering new compositions with desirable properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The current study reports the analysis of the deformation mechanisms at 600 °C in a two-phase, BCC+B2, refractory complex concentrated alloy (RCCA) Al 0.5 NbTa 0.8 Ti 1.5 V 0.2 Zr. At this temperature, the alloy microstructure is unstable and dynamic coarsening of B2 precipitates is evidenced during the mechanical testing. After true plastic strain of 0.030 at strain rate of 10−4 s−1, the deformation becomes highly localized in wavy bands reflecting the profusion of cross-slip. Scanning transmission electron microscopy (STEM) observations highlight the presence of paired a/2⟨111⟩ dislocations that shear the B2 precipitates in a cooperative process. In addition, some chemical segregation effect is observed along the narrow dislocation bands likely induced to decrease the antiphase boundary (APB) energy of the system. [Display omitted] [ABSTRACT FROM AUTHOR]

Microstructure and compression properties of (Hf 0.73 Ta 0.27) 100-X Mo X alloys, where X = 0, 5, 16, 21 and 30 at. %, are reported. The alloys were prepared by vacuum arc melting and hot isostatically pressed at 207 MPa for 3 h at 1400 °C. The alloys with 0, 5, 16 and 21 at. % Mo contained two phases, BCC and HCP, while the alloy with 30 at. % Mo contained three phases, BCC, HCP and cubic Laves (C15). The Hf 73 Ta 27 , Hf 69 Ta 26 Mo 5 and Hf 61 Ta 23 Mo 16 alloys experienced eutectoid transformation with the formation of a mixture of Hf-rich HCP and Ta-rich BCC phases. The volume fraction of the eutectoid regions decreased from 55% to 0% with increasing Mo from 0 to 21 at.%. The alloy with 30% Mo (Hf 51 Ta 19 Mo 30) experienced eutectoid transformation with the formation of a mixture of the Mo-rich cubic Laves (C15) phase and Hf-rich HCP phase. The volume fraction of the eutectoidally transformed regions in Hf 51 Ta 19 Mo 30 was ~20%. Among the studied alloys, Hf 73 Ta 27 had the highest room temperature yield stress of 1738 MPa. The yield stress continuously decreased with increasing Mo concentration to 1468 MPa at 16% Mo and then increased to 1672 MPa at 30% Mo. The room temperature ductility strongly depended on the amount of Mo. The maximum true fracture strain of 0.137 was achieved in Hf 69 Ta 26 Mo 5 , but it rapidly decreased to 0.027 in Hf 51 Ta 19 Mo 30. All the studied alloys were ductile at 1000–1400 °C and no fracture was observed after 0.7 true strain. The yield stress increased almost linearly with increasing the amount of Mo at these high temperatures. • Hf73Ta27, Hf69Ta26Mo5 and Hf61Ta23Mo16 are two-phase alloys consisting of Ta and Mo rich BCC and Hf-rich HCP phases. • Additions of Mo stabilizes and increases the volume fraction of the BCC phase in these three alloys. • Hf58Ta21Mo21 and Hf51Ta19Mo30 contain three phases: Ta-rich BCC, Hf-rich HCP and Mo-rich cubic Laves phases. • The HCP and Laves phases are present inside eutectoidally transformed areas in Hf51Ta19Mo30. • Hf73Ta27 is the strongest at 25 °C but the softest at T ≥ 1000 °C among the studied alloys. • At 25 °C, Hf61Ta23Mo16 has minimum true yield and peak stresses. • At 1000 °C, 1200 °C and 1400 °C, yield stress increases linearly with increasing the concentration of Mo. [ABSTRACT FROM AUTHOR]

C103 is a refractory-based niobium alloy for high temperature applications in aerospace. This paper aims to evaluate the compatibility of C103 alloy as a choice of material for laser-based additive manufacturing (AM) technologies. Crack free AM specimens with relative densities of greater than 99% were successfully fabricated via laser powder bed fusion (L-PBF) method with horizontal and vertical build orientations. Static strength levels of the stress-relieved specimens exceeded that of their wrought counterparts. Higher yield and ultimate tensile strength of AM specimens were attributed to high dislocation density and the formation of dislocation cell structures, which was revealed by transmission electron microscopy. Loss of ductility in AM specimens was partially due to high density of dislocations (i.e., shorter mean free path for dislocations), while the presence of defects may also have compromised the total elongation to failure. Electron backscattered diffraction analysis revealed formation of sharp 001 | | B D rod texture in AM samples, which resulted in anisotropic mechanical behavior. Uniaxial tensile and compressive deformation led to the development of (001) <110> parallel to tensile direction and <111> parallel to build direction, respectively. [ABSTRACT FROM AUTHOR]

• TiZrHfV x Nb x Ta x BCC high-entropy alloys were designed by Bo-Md alloy design method. • TiZrHfV 0.1 Nb 0.1 Ta 0.1 exhibits strain-hardening with deformation-induced phase transformation from BCC to HCP. • TiZrHfV 0.1 Nb 0.1 Ta 0.1 reveals the stepwise transformation process, β to α″ at early strain and β to α′ or α″ to α′ at high strain. [Display omitted] Refractory high-entropy alloys (RHEAs) show a great potential as structural materials due to their remarkable properties, such as high strength and thermal stability. Despite these advantages, the RHEAs still suffer from the limited ductility at room temperature. To overcome the strength-ductility trade-off, the new RHEA design strategy, transformation-induced plasticity (TRIP) HEAs, have been developed by applying metastability-engineering approach. With the newly developed RHEAs design strategy, we designed metastable RHEAs, TiZrHfVxNbxTax (x = 1.0, 0.5 and 0.1) with decrease of the β-stabilizing elements (V, Nb, and Ta) to promote the transformation-induced ductility and work-hardening capability. The proper addition of the β-stabilizing elements (V, Nb, and Ta) into the ternary equi-molar TiZrHf alloy (TiZrHfV 0.1 Nb 0.1 Ta 0.1) leads to stress/strain-induced phase transformation from the body-centered cubic (BCC) to hexagonal-close-packed (HCP) α′ and/or orthorhombic α″ phases. Detailed studies on the phase transformation sequence during the deformation were carried out using X-ray diffraction (XRD), electron back-scattered diffraction (EBSD), and transmission-electron microscope (TEM), which revealed the stepwise phase transformation that is responsible for the strong strain hardening behavior. Furthermore, we further suggested the new transformation-induced-plasticity (TRIP) HEA design strategy, applying the Bo-Md approach. [ABSTRACT FROM AUTHOR]