Tantalum Content Research Articles

Superhard Ta–O–N coatings produced on titanium using induction physical vapor deposition

Due to simulation by the finite element method, the temperature fields in the “inductor – tantalum target – titanium product” system were calculated depending on the inductor current and exposure time during induction physical vapor deposition (IPVD). The conditions under which the tantalum target was heated to the evaporation temperature in vacuum were determined. It was experimentally established that after IPVD at an operating current IO = 500–800 A (inductor current IIND from 4.9 to 6.7 kA), electrical power consumption PC = 1.67–3.32 kW, and exposure time t = 30–300 s, there was an in increase in the content of tantalum C[Ta]= 21.6–42.1 at.%, oxygen C[O]= 16.5–43.5 at.%, and nitrogen C[N]= 2.6–9.4 at.% on the surfaces of titanium samples. According to the XRD data, the resulting thin-layered structure on the titanium samples was mainly presented by the following crystalline phases: tantalum oxides (Ta2O5, Ta2O), tantalum oxynitride/nitride (TaON, TaN), titanium oxides (TiO2, TiO), and metal phases (α-Ta, α-Ti). It was found that during IPVD hard H = 18.85–39.17 GPa and superhard H = 46.18–89.88 GPa tantalum-containing coatings were obtained on the surfaces of titanium samples. The highest resistance to plastic deformation (by 50–70%) and wear resistance of titanium samples with tantalum-containing coatings was observed in the case of short exposure t < 60 s during IPVD.

Solvent-Impregnated Sorbents for Tantalum from Niobium Separation Using a Fixed-Bed Column.

Reactor-grade niobium steel is used as a construction material for nuclear reactors. In this case, the presence of tantalum, which is characterized by a 20 times higher active cross section for capturing thermal neutrons than the cross section of niobium (181Ta: 21.3 barn), cannot exceed 100 ppm. Analytical methods for quality and new separation method development control need very pure niobium matrices—niobium compounds with a low tantalum content, which are crucial for preparing matrix reference solutions or certified reference materials (CRMs). Therefore, in this paper, a new, efficient method for separating trace amounts of Ta(V) from Nb(V) using extraction chromatography with the use of sorbents impregnated with methyl isobutyl ketone MIBK solvent is proposed. Various types of MIBK-impregnated sorbents were used (AG® 1-X8 Anion Exchange Resin, AMBERLITE™ IRC120 Na Ion Exchange Resin, SERVACEL® Cellulose Anion Exchangers DEAE 52, active carbons of various grain size, carbonized blackcurrant pomace, carbonized chokeberry pomace, bentonite, and polyurethane foam in lumps). The highest tantalum removal efficiency was determined using active coal-based materials (>97%). The separation effectivity of tantalum from niobium was also determined in dynamic studies using a fixed-bed column with MIBK-impregnated active carbon. Solutions of various Nb:Ta weight ratios (1:1, 100:1, 1000:1) were used. The most impressive result was obtaining 70 mL of high purity niobium solution of tantalum content 0.027 ppm (in relation to Nb) with 88.4% yield of niobium from a solution of Nb:Ta, weight ratio 1000:1 (purge factor equaled 35,000). It proves the presented system to be applicable for preparation of pure niobium compounds with very low contents of tantalum.

Influence of tantalum composition on mechanical behavior and deformation mechanisms of TiZrHfTax high entropy alloys

The effects of metastability engineering on tuning deformation behavior and deformation mechanisms in TiZrHfTaX (x = 1.00, 0.80, 0.60, 0.50) refractory body-centered cubic (BCC) high entropy alloys were investigated, with specific emphasis on elucidating the underlying interplay between phase stability, mechanical property, and deformation twins. It was found that in proper thermomechanical treated samples, a variation of tantalum content can effectively tune the activation of various deformation mechanisms. Detailed electron back-scattered diffraction and transmission electron microscopy analyses revealed for the first time that {332}<113̅>BCC twinning, deformation-induced α’’ phase, {111}α’’ type I and <2̅11>α’’ type II twinning can be sequentially activated in TiZrHfTaX (x = 1.00, 0.80, 0.60, 0.50) high entropy alloys with decreasing the content of tantalum. The comprehensive strengthening effect of transformation induced plasticity and twinning induced plasticity, was discussed and attributed as the pivotal factor for the improved work hardening capability and mechanical performances, especially for alloys with lower tantalum contents. Consequently, we extended the conventional bond order and d‐orbital energy level diagram that was originally developed for body-centered cubic titanium alloys for deformation mechanism evaluation to body-centered cubic high entropy alloys on the basis of current results, which sheds light on the design of ductile body-centered cubic high entropy alloys with expected deformation mechanisms and optimized mechanical performance.

Effect of Tantalum Concentration on the Microstructure and Mechanical Properties of Novel W-Ta-Re Alloy

Effect of Tantalum Concentration on the Microstructure and Mechanical Properties of Novel W-Ta-Re Alloy

Crystallization of bronze-like perovskite in potassium tantalum germanate glasses: Glass ceramic preparation and its optical properties

Structural, optical and photoluminescent properties of Europium-doped potassium germanate glassy materials were investigated in terms of tantalum content and nanoscale crystallization of potassium tantalate K2Ta8O21. For such purpose, an Eu3+-doped germanate glass, Eu3+-doped tantalum germanate glass and Eu3+-doped tantalum germanate glass-ceramics were prepared and characterized by X-ray diffraction, Raman, UV–Vis–NIR and photoluminescence spectroscopies. Eu3+ emission properties were used as a sensitive probe to investigate its symmetry site into the host structure. The chemical microenvironment around Eu3+ ions have changed with tantalum concentration as well as bronze-like tantalum perovskite K2Ta8O21 phase crystallization into the glasses. The ions preferably occupy sites of higher symmetry and lower energy phonons, which attest the ions migration to the tantalum-rich crystalline environment. The correlation between the phonon side band (PSB) spectra and Raman vibrational modes, 5D0 → 7F2/5D0 → 7F1 ratio and experimental Eu3+5D0 lifetime support this assumption. The highest photoluminescence intrinsic quantum yield (QEuEu) is calculated for glass-ceramics heat-treated for 20 h by using Judd-Ofelt theory, indicating reduction of nonradiative decay processes due to the lower local phonon energy. The luminescence quenching for longer crystallization times demonstrates the importance of optimizing thermal treatment parameters to ensure proper distance between optically active ions. Accordingly, tantalum germanate glass-ceramics are promising for photonic devices as red laser, solid-state lighting and energy conversion applications, as well as for nonlinear optical applications considering the perovskite bronze-like crystallization.

Synthesis and sintering of tungsten tetraboride and tantalum-bearing tungsten tetraboride under ultra high temperature and high pressure

As a highly controversial candidate superhard material, tungsten tetraboride (WB4) has attracted widespread attention due to its desirable mechanical properties. Here, single phase WB4 ceramics and WB4-based materials containing tantalum (Ta) have been successfully synthesized by a high-pressure reaction at 5.0 GPa and 1400 °C - 3000 °C. We systematically investigated the effect of processing temperature on the phase stability of WB4 and doping with different amounts of Ta on mechanical properties and thermal stability. The results confirm that increasing treatment temperature is beneficial to the synthesis of the WB4 phase, and high pressure can effectively inhibit the decomposition of WB4. Additionally, the hardness value of the WB4-based ceramic is found to decrease with the increase of the doped tantalum content. However, the hardness of WB4 is less than W0.9Ta0.1B4 (10 at. % Ta), indicating that a small amount of Ta doping is positive for the mechanical properties of WB4. The results show that the W0.9Ta0.1B4 ceramic synthesized at a temperature of 2800 °C and 5.0 GPa has excellent overall properties. Its Vickers hardness is 42 GPa (0.49 N), and its oxidation resistance temperature is 542 °C, which is much higher than the hardness and thermal stability of tungsten carbide (WC). This work can provide practical guide for the design of new transition metal boride superhard materials, and point out the visual direction for exploring the next generation of superhard materials.

Twin‐Peak Electrocaloric Effect in Ferroelectric Phase of Diffusive SrBi1.98La0.02Nb2−xTaxO9 Materials

Dual‐peak electrocaloric effect (ECE) exists over a broad range significantly below Curie temperature in co‐mixed SrBi1.98La0.02Nb2–x Ta x O9 (SBLNT) compositions exhibiting diffused phase transition. The indirect measurement method is used to study the influence of defect‐modified potential barrier distributions on the ECE of lead‐free SBLNT ceramics. Variation in isothermal entropy change and adiabatic temperature change with varying tantalum (Ta) content signify effects of morphology and impact of altered potential barriers in the parent lattice without involving structural phase transition. A unique appearance of twin‐peak positive–negative ECE in equally doped SrBi1.98La0.02Nb‐TaO9 matrix is attributed to doping‐induced formation of polar nanoregions and distinct dipole switching mechanisms controlled by evenly distributed Nb–Ta potential barriers in SBLNT lattice. Higher doping concentration reduces porosity and promotes electric polarization, thereby the relative cooling power gradually increases with increasing Ta at Nb sites.

Influence of tantalum concentrations on the magnetic properties of amorphous CoFeTaB thin-films

The structural and magnetic properties of amorphous CoFeTaB thin-films were studied as a function of tantalum concentration. The investigations of the structural properties of amorphous CoFeTaB thin-films were undertaken to confirm layer thickness, interface roughness, and their amorphous structure. Temperature dependent magnetic characterizations were performed in order to extract Curie temperatures and their saturation magnetizations of each structure. Curie temperatures and saturation magnetizations of amorphous CoFeTaB thin-films show similar behaviour, where both quantities decrease with increasing tantalum concentration in thin-film structures. These reductions can be attributed to a result of a transfer of electrons from the metalloid atoms to the unfilled d holes of the 3d transition metals, and diffusion of tantalum atoms towards the ferromagnetic layer during thin-film deposition. Investigation of tuning both Curie temperature and magnetization by the addition of a non-magnetic transition metal is crucial for the development of spintronic applications.

Structure and properties of Ta-Zr alloy obtained by high-speed melt quenching from liquid state

The paper considers the effect of Ta-Zr binary system splat quenching implemented by the method of pendant drop melt extraction. The study was conducted using two mixtures of tantalum and zirconium elementary powders with a content of 60 and 6 % of tantalum respectively. After mixing, the compositions were pressed at 250 MPa in a steel mold on a hydraulic press. Sintering was carried out in a vacuum furnace at 1350 °C and a pressure of 10-3 Pa. Splat quenching was carried out in a vacuum at 2•10-2 Pa using electron-beam heating and a spinning disk absorber. Resulting fiber thickness was 15 to 80 gm. The results of testing splat-quenched Ta—Zr alloy discrete fibers and samples formed as a result of rod stock melting by electron-beam heating (as-cast) were studied and compared. It was found that the structure of splat-quenched fibers of the alloy with a Ta content of 6 wt.% consists of 5—10 gm needle-shaped grains, and the alloy with a Ta content of 60 wt.% has a columnar dendritic structure. A study of tantalum and zirconium distribution across the fiber cross-section showed that cooling rate reduction to less than 105 K/s leads to monotectoid transformation in the alloy with a tantalum content of 60 wt.%. It was found that for an alloy with 6 wt.% Ta the fiber microhardness value is 1.5 times higher in comparison with the same alloy without quenching, and for an alloy with Ta 60 wt.% it is higher by a factor of two.

X-ray contrast study of a medical nickel-free shape memory alloy TiNbTaZr

The study of the X-ray contrast of a nickel-free shape memory alloy TiNbTaZr for medical purposes of various compositions was carried out by theoretical calculation of the linear attenuation coefficient and the evaluation of the linear attenuation coefficient by the flaw detection method due to obtaining a contrast image. From the data obtained, it can be argued that all the obtained Ti-Nb-Ta-Zr materials have a radiopacity, the more the higher the content of niobium and tantalum.

Predicting the composition of primary carbides of the multicomponent system Ni-5Cr-9Co-6Al-8.3W-4Re-4Ta-1Mo-1.5Nb-0.15C

Abstract. Problem. Development of new and optimization of existing casting alloys for the manufacture of blades of gas turbine engines for various purposes is an important scientific and technical problem. Given the sensitivity of the structural components to the concentration of alloying elements, there are difficulties in assessing the expected set of properties of the blades from the optimization of the chemical composition or structural state of alloys. Goal. The aim of this work is to study the specifics of the influence of alloying elements on the distribution of primary carbides in the structure, their topology, morphology and their composition for a multicomponent system such as Ni-5Cr-9Co-6Al-8,3W-4Re-4Ta -1Mo-1 , 5Nb-0.15C using the calculation method of CALPHAD prediction (passive experiment) in comparison with the data obtained by electron microscopy (active experiment). Methodology. Modeling of thermodynamic processes occurring during crystallization (cooling) or heating in the structure of alloys was carried out by the CALPHAD method. Results. The results of thermodynamic calculations of the chemical composition of carbides are presented in comparison with experimental data obtained by electron microscopy on a microscope REM-106I with a system of energy-dispersion X-ray spectral microanalysis. Originality. It is shown that when the total concentration of carbide-forming elements increases, the chemical composition of carbides also becomes more complicated. At a concentration of more than 2% of the mass. But in the alloy, in the carbide of MS, the content of tantalum prevails over the content of niobium, it also leads to a decrease in the concentration of tungsten and molybdenum in the carbide. It was found that when the concentration of niobium is more than 3 wt%. in the alloy, its content in the primary carbide exceeds the content of tantalum and the carbide becomes based on Ta. Practical value. On the basis of an integrated approach, computational and experimental, for multicomponent heat-resistant alloys, new regression models are obtained that allow to adequately predict the chemical composition of carbides by the chemical composition of the alloy. which is confirmed by the obtained experimental data.

Dielectric performance of pyrochlore-type Bi2MgNb2-xTaxO9 ceramics: The effects of tantalum doping

The solid solutions based on the pyrochlore-type system Bi2MgNb2-xTaxO9 were formed in the compositional range х = 0–2.0 (Bi1·6Mg0·8Nb1.6-tTatO7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi2MgNb2-xTaxO9 (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi1·6Mg0·8Ta1·6O7.2). The increase in the Ta5+ concentration led to the decrease in the dielectric permeability from 104 (Bi1·6Mg0·8Nb1·6O7.2) to 20 (Bi1·6Mg0·8Ta1·6O7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.

Refractive Indexes and Spectroscopic Properties to Design Er3+-Doped SiO2-Ta2O5 Films as Multifunctional Planar Waveguide Platforms for Optical Sensors and Amplifiers.

This paper reports on the news about refractive index measurements and spectroscopic features of thin films, which can be applied as optical planar waveguides, focusing on their manufacturing processes, designs, and possible applications as optical amplifiers and sensors. Er3+-doped SiO2–Ta2O5 planar waveguides, with Si/Ta ratios of 90:10, 80:20, 70:30, 60:40, and 50:50, were prepared by a soft sol–gel process. Multilayer films were deposited by the dip-coating technique onto 10 μm SiO2–Si (100) p-type silicon and Si (100) silicon easily and successfully. The mechanisms of the densification process, porosity, and hydroxy group or water molecule occurrence have been accompanied by m-line and vibrational spectroscopy analyses. The thickness and refractive index values were used to understand better the influence of temperature and annealing time on the densification of the bulk films and the reduction of the pore volume as the tantalum oxide concentration increases. The refractive index shows the density of the films, and by the atomic force microscopy (AFM) technique, the films showed low surface roughness, achieving relatively high light confinement within the waveguide structure, and negligible optical loss due to surface scattering. Nanoparticle crystallization of Ta2O5 with size distribution ranging from 2.0 to 15.0 nm embedded in SiO2 was observed with size depending on annealing time and tantalum concentration. Intense and broadband emission positioned at 1550 nm, which is attributed to the 4I13/2 → 4I15/2 transition of Er3+ ions, was observed for all planar waveguides under excitation at 271, 272, and 278 nm. Depending on the porosity degree, the adsorption of H2O molecules occurs, changing the refractive index and contributing to the deactivation of excited states of Er3+ ions, making them an optical platform for use as an optical sensor for different species. Besides, the densified waveguides containing 20 or 30 mol % Ta exhibit high potential for applications as broadband optical amplifiers for wavelength division multiplexing (WDM), optical sensing, or augmented reality.

Role of tantalum concentration, processing temperature, and strain-rate on the mechanical behavior of copper-tantalum alloys

Microstructural instability in traditional nanocrystalline metals limits the understanding of the fundamental effect of grain size on mechanical behavior under extreme environmental conditions such as high temperature and loading rates. In this work, the interplay between Ta concentrations and processing temperature on the resulting microstructure of a powder processed, fully dense Cu-Ta alloy along with their tensile and compressive behavior at different strain rates are investigated to probe the possibility of manipulating or tuning microstructurally dependent parameters to control the flow-stress upturn phenomenon. Consequently, the results reveal that there is a crucial length scale, i.e., small grain size and appropriate cluster spacing, below which such upturn is damped out. The observation of changes in flow stress upturn behavior is consistent with the observed changes in measured high-rate plasticity, which enhances below the critical length scale. Furthermore, tension-compression asymmetry also tends to be suppressed in nanocrystalline Cu-Ta alloys, while it becomes evident as the grain size increases to an ultrafine regime. Overall, this work presents a systematic approach to control or engineer reduced high strain rate flow stress upturn behavior in metallic alloys, for high-rate applications.

ВЛИЯНИЕ ПАРАМЕТРОВ ГОРЯЧЕГО ИЗОСТАТИЧЕСКОГО ПРЕССОВАНИЯ НА СТРУКТУРУ И СВОЙСТВА СПЛАВОВ НА ОСНОВЕ КОБАЛЬТА, ПОЛУЧАЕМЫХ МЕТОДОМ СЕЛЕКТИВНОГО ЛАЗЕРНОГО СПЛАВЛЕНИЯ

The paper examines the microstructure and mechanical properties of experimental compositions of a heat-resistant cobalt superalloy, produced by the method of selective laser melting (SLM). Strengthening in the studied materials is achieved by the formation of a fine-celled structure of a solid solution based on cobalt and highly dispersed intermetallic and carbide phases. By the EMPA (electron micro probe analysis) method, the ratio of chemical elements in these phases was evaluated and their type was determined. It was estimated that the tantalum content determines the strength properties of the alloy. It was found that dispersed particles formed during the synthesis have a positive effect on the creep resistance at 800 °C temperature, whereas at 1100 °C temperature the destruction occurs along the boundaries of the persisted structure of synthesis, regardless of the alloying and morphology of structural components.

Effect of Tantalum Content on Microstructure and Mechanical Properties of Cocrnitax Medium Entropy Alloys

Effect of Tantalum Content on Microstructure and Mechanical Properties of Cocrnitax Medium Entropy Alloys

Effect of tantalum solid solution additions on the mechanical behavior of ZrB2

Mechanical properties and microstructure were compared for zirconium diboride and two zirconium diboride solid solutions containing 3 and 6 at% tantalum diboride. X-ray diffraction indicated that the ceramics were nearly phase-pure and that tantalum dissolved into the ZrB2 lattice to form (Zr,Ta)B2 solid solutions. Microstructural analysis indicated that samples achieved nearly full relative density with average grain sizes that ranged from 3−5 μm. The three compositions had similar values of Young’s modulus (510−531 GPa), shear modulus (225−228 GPa), Vickers hardness (15.2–16.4 GPa), and flexural strength (391−452 MPa). Fracture toughness ranged from 2.6 to 3.7 MPa m1/2 and with increasing tantalum content, the fracture mode changed from predominantly intergranular to predominantly transgranular. Diboride solid solution materials had comparable properties to the single metal diboride, but differences in microstructure, secondary phases, and strain state among the three ceramics partially obscured the actual effects of the solid solution on fracture behavior.

The Yichun Ta-Sn-Li Deposit, South China: Evidence for Extreme Chemical Fractionation in F-Li-P-Rich Magma

Abstract The Yichun Ta-Sn-Li deposit occurs within the 9.5-km2 Yashan Igneous Complex, which is made up of felsic, peraluminous intrusions including two-mica, Li-mica, and topaz-lepidolite granites. The topaz-lepidolite granite forms a thin sheet confined to the upper part of the Li-mica granite and is separated from the host metasediments by a marginal pegmatite. The topaz-lepidolite granite is characterized by the presence of snowball textures in which quartz, K-feldspar, and topaz phenocrysts contain albite laths and rare lepidolite and columbite arranged along growth zones. These textures indicate simultaneous crystallization of these phases and attest to the subsolvus character of the topaz-lepidolite granite. Minor and accessory phases in the topaz-lepidolite granite include amblygonite, cassiterite, columbite-tantalite, microlite, wodginite, zircon, monazite, and pollucite. The topaz-lepidolite granite is characterized by low SiO2 (68.6–69.7 wt %), TiO2 (0.01 wt %), Fe2O3(total) (0.15–0.33 wt %), MgO (0.01–0.05 wt %), and CaO (0.15–0.19 wt %), whereas Al2O3 (17.97–18.26 wt %), Li2O (0.90–2.09 wt %), P2O5 (0.43–0.54 wt %), and F (1.09–2.30 wt %) are all very high. Na2O ≫ K2O due to both high Na2O (5.18–6.30 wt %) and low K2O (2.63–3.05 wt %). The topaz-lepidolite granite contains extremely high concentrations of Rb (&amp;gt;3,300 ppm), Cs (&amp;gt;340 ppm), Sn (&amp;gt;140 ppm), and Ta (&amp;gt;120 ppm), with very low Zr/Hf and Nb/Ta. Gaps of &amp;gt;3.5 wt % SiO2 and &amp;gt;2.5 wt % Al2O3 together with large increases in F, Li2O, and P2O5 between the Li-mica and topaz-lepidolite granites are not compatible with an evolution by Rayleigh fractionation processes. It is proposed instead that early crystallization of the Li-mica granite led to the formation of an Si-poor, Al-, Na-, and flux-rich boundary layer whose low viscosity and density allowed effective separation from the remaining melt and crystals. This melt concentrated incompatible elements including tin and tantalum and accumulated in the upper part of the magma chamber where it crystallized the sheet-like bodies of marginal pegmatite and topaz-lepidolite granite. The uppermost part of the topaz-lepidolite granite consists of aplite and overlying quartz lepidolite rock, which is composed mainly of quartz, lepidolite, albite, K-feldspar, topaz, amblygonite, cassiterite, and columbite. Compared to the topaz-lepidolite granite the aplite and quartz-lepidolite rock shows a number of complementary element enrichments and depletions. The quartz-lepidolite rock is enriched in Fe, Mn, K, Rb, Cs, Li, F, Ba, Sr, Ti, Zr, Nb, W, Th, and Sm. The formation of the quartz-lepidolite rock is interpreted to result from separation of an F- and Li-rich alkaline phase, which accumulated beneath the already crystallized marginal pegmatite. Complementary to this was crystallization of the strongly peraluminous sodic aplite with the highest tantalum contents measured in this study.

Influence of Ta on the oxidation resistance of WB2−z coatings

Ternary W1−xTaxB2−z is a promising protective coating material possessing enhanced ductile character and phase stability compared to closely related binaries. Here, the oxidation resistance of W1−xTaxB2−z thin films was experimentally investigated at temperatures up to 700 °C. Ta alloying in sputter deposited WB2−z coatings led to decelerated oxide scale growth and a changed growth mode from paralinear to a more linear (but retarded) behavior with increasing Ta content. The corresponding rate constants decrease from kp* = 6.3 ⋅ 10−4 µm2/s for WB2−z, to kp* = 1.1 ⋅ 10−4 µm2/s for W0.66Ta0.34B2−z as well as kl = 2.6 ⋅ 10−5 µm/s for TaB2−z, underlined by decreasing scale thicknesses ranging from 1170 nm (WB2−z), over 610 nm (W0.66Ta0.34B2−z) to 320 nm (TaB2−z) after 10 min at 700 °C. Dense and adherent scales exhibit an increased tantalum content (columnar oxides), which suppresses the volatile character of tungsten-rich as well as boron oxides, hence being a key-factor for enhanced oxidation resistance. Thus, adding Ta (in the range of x = 0.2–0.3) to α-structured WB2−z does not only positively influence the ductile character and thermal stability but also drastically increases the oxidation resistance.

The niobium and tantalum concentration in the mantle constrains the composition of Earth’s primordial magma ocean

The bulk silicate Earth (BSE), and all its sampleable reservoirs, have a subchondritic niobium-to-tantalum ratio (Nb/Ta). Because both elements are refractory, and Nb/Ta is fairly constant across chondrite groups, this can only be explained by a preferential sequestration of Nb relative to Ta in a hidden (unsampled) reservoir. Experiments have shown that Nb becomes more siderophile than Ta under very reducing conditions, leading the way for the accepted hypothesis that Earth's core could have stripped sufficient amounts of Nb during its formation to account for the subchondritic signature of the BSE. Consequently, this suggestion has been used as an argument that Earth accreted and differentiated, for most of its history, under very reducing conditions. Here, we present a series of metal-silicate partitioning experiments of Nb and Ta in a laser-heated diamond anvil cell, at pressure and temperature conditions directly comparable to those of core formation; we find that Nb is more siderophile than Ta under any conditions relevant to a deep magma ocean, confirming that BSE's missing Nb is in the core. However, multistage core formation modeling only allows for moderately reducing or oxidizing accretionary conditions, ruling out the need for very reducing conditions, which lead to an overdepletion of Nb from the mantle (and a low Nb/Ta ratio) that is incompatible with geochemical observations. Earth's primordial magma ocean cannot have contained less than 2% or more than 18% FeO since the onset of core formation.