Increasing Nb Content Research Articles

Influence of Nb concentration on the structure, stability, and electronic and mechanical properties of D022 Al3Ti by first-principles calculations and experiments

The crystal structure, stability, and electronic and mechanical properties of D022 Al3Ti-Al3Nb pseudobinary solid solution were systematically investigated by a first-principles method and experiment. The calculated formation energies demonstrated that all the Al3(Ti1-xNbx) (0 ≤ x ≤ 1) solid solutions considered are thermodynamically stable and that the stability decreases monotonously with increasing Nb concentration. Lattice parameter a of Al3(Ti1-xNbx) exhibits strong negative deviation from a linear composition dependence, while the trend is reversed for lattice parameter c. Study of the mechanical properties (including the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature) reveals that Nb atom substitution in Al3Ti enhances the mechanical stability and strength. The analysis of calculated electronic structures reveals that the metallic behavior of Al3(Ti1-xNbx) depends on the Nb concentration. Experimentally, Al3(Ti1-xNbx) was synthesized and was characterized by means of X-ray diffraction and scanning electron microscopy. Rietveld refinement results confirmed that the lattice parameters are in the range of our previous calculated values. The differences between the calculated and experimental lattice parameters are also discussed in detail.

Effect of niobium addition in FeCoNiCuNbx high-entropy alloys

Abstract

Magnetoresistance and thermoelectric transport in EuTi1-xNbxO3

We report the impact of Nb substitution in EuTi1-xNbxO3 (x = 0, 0.01, 0.03, 0.06 and 0.1) on the magnetization, resistivity, magnetoresistance (MR), thermopower (S) and thermal conductivity (κ) over a wide temperature range (T = 390–2.5 K). Partial replacement of Ti by Nb weakens the antiferromagnetic coupling between Eu-4f spins, drastically reduces low-temperature resistivity and induces insulator-metal transition above the Neel temperature. MR is negative and decreases from 20% for x = 0.01–0.5% for x = 0.1 at 2.5 K but the sign of MR changes to positive at higher temperatures for x = 0.06 and 0.1. The negative MR is suggested due to spin-dependent scattering of Ti-3d conduction electrons by localized Eu-4f spins and we fitted the experimental data to a theoretical model. The negative sign of S asserts electron doping. The values of S and κ systematically decrease with increasing Nb content. κ(T) is analyzed using the Debye-Callaway model.

Effects of Nb on Superelasticity and Low Modulus Properties of Metastable β-Type Ti-Nb-Ta-Zr Biomedical Alloys

In this work, a series of Ti-xNb-2Ta-3Zr (x = 25, 30, 35, 40 wt.%) alloys are designed, and the transformation of β-phase to α″ martensitic, β-phase stability, microstructure, mechanical properties and corrosion performance of these alloys are investigated. The phase analysis shows as the Nb content increases, the α″ phase in these alloys decreases, while the intensity of the two main peaks β(110) and β(211) of Ti-40Nb-2Ta-3Zr alloy is reduced. These results can be attributed to the variation of β-phase stability caused by Nb element of alloy and cold rolling process. The mechanical properties test shows that the elastic modulus (52 GPa) of the Ti-35Nb-2Ta-3Zr alloy is the lowest and the elongation (18.8%) is the maximum. In addition, the alloy is susceptible to β-phase elastic deformation and stress-induced martensitic transformation resulting in the highest recovery strain of the alloy (66.87%). The polarization curves show that the Ti-35Nb-2Ta-3Zr alloy has the highest corrosion potential (− 0.34 V) and the lowest corrosion current density (0.21 μA cm−2) exhibiting the best corrosion resistance.

Microstructure, phase composition and mechanical properties of new, low cost Ti-Mn-Nb alloys for biomedical applications

A group of new Ti-7Mn-xNb alloys with Nb contents varying from 0 to 10 wt% were fabricated by conventional press and sinter powder metallurgy processing from blended elemental powders. The effect of Nb content on the sintering and phase stability as well as microstructure and mechanical characteristics of the alloys was investigated. Microstructural studies and phase analysis showed a two-phase microstructure composed of α and β phases in all alloys. However, addition of Nb up to 10 wt% enhanced the stability of the β phase hindering the transformation to α during furnace cooling and increasing the proportion of retained β. Sintering and densification of the Ti-7Mn-xNb alloys was enhanced by the fast diffusing Mn and negatively impacted by the slow diffusing Nb due to the reliance of solid sintering processes on diffusion. Nonetheless, the relative densities of the Ti-7Mn-xNb alloys with up to 10 wt% Nb were higher than that of the sintered CP-Ti. The compressive strength and hardness of the alloys varied in the range of (1842–2127 MPa) and (341–375 Hv) respectively. It was also observed that with increasing Nb contents, the elastic modulus decreased, while compressive strain increased due to stabilisation of a greater proportion of the β phase. The results showed that the low-cost Ti-7Mn-xNb alloys possess superior properties to those of CP-Ti and a number of other Ti based alloys developed for biomedical implant applications.

Effect of niobium doping on the dielectric and nonlinear current–voltage characteristics of [formula omitted] ceramics

The effect of Nb5+ on the crystallographic structure, electrical properties and the dielectric behaviour of Na0.5La0.5Cu3Ti(4−x)NbxO12 ceramics prepared via the solid-state reaction method were investigated. Rietveld refinement of the powder diffraction data confirmed a cubic, single phase structure with space group Im3. A low frequency Debye-type relaxation is observed for the Nb-doped samples in addition to the usually observed high frequency relaxation. The grain boundary resistance decreased significantly with increasing Nb5+ dopant. The reduced grain boundary resistivity was found to be responsible for the decrease in the potential barrier at the grain boundary as observed from the overall variation of current density-electric field (J–E) characteristics. The dielectric constant at low Frequency is increased with increasing Nb content. The dielectric behaviour of the compound is explained on the basis of the Internal Barrier Layer Capacitance (IBLC) model.

An application of differential injection to fabricate functionally graded Ti-Nb alloys using DED-CLAD® process

The aim of this paper is twofold: firstly, to compare the microstructural and the mechanical properties of Ti-44Nb samples manufactured pre-alloyed powder and differential injection, and secondly, to demonstrate the feasibility of the differential injection method included in the DED-CLAD® process, to manufacture functionally graded Ti-Nb alloys. Functionally graded materials (FGM) are promising new materials which are perfectly adapted to custom-made parts with various properties for specific applications. In FGMs, titanium and niobium ratios were modified in different steps to create the variation in alloy composition owing to a double-powder feeder. Mechanical analysis and SEM observations show the variation along the deposition depending on the chemical composition. Chemical analysis revealed the homogeneous repartition of the powder mixture as well as the nominal composition of each deposition. Mechanical tests showed a decrease of the microhardness with the increase of Nb content. The elastic modulus was found to be the lowest for Ti-40Nb.

Effect of alloying elements on the precipitation of δ phase in Ni-Nb-Cr-Mo alloys

Effect of alloying elements on precipitation of δ phase in Ni-Nb-Cr-Mo alloys has been studied using thermodynamic method. It is found that an increase of Al/Ti ratio or (Al+Ti) content will inhibit the precipitation of δ phase obviously. When the Al/Ti ratio (exceed 1) or (Al+Ti) content increases, Nb content in δ phase increases, while the Mo content decreases. The mole fraction of δ phase increases obviously with increase of Nb content, however, it is nearly not affected by B content. Artificial neural network (ANN) method is tentatively applied to predict the multielement additions on mechanical properties in these alloys. A suitable composition of Al, Ti and B is suggested which obviously promotes stress-rupture life. The theoretical simulation results are finally verified through experiments.

Synthesis and Characterization of AlCoCrFeNiNbx High-Entropy Alloy Coatings by Laser Cladding

AlCoCrFeNiNbx (x in molar ratio x = 0, 0.25, 0.5, 0.75, and 1.0) high-entropy alloy (HEA) coatings were manufactured on 304 stainless steel by laser cladding. The constituent phases, microstructures, chemical composition, micro-hardness and wear resistance of the HEA coatings were investigated respectively by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), a Vickers hardness tester and a friction/wear testing machine. It was found that an AlCoCrFeNi alloy coating without Nb consisted of body-centered-cubic (BCC) and order BCC (B2) phases, while the AlCoCrFeNiNbx (x > 0) alloy coatings consisted of BCC, B2 and Laves phases. Microstructures of the AlCoCrFeNiNbx alloy coatings evolved from equiaxed grain (x = 0) to hypoeutectic (0.25 ≤ x < 0.75), then to full eutectic (x = 0.75), and finally to hypereutectic (x > 0.75). With increasing Nb content, the Vickers hardness values increased. AlCoCrFeNiNb0.75 alloy coating with a fully eutectic microstructure demonstrated the best wear resistance among the AlCoCrFeNiNbx (x ≥ 0) alloy coatings.

CoCrFeMnNi high-entropy alloys reinforced with Laves phase by adding Nb and Ti elements

Abstract

Effect of Nb-doping on the structural and electrical properties of Ba0.97La0.02Ti1-xNb4x/5O3 ceramics at room temperature synthesized by molten-salt method

We, herein, report on the structural and electrical properties of Ba0,97La0.02Ti1-xNb4x/5O3 ceramics prepared by the molten-salt method. The XRD patterns of the samples at room temperature showed a single perovskite phase having a tetragonal structure with a P4/mmm space group. The electrical properties were studied by complex impedance spectroscopy in the frequency range (1–107Hz) at room temperature. The real and imaginary parts of impedance decreased with increasing Nb concentration. The complex impedance plot exhibited a single impedance semicircle for each compound, identified over the high and medium frequencies followed by a linear section, at low frequency, is explained by Warburg's element. The study of the normalization curves of impedance and electrical modulus showed a mismatch of both peaks, which suggests the presence of short-range motion of charge carriers. The electrical conductivity decreased with increasing niobium amount and the experimental data were fitted using Jonscher's universal power law. The exponent n is slightly superior to 1, indicating that the hopping occurs between neighboring sites.

Electronic Properties of SbTa1-xNbxO4: Phase-Related Distortions

We have studied the effects of structural symmetry related distortions on the band gaps and overall energetics of SbMO4 compounds (M = Ta, Nb). Evolution of the electronic structures was studied as one transitions gradually from a system of 4d bands to 5d bands, and whether this has any effect on the Sb oxidation state or its lone pair s electrons. From the atomic point of view, electronic correlation energy is higher in Ta 5d than in Nb 4d, and Nb 4d bands are higher in energy than Ta 5d. These were examined in terms of oxidation states in their respective solids and mixed alloys. We look at the effects of increasing Nb concentration on the electronic structures of SbTa1-xNbxO4 alloys with emphasis on polyhedral interactions and overall cell distortions. We decouple the effects caused by the presence of different nd cations from those caused by distortions and reorientation of various polyhedra.

Hydrodeoxygenation of phenol over Ni/Ce1-xNbxO2 catalysts

This work investigated the effect of different niobium concentration on the performance of Ni/Ce1-xNbxO2 catalyst for hydrodeoxygenation of phenol in gas phase at 300 °C. Also, the structural modification promoted by Nb addition was studied by several techniques. In situ X-ray diffraction and Raman analyses indicated that high niobium concentration lead to Nb2O5 oxide with hexagonal structure, while in situ X-ray photoelectron spectroscopy (XPS) experiments indicated an enrichment of the surface with Nb. In general, the incorporation of niobium changed the lattice parameter of ceria and promoted the formation of oxygen vacancies as observed by XPS and X-ray absorption near edge structure (XANES) analyses, which favored the activity of the sample. Increasing Nb content increased the selectivity to deoxygenated products (benzene). The superior benzene selectivity is likely due to the higher interaction between the oxygen of phenol molecule and the oxophilic sites represented by the Nb5+ cations of the support that promotes the hydrogenation of carbonyl group.

Nanotubes from the Misfit Compound Alloy LaS-NbxTa(1–x)S2

Misfit layered compounds (MLC) with the composition (LaS)1.15TaS2 (for simplicity denoted as LaS-TaS2) and LaS-NbS2 were prepared and studied in the past. Nanotubes of LaS-TaS2 could be easily synthesized, while tubular structure of the LaS-NbS2 were found to be rather rare in the product. To understand this riddle, quaternary alloys of LaS-NbxTa(1–x)S2 with ascending Nb concentration were prepared herein in the form of nanotubes (and platelets). Not surprisingly, the concentration of these quaternary nanotubes shrank (and the relative density of platelets increased) with increasing Nb content in the precursor. The structure and chemical composition of such nanotubes was elucidated by electron microscopy. Conceivably, the TaS2 in the MLC compounds LnS-TaS2 (Ln = lanthanide atom) crystallizes in the 2H polytype. High resolution transmission electron microscopy showed however that, invariably, MLC nanotubes prepared from 80 at% Nb content in the precursor belonged to the 1T polytype. Raman spectroscopy of i...

Effect of niobium on microstructure and precipitation in As-annealed Sanicro 25 steel

ABSTRACT The chemical compositions of Sanicro 25 stainless steel added with different concentrations of niobium (Nb) (0, 0.29, 0.58, and 0.86 wt-%) were investigated. X-ray diffraction analysis showed that the matrix of all the prepared materials was austenite. The average grain size decreased from 426 to 90 µm with the increase in Nb content. Electron backscatter diffraction and energy dispersive X-ray spectroscopy analyses showed that Cr23C6 was the main precipitate in the Nb-free steel, whereas Nb(C, N) were the main precipitates in Nb-containing steel. The increase in Nb content also increased the volume fraction and average particle size of the precipitates.

Composition Optimization of Nb-Ti Microalloyed High Strength Steel

Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microalloyed steel were investigated. The results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect. Hence, the yield strength increased. Besides, the increased strength by the combined increase of Nb and Ti was similar to that observed for the increase in Ti content alone. This increase in strength was attributed widely to the increase in the Ti content alone rather than Nb. Moreover, the increase in Nb content beyond 0.036 wt% exerted no significant effect on the strength of Ti-Nb microalloyed steels, in which more Ti could be added to further improve the strength of steels.

Effect of alloying elements on the microstructure, coefficient of friction, in-vitro corrosion and antibacterial nature of selected Ti-Nb alloys

Ti-Nb alloys having compositions Ti-24Nb-4Zr-8Sn, Ti-29Nb-13Ta-7Zr and Ti-35Nb-4Sn were synthesized by vacuum arc melting followed by ageing at 1273 K for 6 h in Ar atmosphere. The alloys were characterized by field emmision scanning electron microscopy (FESEM), transmission electron microscopy (TEM), micro X-ray diffraction, friction coefficient and nanohardness tests. In-vitro corrosion of the alloys were studied by potentiodynamic polarisation in body simulated fluid at 37 ○C. The alloy surfaces were exposed to staphylococcus aureus bacteria under controlled environment and the retention of the bacteria was examined by confocal laser scanning microscopy. The alloys had different ratios of α, β phases, different nature of inclusion, and dislocation pile-up resulting in high hardness. Alloys also had good resistance to in-vitro corrosion, low coefficient of friction and were resistant to bacterial growth as compared to commercially pure (CP) Ti. The coefficient of friction was lowest for Ti-24Nb-4Zr-8Sn and it increased with increasing Nb content. The resistance to bacterial growth on the surface increased with increasing volume fraction of β phase in the alloy.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts.

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H2 O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Microstructure and mechanical properties of FeCoCrNiNbX high-entropy alloy coatings

The microstructure and mechanical properties of high-entropy alloy (HEA) coatings by plasma cladding on Ti substrate are studied by the analysis of a series of FeCoCrNiNbX (X = 0.25, 0.6, 0.8) HEA coatings, to reveal the additional elemental Nb effects. The results show that the HEA coatings exhibit typical dendritic structure, and consist of BCC and FCC structures. The wear resistances of FeCoCrNiNbX HEA coatings increase firstly and then decrease with the increase of Nb content, agreeing with the hardness of coatings. Compared with the other HEA coatings from the previous work, the friction coefficients of the FeCoCrNiNbX HEA coatings are relatively lower and more stable, indicating the potential applications, such as for wear protection films of gear. The FeCoCrNiNb0.6 HEA coating shows high hardness and good wear resistance due to the reasonable volume fraction between BCC and FCC phases. The volume fraction of BCC and FCC structures in HEA coating are predicted not only using the valence electron concentration (VEC), but also by calculating parameters of ΔHmix and δ depending on the mixing enthalpy and the atomic size difference.

Effect of Nb Solute Concentration on Crystallite Size Refinement and Strength Enhancement in Mechanically Alloyed Cu-Nb Alloys

Cu-Nb alloys with a Nb concentration range of 0 wt%∼30 wt% were prepared by mechanical alloying (MA) at room temperature. The effects of Nb content on the crystalline refinement process and mechanical properties of the immiscible Cu-Nb system were investigated by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive X-ray detection (EDX), transmission electron microscopy (TEM) and microhardness measurement. Results show that the completed dissolution of Nb in Cu can be achieved in the samples with Nb content less than ∼11 wt% after 100 h milling, although the equilibrium solubility level is nearly zero. The grain size refinement capability of MA-ed Cu-Nb powder is enhanced with increasing Nb content up to 30 wt%. This is because the susceptibility to recovery process becomes reduced, when the amount of Nb solutes segregated into the dislocation of Cu phase is increased. Cu-30wt%Nb powder milled for 100 h, the average Cu grain size is only ∼6 nm. The microhardness of the samples shows an enhancement with increasing Nb concentration. The main strengthening mechanisms of the MA-ed Cu-Nb alloys are linked to the grain size reduction and the dissolution of Nb into Cu matrix.