Spherical Nb Research Articles

Chemical Vapor Synthesis of Ultrafine Niobium Powder Via Sodiothermic Reduction of Chloride

Chemical vapor synthesis was used to prepare Nb nanopowders with NbCl5 vapor as a raw material and Na vapor as a reductant. Thermodynamic analysis indicated that Na had a strong ability to reduce NbCl5. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive spectrometry (EDS), and transmission electron microscopy (TEM) were used to analyze and characterize the product powders. These powders contained two phases: Nb and NaCl. Pure-phase Nb powders were synthesized by vacuum evaporation. The spherical Nb particles had an average size of 30 to 120 nm. In addition, the effects of operating conditions, such as the evaporation temperature, the reaction temperature, and the argon flow rate, on the Nb particle size were investigated.

Microstructural Evolution, Precipitation and Mechanical Properties of 27Cr-4Mo-2Ni Super-Ferritic Stainless Steels

Good mechanical properties and excellent pitting corrosion resistance for 0.8 mm-thick 27Cr-4Mo-2Ni super-ferritic stainless steels produced by one-stage cold rolling or two-stage cold rolling together with intermediate annealing processes are achieved. The microstructural evolution, precipitation and their effects on mechanical properties and corrosion resistance are investigated in terms of optical microscopy, scanning electron microscopy, electron backscattered diffraction pattern and transmission electron microscopy. The results demonstrated that the as-received hot-rolled plates consist of single ferrite grains characterized by α-fiber and γ-fiber orientations. A few Laves phases close to Nb(C, N) are formed in the recrystallized sheets solution-treated at 1050°C. After cold-rolling and finally annealing, fine recrystallized grains characterized by weaken γ-fiber orientation, are accomplished. The formation of Laves phases near the spherical Nb(C, N) makes large Nb(C, N) particles change into small granules. Corrosion resistance is more sensitive to Laves phases than mechanical properties. Small grain size improves strength and ductility, while it has a negative influence on resistance to pitting corrosion. Finer grains and a few more Laves phases are gained in steels processed by a one-stage cold-rolling process. The percentage elongation, yield strength (0.2% proof stress), ultimate tensile strength and average corrosion rate of final sheets produced by a one-stage cold-rolling process are 27.3%, 520 MPa, 641 MPa and 0.033 mm/a, respectively, and the values for two-stage cold-rolling process are 24.4%, 494 MPa, 610 MPa and 0.022 mm/a, respectively.

Effect of Annealing Conditions on the Dissolution Process of Complex Carbonitrides in a V-Nb-Ti Microalloyed Steel

The present work has been undertaken to assess the evolution of dissolution process of large dendritic precipitates in a V-Nb-Ti microalloyed steel. The study was performed by reheating the samples at 1250°C, simulating the industrial reheating practices at laboratory scale and in situ, following industrial profile; afterwards the samples were quenched in 10%NaCl aqueous solution. The characterization of the material was carried out by scanning electron microscopy accompanied with dispersive energy spectrometry, and chemical analysis by inductively coupled plasma optical emission spectrometry. The results showed a partial dissolution of dendritic precipitates. This process ocurred by a progressive dissolving the Nb-rich shells formed over cuboidal particles and primary arm of well-faceted dendritic precipitates, and by fragmentation and spheroidization of secondary branches. These processes gave rise to spherical Nb rich precipitates and cuboidal particles at the reheating conditions used in this study. Both type of particles contain vanadium.

Energy Density Dependence of Bonding Characteristics of Selective Laser-Melted Nb–Si-Based Alloy on Titanium Substrate

Spherical Nb–20Si–24Ti–2Cr–2Al pre-alloyed powders were processed by selective laser melting (SLM) on Ti6Al4V substrates with different energy densities. A series of single tracks and single layers were produced using different processing parameters, including powder size, laser power, scanning speed and hatch distance. Results showed that the pre-alloyed powders ranging from 45 to 75 μm were more applicable to SLM with less balling tendency, in comparison with those between 75 and 180 μm. The increase in linear energy density (LED) resulted in the decrease in contact angle and the increase in the width of single track as well as its penetration depth into the substrate. Smaller hatch distance leaded to a larger remelted part of the former track and a higher volumetric laser energy density. With a thickness of 75.6 μm, an interfacial intermediate layer, enriched in Ti and depleted in Nb, Si, Cr and Al, was formed between the SLM part and the Ti6Al4V substrate. The mechanisms of the elimination of balling phenomenon by employing a higher LED and the interfacial bonding characteristics between Nb–Si-based alloys via SLM and the Ti6Al4V substrate were discussed.

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10 nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.

Synthesis and characterization of NbCr2 Laves phase produced by spark plasma sintering

Abstract

Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen Storage Properties of Mg-Based Nanocomposite

With the intention of improving the hydrogenation/dehydrogenation kinetics of Mg, Mg-7.5 wt % Nb nanocomposite has been prepared by hydrogen plasma-metal reaction approach. The spherical Nb nanoparticles (NPs) of 12 nm are uniformly decorated on the surface of Mg NPs. The Mg–Nb nanocomposite can quickly uptake 4.0 wt % H2 in 10 min and reach a saturation value of 5.7 wt % H2 in 60 min at 473 K. Furthermore, it can also release 4.0 wt % H2 in 60 min at 573 K. The reversible hydrogen storage capacity is as high as 7.0 wt % at 673 K. Nb NPs transform into NbH during hydrogenation and recover after dehydrogenation process. They restrain the growth of Mg and work as catalysts to accelerate the hydrogen transportation in the Mg-based nanocomposite by decreasing the activation energies of hydrogenation/dehydrogenation to 70.9 and 86.4 kJmol–1, respectively. The catalytic mechanism of Nb NPs is explained in terms of spillover, d-electrons and electronegativity effects. The nanosizing effects of both Mg and Nb and...

Precipitate strengthening in nanostructured metallic material composites

Nanostructured metallic material (NMM) composites are a new class of materials that exhibit high structural stability, mechanical strength, high ductility, toughness and resistance to fracture and fatigue; these properties suggest that these materials can play a leading role in the future micromechanical devices. However, before those materials are put into service in any significant applications, many important fundamental issues remain to be understood. Among them, is the question of the strengthening of NMM using second phase particles and if the addition of precipitates will strengthen the structures in the same manner as in bulk crystalline solids. This issue is addressed in this work by performing molecular dynamics simulations on NMM with precipitates of various sizes and comparing the results with the same structure without precipitates. In this view, Cu/Nb bilayer thin films with spherical Nb particles inside the Cu layer were examined using molecular dynamics simulations and show a significant improvement on their mechanical behavior, compared to similar structures without particles. Furthermore, an analytical model is developed that explains the strengthening behavior of an NMM that has precipitates inside one layer. The theoretical results show a qualitative agreement with the finding of the atomistic simulations.

A Novel Process for Making Spherical Powders of High Nb Containing TiAl Alloys

A novel process was developed to make micro-fine spherical high Nb containing TiAl alloyed powders in quantity. Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y(at%) ingot prepared by vacuum induction melting was heat-treated for homogenization, and then machined with a crusher. The chippings were refined by fluidized bed jet milling, and subsequently spheroidized with radio frequency (RF) argon plasma. The effects of processing parameters on powder characteristics were studied. By fluidized bed jet milling, the particle size and distribution uniformity both decline with the rise of classifier frequency. Above 5800rpm, the number-average diameter is lower than 20μm with a bimodal particle size distribution. The powders consist of coarse plate-shaped particles and some finer flaky debris, and with the further rise of the frequency, the amount of debris gradually increases. The plasma-spheroidized powders are found to possess good sphericity and composition homogeneity with phase dominated by supersaturated α2-Ti3Al. The granulometric characteristics of the spherical powders strongly depend on those of the feed powders. For the jet milled powders below 5800rpm, due to the relatively lower size distribution uniformity, the fine particle fraction reduces after spheroidization, which causes an obvious improvement of distribution uniformity and the corresponding increase of number-average diameter

The Effect of Cold-Drawing on Recovery Stress of Ni-Ti-Nb Shape Memory Alloy

In this paper, self-made tension machine was used to explore the evolution of recovery stress in cold drawing for NiTiNb alloy. The microstructure evolution in different cold drawing conditions was observed. The results show that due to the pre-strain temperature (213K) higher than Ms, the peak values of recovery stress are relatively poor (lower than 490MPa) after cold drawing. After hot drawing, the original spherical Nb particles were changed to thin piece of nano lines and the contribution of Nb nano lines (BCC structure) in NiTi matrix (B2) was greatly improved.

Transformation Behavior of Precipitates in a W-alloyed 10 wt pct Cr Steel for Ultra-supercritical Power Plants

Thermodynamic calculation, thermal analysis, and identification and observation of precipitates have been carried out on a W-alloyed 10 wt pct Cr steel by means of ThermoCalc program, differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) with an energy dispersive X-ray spectrometer, respectively. Several critical phase transformation points were determined by combining experimental results with calculations. Two individually stable phases of Nb(C, N) and VN, instead of one single phase MX (X: C, N), M(23)C(6) or Laves phases, were predicted in the calculated equilibrium phase diagram. An unstable elongated M(7)C(3) with relatively higher Cr was detected unexpectedly in the normalized and tempered steel. Two kinds of spherical Nb(C, N) with different size were recognized as the primary and the secondary precipitates of Nb(C, N) which contain different V contents. It was observed that one kind of complex precipitate in a special V-wing shape was resulted from two plate-like VN adhering to coarse primary spherical Nb(C, N).

Precipitation behaviors of X70 acicular ferrite pipeline steel

The morphology, structure, and chemical composition of precipitates in the final microstructure of Nb-V-Ti microalloyed X70 acicular ferrite pipeline steel were investigated using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). Precipitates observed by TEM can be classified into two groups. The large precipitates are complex compounds that comprise square-shaped TiN precipitate as core with fine Nb-containing precipitate nucleated on pre-existing TiN precipitate as caps on one or more faces at high temperature. In contrast, the fine and spherical Nb carbides and/or carbonitrides precipitate heterogeneously on dislocations and sub-boundaries at low temperature. From the analysis in terms of thermodynamics, EDS and chemical composition of the steel, NbC precipitation is considered to be the predominant precipitation behavior in the tested steel under the processing conditions of this research.

Anisotropic Microstructure and Hydrogen Permeability of Nb-Ti-Ni Duplex Phase Alloy Prepared by Hot Forging-Rolling Technique

AbstractIt has been demonstrated that the as-cast Nb40Ti30Ni30 duplex phase alloy, which consists of the primary (Nb, Ti) solid solution and the fine lamellar type eutectic {TiNi+(Nb, Ti)} phase, shows higher hydrogen permeability ¶ than that of pure Pd without the hydrogen embrittlement at 673K. In this alloy, the eutectic phase contributes to the suppression of the hydrogen embrittlement, while the primary one does mainly to the hydrogen permeation. It is important to note that even if the eutectic microstructure disappears and is replaced by the small spherical (Nb, Ti) phase embedded in the TiNi matrix by rolling and subsequent annealing, its high hydrogen permeability and large resistance to the hydrogen embrittlement are sustained. Furthermore, the present authors have observed that the primary (Nb, Ti) phase is largely elongated along the rolling direction. Consequently, it is expected that rolled and annealed Nb-Ti-Ni alloys show the large anisotropy of the microstructure and the hydrogen permeability. In the present work, the microstructure and hydrogen permeability ¶nof the Nb40Ti30Ni30 alloy after rolling and annealing treatments are examined in order to develop highly hydrogen permeable alloys utilizing the anisotropic microstructure.The primary (Nb, Ti) phase and the very fine eutectic {TiNi+(Nb, Ti)} phase are observed in the as-cast Nb40Ti30Ni30 alloy by scanning electron microscopy (SEM). The X-ray diffractometry (XRD) indicates that this alloy consists of the B2-TiNi intermetallic compounds and the bcc- (Nb, Ti) solid solution. The primary (Nb, Ti) phase is largely elongated along the rolling direction by forging and subsequent rolling at 1173 K. On the other hand, the eutectic microstructure becomes unclear with increasing the rolling reduction. The composite alloys in which the (Nb, Ti) phase is elongated along to the rolling direction are obtained by 69 % rolling reduction. ¶n of this sample is four times higher than that of as-cast one. On the other hand, ¶ for the sample vertical to the rolling direction is reduced to about one third of that of as-cast one. Thus, the hydrogen permeability of the Nb40Ti30Ni30 alloy varies about ten times or more depending on the direction of the primary¡]Nb, Ti¡^ phase. We conclude that it is possible to produce highly hydrogen permeable Nb-Ti-Ni alloys using the anisotropy of the microstructure prepared by hot forging and rolling.

Coherence and fluctuation in 3D networks: the superconducting behavior of Nb particles embedded in a Cu matrix

We show the preparation and properties of granular samples formed by a regular distribution of spherical Nb particles, about 2 μm in diameter, embedded in a Cu matrix. Measurements of magnetization, resistance and V× I curves show clearly the combined superconducting behavior of the weak coupling and bulk regimes. Our results are interpreted using an effective medium model for the Josephson coupling in a 3D array of grains. In special we have observed the predicted small oscillations in the critical current, measured as a function of the applied magnetic field.

Spherical Nb single crystals containerlessly grown by electrostatic levitation

Spherical Nb (Tm=2750 K) single crystals were grown via containerless electrostatic levitation (ESL). Samples became spherical at melting in levitation and undercooled typically 300–450 K prior to nucleation. As-processed samples were still spherical without any macroscopic shape change by solidification showing a uniform dendritic surface morphology. Crystallographic {111} planes exposed in equilateral triangular shapes on the surface by preferential macroetching and spotty back-reflection Laue patterns confirm the single crystal nature of the ESL-processed Nb samples. No hysteresis in magnetization between zero field and field cooling also implies a clean defect-free condition of the spherical Nb single crystals.

A Mössbauer investigation of the homogenisation of Nd 11.8Fe 82.3− xNb xB 5.9 alloys

The effect of niobium addition on the microstructure of Nd–Fe–Nb–B alloys (with 0, 2, 4 and 5 at.% Nb) has been investigated by transmission Mössbauer spectrometry and X-ray diffraction analyses. In the as-cast alloys, the Nb 26Fe 32B 42 (at.%) phase was characterised by a paramagnetic doublet, with an isomer shift IS=−0.04 mm s −1 relative to α-Fe, and a quadrupolar splitting QS=0.22 mm s −1. The contribution of the Nd 2Fe 17 phase was observed in the Mössbauer spectra of the alloys with 4 and 5 at.% Nb. Niobium additions allow a drastic reduction of the homogenisation times during annealing in the 800–1100°C temperature range, in agreement with previous studies. The kinetics of the disappearance of α-Fe in alloys with 2 at.% Nb were determined by Mössbauer spectrometry. The experimental curves were fitted with a simple theoretical model of growth of a spherical Nb–Fe–B precipitate in a spherical α-Fe matrix. The activation energy related to the α-Fe disappearance was estimated to be 112±16 kJ mol −1.

Fracture and fatigue-crack growth behavior in ductile-phase toughened molybdenum disilicide: Effects of niobium wirevs particulate reinforcements

A study has been made of the fracture toughness/resistance-curve (R-curve) and cyclic fatigue-crack propagation behavior in a molybdenum disilicide composite, ductile-phase toughened with nominally 20 vol pct Nb-wire mesh reinforcements (Nb m /MoSi2); results are compared with monolithic MoSi2 and MoSi2 reinforced with 20 vol pct spherical Nb particles (Nb p /MoSi2). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi2, both under monotonic and cyclic fatigue loading conditions. Specifically, the Nb m /MoSi2 composite exhibits R-curve behavior with a steady-state fracture toughness of ∼13 MPa $$\sqrt m $$ , compared to unstable fracture atK c values below 5 MPa $$\sqrt m $$ in unreinforced MoSi2 or Nb p /MoSi2. Such behavior is seen to be associated with extensive crack deflection within the reaction layer between Nb and the matrix, which leads to crack bridging by the unbroken ductile phase. Similarly, resistance to fatigue-crack growth is found to be far superior in the wire-reinforced composite over pure MoSi2 and Nb p /MoSi2. Although crack paths are again characterized by extensive deflection along the Nb/matrix reaction layer, the role of crack bridging is diminished under cyclic loading due to fatigue failure of the Nb. Instead, the superior fatigue properties of the Nb m /MoSi2 composite are found to be associated with high levels of crack closure that result from highly deflected crack paths along the (Nb,Mo)5Si3 reaction layer interface.

Finite element evaluation of elastic strain energies during solid state transformation: Coherent, spherical precipitate in a finite matrix

A finite element method (FEM) has been proposed based on the initial strain approach to evaluate elastic strain energies during solid state transformations. A significant contribution of this work is that how much the image force due to the free surface relaxes the strain energy and this has not been solved for the concentric shell system considered here. Moreover, the finite element method can be used to study precipitation from free surfaces. The method has been utilized to evaluate strain energies for the case of NbH precipitation from the center of the spherical Nb system outwards, and from the surface of the system inwards. Although, the variation in elastic strain energies as a function of transformation was similar in both cases as expected (i.e. parabolic), the finite element analysis revealed that a major portion of the strain energy is stored in the matrix for the center-to-surface path and in the precipitate for the surface-to-center path.

Particle and Phase Thicknesses from XPS Analysis of Supported Bimetallic Catalysts: Calcined Co-Rh/Nb 2O 5

The surface structure and elemental composition of a series of calcined Co-Rh/Nb 2O 5 bimetallic catalysts have been investigated using X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). New formulae for the quantitative analysis of XPS intensities for supported bimetallic catalysts involving up to three separate, layered phases on spherical support particles are used for the first time. These apply an average takeoff angle for photoelectrons from spherical particles whose radii are large compared to the electron attenuation length. Calcined monometallic Co/Nb 2O 5 and Rh/Nb 2O 5, and four calcined bimetallic Co-Rh/Nb 2O 5 catalysts with similar Co loadings (≍ 1.9 wt%) and variable Rh loadings (0.3 to 2.3 wt%) were examined. Reference spectra for pure CoNb 2O 6 (columbite) are also presented here for the first time. The catalysts were prepared by incipient wetness impregnation and calcined at 673 K to generate the oxide precursors. The XPS lineshapes and the Co (2p) spinorbit splitting indicated the presence of two Co species, Co 3O 4 and Co +2, on all calcined Co-containing catalysts. The measured XPS Co/Nb and Rh/Nb atomic ratios for the catalysts were factors of 1.5-2 and 2-4.5, respectively, greater than the bulk atomic ratios, showing that both Co and Rh oxides were surface-enriched. The measured XPS peak intensities were compared to the values predicted from several different structural models of the oxide particles. These models assumed large spherical Nb 2O 5 particles (diameter ≍ 60 nm to match the BET area) with the Co and Rh oxides covering fractions of this support′s surface in several particle arrangements. The best agreement, according to the minimum least squares criteria, was found for a model in which the Co +2 phase is adsorbed on the Nb 2O 5 support surface, whereas the Co 3O 4 forms thick (>2.5 nm) islands covered by Rh2O3 on approximate to 3.6% of the support surface. The thickness of the Rh 2O 3 overlayer increased to 2.6 nm as the Rh/Co bulk atomic ratio increased to 0.72. The Co +2 phase was present at submonolayer concentrations. Between 78 and 90 mole% of the total Co was present as Co 3O 4 in this model. The reduction temperature of the Co 3O 4 strongly decreased as the Rh/Co bulk atomic ratio increased, while the reduction temperature of the Rh 2O 3 was not strongly influenced by the presence of Co 3O 4. Thus, the TPR results are consistent with this bilayer island model, with Rh 2O 3 on top of Co 3O 4, The Co +2 species was not obvious in TPR due to its low concentration.

Effects of Precipitate Shape on High Temperature Strength of Modified 9Cr-1Mo Steels.

A dispersion of precipitates introduces a threshold stress which opposes deformation. The effect of precipitate shape on dispersion strengthening at elevated temperature has been examined. Three kinds of Mod. 9Cr-1Mo steels without Nb(C, N), with spherical Nb(C, N) and with complex Nb(C, N)+V(N, C) were designed to have the same microstructures except for the shape of precipitates. The threshold stresses were measured by tensile and creep tests at 600°C. Two sorts of effects have been observed. The first is that complex precipitates restrain dislocation from climbing even in a condition in which dislocation climbs over a spherical precipitate. Mobile dislocations are considerably trapped at the concavity of the V-wings during deformation. The second effect is that the probability of dislocation trapping at V-wing is larger than that at spherical Nb(C, N). These trapping effects play important roles in the strenghening effect of precipitate.