Elemental Nb Research Articles

Microstructures of Oxidized Primary Carbides on Superalloy Inconel 718

The oxidation behavior of carbides on Inconel 718 surface was investigated. Oxidation tests were performed at the temperature range of 300 to 650°C. Two types of carbide, NbC and TiC were detected, and the amount of NbC was significantly higher than that of TiC. The oxidation of NbC started at the carbide surface and the matrix interface, and propagated into the interior of the carbide and eruption subsequently occurred with increasing oxidation time. In case of TiC, the oxidation occurred at the outer surface of the carbide and the matrix interface, but did not propagated into the interior of the carbide. Oxides formed on the outer surface of the TiC contained a relatively large amount of elemental Nb. The cross-section of oxidized specimen was investigated. NbC was oxidized more easily due to the significantly lower Gibbs free energy of oxidation than that of TiC. Eruption occurred mainly at the NbC because the Pilling-Bedworth ratio of Nb is higher than the critical value of 2.3 where eruption typically occurs.

Preparation of Nb-40Ti Powders by High-Energy Milling

Porous Ti-Nb alloys are promising candidates for biomedical applications. In the present study, alloy powders containing 60 wt-% Nb were prepared by high-energy milling of Nb, Ti, and/or TiH2 powders. The high-energy milling process was carried out in a planetary ball mill. The starting and as-milled materials were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). Elemental (Nb, and Ti) and TiH2 powder mixtures with composition Nb-40wt%Ti were mechanically alloyed for 2 to 30 h. The formation of a BCC Nb(Ti) solid solution by high-energy milling using elemental Ti powder to produce Nb-40Ti was observed after milling for 30 h. A HCP-Ti solid solution was formed after milling for 30 h due to the partial decomposition of titanium hydride powder mixture during high-energy milling.

Preparation of Nb 5Si 3 intermetallic and Nb 5Si 3/Nb composite by self-propagating high-temperature synthesis

Production of niobium silicide Nb 5Si 3 and composite Nb 5Si 3/Nb was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of sample green density, preheating temperature, and initial stoichiometry on the combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained combustion front propagating in a spinning mode. Moreover, phase transformation continues during the cooling of the burned sample, leading to the emergence of an afterglow on the compact. As the sample density and preheating temperature increased, the combustion temperature was found to increase and the flame-front velocity was correspondingly enhanced. XRD analysis shows that the nature of end products is significantly influenced by the starting stoichiometry of sample compacts. It was found that the compact with an initial composition of Nb:Si = 5:3 yielded predominantly α-Nb 5Si 3, whereas the sample made up of Nb:Si = 3:2 produced largely β-Nb 5Si 3. Based upon the data measured from the samples of Nb:Si = 5:3, an activation energy of 259.2 kJ/mol was deduced for the formation of Nb 5Si 3 by SHS. In the synthesis of the Nb 5Si 3/Nb composite, the increase of elemental Nb content in the final composition lowered the combustion temperature and thereby reduced the flame-front velocity. The Nb 5Si 3/Nb composites with a concentration of Nb ranging from 5 to 15 mol.% were directly produced by SHS in this study. XRD analysis of the Nb 5Si 3/Nb composite identifies the formation of Nb 5Si 3 dominated by the α form, along with the existence of elemental Nb.

Amorphization of Ni60Nb20Zr20 by mechanical alloying

Abstract Elemental Ni, Nb and Zr powder mixtures with composition Ni 60 Nb 20 Zr 20 were mechanically alloyed in a ball mill. The evolution of structure of the powders was investigated with a combination of X-ray diffractometry and differential scanning calorimetry and was compared with corresponding melt spun samples. Mechanical alloying of Ni 60 Nb 20 Zr 20 alloy involves two consecutive amorphization reactions. Mechanical alloying first leads to the amorphization reaction between Ni and Zr layers with a strong thermodynamic driving force as well as fast kinetics for Ni–Zr amorphization compared with Ni–Nb amorphization. On further milling the kinetic requirements for a Ni–Nb amorphization reaction is fulfilled and consequently a Ni–Nb amorphous phase forms. The resulting two amorphous phases homogenize at longer milling times.

Supersaturated solid solution of niobium in copper by mechanical alloying

Alloys with both high strength and high conductivity have been produced by mechanical alloying. In the present study, copper was mechanically alloyed with 5, 10 and 20 at.% Nb using a planetary ball mill. The Cu–Nb phase diagram shows a negligibly low mutual solubility in the solid state, but high energy ball milling can largely extend the region of solid state solution. Previously, it was observed that niobium partly dissolves in the copper lattice during milling. The present investigation demonstrates that this limit can be extended to a strongly supersaturated Cu solid solution of up to 10 at.% Nb provided the appropriate mechanical alloying method is applied. The change in the powder microstructure was followed by scanning and transmission electron microscopy (TEM) as well as by X-ray diffraction (XRD) analysis. In the case of Cu–5%Nb and Cu–10%Nb a homogeneous single-phase microstructure was obtained after 30 h of milling. Elemental Nb could no more be detected, indicating the formation of a metastable supersaturated Cu–Nb solid solution.

Reactions in Molten Alkalimetal Polychalcogenides: What Happens in the Melt? A Study of the Reactions in the System K-Nb-S Using Differential Scanning Calorimetry, Infrared Spectroscopy, and X-Ray Powder Diffraction

The reactions of potassium polysulfides with elemental Nb were investigated with different analytical techniques. The amount of the polysulfide applied has no influence onto product formation, i. e. the ratio K2Sx: Nb is not important. The length of the polsysulfide chain, i. e. the value of x in K2Sx determines what product is formed. In sulfur-poor melts, K3NbS4 is observed. Increasing x to 5 - 6, K4Nb2S11 is formed with a structure containing S2 2− anions. Finally, applying a melt with x > 6, K6Nb4S25 is found as the product with a crystal structure containing the S5 2− polysulfide anion. When K2Sx (x < 5) is heated with sulfur in the first step the pentasulfide K2S5 is formed. Immediately after melting of K2S5 a reaction with elemental Nb occurs. The results of FT-IR and X-ray investigations have demonstrated that after oxidation the anion [Nb2S11]4− is formed relatively fast, and after a short time crystalline K4Nb2S11 can be detected. After 24 h the reaction is complete.

Investigations of the formation mechanism of nanostructured NbAl3 via MASHS reaction

The nanostructured NbAl3 intermetallic compound was synthesized using the mechanically-activated self-propagating high-temperature synthesis (MASHS) technique. This process results from the combination of two steps: a short duration ball-milling of a pure elemental Nb+3Al powder mixture followed by a self-propagating high-temperature synthesis (SHS) reaction induced by the Nb+3Al reaction exothermicity. Synchrotron time-resolved XRD coupled with a 2D infrared camera were used to investigate the structural and thermal evolutions during the SHS reaction, and to study in situ the mechanism of NbAl3 formation. The influence of the incoming heat flux and the mechanical activation effect on the phase transformation kinetics induced by the SHS process were studied. Owing to the temporal resolution of 100 and 200 ms between two consecutive diffraction patterns and IR images, respectively, it was shown that solid niobium reacts with liquid aluminium via a heterogeneous nucleation reaction, to form the NbAl3 compound by successive combustion fronts.

A metastable allotropic transformation in Nb induced by planetary ball milling

The present investigation concerns planetary ball milling of elemental Nb in hardened steel and WC-coated media. Continued milling for about 20 h reduces the grain size below 10 nm with a concomitant lattice expansion. The final product obtained after 30–40 h milling is a pure f.c.c. phase. A detailed micro structural/compositional characterization by X-ray diffraction, resistivity measurements and wet chemical analysis proves that the said f.c.c. phase is neither NbN nor NbC. Instead, it is suggested that elemental Nb undergoes an irreversible b.c.c.→f.c.c. allotropic transformation when the grain size reduces to below 10 nm during high energy ball milling. Furthermore, this f.c.c. phase possibly undergoes an impurity stabilized ordering transformation on annealing at 900°C for 2 h. Finally, the role of impurity in causing this allotropic change is examined.

Structural evolution during mechanical alloying and annealing of a Nb-25at%Al alloy

Mixtures of pure elemental Al and Nb powders of Nb-25at%Al composition was mechanically alloyed, and structural evolution during high energy ball milling has been examined. Al dissolved in Nb from the early stage of the ball milling, and amorphization became noticeable after longer than five hours of milling. However the dissolution of Al in Nb was not completed before the amorphization. No intermetallic phase formed during the mechanical alloying. Before complete amorphization, metastable nitride of Nb4.62N2.14 (i.e., β-NbN) with hexagonal structure has formed in nanocrystalline size through nitrogen incorporation from ambient environment. The lattice parameter of Nb increased significantly (up to 3.3433 A after 5 hours of milling) during the milling. Upon annealing above 950 °C, Nb2Al became the dominant feature with the β-NbN, and Nb3Al did not form from the samples milled at ambient environment. Nb3Al appeared only from a sample milled at Ar environment. Structural evolution during mechanical alloying of the Nb-Al system is critically dependent the upon milling environment.

Fabrication and evaluation of Nb/Nb5Si3 microlaminate foils

Alloys of Nb and Nb5Si3, and in particular Nb/Nb5Si3 microlaminates, have potential as high-temperature materials. In this study, microlaminates of Nb and amorphous Nb-37.5 at. pct Si are magnetron sputter deposited from elemental Nb and polycrystalline Nb5Si3 targets. The microlaminates are heat treated at high temperatures to produce crystalline layers of Nb and Nb5Si3 that are flat, distinct, and stable for at least 3 hours at 1200 °C. The layers consist of textured Nb grains and equiaxed submicron Nb5Si3 grains. Initial room-temperature tensile tests indicate that the microlaminates have strengths similar to cast and extruded alloys of Nb and Nb5Si3. The fracture mode of the Nb layers is dependent on the Nb layer thickness, with thin layers failing in a ductile manner and thick layers failing by cleavage. The Nb layers bridge periodic cracks in the Nb5Si3 layers, and using a shear lag analysis, the tensile strength of Nb5Si3 is estimated. The results indicate that microstructurally stable and mechanically robust microlaminates of Nb and Nb5Si3 can be fabricated by sputter deposition with a high-temperature heat treatment. The processing, microstructure, and mechanical properties of these microlaminates are discussed.

Solid state synthesis of NiAl–Nb composites from fine elemental powders

Dense NiAl matrix composites reinforced with ductile Nb particles were synthesized in solid state from the blend of fine elemental Ni, Al and Nb powders. The manufacturing route involved full density consolidation of the starting blend followed by reactive hot pressing (RHP). The maximum processing temperature did not exceed 800°C. The phase composition and microstructure of materials obtained were characterized by X-ray diffraction, SEM/EDX and TEM. The kinetics and sequence of phase formation during synthesis were investigated. No interaction between Nb particles and NiAl matrix was observed during processing or during longer anneals at 800°C. Mechanical properties of the material synthesized were evaluated in compression tests at different temperatures and by microhardness measurements. Nb reinforcement was found to significantly increase the room temperature ductility of NiAl.

The synthesis of NbSi2 by mechanical alloying

The stoichiometric intermetallic compound NbSi2 has been synthesized by mechanical alloying (MA) elemental Nb and Si powders. The alloying process has been investigated by means of x-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was found that the formation of the Nb2Si intermetallic compound occurs abruptly after 65 min of milling without any interruptions during the alloying process. However, short interruptions at a 5 min interval during ball milling result in a gradual reaction for the formation of the NbSi2 compound as well as a new metastable bcc structured solid solution. We conclude that the temperature rise during mechanical alloying plays an important role in initiating the abrupt reaction after an incubation milling time.

MBE growth and characterization of epitaxial TiO 2 and Nb-doped TiO 2 films

Epitaxial TiO 2 and Nb-doped TiO 2 thin films have been grown on (110) TiO 2 rutile substrates by molecular beam epitaxy using elemental Ti and Nb sources along with an electron cyclotron resonance oxygen plasma source. Film composition was measured by X-ray photoelectron spectroscopy. Reflection high-energy electron diffraction and low-energy electron diffraction patterns reveal excellent long-range crystallographic order and a rutile structure for both TiO 2 and Nb-doped TiO 2 films. The high degree of similarity in Ti and Nb core-level X-ray photoelectron diffraction angular distributions establishes that Nb is substitutionally incorporated in the rutile lattice. Analysis of the Nb 3d 5 2 core-level binding energy suggests am Nb 4+ oxidation state.

Intermetallic sheets synthesized from elemental Ti, AI, and Nb foils

Intermetallic sheets synthesized from elemental Ti, AI, and Nb foils

Detailed phase analysis of a 77 at.%Nb-Al system prepared by low-energy ball milling

Elemental powders of Nb (77 at.%) andAl (23 at.%) were alloyed in a low-energy ball mill for 360–1800 ks in hopes of forming a nanocrystalline intermetallic compound, Nb3Al. It was revealed by careful deconvolution of XRD spectra into various peaks of possibly existing phases that the resultant powder is a nanocrystalline multiphase mixture consisting of Nb3Al, Nb2Al, elemental Nb and Al, with their particulate sizes of approximately 5–8 nm, even after 1800 ks milling. The volume fractions of these phases were estimated to be 28.4, 39.5, 30.3 and 1.8 vol.%, respectively. The kinetic energy provided by ball-milling was insufficient to produce a single Nb3Al phase, and even heating to 933 K for 25.2 ks failed the total conversion of a multiphase mixture.

Mechanical alloying of NiNb in different atmospheres

The formation of NiNb alloys prepared by mechanical alloying (MA) in different atmospheric conditions was examined by X-ray diffraction. Ball milling of elemental Ni and Nb powders under an argon atmosphere for alloys with Nb contents up to 15 at.% forms a supersaturated solid solution on the base of Ni. Amorphous alloys are formed if the Nb content is more than 20 at.%. The interaction of metallic powders Ni and Nb was absent and a b.c.c. phase was formed for all compositions up to pure Nb, when MA of NiNb powders was performed in the presence of water, alcohol or acetone in Ar and air atmospheres. This phase, identified as the interstitial phase of oxygen in niobium, has the lattice parameter a = 3.390 A ̊ and an O content of about 34 at.%.

Epitaxial Growth and Characterization of NbxTi1−xO2 Rutile Films by Oxygen-Plasma-Assisted Molecular Beam Epitaxy

AbstractEpitaxial films of NbxTi1−xO2 rutile were grown on TiO2 (110) and (100) at 600 °C by oxygen-plasma-assisted molecular beam epitaxy using elemental Ti and Nb sources. The epitaxial films were characterized by means of reflection high-energy and low-energy electron diffraction (RHEED/LEED), x-ray photoelectron spectroscopy and diffraction (XPS/XPD), ultraviolet photoemission spectroscopy (UPS) and atomic force microscopy (AFM). The epitaxial films grow in a layer-by-layer fashion and have excellent short- and long-range structure order at x≤0.3 on TiO2(110) and at x≤0.15 on TiO2(100). However, the epitaxial films become rough and disorder at higher doping levels. Nb substitutionally incorporates at cation lattice sites, leading to NbxTi1−xO2 solid solutions. In addition, the oxidation state of Nb in the NbxTi1−xO2 films has been determined to be +4.

Powder processing of ductile-phase-toughened NbNb 3Al in situ composites

Model high temperature NbNb 3Al in-situ intermetallic matrix composites have been successfully fabricated using powder metallurgy and arc-melting techniques. Vacuum hot pressing of elemental Nb and Al powders, mixed in the ratio Nb6wt.%Al and synthesized at 1200 °C, yeilded a dual-phase microstructure consisting of equiaxed islands of ductile Nb particles in a brittle Nb 3Al matrix. Subsequent thermal treatment at 1800 °C for 24 h resulted in an alternative composite microstructure with the Nb present in a lamellar morphology following a peritectic transformation, similar to structures obtained via arc-melting and thermal treatment procedures. The precipitation of ductile Nb phase following thermal aging was found to nucleate heterogenously at grain boundaries and to progress along specific growth directions, resulting in a uniform and fine distribution of filamentary Nb within the Nb 3Al matrix. Both equiaxed and lamellar bNb 3Al composite microstructures show improved fracture toughness of about 6 MPa m 1/2 compared with unreinforced Nb 3Al (about 1 MPa m 1/2), principally because of crack bridging and crack renucleation effects induced by the presence of the ductile Nb phase.

Investigation of Amorphization of Nb-Si Alloys by Mechanical Alloying

Amorphous Nb62.5Si37.5 and Nb75Si25 alloys were produced from mixtures of elemental Nb and Si powders by mechanical alloying (MA). The structural changes of mechanically alloyed powders were monitored by x-ray diffraction. It was found that NbSi2 intermetallic compound was formed in the initial stage of MA of Nb62.5Si37.5 or Nb75Si25 powders. With continued milling, the mixtures of intermetallic NbSi2 and elemental Nb transformed into amorphous Nb-Si alloys (Nb62.5Si37.5 or Nb75Si25). The slow interdiffusion of Nb and Si and the slower diffusion of silicon atoms to niobium matrix than that of niobium atoms to silicon matrix were suggested to be responsible for the formation of intermetallic NbSi2. The amorphization of 3NbSi2 + 7Nb mixtures (or NbSi2 + 5Nb mixtures) is believed to be controlled by material transfer, which moves composition of line intermetallic NbSi2 off stoichiometry, and causes the matrix of elemental Nb deformed.

Properties of Reactively Synthesized Titanium Aluminides

AbstractReactive synthesis directly forms compounds from their elemental constituents. In this U.S. Bureau of Mines study the microstructure, hot-hardness and tensile properties of alloys and composites based on TiAl and Ti2AlNb were studied. Ti2AlNb sheets were processed from elemental Ti, Al and Nb foils. The elemental foils were consumed during processing, leaving a microstructure consisting of three phases: O (orthorhombic Ti2AlNb), B2 (ordered cubic Ti), and α2 (Ti3Al). The composite sheet structures were heat-treated to produce a microstructure consisting of O lathes in a B2 matrix. A significant advantage of reactive synthesis is the ease of formation of complex shapes prior to synthesis of the aluminide. TiAl-based alloys and in situ TiAl/boride and TiAl/Ti5Si3 composites were produced from elemental mixtures of Ti and Al with either additions of B or Si powders. The hot-hardness and tensile properties of these alloys and composites are reported. The Kirkendall porosity associated with reactive sintering of TiAl can be eliminated by simultaneously reactive sintering TiAl and Ti5Si3.