Niobium Diffusion Research Articles

Microstructures and mechanical behaviour of bimetallic structures of tungsten alloy (90WNiFe) and nickel alloy (In625) fabricated by wire-arc directed energy deposition

ABSTRACT This study reported the fabrication of bimetallic structures (BS) from tungsten alloy (90WNiFe) and nickel alloy (In625) via wire-arc directed energy deposition (DED)-based additive manufacturing (AM). Its microstructures and mechanical properties were investigated for three different heat input conditions: low (180A), medium (200A), and high (220A). The highest average ultimate tensile strength of 618 MPa was achieved in the low heat input condition, with an average elongation of 49%. Although the ultimate tensile strength of the produced BS is lower than that of 90WNiFe and In625 individually, its yield strength closely resembles that of In625 processed by wire-arc DED. Tensile samples from each heat input condition experienced different fracture locations and exhibited distinct fracture morphologies. It can be concluded that the bonding strength is attributed to the diffusion of chromium, nickel, molybdenum, and niobium from In625 into the γ-(Ni, Fe, W) binding matrix of the 90WNiFe substrate.

Materials characterization of Ti6Al4V to NbZr1 bimetallic structure fabricated by wire arc additive manufacturing

This study investigated the microstructures and mechanical properties of a bimetallic structure (BS) of a titanium alloy (Ti6Al4V) and a niobium alloy (NbZr1) manufactured by wire arc additive manufacturing (WAAM). Three BS thin-walls were deposited with different heat input conditions: low (180 amperes (A)), medium (200 A), and high (220 A). Microstructural characterization at the interface showed that the Niobium (Nb) diffusion on the Ti6Al4V side increased as the heat input was increased. The interfacial microstructure comprised an island area and a dendritic zone with solid solutions of (β Ti and Nb) and (α + β Ti and Nb), respectively. The hardness ranged from 99 to 367 Vickers Hardness (HV), with a considerable decrease at the interface due to the migration of Nb. Maximum ultimate tensile strength of 567 MPa was achieved for the low heat input sample, and the failure occurred at the NbZr1 side. The fractography revealed intergranular and transgranular fracture morphologies, indicating a brittle failure. The results showed that the proposed BS has a good bonding strength, and neither defects (e.g., pores and cracks) nor intermetallic compounds were observed at the interface.

An investigation on thermodynamics and kinetics of η phase formation in Nb-modified iron-nickel base A286 superalloy

In this study, precipitation of η phase (Ni3Ti) in conventional and Nb-modified (Nb-A286) A286 superalloys was evaluated at different aging times and temperatures. The TTP curve of the η phase formation was plotted using thermodynamic analyses, kinetics and microstructural studies. Depending on temperature and heat treatment, the η phase precipitated at the grain boundaries or twin sites, as a result of the γ′ phase or matrix austenite transformation. Heat treatment of conventional A286 superalloy and Nb-A286 was performed within a temperature range of 650 to 900 °C for 2 to 30 h. The η phase transformation was evaluated by scanning electron microscope (SEM) which is equipped to energy dispersive X-ray spectroscopy (EDS) and optical microscopy (OM). In the analyses based on thermodynamic calculations, the interaction of the Gibbs free energy of η phase formation and the diffusion activation energy of the elements, especially titanium and niobium, was considered. The microstructural studies showed that increasing the heat treatment time results in increasing the volume fraction of the η phase. By increasing the aging temperature to 840 and 860 °C for conventional A286 superalloy and Nb-A286 superalloy, respectively, the η phase volume fraction increased, however, further increase led to volume fraction decrease. The results of the thermodynamic analyses showed the tip of the TTP diagrams at temperatures of 860 and 820 °C for the A286 and Nb-A286 alloys, respectively. Investigation of kinetics calculations showed that η phase transformation depends on the diffusion of titanium, nickel, and niobium.

Structure and thermal diffusivity of SiC-NbC ceramics

The structure and thermal diffusivity of SiC - NbC ceramic materials are investigated. It is shown that with a change in the composition in sintered ceramics, the lattice parameter changes, and also in the near-contact region there is a mutual diffusion of silicon and niobium. This indicates the formation of solid solutions. As the niobium carbide content and temperature increase, the thermal diffusivity of SiC - NbC ceramic materials decreases.

Development of precipitation model with the use of the Lattice Boltzmann Method and its application for the rolling process

A new precipitation model based on the Lattice Boltzmann Method with elements of Cellular Automata is presented in the paper. The model consists of three submodels: nucleation, precipitation growth, and diffusion of niobium and carbon.The nucleation model used classical nucleation theory and considers homo- and heterogeneous nucleation. The nucleation rate depends on the number of possible nucleation sites, which are defined by dislocation density, i.e. deformation conditions. The nucleation rate depends also on the Zeldowicz parameter, the factor of condensation rate, precipitation work, temperature, and other variables. The nucleation rate is realized in the model by calculating the probability of nucleon appearance in every cell, free of precipitation, and depends on the nucleation rate, the cell volume, and the time step.The growth or dissolution of the precipitation depends on the model on particle size and prevailing local conditions near the particle. The temperature and concentration define the equilibrium size of the particle.LBM model is applied to solve the Fourier equation for niobium and carbon diffusion. Near the precipitations equilibrium concentration of niobium and carbon is established according to precipitations size and temperature, while free diffusion is considered in the other space.Strain-induced precipitation at different temperatures and with the different constant cooling rate was simulated. Results are coincident with theory and experimental researches reported in the scientific literature.Modeling of precipitation during the rolling process with consideration of recrystallization shows an ability of the model to simulate and study the more complex processes that strain-induced precipitation in single deformation. It can be applied for multi-stages deformation processes to design or optimize them in view of precipitation and microstructure evolution.

An experimental approach to examine fluid-melt interaction and mineralization in rare-metal pegmatites

AbstractNiobium and tantalum, rare metals and high field strength elements (HFSEs) that are essential to modern technologies, are concentrated among others in lithium-cesium-tantalum (LCT) pegmatites and rare metal granites. The most important hosts for Nb-Ta in these types of deposits are the columbite group minerals (columbite-tantalite), but at some ore deposits significant Ta is also contained in wodginite, microlite, and tapiolite. Previous solubility experiments of HFSE minerals have been limited to high temperatures because of the slow diffusivities of HFSEs in granitic melts. An experiment protocol is described herein that allows HFSE mineral solubilities to be determined at lower temperatures, more in line with the estimated solidus temperatures of LCT pegmatites and rare metal granites. This is achieved through the interaction of a melt that is enriched in high field strength elements (e.g., P and Nb or Ta) with a fluid enriched in a fluid-mobile element (FME, e.g., Mn). A starting glass enriched in a slow diffusing HFSE was synthesized, and HFSE mineral saturation is obtained via the diffusion of a FME into the melt via interaction with a fluid. This interaction can occur at much lower temperatures in reasonable experimental durations than for experiments that require diffusion of niobium and tantalum. The solubility product of columbite-(Mn) from the fluid-melt interaction experiment in a highly fluxed granitic melt at 700 °C is the same as those from dissolution and crystallization (reversal) experiments at the same P-T conditions. Thus, both methods produce reliable measurements of mineral solubility, and the differences in the metal concentrations in the quenched melts indicates that the solubility of columbite-(Mn) follows Henry's Law. Results show that columbite-(Mn) saturation can be reached at geologically reasonable concentrations of niobium in melts and manganese in hydrothermal fluids. This experimental protocol also allows the investigation of HFSE mineral crystallization by fluid-melt interactions in rare-metal pegmatites. Magmatic origins for columbite group minerals are well constrained, but hydrothermal Nb-Ta mineralization has also been proposed for pegmatite-hosted deposits such as Tanco, Greenbushes, and granite-hosted deposits such as Cínovec/Zinnwald, Dajishan, and Yichun. This study shows that columbite-(Mn), lithiophilite, and a Ca-Ta oxide mineral (that is likely microlite) crystallized from experiments in fluid-melt systems at temperatures as low as 650 °C at 200 MPa. It is important to note that HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.

Precipitation kinetics of γ″ phase and its mechanism in a Nb-bearing nickel-based superalloy during aging

While understanding the precipitation kinetics of gamma double prime (γ″) phase is of vital importance in Inconel 718 superalloy, kinetics studies by Johnson–Mehl–Avrami–Kolmogorov (JMAK) and thermoanalysis techniques have not been conducted so far, and hence, investigating the mechanism of γ″ formation deserves more attention. In response, in the present work, the precipitation kinetics of γ″ phase was studied by JMAK analysis based on the hardness measurements during aging and differential thermal analysis (DTA) during continuous heating. Average Avrami exponent of 0.64 was obtained, which was interpreted as precipitation on dislocations on the theoretical front. Moreover, the activation energy of precipitation (Q) was found to be close to the activation energy for the diffusion of niobium in the matrix, which was also confirmed by the Kissinger's plot obtained from DTA analysis. As a result, the present work contributes to the kinetics and mechanism of the γ″ phase formation.

Strong and reliable dissimilar brazing titanium to copper using niobium diffusion barrier

Dissimilar brazing of titanium (Ti) and copper (Cu) using Ag-based braze and niobium (Nb) diffusion barrier was investigated. It was found that the addition of a Nb interlayer can effectively block the interaction between Ti substrate and molten braze, thus completely eradicate the formation of brittle intermetallic compounds (IMCs). This leads to a brazed joint consisting of Ti-Nb solid solution, remnant Nb interlayer and Ag-Cu braze. The resultant IMCs free joint exhibits superior structural integrity with bonding strength exceeding that of the Cu substrate. Meanwhile, negligible dissolution of Nb into molten braze and sluggish solid-state diffusion at the Ti/Nb interface keep the Nb interlayer intact, enabling excellent reliability of the joint.

Interfacial Engineering for Gradient Optical Fiber

AbstractGradient functionalization is a widely applied process in materials science that involves generating gradient distribution of composition or structure to yield materials with new functions or improved properties. Here an interfacial engineering is described for construction of gradient fiber, which exhibits gradient distribution of active centers, bonding configuration, and the resultant optical response. It is shown that the interfacial diffusion of protype niobium (Nb) in the core and cladding region of the fiber can be tuned via rational control of the viscosity of melt and the subsequent thermal activation. This leads to gradient distribution of Nb species and active [NbO6]7− centers within 10–20 µm region. As a result, unique optical properties including the tunable energy transfer between active centers and special annular emission beam can be obtained. These results highlight a fundamental principle to develop novel photonic fibers and provide a great step in expanding the scope of gradient materials.

Effects of Postweld Heat Treatment on Microstructure and Properties of Laser-Welded Ti-24Al-15Nb Alloy Joint

The microstructure evolution and mechanical properties of laser-welded joints of Ti-24Al-15Nb alloy under different postweld heat treatment (PWHT) conditions were systematically investigated. The results show that the microstructure is very sensitive to PWHT temperatures. The weld zone consists of B2 and O phases after PWHT. With the PWHT temperatures increasing from 850 to 1000 °C, the amount of O phase decreases gradually, while the grains of O precipitates become coarser. After PWHT, there are some thin acicular O precipitates in heat-affected zone (HAZ), and the decomposition of α2-phase caused by niobium diffusion can be observed in the HAZ. The PWHT can significantly increase the microhardness of joints, resulting from O phase precipitation hardening effect. The tensile strength and elongation of joints can be remarkably improved after PWHT, which was closely related to the strengthening effect of O precipitates and slip transmission between O and B2 phases. In addition, the results indicate that the best mechanical properties can be achieved only when the number and size of O phase and B2 phase are the best match.

Revisiting the Diffusion of Niobium in an As-Cast Nickel-Based Superalloy During Annealing at Elevated Temperatures

While understanding the interdiffusion of Nb in the austenite matrix of Ni-based superalloys is important for studying the precipitation reactions (Laves phase, γ″-precipitates and δ-phase) and homogenization of the as-cast structure, there is no report on the activation energy (Q) and pre-exponential factor (D0). Moreover, for the Ni–Nb diffusion couple, the reported Q values of 202.6 and 257 kJ/mol are not consistent. In the present work, based on the concept of residual segregation index during homogenization treatment and elemental analyses from dendrite cores and interdendritic segregated regions, Q and D0 were respectively determined as ~ 236 kJ/mol and 0.0053 cm2/s for the Ni-based superalloys. Improved analysis of the Ni–Nb system resulted in Q and D0 values of ~ 234 kJ/mol and 0.1618 cm2/s, respectively. Therefore, it was concluded that the activation energy for interdiffusion of Nb in superalloys and Ni–Nb diffusion couple is the same. However, due to the presence of other alloying elements in the superalloys, D0 for Ni–Nb diffusion couple is ~ 30 times that of the superalloys.

Effect of diffusion alloying time on corrosion resistance and surface conductivity of niobium-alloying-modified AISI430 stainless steel for DFAFC bipolar plate

Purpose The purpose of this study is to improve the surface electrical conductivity and corrosion resistance of AISI430 stainless steel (430 SS) as bipolar plates for direct formic acid fuel cell (DFAFC). Design/methodology/approach The niobium diffusion layers have been successfully synthesized on 430 SS substrate by the plasma surface diffusion alloying technique under different diffusion alloying time. Findings The surface morphology of Nb-modified 430 SS prepared under the diffusion alloying time of 2 h is more homogeneous, relatively sleek and compact without surface micropore and other common surface blemishes. The potentiostatic and potentiodynamic polarization measurements manifest that Nb-modified 430 SS prepared under the diffusion alloying time of 2 h enormously ameliorate the corrosion resistance of bare 430 SS compared with other Nb-modified 430 SS samples and its corrosion current density is maintained at −1.4 µA cm−2 in simulated anodic environment of DFAFC (0.05 M H2SO4 + 2 ppm HF + 10 M formic acid at 50 °C). Originality/value The effect of diffusion alloying time on the corrosion resistance and surface conductivity of Nb-modified 430 SS has been carefully studied. The Nb-modified 430 SS samples prepared at the diffusion alloying time of 2 h have the best surface electrical conductivity and corrosion resistance in the simulated anodic environment of DFAFC.

Microstructural Evolution and Phase Transformation of a Ti-5Nb-5Al Alloy During Annealing Treatment

The microstructural evolution and phase transformation of a Ti-5Nb-5Al alloy during isothermal annealing treatment were studied in this paper. The microstructural evolution was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with X-ray energy dispersive spectroscopy (EDS) and electron backscattering diffraction (EBSD). The first-principle calculation by density functional theory method was performed to analyze the effect of niobium diffusion on the α to β transformation. The results showed that the α phase initially grew by merging neighboring grains and then by boundary splitting with increasing the solution time below the β-transus temperature. However, the α phase disappeared absolutely above the β-transus temperature. According to the thermodynamic analysis by DFT calculations, the diffusion of niobium from β to α phase can promote the α to β transformation. After quenching in water, the acicular α’ phase precipitated from β matrix with an orientation relationship of {110}bcc||{0001}hcp and bcc|| hcp, and thus increased the hardness of the alloy.

Molecular Dynamics Investigation of the Effect of Lanthanum on the Diffusivity of Niobium in Austenite

Molecular Dynamics Investigation of the Effect of Lanthanum on the Diffusivity of Niobium in Austenite

Cellular behaviour of bone marrow stromal cells on modified Ti-Nb surfaces

The cellular behaviour of bone marrow stromal cells on titanium surfaces modified by niobium diffusion is presented in order to test their osteogenic differentiation response after culturing for 21days. The surface modification of Ti substrates produced by powder metallurgy was performed through niobium diffusion treatments. Ti-Nb exhibited a β-Ti surface together with a microstructural (β / α+β / α) and compositional (Ti-Nb) gradient which enhances hardness, wear resistance and lowers the elastic modulus making it more similar to the human bone. Furthermore, the Ti-NbNH4Cl by means of the activating agent achieved three times the hardness of Ti together with a porous surface.The in vitro osteogenic differentiation response of bone marrow stromal cells on both Ti-Nb surfaces indicated the positive cell-material interaction. The osteogenic differentiation of cells was successful after 21days, considering the positive response in terms of increased cell viability, lactate dehydrogenase-(LDH) activity, alkaline phosphatase-(ALP) activity expression (osteogenic marker) and bone-like nodules deposition by ST-2 cells as a bone mineralization cue. Therefore, the positive effect of a low elastic modulus Ti-Nb surface and a porous nitride Ti-NbNH4Cl with suitable wettability and average roughness values on the osteogenic differentiation response of bone marrow stromal cells is demonstrated.

Electrochemical Properties of Niobium Modified AISI316L Stainless Steel Bipolar Plates for Direct Formic Acid Fuel Cell

AbstractTo ameliorate the corrosion resistance and surface electrical conductivity of AISI316L stainless steel (316L SS) as bipolar plates for direct formic acid fuel cell (DFAFC), a niobium diffusion layer has been successfully prepared on 316L SS substrate via the plasma surface diffusion alloying (PSDA) technique. The niobium modified AISI316L stainless steel (Nb‐316L SS) has a compact and successive niobium diffusion layer with a lower interfacial contact resistance (ICR) compared with bare 316L SS. The electrochemical behaviors of Nb‐316L SS in simulated varied anode (0.05M H2SO4 + 2 ppm HF + xM formic acid (x = 2, 5, 10 and 15) solution at 50 °C) and cathode (0.05M H2SO4 + 2 ppm HF solution at 50 °C, bubbled with air) DFAFC environments are systematically studied. The results show that the corrosion resistance of 316L SS is distinctly bettered in both anode and cathode environments of DFAFC after the PSDA modification. After 4 h potentiostatic polarization tests, the ICR of bare and Nb‐316L SS have increased at the clamping force of 140 N cm−2, but the increasing amplitude of the ICR for Nb‐316L SS is much smaller than that of bare 316L SS.

Study of Niobium Diffusion and Clusterization in hcp Zr-Nb Dilute Alloys

We perform atomistic simulations aimed on study of diffusion of constituents and niobium precipitation in hcp Zr-Nb. We report diffusivities of Zr and Nb in hcp Zr-Nb alloys computed for the models containing up to 5 at.% of niobium. The calculated diffusivity of niobium rises with increase of its content in the alloy. The simulations also show that for a studied concentration range addition of niobium slightly enhances self-diffusion of zirconium in the alloys. The work is also devoted to description of niobium incorporation and clusterization in hcp zirconium. It is confirmed that for a single niobium atom incorporated in hcp zirconium lattice the octahedral position is the most favorable. We estimated the energy describing niobium cluster formation in pure hcp zirconium. According to the simulation results, we can suggest that the minimum niobium cluster size that can be expected in hcp Zr corresponds to about 80 atoms.

Photoelectrochemical water splitting with a SrTiO3:Nb/SrTiO3 n+-n homojunction structure.

A very limited knowledge exists about the effect of non-uniform doping of epitaxially grown strontium titanate thin film electrodes on their photoelectrochemical performance in water splitting. In this work, water splitting photoanodes featuring an n+-n homojunction were fabricated by the pulsed laser deposition technique, where epitaxial SrTiO3 thin films were grown on Nb doped n+-SrTiO3 single crystalline substrates. Thermal diffusion of niobium from doped substrates into the deposited thin films formed an n+-n homojunction, which was profiled by angle-resolved XPS and cross-sectional STEM-EDX techniques. This homojunction was found to make a significant impact on the incident photon-to-current efficiency of photoanodes by affecting their depletion width, which was in agreement with the theoretical simulations.

First-principles study of the effect of lanthanum on the niobium diffusion in fcc Fe

The effect of La on the diffusion of Nb in fcc Fe has been investigated using the first-principles calculations based on the density functional theory. The binding energies of Nb–vacancy, La–vacancy and La–Nb pairs have been calculated. The interactions of Nb–vacancy and La–Nb are attractive in 1nn and 2nn configurations (nn: nearest–neighbor). La atom attracts strongly with the 1nn vacancy, but has a weakly repulsive interaction with the 2nn vacancy. We consider four different Nb jumps in the presence of La atom to investigate the Nb diffusion in terms of vacancy formation and migration energy. The results suggest that La increases the diffusion activation energy of Nb in fcc Fe matrix, and is helpful to decelerate the Nb-diffusion-involved phase transformation process.

Changes in the structure of materials in brazing niobium with sapphire using a glass–ceramic solder

The changes in the structure of joints between sapphire and niobium produced by high-temperature brazing using a CaO–Al2O3 brazing alloy are investigated. The effect of temperature on the recrystallization processes in niobium and its alloys with zirconium on the formation of the second phase in the CaO–Al2O3 system and the diffusion of niobium in the glass–ceramic solder is investigated. The optimum conditions of brazing the niobium alloy with sapphire are determined.