Niobium Diffusion Research Articles

Microstructural evolution and tensile behavior of Ti2AlNb alloys based α2-phase decomposition

The formation mechanism of the fine plate-like O-phases within α2-phases and tensile behavior of an isothermally forged Ti–22Al–25Nb (at%) orthorhombic alloy at 1040°C during heat treatment were investigated. The investigation indicated that the alloys were heat-treated in O+B2 phase region after α2+B2 phase region isothermally forging, the equiaxed α2-phase was not stable and decomposed into O+α2 phases. The α2 phases formed during isothermal forging process have higher concentration of Nb and begun to decompose during O+B2 phase region heat treatment. And then the α2 phases separated into Niobium-lean and Niobium-rich regions through the Niobium diffusion: α2→α2 (Nb-lean)+O (Nb-rich). Nb-rich regions with composition similar to Ti2AlNb transformed to the O-phase, while the Nb-lean regions remained untransformed and retained the α2-phase. The deformation behavior and fracture mechanism of Ti–22Al–25Nb alloy at room temperature were discussed. The deformation behavior and microstructural evolution of this alloy at different temperatures and stain rates were also investigated using uniaxial tensile test.

Effects of post-weld heat treatment on microstructure and mechanical properties of linear friction welded Ti2AlNb alloy

Linear friction welds of a Ti2AlNb-based alloy were subjected to post-weld heat treatment (PWHT) at 815°C×1h (PWHT1) and at 1000°C×1h+815°C×2h (PWHT2) followed by air cooling. The microstructure and mechanical properties of the joints under as weld and PWHT conditions were investigated. The results indicate that in the as weld joint, the weld zone mainly consists of B2 phase while the quantity of α2 and O phase is relatively small, and most O phase gradually turns into B2 phase when the amount of α2 phase reduces as well in thermo-mechanically affected zone (TMAZ). After PWHT1, O precipitates within recrystallized B2 grains can be found in weld zone and the decomposition of α2 phase caused by niobium diffusion is found in TMAZ. PWHT1 have significantly enhanced the mechanical properties due to the O phase precipitation hardening and refined grain strengthening. And the coarsening of the microstructure in weld zone under PWHT2 is evident, which consequently reduces the elongation.

Elemental Diffusion Behaviour of Biomedical Grade Titanium Alloy through Thermal Oxidation

Major issues related to implant failure are wear debris and metal ions release where Titanium-Aluminium-Niobium alloys still face those problems despite of better biocompatibility. Surface modification is one of the alternatives in order to reduce those wear as well as ion release problems to the host tissue. In this study, experiments were carried out to investigate the element diffusion behaviour of Ti-6Al-7Nb alloy through thermal oxidation in order to obtain coating on the surfaces for diminishing those effects. Thermal oxidation was carried out at 650°C for three different durations 6, 12 and 24 hours. It is found that at prolong time, Niobium diffusion occurs where short duration Aluminium dominates. This suggests that longer heating time promotes heavy metal diffusion by restricting diffusion of light metal and hence, dominates the heavy metal oxide layer formation. The oxide layer formed on the substrate may lead to increase the lifespan of the implant and reduces the harmful effects caused by wear debris or toxic ion from metal alloys.

Effects of combined silicon and molybdenum alloying on the size and evolution of microalloy precipitates in HSLA steels containing niobium and titanium

The effects of combined silicon and molybdenum alloying additions on microalloy precipitate formation in austenite after single- and double-step deformations below the austenite no-recrystallization temperature were examined in high-strength low-alloy (HSLA) steels microalloyed with titanium and niobium. The precipitation sequence in austenite was evaluated following an interrupted thermomechanical processing simulation using transmission electron microscopy. Large (~105nm), cuboidal titanium-rich nitride precipitates showed no evolution in size during reheating and simulated thermomechanical processing. The average size and size distribution of these precipitates were also not affected by the combined silicon and molybdenum additions or by deformation. Relatively fine (<20nm), irregular-shaped niobium-rich carbonitride precipitates formed in austenite during isothermal holding at 1173K. Based upon analysis that incorporated precipitate growth and coarsening models, the combined silicon and molybdenum additions were considered to increase the diffusivity of niobium in austenite by over 30% and result in coarser precipitates at 1173K compared to the lower alloyed steel. Deformation decreased the size of the niobium-rich carbonitride precipitates that formed in austenite.

Niobium diffusion modified austenitic stainless steel bipolar plate for direct methanol fuel cell

An austenitic stainless steel with a niobium diffusion protective layer is evaluated for bipolar plate of direct methanol fuel cell (DMFC). Corrosion resistance of niobium diffusion modified AISI 304 stainless steel (niobized 304 SS) is investigated in simulated DMFC cathodic environment (0.5 M H2SO4 + 2 ppm HF + 0.1 M methanol solution at 50 °C) and anodic environment (0.5 M H2SO4 + 2 ppm HF + (1 M, 10 M and 20 M) methanol solution at 50 °C), respectively. Potentiodynamic, potentiostatic as well as electrochemical impedance spectroscopy tests show that, comparing with a bare 304 SS, the corrosion current density of niobized 304 SS is reduced greatly while the polarization resistance is raised in the simulated DMFC cathodic environment. Corrosion tests in the simulated anodic environment are applied to examine the effect of methanol on the corrosion behaviour of niobized 304 SS. It is interesting to find that the niobized 304 SS shows better corrosion resistance in the higher methanol concentration solutions.

Metamorphic reaction rates at ∼650–800 °C from diffusion of niobium in rutile

The ability to quantify the rates at which metamorphic reactions occur is critical to assessing the extent to which equilibrium is achieved and maintained in a variety of dynamic settings. Here we investigate the kinetics of rutile replacement by titanite during amphibolite-facies overprinting of eclogite, garnet amphibolite and anorthosite from Catalina Island, CA, the Tromsø Nappe, Norway, the North Qaidam terrane, China, and the Guichicovi Complex, Mexico. Trace element concentration profiles across rutile rimmed by titanite, as determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), reveal Nb zoning in rutile that we interpret as the result of Nb back-diffusion from the rutile–titanite boundary. We present new field-based reaction rates calculated from grain boundary velocities, which in turn were calculated using a 1-D diffusion model for Nb back-diffusion into rutile during titanite replacement over the temperature range 670–770°C. Our data are consistent with or slightly faster than previous estimates of field-based reaction rates for regional metamorphism, and extend the temperature and compositional range over which regional metamorphic reaction rates are known.

A molecular-dynamics simulation of grain-boundary diffusion of niobium and experimental investigation of its recrystallization in a niobium-copper system

A model of high-angle general-type grain boundaries is developed and grain-boundary diffusion of niobium in a Nb–Cu system is investigated using the method of molecular dynamics. The results of predictions of the effect of a copper impurity on grain-boundary self-diffusion of niobium are compared with the experimental data on diffusion annealing of multi-layered composites containing Cu/Nb boundaries. Recrystallization of polycrystalline niobium is investigated under the conditions of grain-boundary diffusion of copper.

Diffusion and segregation of niobium in fcc-nickel

Niobium is one of the major alloying elements, among the refractory elements, contributing to the strengthening of superalloys. Consequently, data about its behavior and its migration mechanism in fcc-Ni are essential knowledge to understand and control the strengthening in such alloys. We present in this work Nb interactions, solubility and diffusion in Ni performed by using the GGA approximation of the density functional theory. The substituted site is found to be the most favorable configuration in comparison to the tetrahedral and octahedral sites. The effect of temperature on solubility is discussed taking into account the thermal expansion of the lattice parameter and the vibrational contribution. Its diffusion mechanism is also discussed and compared to the literature. We finally discuss the segregation of Nb atoms on a Σ5-(012) symmetric tilt grain boundary.

Redistribution of components in the niobium-silicon system under high-temperature proton irradiation

The redistribution of components in the niobium-silicon system during magnetron-assisted sputtering of niobium, vacuum annealing, and high-temperature proton irradiation is studied. It is established that, during magnetron-assisted sputtering followed by vacuum annealing, silicon penetrates through the metal film to the outer boundary of the film. Under high-temperature proton irradiation, the suppression of the diffusion of niobium into silicon is observed. This effect is attributed to the high concentration of radiation vacancies in the region of the Nb/Si interphase boundary.

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900–1000 °C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates δ-Ni3Nb and/or (Ni, Mo)6C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of δ-Ni3Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The δ-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)6C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.

Assessment of niobium segregation energy in migrating ferrite/austenite phase interfaces

Abstract The effects of physical parameters on the total dissipation of Gibbs energy inside migrating ferrite/austenite interfaces have been re-investigated using Odqvist's solute drag model. It is shown that the mobility of the ferrite/austenite phase interface and the diffusivity of niobium in the interface have a strong effect on the Gibbs energy dissipation. As the phase interface mobility was recently reassessed, it is clear that the Gibbs energy of niobium segregation and the diffusivity of niobium in the interface must also be reassessed. In this paper both parameters are reassessed in the Fe–C–Nb system using new physical parameters and experimental results from ultra-low carbon steels.

Contribution of precipitate on migrated grain boundaries to ductility-dip cracking in Alloy 625 weld joints

We investigated the crack properties in Alloy 625 weld metals and their characteristics using experimentally designed filler wires fabricated by varying the niobium and manganese contents in the flux with the shield metal arc welding (SMAW) process. The fast diffusivity of niobium on the migrated grain boundary (MGB) under strong restraint tensile stress, which was induced by the hardened matrix in weld metal containing high niobium and manganese, accelerated the growth of niobium carbide (NbC) in multipass deposits. Coalescence of microvoids along with incoherent NbC and further propagation induced ductility-dip cracking (DDC) on MGB.

HRTEM studies of NiNbZr + Ag amorphous‐nanocrystalline composites

Amorphous powder of composition corresponding to Ni60Ti20Zr20 (in at%) was obtained by ball milling in a high-energy mills starting from pure elements. Formation of the amorphous structure was observed already after 20 h of milling, although complete amorphization occurred after 40 h. The microhardness of powders increased from about 30 HV for pure elements to above 400 HV (1290 MPa) after 40 h of milling. Transmission electron microscopy (TEM) allowed to identify nanocrystalline inclusions of intermetallic phases of size 2-10 nm. Uniaxial hot pressing was performed in vacuum at temperature below the crystallization T(x) it is 510 degrees C and pressure of 600 MPa, Mixed amorphous powders and nanocrystalline silver powders were used to form a composite, in which microhardness was near 970 MPa HV and 400 HV for the amorphous phase and nanocrystalline silver, respectively. The compression strength of the composite containing 20 wt% of nanocrystalline Ag powder was equal to 600 MPa and plastic strain was 2%. Microstructure studies showed low porosity of composites of less than 1%, uniform distribution of the silver phase and a transition zone between both components, about 150 nm thick, where diffusion of nickel, niobium and zirconium into silver was observed. High-resolution TEM allowed identifying the structure of nanocrystalline inclusions in the amorphous matrix after hot pressing as either Ni(3)Zr or Ni(17)Nb(3). The identification was performed basing on measurements of angles and interatomic distances using inverse Fourier transformed images with enhanced contrast using Digital Micrograph computer program.

Gamma double prime precipitation kinetic in Alloy 718

Gamma double prime (γ′′), precipitation was studied in Alloy 718 using isothermal and isochronal aging heat treatments applied between 943 and 1003 K. It is shown, that the coarsening behavior of γ′′ precipitates follows the coarsening kinetic predictions of the Lifshitz–Slyozov–Wagner (LSW) theory. The activation energy for γ′′ growth has been determined as equal to 272 kJ mol −1 and seems to be controlled by volume diffusion of niobium in the matrix. The energy of the γ′′/matrix interface, Γ, has been found to be 95 ± 17 mJ m −2 by assuming that the γ′′ precipitates adopt a disk shape which minimizes the total energy. This energy includes a volume distortion term calculated from the Eshelby inclusion theory and a surface component which is assumed to be isotropic. This interfacial energy is discussed and compared with the energy of γ′/matrix and γ′′/matrix interfaces in other superalloys. The constant K′′ of the LSW law time dependence has been calculated using the value of interfacial energy and the activation energy of γ′′ precipitates coarsening and is found to be in good agreement with our experimental values.

Study of Diffusion and Reaction Diffusion in the Fe-C-Nb System

Experiments of niobium diffusion at infinite dilution and Nb reaction-diffusion in pure iron and in ferrites with different amounts of carbon were performed, for comparison, in order to understand the influence of carbon on the diffusion process in the Nb-Fe system. A proportional decrease of the diffusion coefficient with the increasing amount of carbon was found. This effect seems to be stronger than in the self-diffusion analyzing the literature; moreover SIMS measurements in niobium- implanted samples show a redistribution of carbon during the first steps of the diffusion process. For those reasons, a stronger effect of carbon-niobium interaction over the carbon-vacancy interaction seems to be responsible for the decrease in the diffusion coefficient.

Niobium and titanium diffusion in the high niobium-containing Ti–54Al–10Nb alloy

Titanium and niobium diffusion in the ternary Ti–54Al–10Nb alloy (the composition are in at.%) is investigated in an extended temperature interval by the radiotracer technique. The mechanical sectioning was performed by grinding or sputtering in order to measure the tracer diffusivities at higher and lower temperatures, respectively. Almost linear Arrhenius dependencies are established for Nb (Ti) diffusion with the pre-exponential factor and the activation enthalpy of 1.9×10 −5 (4.0×10 −4) m 2 s −1 and 280 (304) kJ mol −1, respectively. Niobium is a slower diffuser in comparison to Ti in the ternary as well as in the binary (α 2-Ti 3Al and γ-TiAl) titanium aluminides. The heavy alloying of γ-TiAl with 10 at.% niobium, however, enhances significantly both the Ti and the Nb diffusivities. This effect is attributed to an elastic distortion of the L1 0 structure of TiAl induced by the oversized Nb atoms, which may decrease the activation barriers for atomic diffusion.

Influence of magnetization change on solute diffusion in iron

The activation energy for solute diffusion in the ferromagnetic state of iron can be represented by Q p(1 + αs 2), where Q p is the activation energy in the paramagnetic state, s is the ratio of the spontaneous magnetization at T K to that at 0 K. The constant α for self-diffusion and diffusion of cobalt, chromium, molybdenum, niobium and titanium in α-iron has been determined experimentally. Compiling these values of α, we find that the constant α can be expressed as a linear function of ΔM, which is the change of magnetization of iron on dilution with solute elements, if ΔM is taken for the first and second shells of the solute atom.

Tracer Diffusion of Niobium and Titanium in Binary and Ternary Titanium Aluminides

Tracer Diffusion of Niobium and Titanium in Binary and Ternary Titanium Aluminides

Tracer diffusion of niobium and titanium in binary and ternary titanium aluminides

This study presents recent results on titanium (Ti) and niobium (Nb) diffusion in binary (α2-Ti3Al and γTiAl) and ternary high-Nb-containing (γTiAl-10Nb) Ti aluminides. Three different techniques were used to measure self-diffusion and solute (Nb) diffusion. The radiotracer technique combined with mechanical sectioning by grinding was applied at higher temperatures, whereas ion-beam sputtering or secondary ion mass spectrometry was used to analyze the short penetration profiles at lower temperatures. The results measured by these different techniques for different penetration depths are very consistent. Nb is a slower diffuser compared to Ti in the ternary as well as in the binary Ti aluminides. The heavy alloying of γTiAl with 10at.% Nb, however, enhances significantly both the Ti and the Nb diffusivities. The diffusion mechanisms for Ti and Nb in ordered Ti aluminides are discussed.

Flexural Strength and Shear Strength of Silicon Carbide to Silicon Carbide Joints Fabricated by a Molybdenum Diffusion Bonding Technique

The capability to form robust SiC‐to‐SiC joints is needed to enable the fabrication of complex SiC‐based structures. In this work, molybdenum foils are used to develop a diffusion bond with SiC during a vacuum heat treatment at 1500°C for 10 h. The bond consists of a central region of Mo2C with Mo5Si3+carbon and/or Mo5Si3C phases at the SiC interface. Flexure strength determined using a standard 4‐point flexural test method (ASTM C1161) over the temperature range from room temperature to 1100°C showed that the molybdenum foil joined SiC failed at a somewhat lower value (263–50 MPa) than similarly tested monolithic chemically vapor deposited (CVD) SiC without any bond (443–197 MPa). Differences in elastic properties and coefficient of thermal expansion between SiC and the phases produced in the molybdenum foil joint region likely result in the formation of slightly larger flaws in the SiC near the joint, which are fracture initiation sites that result in lower flexural strength values for molybdenum‐joined SiC. Shear strength results obtained using a double‐notched specimen are also compared for the joined and monolithic specimens, which provides fracture strength data that can be used for comparisons. The shear strength values for the molybdenum‐joined SiC (264–61 MPa) are found to be within the data scatter of the monolithic material (397–31 MPa). These results indicate that a molybdenum foil diffusion bonding technique can be used to produce joints that are as strong as continuous bars of monolithic SiC in shear‐type loading, but they have lower flexural strength values than monolithic SiC. However, the flexure strength values for molybdenum foil‐joined SiC are within the range of values or higher than values reported in literature for the joining of SiC using reaction forming, reaction‐bonding, polymer precursors, and niobium diffusion bonding.