Nb Ti System Research Articles

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

Experimental Investigation and Thermodynamic Description of the Co–Nb–Ti System

Experimental Investigation and Thermodynamic Description of the Co–Nb–Ti System

Experimental investigation of isothermal sections at 1373 and 1473 K in the Co–Nb–Ti system

The isothermal sections at 1373 and 1473 K in the Co–Nb–Ti system were constructed using annealed diffusion couples and equilibrated alloys by using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), electron probe microanalysis (EPMA), and X-ray diffraction (XRD) techniques. Three three-phase regions, Co2(Nb,Ti) + Co7Nb6 + CoTi, Co7Nb6 + CoTi + bcc(Nb,Ti), and CoTi + liquid + bcc(Nb,Ti) were determined. Co3Nb and α-Co2Ti with a C36 crystal structure, and Co2Nb and β-Co2Ti with a C15 crystal structure at 1373 and 1473 K formed continuous solid solutions Co3(Nb,Ti) and Co2(Nb,Ti), respectively. The solubilities of Ti in Co7Nb6 and Nb in CoTi are determined to be ~11.4 at% and ~22.3 at% at 1373 K, and ~12.5 at% and ~24.8 at% at 1473 K, respectively. No ternary compounds were observed.

Calculation of Existence Domains and Optimized Phase Diagram for the Nb-Ti Binary Alloy System Using Computational Methods

We present the thermodynamic modeling for the Nb-Ti binary alloy system using CALPHAD (CALculation of PHAse diagram) and CE-CVM (Cluster expansion-cluster variation method) focusing on the solid solution part of the system. Existence domains for the various invariant reactions between the interaction parameters of the system were calculated between BCC and HCP phases. Five different regions viz. Simple Isomorphous, Monotectoid, Simple Isomorphous with congruent maxima, Monotectoid with congruent maxima and Syntectoid were identified in the calculated existence domains, each of which has its own characteristic reactional topology. The calculated existence domains are in good agreement with the earlier thermodynamic assessments made on the Nb-Ti system. Further, we have used the first-principles calculation to generate the thermochemical data for BCC and HCP phases for the Nb-Ti system. Moreover, we have also calculated the optimized phase diagram using the calculated existence domains, earlier experimental data, and the generated thermochemical data for the Nb-Ti system.

A thermodynamic description of the C–Nb–Ti system over the whole composition and temperature ranges and its application in solidification microstructure analysis

An understanding of the thermodynamic and phase diagram data for the C–Nb–Ti system is essential for the development of cermets, cemented carbides as well as Nb-based superalloys. Based on a critical evaluation of the literature data, the C–Nb–Ti ternary system has been reviewed and assessed by means of the CALPHAD technique. A self-consistent thermodynamic description for the C–Nb–Ti system has been developed over the whole composition and temperature ranges. Comprehensive comparisons between the calculated and measured phase diagrams available in the literature show that the reliable experimental information can be satisfactorily accounted for by the present thermodynamic description. The as-cast microstructures of several Nb-based superalloys reported in the literature have been analyzed by using Scheil solidification simulations based on the presently obtained thermodynamic database. The improvement mechanism of room-temperature hardness and high-temperature yield stress can be successfully interpreted according to the simulated amount of (Nb,Ti)C carbide, the effective strengthening particles in the as-cast Nb-based superalloys, which indicates a powerful tool of thermodynamic calculations in materials development.

The Al–B–Nb–Ti system. VI. Experimental studies and thermodynamic modeling of the constituent Al–B–Nb system

• Experimental analysis of 20 distinct Al–Nb–B alloys by XRD, EDS, WDS and DTA techniques. • Thermodynamic modeling of the complete Al–Nb–B system by the CALPHAD approach. • Thermodynamic calculations of phase equilibria in comparison to experimental data. Phase equilibria and phase transformations in the ternary Al–B–Nb system were investigated by XRD, SEM/WDS and SEM/EDS techniques, DTA, and optic pyrometry. No ternary compound, including the ternary phases reported in literature, was found. It was experimentally measured that the Al–Nb phases ε (NbAl 3 ), σ (prototype σCrFe) and δ (prototype Cr 3 Si) have evident ternary extensions with maximal solubility of boron amount to 1.2, 10.4 and 3.2 at.%, respectively. The Al solubility in niobium borides is also significant. The obtained results were used to elaborate the thermodynamic description of this ternary system based on recently published thermodynamic models of the constituent binary subsystems using the CALPHAD approach (Thermo-Calc/PARROT). The Al–B–Nb reaction scheme, projections of the liquidus and solidus surfaces as well as a number of isothermal sections and isopleths were calculated using the proposed thermodynamic description and compared with the experimental results. The calculations were shown to reproduce adequate the experimental data. The main features of the phase equilibria in the range of melting/solidification are the existence of two quasibinary sections, NbAl 3 –NbB and NbB 2 –AlB 12 , eutectic L E ↔ ε + σ + (NbB) at 1850 K and cascades of invariant U-type reactions.

Experimental investigation of the transverse resistivity in Nb3Sn wires through ac susceptibility

In this paper we compare two different experimental methods to estimate the transverse resistivity in superconducting NbTi and Nb3Sn multifilament wires. The first method is based on direct measurements of the resistance through micro wires bonded on the cross section of the strand. The second method exploits the physical meaning of the ac susceptibility of the multifilament wires. Both methods have been previously used: the first on Nb3Sn and NbTi strands, while the second has been applied only to NbTi wires. Here we first compare and analyze results in a NbTi system, then we extend the ac susceptibility method to Nb3Sn wires. The advantages and the limits of both techniques, when used on wires with different layouts, are analyzed. The results of the transverse resistivity obtained by using the two experimental methods are finally compared to the estimation based on a numerical approach.

Thermodynamics, surface properties and microscopic functions of liquid Al–Nb and Nb–Ti alloys

The energetics of mixing in Al–Nb and Nb–Ti liquid alloys have been analysed through the study of surface properties (surface tension and surface segregation), dynamic properties (chemical diffusion) and microscopic functions (concentration fluctuations in the long-wavelength limit and chemical short-range order parameter) in the frame of statistical mechanical theory in conjunction with quasi-lattice theory (QLT). The phase diagram of the Al–Nb system exhibits three intermetallic compounds in the solid state, while a relatively simple phase diagram of the Nb–Ti system indicates nearly ideal mixing behaviour in the liquid phase and two stable solid phases. Since the structure of a liquid alloy is in some respects similar to that of a crystal, the complex formation phenomenon in Al–Nb melts has been analysed by a quasi-lattice structural model for chemical complexes. The energetics and structural arrangement in Nb–Ti melts have been studied by the quasi-chemical approximation (QCA) for the regular solution model.

The Al–B–Nb–Ti system: V. Thermodynamic description of the ternary system Al–B–Ti

The thermodynamic description of the ternary system Al–B–Ti is obtained by modelling the Gibbs energy of all individual phases in the system using the CALPHAD approach. The model parameters have been evaluated using the computer optimization technique PARROT based on the available descriptions of the constituent binary systems Al–B, B–Ti and Al–Ti recently published and relevant experimental information on phase equilibria for the ternary system. For the ternary system Al–B–Ti the reaction scheme, projection of the liquidus, solidus and solvus surfaces, selected vertical and isothermal sections are calculated using the proposed thermodynamic description. An acceptable agreement between the calculations and experimental data is achieved.

The Al–B–Nb–Ti system: IV. Experimental study and thermodynamic re-evaluation of the binary Al–Nb and ternary Al–Nb–Ti systems

The thermodynamic description of the entire ternary Al–Nb–Ti system is obtained by CALPHAD modelling of the Gibbs energy of all individual phases, taking into account experimental data on phase equilibria and thermodynamic properties published and complemented by own experiments. The description includes a re-evaluation of the constituent binary Al–Nb system. Selected equilibrium calculations were performed with the Thermo-Calc software using the proposed description. They are shown to well reproduce experimental data on both, phase equilibria and thermodynamic properties in the entire Al–Nb–Ti system.

The Al–B–Nb–Ti system

The thermodynamic description of the binary system Al–Ti is obtained by modelling the Gibbs energy of all individual phases using the CALPHAD approach. The model parameters have been evaluated by means of the computer optimization module PARROT, available within Thermo-Calc, taking into account recent experimental data and critically assessed information on phase equilibria and thermodynamic properties. The calculations are in close agreement with the experimental data on both phase equilibria and thermodynamics in the entire Al–Ti system.

Influence of titanium admixtures on the structure and physicomechanical properties of niobium

On the basis of experimental investigations, we analyze the effect of temperature on the phase composition and microstructure of VN-10 alloy. We study some mechanical (σu, σ0.2, and δ), including long-term strength, and tribological (mass wear) characteristics of a niobium-titanium alloy. We have established that alloys of the Nb-Ti system are characterized by the dispersion-soluble type of strengthening. In the course of oxidation of the alloy in air, oxides of complex structure and texture are formed. Under dry friction, the alloy is more wear resistant after oxidation than after annealing.

The Al–B–Nb–Ti system: II. Thermodynamic description of the constituent ternary system B–Nb–Ti

The thermodynamic description of the ternary system B–Nb–Ti is obtained by modelling the Gibbs energy of all individual phases in the system using the CALPHAD approach. The model parameters have been evaluated by means of a computer optimization technique based on the available descriptions of the constituent binary systems B–Nb, B–Ti and Nb–Ti and relevant experimental information on phase equilibria for the ternary system. The thermodynamic descriptions of both systems with boron, B–Nb and B–Ti, were published in the first part of the work. For the ternary system B–Nb–Ti several vertical and isothermal sections as well as the projection of the liquidus and solidus surface are calculated using the elaborated thermodynamic description. A tolerable agreement between the calculations and experimental data is achieved.

The Al–B–Nb–Ti system: I. Re-assessment of the constituent binary systems B–Nb and B–Ti on the basis of new experimental data

Aiming to obtain a reliable description of the quaternary Al–B–Nb–Ti system, the thermodynamic descriptions of the constituent binary systems B–Nb and B–Ti are revised by modelling of the Gibbs energies of all individual phases using the CALPHAD approach and recent SGTE descriptions of the Gibbs energies of phases for pure elements. The model parameters have been evaluated taking into account the data on thermodynamic properties and phase equilibria reported in recent publications and obtained by own key measurements. The phase diagrams and the thermodynamic properties calculated with the evaluated parameters are in good agreement with the corresponding experimental data, and the thermodynamic descriptions of the systems become more suitable for high-order systems.

Isothermal section at 950 °C of the Co–Nb–Ti system

The isothermal section at 950 °C of the Co–Nb–Ti system was investigated using diffusion couples and six ternary alloys. The specimens were examined by means of optical microscopy, scanning electron microscopy and electron probe microanalysis. No ternary compound was found. Nine three-phase equilibria were observed. The binary intermediate phases NbCo 3 and Nb 2Co 7, the existence of which is still controversial in the literature, were observed at 950 °C. The solubilities of Ti in Nb 2Co 7, NbCo 3, λ-NbCo 2 and μ-Nb 6Co 7 were determined to be 1.9, 16.4, 14.4 and 17.4 at.%, respectively. The solubilities of Nb in TiCo 3, TiCo 2(h), TiCo 2(c), TiCo and Ti 2Co were determined to be about 5.7, 11.0, 2.6, 13.6 and 3.1 at.%, respectively.

Phase Equilibria of the Cu—Nb—Ti System at 850 °C.

Abstract The phase equilibria of the Cu–Nb–Ti system at 850 °C were investigated by a high-efficiency diffusion couple approach. The samples were studied by means of optical microscopy, scanning electron microscopy and electron probe microanalysis. At 850 °C, the phase TiCu2, the stability range of which is controversial in the literature, was observed. The solubilities of Nb in Ti2Cu, TiCu and Ti3Cu4 were determined to be 3.5, 3.1 and 4.6 at.%, respectively. The solubility of Nb in TiCu4 was negligibly small. The following three-phase equilibria were observed: Ti2Cu + TiCu + (Nb,Ti), TiCu + Ti3Cu4 + (Nb,Ti) and Ti3Cu4 + TiCu4 + (Nb,Ti).

Enhanced T/sub c/ and H/sub c2/ near the metal/insulator transition

We have studied the behavior of H/sub c2/ for the Nb/sub 1-x/Ti/sub x/ system as a function of disorder (i.e, x). To account for the observed behavior we substituted expressions for /spl gamma/ and T/sub c/, previously derived for disordered systems, into the GLAG expression for H/sub c2/. This extension accounts very well for the data for the NbTi system and, accordingly, we are studying its application to data for several other superconducting systems.

Phase equilibria of the Cu–Nb–Ti system at 850 °C

The phase equilibria of the Cu–Nb–Ti system at 850 °C were investigated by a high-efficiency diffusion couple approach. The samples were studied by means of optical microscopy, scanning electron microscopy and electron probe microanalysis. At 850 °C, the phase TiCu 2, the stability range of which is controversial in the literature, was observed. The solubilities of Nb in Ti 2Cu, TiCu and Ti 3Cu 4 were determined to be 3.5, 3.1 and 4.6 at.%, respectively. The solubility of Nb in TiCu 4 was negligibly small. The following three-phase equilibria were observed: Ti 2Cu + TiCu + (Nb,Ti), TiCu + Ti 3Cu 4 + (Nb,Ti) and Ti 3Cu 4 + TiCu 4 + (Nb,Ti).

Phase equilibria of the Fe–Nb–Ti system at 900 °C

The isothermal section at 900 °C of the Fe–Nb–Ti system was determined using the diffusion couple approach with the aid of equilibrated alloys. The specimens were examined by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis. No ternary compound was found. The isothermal section consists of five single-phase regions, six two-phase regions and two three-phase regions. The solubility of Nb in TiFe were determined to be 6 at.%, and the solubility of Ti in μ-Nb 19Fe 21 to be 23.6 at.%. NbFe 2 and TiFe 2 form a continuous solid solution (Nb, Ti)Fe 2.

Thermodynamic assessment of the Nb-Ti system

The thermodynamic properties of the Nb-Ti system have been evaluated by using a regular solution model to describe the Gibbs energies of various phases including both equilibrium and metastable phases. A set of thermodynamic parameters more consistent with most of the selected experimental data than previous assessments has been obtained. Stable and metastable phase equilibria, T 0 loci, metastable chemical spinodal curve of β phase and thermodynamic properties are calculated with the optimized parameters. With the thermodynamic parameters obtained in this work, authors discuss the possible occurrence range of ω phase.