Nb Phase Research Articles

Phase evolution during high energy ball milling of immiscible Nb–Zr alloys

Mechanical alloying (MA), a solid-state processing technique used extensively to synthesize metastable phases, was employed to synthesize solid solution and amorphous phases in Nb-rich Nb–Zr powder blends. These metastable phases could be synthesized by MA, under different processing conditions, even though the heat of mixing between Nb and Zr is positive, which makes alloying them difficult. The effect of alloy composition, milling time, and the ball-to-powder weight ratio (BPR) were varied and their effect studied on phase evolution and microstructure in the milled powders. The composition of the milled powders was varied starting from a low value of 5 to about 50at.% Zr. At a Zr content below 40at.%, amorphization was achieved at a higher BPR of 30:1, i.e. more milling energy. The formation of an amorphous phase at Zr contents lower than 40% was achieved for the first time in this work and confirmed using TEM. However, this amorphous phase crystallized rapidly on continued milling (mechanical crystallization) to form an FCC phase. Additionally, milling of powders with low Zr contents primarily resulted in the formation of Nb-based Nb–Zr solid solutions.

Solid Solution Cermet: (Ti,Nb)(CN)–Ni Cermet

Solid solution powders without W, (Ti,Nb)(CN) powders with a B1 structure (NaCl like), were synthesized by high energy milling and carbothermal reduction in nitrogen. The range of molar ratios of Ti/Nb for forming complete (Ti,Nb)(CN) phase was broader than that of Ti/W for the (Ti,W)(CN) phase because carbide or carbonitride of Nb had a B1 crystal structure identical to Ti(CN) while WC had a hexagonal crystal structure. The results revealed that the hardness of (Ti,Nb)(CN)-Ni cermets was higher than that of (Ti,W)(CN)-Ni cermets. The lower density of the (Ti,Nb)(CN) powder contributed to the higher hardness compared to (Ti,W)(CN) because the volumetric ratio of (Ti,Nb)(CN) in the (Ti,Nb)(CN)-Ni cermets was higher than that of (Ti,Nb)(CN) in the (Ti,W)(CN)-Ni cermets at the same weight ratio of Ni. Additionally, it was assumed that intrinsic the properties of (Ti,Nb)(CN) could also be the cause for the high hardness of the (Ti,Nb)(CN)-Ni cermets.

Microstructure and Microsegregation of an Inconel 625 Weld Overlay Produced on Steel Pipes by the Cold Metal Transfer Technique

Abstract The aim of this work was to investigate the development of microstructure and variations in chemical composition in commercial Inconel 625 coatings on a ferritic-pearlitic steel overlaid by the CMT method.The investigation showed that microsegregation occurring during the weld overlay solidification makes the dendrite cores to be richer in Ni, Fe and Cr and in the between dendrite arms in Mo and Nb. Niobium shows the strongest tendency to segregation during solidification; molybdenum tends to segregate less and chromium has the lowest tendency to segregation. Although Inconel 625 is a solid solution strengthened alloy, Nb and Mo-rich phases are formed in the between dendrite arms of weld overlays.

Stability Domains of NbC and Nb(CN) During Carbothermal Reduction of Niobium Oxide

The Gibbs free energies of formation of Nb(CO) and Nb(CN) were calculated using ab initio simulation. Phase stability diagrams of the Nb2O5–NbO2–Nb(CO)–NbC/Nb(CN) system were constructed with respect to carbon activity, oxygen pressure, and the characteristics of solution formation. The constructed diagrams suggest that Nb2O5 can be reduced without the formation of NbO owing to the formation of an Nb(CO) solid solution phase. The Nb(CO) phase might not be formed under a high partial pressure of nitrogen. This study demonstrates the absences of NbO and Nb(CO) during, respectively, the syntheses of NbC and Nb(CN) from Nb2O5. The constructed stability diagrams could also represent the conditions for syntheses of niobium carbide or stoichiometric niobium carbonitride by the carbothermal reduction of Nb2O5 under argon or nitrogen.

Specific heat of Nb3Sn: The case for a single gap

The important influence of multiple gaps in the superconductivity of MgB2 and Fe-based compounds, especially because of the possibility that manipulation of a second gap can significantly raise the upper critical field Hc2, has refocused attention on Nb3Sn because anomalies in both specific heat and point-contact tunneling studies have led to the proposal that Nb3Sn is also a two-gap superconductor. Here, we search for evidence of the second gap in a careful study of the influence of the homogenization temperature on the sample uniformity. We show that it is very difficult to fabricate samples that are both homogeneous and stoichiometric. We find so-called “second-gap” anomalies disappear only after high temperature and long-term annealing. Such a well-annealed sample shows only a strong, electron-phonon-coupled, single-gap behavior. In contrast, samples reacted and annealed at lower temperatures, as in the earlier two-gap studies, show small chemical composition variations of the A15 phase. We propose that the second gap sightings are actually due to variation of Tc within very difficult-to-fully homogenize samples. A curiosity of the A15 Nb3Sn phase is that almost any mixture of Nb and Sn tries to form a stoichiometric A15 composition, but the residue of course contains off-stoichiometric A15, Nb, and other phases when the Nb:Sn ratio departs from the true 3:1 stoichiometry.

Microtexture Development of Niobium in a Multilayered Ti/Al/Nb Composite Produced by Accumulative Roll Bonding

Multilayered Ti/Al/Nb composites were produced by the accumulative roll bonding (ARB) process utilizing pure Ti, Al, and Nb element sheets. Up to four cycles of ARB were applied to the composites. The microstructure and texture evolution on the Nb phase were studied by X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscopy, and electron backscattered diffraction. Nb and Ti layers necked and fractured as the number of ARB passes increased. After four ARB cycles, a nearly homogeneous distribution of Nb and Ti layers in Al matrix was achieved. As-received Nb sheet exhibited a fully lamellar structure and had a strong cold-rolling texture. After subjecting to ARB, slight grain refining was observed and the high-angle boundary fraction was increased. The intensity of the α-fiber was weakened, while that of the γ-fiber was strengthened during ARB. The texture evolution was attributed to partial recrystallization during the ARB process as a result of adiabatic heating.

Synergy of Cr concentration and withdraw rate on microstructure evolution of directionally solidified Nb–14Si–24Ti alloys

The synergy of Cr concentration and withdraw rate on the microstructure of directionally solidified and heat treated Nb–14Si–24Ti–xCr (x = 0, 2, 8, 10, 14 at-%) alloys was investigated. The phases present in the directionally solidified alloys were sensitive to Cr concentration and withdraw rate. The addition of Cr changed the solidification path of ternary alloys. High Cr concentration stabilised the Nb5Si3 and facilitated the formation of Cr2Nb during the directional solidification process. However, the effect of Cr was suppressed by the increase in withdraw rate. The competition between the effect of Cr addition and withdraw rate determined the microstructure of alloys and resulted in a four-phase microstructure. During heat treatment, the Nb3Si phases in Nb–14Si–24Ti–0Cr alloys were stable, while in the Cr contained alloys, the Nb3Si decomposed into NbSS/Nb5Si3 eutectoid through the eutectoid reaction (Nb3Si→NbSS+Nb5Si3).

Surface Microstructure Modification in Square Extruded Al-Nb Powder Composites by Shot Peening

75%Al-25%Nb powder composites, fabricated by square shape cold extrusion, were subject to shot peening treatment with full coverage. Shot peening results in a high number of intense local deformations, with a surface roughness in our case of about l6gm. Due to the high local deformation down to nano-scale surface grain refinement and strain accumulation was generated. Previous texture characterization was performed by neutron diffraction and laboratory X-rays (Cu Kα radiation). The first method took advantage of the high penetration power and averaging capabilities and the second method was further used taking advantage of the low penetration to characterize surface microstructure modification. Peak broadening, before and after shot peening, was analyzed by MAUD software and domain sizes and microstrains were calculated for both phases. Simultaneous EBSD and EDS scans, on 30 nm step sizes, were performed on a FESEM Quanta 200 + TSL-EDAX, showing the highly heterogeneous microstructure developed because of shot peening. Protrusions, due to particle impacts, are clearly seen on EBSD maps. Results mainly revealed that, for Al phase, domain sizes decrease, while microstrains and dislocation densities consistently increase after the materials have been subjected to SP. For Nb phase the visible effect of SP is an increment of microstrains, and related dislocation densities, but keeping the domain sizes almost constant.

Phase transformation and decomposition mechanisms of the βo(ω) phase in cast high Nb containing TiAl alloy

High Nb containing TiAl (Nb–TiAl) alloys are thought to be used at higher temperatures than the conventional TiAl alloys, typically up to 900°C. However, the βo and ω-related phases are usually induced by Nb-segregation in cast alloys. It is important to reveal the transformation mechanisms of the βo and ω-related phases at elevated temperatures for better design of the alloy and optimizing the heat treatment process. In this study, the interconversion mechanism between the ordered ω and βo phases and the decomposition process of the βo phase in the as-cast Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at.%) alloy are studied. The ωo particles grow up by richening Nb and rejecting W to the surrounding βo matrix. The βo and ωo phase are separated after annealing at 900°C, whereas the ωo phase dissolves into the βo matrix after annealing at 950°C. The βo and ωo phases undergo βo→γ and ωo (B82 structure)→D88-ω (D88 structure)→γ transformation respectively during annealing, and the βo→γ transformation can be achieved in several ways. The ωo particles with a high Nb content act as pinning points at the βo/γ boundaries during the βo→γ transformation. The corresponding mechanisms of the transformations mentioned above are discussed.

Transformation behavior of the γU(Zr,Nb) phase under continuous cooling conditions

The selected alloy for designing a high-density monolithic-type nuclear fuel with U–Zr–Nb alloy as meat and Zry-4 as cladding, has to remain in the γU(Zr,Nb) phase during the whole fabrication process. Therefore, it is necessary to define a range of concentrations in which the γU(Zr,Nb) phase does not decompose under the process conditions.In this work, several U alloys with concentrations between 28.2–66.9at.% Zr and 0–13.3at.% Nb were fabricated to study the possible transformations of the γU(Zr,Nb) phase under different continuous cooling conditions. The results of the electrical resistivity vs temperature experiments are presented.For a cooling rate of 4°C/min a linear regression was determined by fitting the starting decomposition temperature as a function of Nb concentration. Under these conditions, a concentration of 45.3at.% Nb would be enough to avoid any transformation of the γU(Zr,Nb) phase. In experiments that involve higher cooling conditions, it has been determined that this concentration can be halved.

Effect of Nb and Nb5Si3 Powder Size on Microstructure and Fracture Behavior of an Nb-16Si Alloy Fabricated by Spark Plasma Sintering

The influence of Nb and Nb5Si3 powder sizes (Nb: 83.8, 37.4, 4.9 μm; Nb5Si3: 86.4, 43.6, 2.9 μm) on microstructure and fracture behavior of an Nb-16Si alloy fabricated by spark plasma sintering (SPS) was investigated. The results revealed that all as-sintered samples consisted of Nb and Nb5Si3 phases, with no new phases formed during SPS. The morphologies of the Nb and Nb5Si3 phases in the as-sintered samples depended on the original Nb and Nb5Si3 powder sizes. Large Nb5Si3 powders 86.4 and 43.6 μm in combination with Nb powder of described sizes resulted in an Nb matrix plus Nb5Si3 island biphase microstructure, which supplied excellent fracture toughness. The microstructure made from the finest Nb powder (4.9 μm) and the largest Nb5Si3 powder (86.4 μm) had a high fracture toughness of 12.4 MPa m1/2 through a mixed fracture mechanism of dimple, tear, and cleavage from the small Nb grains. When the Nb5Si3 powder size was decreased to 2.9 μm, the sample tended to form an Nb5Si3 matrix plus Nb island biphase microstructure, which exhibited a poor fracture toughness value of between 5.6 and 8.0 MPa m1/2 due to the crack propagation mainly within the brittle Nb5Si3 phase.

Effects of Nb addition on microstructure and mechanical properties of TiNiNb alloys fabricated by elemental powder sintering

Porous TiNi binary and TiNiNb ternary alloys of four compositions (Ti50Ni47.5Nb2.5, Ti50Ni45Nb5, Ti50Ni42.5Nb7.5, and Ti50Ni40Nb10) were fabricated by the elemental powder sintering process. The effects of Nb addition on microstructure and mechanical properties of TiNi(Nb) alloys were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and tensile tests, respectively. With the increase of Nb contents, the TiNi matrix as the main phase is always retained, while the intensity of its diffraction peak gradually became weak with the appearance of β-Nb and TiNb phases. Most Nb particles are well-distributed in the TiNi matrix and (Ti,Nb)2Ni phase is found in the binding domain between Nb phase and TiNi phase. The tensile strength and elastic modulus of TiNiNb alloys increase with the increase of Nb contents, due to the solid solution strengthening effect of Nb-rich particles, the enlarged sintering neck and the strengthened bond between particles. Consequently, Nb plays a crucial role in adjusting composition and improving microstructure and mechanical properties of TiNiNb alloys.

Carbonatite-hosted niobium deposit at Aley, northern British Columbia (Canada): Mineralogy, geochemistry and petrogenesis

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian. Primary Nb mineralization consists of pyrochlore (commonly comprising a U–Ta-rich and F-poor core) and ferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases and associated heavy minerals (apatite±magnetite±zircon±baddeleyite) precipitated early in the magmatic history and probably formed laterally extensive cumulate layers up to at least 1.5m in thickness. Fractionation of copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituent minerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomatic rocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb in these rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive margin of North America became the zone of plate convergence in the Cretaceous, the host carbonatites were strongly deformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, folding and, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals were profoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistry resulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, and development of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At these late evolutionary stages, Nb was mobilized only to a very limited extent and sequestered in a variety of minerals (fersmite, euxenite, Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystals associated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomite carbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-element composition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rocks hosting the Aley deposit and were capable of transporting F−, (PO4)3−, U, Th and rare-earth elements, but not Nb.

Steady-state dynamical phase diagram calculation of U–Nb binary system under irradiation: Ballistic effect

A kinetic model involved in ballistic effects has been used for studying the steady-state configuration under irradiation, with considering the irradiation-enhanced diffusion. By applying this model to all the solid phases, the dynamical phase diagram of U–Nb system under irradiation is calculated and critically compared with the assessed thermodynamic equilibrium phase diagram. The calculated results indicate that the irradiation has little effect on the phase relationships of U–Nb system at high temperatures. However, it leads to the close of the miscibility gap of U–Nb system at relatively low temperatures. Meanwhile, the high-temperature stable γ(U,Nb) phase is stabilized by the irradiation, which is in agreement with the available experimental results.

Tribological Properties of Double-Glow Plasma Surface Niobizing on Low-Carbon Steel

The niobized layer was formed on Q235 low-carbon steel by double-glow plasma surface niobizing to improve its wear resistance. The microstructure, phase composition, and microhardness were determined. The friction and wear properties of the niobized samples and the untreated alloys were tested on a ball-on-disk tribometer by rubbing against GCr15 and silicon nitride (Si3N4) balls at room temperature and 400°C, respectively. The results indicated that the alloyed layer that contained a sediment layer and diffusion layer is about 35 μm in thickness, metallurgically adhered to the base metal. Niobium content was gradually decreased along the depth direction from the surface, which was similar to the change in the microhardness. The alloying layer mainly consisted of Nb, Fe2Nb, and FeNb phases. Under unlubricated sliding conditions, the friction coefficients and the specific wear rates were lower than those of the untreated carbon steel at room and high temperatures. The wear mechanism of the niobized specimen at room temperature is dominated by slightly abrasive wear, whereas the predominant wear mechanism is abrasive wear and fatigue delamination at high temperature.

Microstructures and room temperature fracture toughness of Nb/Nb5Si3 composites alloyed with W, Mo and W–Mo fabricated by spark plasma sintering

Microstructures and room temperature fracture toughness of Nb/Nb5Si3 composites alloyed with W, Mo and W–Mo fabricated by spark plasma sintering were investigated. The microstructures were examined using scanning electron microscope (SEM). X-ray diffraction (XRD) was performed on the bulk specimens for identification of phases. The chemical species were analyzed using electron-probe micro-analysis (EPMA). Results indicated that the microstructures of Nb/Nb5Si3 composites alloyed with W or Mo is unaltered, including primary Nb and eutectic mixtures of Nb and Nb5Si3, and the coarse and fine eutectic mixtures. The W and Mo elements solid solution in Nb and Nb5Si3 phase are detected. But that alloyed with W and Mo together, The microstructures are change obviously, including Nb phase, the solid solubility phases of W and Mo atoms in Nb, and the solid solubility phases of Nb atoms in W are also found, but the solid solubility phenomenon of Nb5Si3 phases is not detected. The microhardness of Nb and Nb5Si3 phases increases obviously because of solid solution strengthening. The Nb/Nb5Si3 composite alloyed with W and Mo together hashing high fracture toughness is attributable to the big eutectic Nb and interface of eutectic phases, which can bear large deformation to absorb the crack energy and form the decohesion between eutectic phases.

The liquidus surface of the Cr–Al–Nb system and re-investigation of the Cr–Nb and Al–Cr phase diagrams

The liquidus surface and corresponding reaction scheme of the ternary Cr–Al–Nb system were determined experimentally. The solidification paths of a series of more than 40 ternary alloys were deduced from investigation of their as-cast microstructures and measurement of all reaction temperatures applying scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), and differential thermal analysis (DTA). The hexagonal C14-type Laves phase Nb(Cr,Al)2, which is not stable in any of the binary boundary systems and which is the only ternary compound, forms the most extended primary crystallization field of the ternary system dominating the centre of the liquidus surface. A ternary eutectic was found near the Al–Nb boundary composed of the three intermetallic phases C14+Nb2Al+NbAl3. Besides the ternary liquidus surface, the solidus and liquidus curves of the Cr–Nb boundary system and of the Cr-rich part of the Al–Cr system were determined resulting in revised binary phase diagrams.

Aging response of transient liquid phase bonded wrought IN718 superalloy: influence of post-bond heat treatment

Aging response of transient liquid phase Nb bearing wrought IN718 nickel base superalloy is studied. The aging behaviour of the joint is influenced by low Nb+Al+Ti content of isothermal solidification zone (ISZ) and formation of Nb–Cr–Mo based boride precipitates in diffusion affected zone (DAZ). It was shown that applying a post-bond heat treatment which was able to eliminate the diffusion induced boride precipitates in DAZ and increase the Nb+Al+Ti content of the ISZ improved the aging response and shear strength of the joint.

Development of a new 718-type Ni-Co superalloy family for high temperature applications at 750∘C

Alloy718 has been used for many years in the aerospace industry due to its unique mechanical properties and good processing characteristics, especially its workability. However, the temperature limit of Alloy718 is about 650 ∘ C because of the thermal instability of the main strengthening phase γ′′ -Ni3 (Nb,Ti,Al). Numerous attempts have been made to develop a new wrought 718-type alloy for high temperature applications. The approach was to increase the stability, i.e. the solvus temperature of the γ′ -phase (Tγ′ ,s ). However, this affected workability as the solvus temperature of the δ-phase (Tδ,s ) did not increase accordingly so that the window for fine grain forging Tδ,s -Tγ′ ,s became smaller. In this paper the development of a new γ′ /γ′ -alloy on the basis of Alloy718 is presented, where the microstructure is stable at 800 ∘ C, mechanical properties are similar to Alloy718, yet do not deteriorate beyond 650 ∘ C, and the forging window is wider than the one of Alloy718, allowing for good workability. This was essentially achieved by the addition of about 17%–30% Co in combination with an Al/Ti-ratio of more than 5.0 and an Al-content of about 1.6%–2.2%. The key role of cobalt is to stabilize the δ-phase, allowing for solvus temperatures in excess of 1100 ∘ C. Consequently, the stability of the γ′ -phase can be increased by further addition of aluminium. At the same time the Ti-content is reduced to prevent formation of the η-(Ni,Co)3 (Ti,Al,Nb) phase. Besides discussion of the alloy development concept, information on microstructure evolution and mechanical properties will be given.

Interfacial orientation and misorientation relationships in nanolamellar Cu/Nb composites using transmission-electron-microscope-based orientation and phase mapping

A transmission-electron-microscope-based orientation mapping technique that makes use of beam precession to achieve near-kinematical conditions was used to map the phase and crystal orientations in nanolamellar Cu/Nb composites with average layer thicknesses of 86, 30 and 18nm. Maps of high quality and reliability were obtained by comparing the recorded diffraction patterns with pre-calculated templates. Particular care was taken in optimizing the dewarping parameters and in calibrating the frames of reference. Layers with thicknesses as low as 4nm were successfully mapped. Heterophase interface plane and character distributions (HIPD and HICD, respectively) of Cu and Nb phases from the samples were determined from the orientation maps. In addition, local orientation relation stereograms of the Cu/Nb interfaces were calculated, and these revealed the detailed layer-to-layer texture information. The results are in agreement with previously reported neutron-diffraction-based and precession-electron-diffraction-based measurements on an accumulated roll bonding (ARB)-fabricated Cu/Nb sample with an average layer thickness of 30nm as well as scanning-electron-microscope-based electron backscattered diffraction HIPD/HICD plots of ARB-fabricated Cu/Nb samples with layer thicknesses between 200 and 600nm.