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The effect of the cooling rate on the structure and properties of hypoeutectic Nb–Si–Al alloys obtained by aluminothermy in an argon atmosphere studied by scanning electron microscopy, X-ray phase analysis, and energy-dispersive analysis. The indentation method is used to measure the mechanical properties of the obtained alloys: the fracture toughness and hardness. The cooling rate of the alloy is regulated by replacing the crucible material: the cooling rates of the alloy differ by almost an order of magnitude. The difference in the cooling rates does not affect the phase composition of the obtained alloys: in all cases, a solid solution of silicon and aluminum in niobium (Nbss) and β-Nb5Si3 silicide are formed. The microstructures of the obtained alloys are similar, but differ in dispersion. The formation of a finer structure led to an increase in the mechanical properties of the alloy compared to those of the alloy with a lower degree of structure dispersion. [ABSTRACT FROM AUTHOR]

We report on the electrical characterisation of a series of thin amorphous chromium oxide (CrOx) films, grown by dc sputtering, to evaluate their suitability for use as on-chip resistors in nanoelectronics. By increasing the level of oxygen doping, the room-temperature sheet resistance of the CrOx films was varied from 28 Ω/  to 32.6 kΩ/ . The variation in resistance with cooling to 4.2K in liquid helium was investigated; the sheet resistance at 4.2K varied with composition from 65 Ω/  to above 20GΩ/ . All of the films measured displayed linear current-voltage characteristics at all measured temperatures. For on-chip devices for quantum phase-slip measurements using niobium-silicon nanowires, interfaces between niobium-silicon and chromium oxide are required. We also characterised the contact resistance for one CrOx composition at an interface with niobium-silicon. We found that a gold intermediate layer is favourable: the specific contact resistivity of chromium-oxide-to-gold interfaces was 0.14 mΩcm2, much lower than the value for direct CrOx to niobium-silicon contact. We conclude that these chromium oxide films are suitable for use in nanoscale circuits as high-value resistors, with resistivity tunable by oxygen content. [ABSTRACT FROM AUTHOR]

The CoFeBSiNb soft magnetic metallic glasses with strong glass forming ability were selected as microwave absorption materials for the first time. The metallic glass powders (MGPs) were prepared through milling and hydrochloric acid treatments. The effects of mass fraction of MGPs in wax composites and corrosion treatment on the microwave absorbing properties were investigated in details. The results demonstrated that the complex permittivity and permeability of the composites increased gradually as the MGPs mass fraction increased. Through chemical corrosion, the complex permittivity of the metallic glasses apparently increased, which due to the strengthened interfacial polarization caused by the porous surface structures. As a result, the minimum value of reflection loss (RL) for the composites containing 60 wt% of MGPs corroded for one hour under pH = 2 reached −45.2 dB at 14.64 GHz with a thickness of only 2.3 mm and the bandwidth reached 14 GHz for RL < −10 dB or 6.2 GHz for RL < −20 dB. [ABSTRACT FROM AUTHOR]

Presents a study that examined the effect of nitrogen ion implantation on the formation of amorphous ribbons of niobium silicon alloys ( Nb[sub3]Si). Effect of nitrogen ions on A15 phase formation of Nb[sub3]Si during annealing; Methodology; Analysis of the crystal structure of the implanted surface of Nb[sub3]Si.

Investigates the formation of niobium (Nb)-silicon (Si) from Nb/Si using relative frequency sputtering and rapid thermal processing. Correlation of the variation of the sheet resistance versus annealing temperature and grain growth; Application of x-ray powder diffraction and Auger depth profiling; Effect of contamination by oxygen on the silicide growth.

The effect of yttrium on the microstructure and high-temperature oxidation performance of Nb-15Si-24Ti-4Cr-2Al-2Hf (at.%) alloys was investigated. Alloys containing different yttrium contents were designed, aiming at maximizing the beneficial effect of yttrium addition and avoiding the possible adverse effect of over-doped yttrium. Results showed that Y was primarily partitioned in γ-Nb 5 Si 3 , relative to Nb solid solution (Nbss), Nb 3 Si and α-Nb 5 Si 3 . Microstructural refinement and oxidation resistance enhancement were achieved by yttrium addition up to 0.3 at.%, accompanied by the transition towards parabolic oxidation kinetics. An adverse effect of over-doped yttrium (0.5 at.% addition) by coarsening the microstructure and deteriorating the oxidation resistance was confirmed. A relatively continuous glassy SiO 2 layer was generated in the oxide scale formed on the Y-containing alloys with enhanced oxidation resistance. [ABSTRACT FROM AUTHOR]

In the present work, the role of Zr addition on the microstructure and phase formation of hypoeutectic Nb−16 at. % Si alloy has been investigated. The results showed that both binary and alloy with 2 at. % Zr resulted in two phase microstructures composed of Nb ss and Nb 3 Si phases. In contrast, the alloys with 4 at. % Zr and 6 at. % Zr revealed two phase microstructures composed of Nb ss and α−Nb 5 Si 3 phases. The orientation relationship (OR) obtained between eutectoid lamellar structure comprising of Nb ss and α−Nb 5 Si 3 phases is (110) Nb//(110) Nb 5 Si 3 . The equilibrium microstructures consisting of Nb and α−Nb 5 Si 3 phases were obtained in as cast condition when the Zr concentration is above 2 at.%. The addition of Zr accelerated the dissociation kinetics of Nb 3 Si phase in to Nbss and α−Nb 5 Si 3 phases during solidification. The formation of α−Nb 5 Si 3 phase in the as cast condition eliminates heat treatment required for decomposition of Nb 3 Si phase in Nb-Si alloys. [ABSTRACT FROM AUTHOR]

A series of mesoporous Nb-silicates with varying Nb loading from 2.2 to 22.0 wt% were synthesized by the Evaporation-Induced Self-Assembly (EISA) method using Pluronic P123 as mesopore forming agent. High dispersion of NbOx species was achieved up to a Nb loading of ∼22 wt% without any formation of bulk crystalline Nb 2 O 5 . Small angle X-ray scattering, TEM and N 2 physisorption confirm mesoporosity in these materials with short-range ordering. The BET surface area and pore volume range from 779 to 978 m 2 /g and 0.76–1.21 cm 3 /g, decreasing at higher Nb loadings. Isolated NbO 4 and oligomeric NbOx species were evident from diffuse reflectance UV-Vis spectra. For the epoxidation of cyclohexene with H 2 O 2 , the Nb-EISA catalysts generally perform better in terms of activity, epoxidation selectivity and H 2 O 2 utilization compared to other mesoporous niobium silicates. Further, a significant improvement in the epoxide selectivity was achieved with Nb-EISA samples calcined in N 2 during synthesis such that a thin C layer is left behind. The C layer may induce pore hydrophobicity that prevents the ring-opening reaction of the primary epoxide product. [ABSTRACT FROM AUTHOR]

High-velocity shock loading was applied over an elemental Nb and Si granular mixture (atomic ratio of 5:3) using the flyer plate technique to produce niobium silicide compounds, with flyer velocities of up to 2.23 km/s and 2.63 km/s employed in two separate treatments. Although it was ascertained that both velocities were sufficient to initiate a chemical reaction between the reactant mixtures, the extent of the shock-induced chemical reaction varied with the impact velocity. Whereas the lower impact velocity (2.23 km/s) led to a partial chemical reaction and localized products of NbSi 2 , Nb 3 Si, as well as unreacted Nb and Si, the higher impact velocity (2.63 km/s) brought about a complete shock-induced chemical reaction producing single-phase Nb 5 Si 3 bulk product that matched the atomic ratio of Nb/Si in the mixture. [ABSTRACT FROM AUTHOR]

A Mo-Si-B coating was prepared on an Nb-Si based alloy by detonation-gun spraying of Mo followed by co-deposition of Si and B via pack cementation. The microstructural evolution and interdiffusion behavior of the coating/substrate system were investigated by annealing for different times at 1250 °C in argon atmosphere. The results show that the interdiffusion zone has a two-layer structure consisting of (Mo, X ′ ) 5 Si 3 (X ’  = Nb, Ti, Cr, Hf) and (Nb, X) 5 Si 3 (X = Ti, Cr, Hf), respectively. The growth of the transitional layer, which obeys a parabolic rate law, is dominated by the inward diffusion of Si from the outer layer to the substrate. The interdiffusion coefficients of Si in the transitional layers are calculated using a modified method based on the Boltzmann-Matano analysis. [ABSTRACT FROM AUTHOR]

The concentration of Si plays a crucial role in the ductile-brittle transition of Nb solid solution (NbSS) alloys, and an appropriate annealing treatment contributes to controlling the Si concentration in NbSS alloy. In this paper, Nb-1Si (at%) alloy is arc-melted and annealed at different temperature (1300, 1400, and 1500 °C) for 10 h. After annealing, both the strength and ductility increase. Particularly, after annealing at 1300 °C, the fracture feature transforms from cleavage to dimples, and the alloy possesses good strength and retains decent ductility due to the low Si concentration and small-sized intragranular Nb3Si particles precipitating in the NbSS. The significant effect of annealing on the fracture mechanism of Nb-1Si provides guidance for the design of Nb-Si based alloys. [ABSTRACT FROM AUTHOR]

The hardness, phase and microstructural development of Nb-18Si-24Ti-2Cr-2Al (at.%) alloys processed by electron beam surface melting (EBSM) and subsequent heat treatments were investigated. The EBSM experiments were performed using an electron beam based 3D printing system. Results showed that Nbss and Nb 3 Si phases were obtained via EBSM with a significantly refined microstructure. The eutectoid reaction of Nb 3 Si → Nbss + αNb 5 Si 3 was triggered by heat treatments (HT) at 1200 °C or 1450 °C for 5 h. The growth and the coarsening of αNb 5 Si 3 grains were promoted with a higher HT temperature. The hardness of the EBSM alloy was remarkably reduced by HT. [ABSTRACT FROM AUTHOR]

Peculiarities of the structure of a refractory eutectic alloy of the Nb - Si system, formed by the method of directed crystallization with liquid-metal coolant, have been studied. Characteristic zones of microstructure of the ingot obtained upon directed crystallization are considered, the alloy composition is analyzed, and volume fractions of phases in the Nb - Si composite are determined. [ABSTRACT FROM AUTHOR]

Nb-20Si-6Mo (at.%) alloy was prepared by arc melting under Ar-H 2 gas mixture atmosphere (Melt Hydrogenation Technology). It indicated that the microstructures of Nb-20Si-6Mo alloy change significantly after the melt hydrogenated treatment. The morphologies of primary Nb 5 Si 3 phase change from polygon to dendrite shapes with the increase of H content. The room temperature compression at same strain rate indicates that the σ max decreases with the increase of hydrogen content. High temperature (1400 °C) compression at same strain rate indicates that H addition can cause the peak strain to take place in advance at high deforming temperature. The high temperature compression stress tends to firstly increase and then decrease due to the effect of H addition. The reason for the effects of hydrogenation on the high temperature compression stress of Nb-20Si-6Mo alloy has been discussed. [ABSTRACT FROM AUTHOR]

Nb-18Si-24Ti-2Cr-2Al-2Hf (at.%) alloy was fabricated by selective laser melting (SLM). Results showed that an ultra-fined microstructure was achieved by SLM. After oxidation at 1250 °C for 50 h, the weight gains of Nb-Si based alloys via SLM and vacuum induction melting (VIM) were 96.48 mg/cm 2 and 198.91 mg/cm 2 , respectively. Compared with the VIM alloy, the SLM alloy exhibited a slower oxidation kinetic rate, featured by the formation of a more compact oxide scale without spallation. The improvement in the oxidation resistance was attributed to the microstructure refinement. [ABSTRACT FROM AUTHOR]

In this paper, a forward and backward feed propagation artificial neural network (BP ANN) was developed to research the quantitative relationship between silicide and fracture toughness of Nb-Si alloys. The alloys were produced by directional solidification and heat treatment. The toughness was measured by a three-point bending method used for ANN output. Five characteristic factors used for ANN input were abstracted and measured. The sequence of factors is silicide volume fraction > γ-Nb 5 Si 3 > silicide shape > silicide size > silicide continuity by sensitivity analysis. As a result of this study, the ANN model was found to be successful for predicting toughness with high accuracy and good generalization ability within the range of 9.2–26.1 MPa m 1/2 . The quantitative formulas of silicide feature parameters and fracture toughness were established by transfer function, weight matrix and threshold of ANN model. The effect of each parameter and interact influence of two parameters on the fracture toughness were studied, and the technological parameters of the alloy were optimized by artificial neural network model. [ABSTRACT FROM AUTHOR]

The microstructural characterization, fracture toughness and oxidation resistance of Nb-15Si-24Ti-4Cr-2Al-2Hf (at%, V-free) alloys and Nb-15Si-24Ti-4Cr-2Al-2Hf-1V (at%, 1V) alloys were investigated. Results showed that the V-free alloys and 1V alloys both consisted of Nbss, αNb 5 Si 3 and γNb 5 Si 3 phases, and V was primarily partitioned in Nbss phases. Compared with the V-free alloys, the 1V alloys were featured by a better continuity of Nbss, finer average diameter of silicides and lower volume fraction of γNb 5 Si 3 . The 1 at% addition of V enhanced the fracture toughness of Nb-Si based alloys from 9.87 to 12.98 MPa m 1/2 at room temperature. The fracture surfaces of 1V alloys were more undulated, compared with those of the V-free alloys. The 1 at% addition of V significantly improved the oxidation resistance of Nb-Si based alloys, by reducing the weight gain from 264.7 to 148.5 mg/cm 2 after oxidation at 1300 °C for 100 h. The transition from a paralinear oxidation kinetic to a parabolic oxidation kinetic was triggered by the addition of V. [ABSTRACT FROM AUTHOR]

Mo-Si-B alloys composed of two intermetallic compound phases (Mo 5 SiB 2 and Mo 3 Si) and a molybdenum solid solution matrix phase have been investigated for use as high-temperature structural materials due to their high melting point and good creep resistance. However, despite these advantages, Mo-Si-B alloys are difficult to use in practical applications because they have insufficient fracture toughness at room temperature. So, in many researches, microstructure control and the addition of other elements in the α-Mo matrix phase are conducted as an effective way to improve the fracture toughness. In this study, niobium (Nb) was added to a Mo-Si-B alloy by a powder metallurgical method to improve the mechanical properties. First, the Mo and Nb powders were pulverized by high-energy ball milling. Then, the synthesized intermetallic compound powders, which were fabricated by continuous heat treatment under a H 2 atmosphere, were mixed with ball-milled Mo and Nb powder. Pressureless sintering was conducted at 1400 °C for 3 h under a H 2 atmosphere. The Vickers hardness and fracture toughness were measured to investigate the mechanical properties of the sintered Mo-Si-B and Mo-Nb-Si-B alloy. The Vickers hardness was about 425 Hv for a Mo-Nb-Si-B alloy, which was lower value of 165 Hv compared to Mo-Si-B alloy (590 Hv). On the other hand, the fracture toughness of the Mo-Nb-Si-B alloy (14.5 MPa·√m) greatly increased compared to that of the Mo-Si-B alloy (12.6 MPa·√m). [ABSTRACT FROM AUTHOR]

The effect of gallium addition on the solidification microstructure and micromechanical properties of constituents was investigated in two sets of Nb 16Si(1Ga) (in at.%) and Nb 16Si 22Ti 2Cr 2Al 2Hf(5Ga) (all in at.%) alloys. The addition of gallium was found to promote the formation of Nb 5 Si 3 and (Nb,Ti) 5 Si 3 phases in Nb 16Si 1Ga and Nb 16Si 22Ti 2Cr 2Al-2Hf 5Ga alloys, respectively. Evaluation of micromechanical properties of constituents were done by nanoindentation and micropillar compression testing. Nanoindentation showed that gallium addition reduces hardness and Young's modulus for both Nb ss and silicide phases (Nb 3 Si and (Nb,Ti) 5 Si 3 ); while it enhances fracture toughness of silicide phases. However, micropillar compressions showed that the addition of gallium decreases the compressive strength of the Nb ss phase while increasing the strength of the silicides. [ABSTRACT FROM AUTHOR]

In this work, as-quenched FeCuNbSiB amorphous strips were treated by conventional one-step and two-step annealing processes, respectively. And the magnetic properties (core loss and dynamic coercivity), crystallization kinetics and microstructures of annealed samples were investigated. Results show that two-step annealing process can effectively improve magnetic properties and optimize microstructures. More uniform grain size distribution was achieved by two-step annealing process compared with one-step annealing process, owing to promoting nanocrystallite nucleation and inhibiting further grain growth by two-step annealing process. And some nanocrystallites tend to be adjacent to grain boundary rather than be surrounded by remaining amorphous in all surfaces of nanocrystallines. Nevertheless, too much low-angle grain boundaries in nanocrystalline ribbons would lead to deteriorate magnetic properties. What interesting is that the relation, between dynamic coercivity and external field, associated with nanocrystallization temperature, could be a practical method to characterize the change mechanism of magnetization, and needs to be confirmed by further theoretical and experimental investigation. [ABSTRACT FROM AUTHOR]

Nb Si-based alloy, which primarily consists of niobium solid solution (Nb SS ) and α-Nb 5 Si 3 , is a promising ultra-high temperature material. The overall fracture toughness of this alloy is limited by the brittle α-Nb 5 Si 3 phase. Alloying Ti, which can improve the toughness of Nb SS , is typically introduced into Nb Si-based alloy. The overall toughening effect of the alloying elements on multi-composites is dependent on the alloying effect on each component. However, the influence of Ti on the brittleness of α-Nb 5 Si 3 is unclear, which makes the beneficial effect of Ti on the mechanical properties of Nb Si-based alloy ambiguous. By calculating the ideal cleavage energy and cleavage strength using a first-principles method, we find that a low amount of Ti will improve the toughness of Nb 5 Si 3 , while Ti-rich Nb 5 Si 3 becomes more brittle, which is in good agreement with experimental results. The concentration-sensitive effect of alloying Ti is induced by the variation of its substitution site and can be well explained by the electronic structure analysis. Our findings demonstrate that the alloying amount of Ti should be carefully selected in the Nb Si-based alloy. [ABSTRACT FROM AUTHOR]

A B-modified silicide coating was prepared on the Nb-Si based alloy by Si-TiB 2 co-deposition at 1300 °C for 10 h. The results show that the two-layer coating is composed of a (Nb, X)Si 2 (X represents Al, Cr, Ti and Hf) + NbTiB 4 outer layer and a (Nb, X)Si 2 + Cr 2 Nb inner layer. Static oxidation tests indicate that the mass gain of B-modified silicide coating was 2.39 mg/cm 2 after oxidation at 1250 °C for 100 h. The good oxidation resistance is attributed to the formation of a continuous and dense scale mainly consisting of glassy borosilicate. [ABSTRACT FROM AUTHOR]

Y and Ge modified silicide coating was prepared on the Nb-Si based alloy by Si–Ge–Y co-deposition at 1300 °C for 10 h. It is observed that the coating is composed of an outer layer and a transitional layer. The mass gain of Si–Ge–Y coating is 2.78 mg cm −2 after oxidation at 1250 °C for 100 h. The results indicate that Y refines grains, which can promote the rapid formation of protective SiO 2 and GeO 2 scale. Therefore, the oxidation resistance of the coating is improved. [ABSTRACT FROM AUTHOR]

FeCuNbSiB nanocrystalline alloys were prepared by using conventional one-step annealing process and two-step annealing process. It was found that two-step annealing process can effectively improve soft magnetic properties and optimize microstructure. By separately controlling the formation of Cu clusters and further optimize the nanocrystalline structure, Bcc α-Fe with the grain size of 13 nm is formed in samples pretreated at 400 °C and nanocrystallized at 560 °C for 1 h. The samples exhibit excellent magnetic properties, such as lower coercive force of 0.7 A/m, higher initial permeability of 9.16 × 10, lower core loss of 0.18 W/kg at 0.7 T and 400 Hz, and 0.5 W/kg at 0.7 T and 1 kHz, respectively. The microstructure and magnetic structure evolution during different annealing processes were investigated. Correlation among the magnetic properties, magnetic structures and the microstructures changes in two different crystallization processes was studied systematically. [ABSTRACT FROM AUTHOR]

The effect of vacancy on mechanical properties of α-Nb5Si3 is systematically investigated by first-principles calculations. Four different mono vacancies in this alloy are considered in detail. The vacancy formation energy, formation enthalpy, elastic modulus, hardness, B/G ratio, thermodynamic properties and electronic structure of α-Nb5Si3 with different vacancies are discussed. The calculated vacancy formation energies show that Nb vacancies are more stable than that of Si vacancies, and a-Nb5Si3 prefers to form the Nb-va2 vacancy. Those vacancies weaken the volume, shear deformation resistances and reduce the elastic stiffness. However, those vacancies result in brittle-to-ductile transition and a-Nb5Si3 with Si-va1 mono vacancy exhibits ductile behavior. The calculations of electronic structure reveal that these vacancies change the localized hybridization between Nb-Si and Nb-Nb atoms, which are the origin of brittle-to-ductile transition. Finally, we conclude that vacancy is beneficial for improving the ductility of Nb5Si3. [ABSTRACT FROM AUTHOR]

The Laves phase strengthened ferritic steel Crofer 22 H has been proposed as construction material for interconnects in solid oxide fuel cells. The background of the present study relates to the further qualification of this steel, especially with respect to a possible optimization of amount and composition of the strengthening Laves phase precipitates. For this purpose the chemical composition of the Laves phase in a number of high purity model alloys as well as in Crofer 22 H equilibrated at temperatures between 700 and 1100 °C was measured by EDX/WDX and atom probe tomography (APT). The obtained chemical compositions were used for a qualitative estimation of the site occupancy for Fe, Cr, Nb, W and Si in the Laves phase unit cell. Additionally, the results from APT measurements indicate the important role of impurities such as e.g. titanium in the Laves phase formation. Finally, the experimental results were compared with Thermocalc calculations using the database TCFE 7. This revealed that within the temperature range 800-900 °C a qualitative description of phases is possible, however, substantial differences existed particularly for the steel Crofer 22 H at and below 700 °C and above 950 °C. [ABSTRACT FROM AUTHOR]

Monolithic amorphous Fe73.5Nb3Si13.5B9Cu1/Fe74.5Nb3Si13.5B9 bilayer ribbons were obtained by double-nozzle melt-spinning and subsequently annealed to produce a composite with a tailored nano/micro-crystalline structure. The overall magnetic behavior is characterized by butterfly-shaped high field hysteresis loops and positively biased low field ones. The main questions we wish to address here are whether the global magnetic behavior of the bilayer can be separated into the individual contributions of each layer and the magneto-coupling between them can be well understood. For that purpose, we performed first-order reversal curve analysis, which enabled us to distinctly identify two phases, of ultra-soft and semi-soft magnetic natures, whose mutual predominant interaction is the magnetostatic coupling. [ABSTRACT FROM AUTHOR]

The influence of phase transformation conditions (α/β solution treatment plus aging, β solution treatment plus aging, and β solution treatment plus duplex aging) on the microstructure and tensile properties of Ti-3Al-15Mo-3Nb-0.2Si alloy was studied. Microstructure observation indicates that the primary α phase, which formed during α/β solution treatment, can effectively limit the growth of the β grains. The alloy was heat treated in the α/β solution, which together with aging, formed smaller grains and finer precipitates in the sample and showed good tensile ductility with a 15% elongation. Aging temperatures from 450 to 600 °C were used to study the effects of aging treatment on the alloy's microstructure and tensile properties. Precipitation phases appeared in particular positions (α, α, and α) during the aging process which depended on phase transformation conditions and caused variations in the alloy's tensile properties. The alloy treated by duplex aging after β solution had a higher ultimate tensile strength at 1370 MPa than that of other samples, resulting from the finer α precipitates transformed from the ω phases. The relationship of tensile properties and phase transformation was investigated by observing the material fracture, and small and dense dimples were seen in the tensile specimen treated by α/β solution plus aging treatment which led to excellent ductility. Deep dimples in the duplex aging specimen resulted in beneficial high strength. [ABSTRACT FROM AUTHOR]

With the guidance of the calculated phase diagram of the Nb–Ti–Si–B system, three alloys (M1: Nb–20Ti–14Si; M2: Nb–20Ti–14Si–4.5B; M3: Nb–24Ti–14Si–4.5B (at.%)) were selected to investigate the influence of alloying elements B and Ti on their microstructural formation. The alloys were prepared by arc-melting and then directionally solidified (DS) at a withdrawing rate of 10 μm/s. The DS microstructures in the near-solidification front zone, the mushy zone, and the steady-state zone were characterized by SEM, EPMA and XRD analyses. The experimentally observed solidified microstructures were then compared with the predicted solidification path calculated from the Scheil model. The good agreement between the experimental observations and the predicted solidification paths suggests that the computational models can be effectively used for the design of the Nb–Ti–Si–B alloys. [ABSTRACT FROM AUTHOR]

Isothermal oxidation behavior of the single layer NbSi 2 and the double layered (NbSi 2 + Nb 5 Si 3 ) silicide coated Nb–1Zr–0.1C (wt.%) alloys has been studied between 800 and 1300 °C in static laboratory air. The single layer coated alloy shows a lower weight gain as compared to the double layer coatings at all temperatures. The formation of amorphous silica scale embedded with Nb 2 O 5 oxide particles is observed beyond 1100 °C, and a superior oxidation resistance is detected between 1100 and 1300 °C. A kinetic equation relating the thickness of the coating layer with the various process parameters for pack siliconizing of Nb-alloy has been established. [ABSTRACT FROM AUTHOR]

Transition metal silicides are of great interest as high-temperature structural materials. Recent investigations aimed at the development of high-temperature Niobium-base structural alloys where silicide phases play a major role. Tantalum, in spite of its high atomic mass, is a potential micro-alloying element in these alloys to provide higher strength at high temperatures. In this work the invariant reactions of the Ta-Si system were reviewed via microstructural characterization of as-cast alloys. Samples were prepared by arc melting and their microstructures were characterized by scanning electron microscopy and x-ray diffractometry. The results confirmed the solid phases and the nature of the invariant reactions in this system. Few corrections in the currently accepted phase diagram are suggested: (1) the composition of the liquid phase that participates in peritectic formation of TaSi should be near 32 at.% Si instead of 28 at.% Si; (2) the inclusion of the βTaSi ↔ αTaSi + L metatectic reaction involving the βTaSi, αTaSi and L phases. [ABSTRACT FROM AUTHOR]

This letter attempts to comment on the article by Chen et al. [Mater. Sci. Eng. A 688 (2017) 416] in the aspects of microstructure and tensile properties of ductile iron with niobium additions. [ABSTRACT FROM AUTHOR]

The influence of B element on structure and mechanical properties of NbSi 2 is investigated by using the first-principles approach. To explore the solution strengthening mechanism of B element, the doped sites including the octahedral interstice site (OI) and tetrahedral interstice site (TI) are considered. It is found that B element prefers to occupy the OI site in comparison with the TI site. B element occupied OI site can slightly improve the volume deformation resistance for NbSi 2 . Importantly, B element leads to brittle-to-ductile transition. The reason is that the addition of B element can weaken the charge interaction between the Nb and Si atoms. [ABSTRACT FROM AUTHOR]

Nb-Si alloys have been widely considered as the potential candidate materials for turbine applications. Optimal amounts of B and Si addition to Nb-18Ti-14Si (at%, F1) were identified based on Nb-Ti-Si-B phase diagram. Nb-18Ti-14Si-4.5B (F2) had been obtained desirable (Nb) + T2 eutectic and significantly improved fracture toughness from 7.85 (F1) to 11.98 MPa m1/2. With extra 3Si addition, F3 (Nb-18Ti-17Si-4.5B) consisted of primary T2 and (Nb) + T2 eutectic, it retained comparable fracture toughness of 11.76 MPa m1/2, even excessive amounts of primary T2 do not compromise fracture toughness. [ABSTRACT FROM AUTHOR]

First-principles calculations were carried out for the Nb3Si, αNb5Si3, βNb5Si3 and NbSi2 intermetallic compounds and their constituent elements. The enthalpies of formation were computed for all the intermetallic compounds, as well as the eutectoid transformation temperature for Nb3Si → Nb + αNb5Si3, the transition temperatures of βNb5Si3 to αNb5Si3, and the range of stability of the NbSi2. There was good agreement between the results of the present study and the Nb-Si phase diagram. The elastic constants, bulk, shear and Young's moduli, Poisson's ratio and Debye temperature were also calculated for the intermetallic compounds and Nb and Si. The Cauchy pressures, Pugh's index and Poisson's ratio indicate that the high temperature phases (βNb5Si3 and Nb3Si) should be more ductile (with Nb3Si being the most ductile) than the αNb5Si3 and NbSi2 (with the latter being the most brittle). [ABSTRACT FROM AUTHOR]

Two kinds of niobium(V)-silica catalysts for the selective epoxidation were synthesised by post-synthesis modification of non-ordered mesoporous silica supports, starting from niobocene dichloride via solvent-less organometallic precursor dry impregnation or conventional liquid-phase grafting technique. Grafted Nb/SiO2 solids were used as catalysts, in the presence of aqueous H2O2, for the epoxidation of unsaturated cyclic and terpenic compounds of interest for fine and specialty chemistry, in particular: cyclohexene, 1-methylcyclohexene, limonene, carveol, α-terpineol, isopulegol, carvotanacetol, carvone, as well as squalene and isopulegyl acetate. These catalysts showed high yields (up to 73%) and excellent chemoselectivities to the desired epoxides (up to 98%), also in short reaction times (down to 1 h). [ABSTRACT FROM AUTHOR]

Highlights: [•] The coating has the three-layer structure, with a thick (Nb, X)Si2 outer layer. [•] Some “oxide pegs” formed at the interface of the oxide scale and coating. [•] The coating exhibits better cyclic oxidation resistance than the Si–Al coating. [ABSTRACT FROM AUTHOR]

Highlights: [•] Microstructure of Nb/Nb5Si3 composite alloyed with W and Mo is change obviously. [•] W and Mo elements can solid solution in Nb and Nb5Si3 phase respectively. [•] Alloyed with W and Mo together, the solid solubility of Nb5Si3 phases is undetected. [•] The Nb/Nb5Si3 composite alloyed with W and Mo together has high fracture toughness. [Copyright &y& Elsevier]

We are studying the properties of Niobium Silicon amorphous alloys as a candidate material for the fabrication of highly sensitive kinetic inductance detectors (KID), optimized for very low optical loads. As in the case of other composite materials, the NbSi properties can be changed by varying the relative amounts of its components. Using a NbSi film with T $$_c $$ = 1 K we have been able to obtain the first NbSi resonators, observe an optical response and acquire a spectrum in the band 50-300 GHz. The data taken show that this material has very high kinetic inductance $$L_k$$ and normal state surface resistivity $$\rho _n$$ . These properties are ideal for the development of KID. More measurements are planned to further characterize the NbSi alloy and fully investigate its potential. [ABSTRACT FROM AUTHOR]

As shown by the Planck mission (Planck Collaboration. Astronomy and astrophysics. arXiv1303.5071P, ), background limited bolometers in a space environment are very sensitive to high energy particles. In order to not degrade their sensitivity, it is necessary to reduce this unwanted signal. To achieve this goal, a good understanding of the detector's thermal architecture is mandatory. To investigate this question, we used an $$\alpha $$ particle source in front of our niobium silicon (NbSi) alloy Transition edge sensors (TES). The number of time constants required to fit the data and the way these time constants behave as we change the bias power gave us a good insight on the TES thermal architecture. Indeed we expect a decrease of the detector time constant due to the electro-thermal feedback properties. We will first present some standard characterizations of NbSi TES using a simple thermal model and how they indicate the presence of multiple thermal decouplings. Then we will show the results of the $$\alpha $$ particles measurements and how we used them to build our thermal model for Complex Impedance fitting. All this work has been done for the QUBIC experiment, a B-modes instrument. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the effect of adding 2 to 8 at.%Sn on the microstructure of Nb–25Ti–8Hf–2Cr–2Al–16Si. The samples were synthesised by casting and were heat treated for 5 days at 1200 °C. The as-cast and heat-treated microstructures were characterised using X-ray diffraction, scanning electron microscopy and microprobe analysis. The results revealed strong microstructural changes when Sn content exceeded 2 at.% Sn. The two-phase (Nb,Ti)SS/γ-M5Si3 composite evolves towards a quaternary phase equilibrium: (Nb,Ti)SS saturated with Sn/α-M5Si3/(Nb1−x Ti x )3(Sn1−y Ti y )/γ′-M5Si3. The γ′-M5Si3 differs from the γ-M5Si3 through its Hf and Ti contents. The (Nb,Ti)SS fraction decreases strongly, benefitting the (Nb1−x Ti x )3(Sn1−y Ti y ) fraction. Therefore, adding greater than 2 at.% Sn addition may dramatically affect the mechanical properties of Nb/Nb5Si3 composites because the solid solution (Nb,Ti)SS has been shown to dominate the toughness of such materials. [Copyright &y& Elsevier]

This paper reports the effects of adding from 2 to 8 at.% tin on the oxidation behaviour of Nb/Nb5Si3 composites at 815 °C and at higher temperatures (1100 and 1200 °C). The role of tin in the elimination of pesting and in the oxidation process at high temperatures was established. The consumption of elements with a higher affinity for oxygen than Sn induces the accumulation of tin at the oxide/internal oxidation zone boundary. Low melting point phases (NbSn2 and/or pure Sn) form at 815 °C, whereas a layer of M5Si3 and Nb5SiSn2 forms at 1100 and 1200 °C. Once these products are formed, they generate an oxygen diffusion barrier and allow the elimination of pesting. However, for long oxidation processes at 1100 °C, the oxidation rate of Nb/Nb5Si3 composites containing tin should be higher than that for tin-free composites. Moreover, some oxidation results have suggested that the presence of A15-(Nb,Ti)3(Sn,Ti) in the microstructure of composites with at.%Sn > 2 can severely impact the low temperature fracture toughness of these composites. [ABSTRACT FROM AUTHOR]

Highlights: [•] The DS samples were fabricated by ECCDS. [•] With increasing withdrawal rate, the microstructure became finer. [•] With increasing withdrawal rate, KQ first decreased and then increased. [•] With increasing withdrawal rate, the tensile strength (1250°C) increased. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with phase constitutions, microstructural evolutions, and mechanical properties of Nb–16Si–xFe in-situ composites (where x = 2, 4, 6 at.%, referred as to 2Fe, 4Fe and 6Fe alloys, hereafter) prepared by arc-melting. It is found that with additions of Fe, Nb4FeSi silicide arises and microstructures of as-cast samples are consisted of dendritic-like NbSS phase, Nb3Si block, and Nb4FeSi matrix in the 2Fe and 4Fe alloys, and of the dendritic-like NbSS phase and Nb4FeSi matrix in the 6Fe alloy. When heat-treated at 1350 °C for 100 h, part of the Nb3Si phase decomposes in the 2Fe and 4Fe alloys, and the 6Fe alloy shows no change in microstructure as compared with the as-cast one. The Nb4FeSi silicide is found to be brittle, its fracture toughness and elastic modulus are first obtained, having values about 1.22 MPa m1/2, and 310 GPa, respectively. The fracture toughness of the bulk as-cast and heat-treated Nb–16Si–xFe samples are changed slightly by the Fe additions, which is in a range of 9.03–10.19 MPa m1/2. It is interesting that at room temperature, strength is improved by the Fe additions, whereas at 1250 °C and 1350 °C the strength decreases. As the Fe content increased from 2 at.% to 6 at.%, for example, the 0.2% yield strength increases from 1410 MPa to 1580 MPa at room temperature, decreases from 479 MPa to 385 MPa at 1250 °C. [Copyright &y& Elsevier]

NbSi alloys were prepared by direct electrochemically reducing four mixed Nb2O5 and SiO2 powders (Nb-10Si, Nb-20Si, Nb-30Si, and Nb-37.5Si) in molten CaCl2 electrolyte at 900° C. The samples were characterized with scanning electron microscope (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). No oxidized phases were remained by XRD tests. Under SEM, Nb phase was scattered in Nb5Si3 phase for the samples of Nb-10Si, Nb-20Si, and Nb-30Si. For the sample of Nb-37.5Si, only Nb5Si3 was obtained after electrolysis. [ABSTRACT FROM AUTHOR]

Abstract: Nb5Si3 intermetallic compound was directly synthesized via an electrodeoxidation process from molten CaCl2 electrolyte. The reaction mechanism was studied by performing a series of quenching investigations, which suggested that the formation of a series of sub oxides and compounds containing calcium and oxygen preceded the formation of Nb5Si3. While Nb2O5 was gradually reduced to its sub oxides, (Nb2O5/NbO2/NbO/Nb), SiO2, on the other hand, got directly reduced to silicon and reacted with Nb to form Nb5Si3. [Copyright &y& Elsevier]

Abstract: Properties of amorphous Fe73Cu1Si16B7Nb3 foil, the Si rich metal alloy, and the foils partly crystallised after annealing, were analysed structurally and magnetically in the temperature range from 4 to 1000K. The Fe (Si) and Fe (B) structures were identified and characterised with the crystallisation temperatures: 750 and 893K, activation energies 460 and 580kJ/mol. The Curie temperatures for amorphous structure: 613K and for crystalline structures: 820, 875, 920 and 980K were determined. It was found and analysed a delay of the sample magnetisation, determined by thermomagnetometry, with respect to structural crystallisation, determined by scanning calorimetry, which was correlated with magnetic hyperfine field, determined by transmission Mössbauer spectroscopy. [Copyright &y& Elsevier]

Abstract: Nb–Ti–Si based alloys are considered as candidates of next-generation high temperature materials (i.e., working temperature >1200°C). Boron has a beneficial effect in enhancing the oxidation resistance and reducing the anisotropy ratio a c/a a of the T2 phase in the Nb–Si alloys. The liquid–solid multiphase equilibria in the Nb–Ti–Si alloys with B additions have been investigated via an approach of integrating thermodynamic modeling with designed experiments. The present study suggests that Ti and Si additions increase the stability of Nb3B2, and the primary region of Nb3B2 will appear in the Nb–Ti–Si–B quaternary, while there is no primary solidification region of Nb3B2 in the constitute binaries and ternaries. The proposed liquidus surface of the Nb-rich Nb–Ti–Si–B system is associated with eight primary solidification regions of Nbss, T2, T1, D88, (Nb,Ti)3Si, NbB, TiB and Nb3B2. The direct eutectic solidification occurs between Nbss and T2 in the Nb-rich region in the Nb–Ti–Si–B liquidus projection, which could provide new opportunities for alloy design based on the Nb–Ti–Si–B system. [Copyright &y& Elsevier]

We present preliminary results on characterisation of voltage biased superconducting bolometers with an NbSi alloy sensor. I( V) curves as well as complex impedance and noise measurements are carried out in a dilution fridge with a pulse tube as a first stage cooler. SQUIDs polarization and addressing are controlled by a cold ASIC in which a SiGe amplifier has been implemented. First characterisations were done on full membrane bolometers and show a noise level of about $10^{-16}\ \mathrm{W}/\sqrt{\mathrm{Hz}}$. Those detectors are developed for the QUBIC experiment, a B-modes telescope that will be deployed in Antactica in 2013. [ABSTRACT FROM AUTHOR]

We describe the transition-edge-sensor (TES) focal plane arrays used for three cosmic microwave background (CMB) polarimetry projects. The first is BICEP2. The focal plane array for BICEP2 has been delivered and the instrument is now taking data at the South Pole. Second is the Keck Array. The focal planes for Keck will be nearly identical to those of BICEP2, but will cover multiples frequencies (96, 150 and 225 GHz). The scheduled deployment of Keck is late 2010. Finally, there is Spider, a high-altitude balloon observatory, which will also cover 96, 150, and 225 GHz and will be deployed in 2012. The goal of all three experiment is detecting B modes in the polarization of the CMB. [ABSTRACT FROM AUTHOR]