Niobium Silicides Research Articles

Microstructural Evolution and Mechanical Properties of a Multicomponent Nb-16Si-22Ti-2Al-2Hf-2Cr Alloy Prepared by Reactive Hot Press Sintering

In this paper, the effect of reactive hot press sintering (RHPS) parameters, such as milling time, hot pressing temperature, and heat treatment, on phase constitution, microstructure, and room temperature mechanical properties of a multicomponent Nb-16Si-22Ti-2Hf-2Al-2Cr alloy prepared from ball milled powder mixture is investigated. Evaluation of the microstructure revealed that all as-sintered and heat-treated samples mainly consisted of particles of Nb and Ti solid solutions (NbSS and TiSS), as well as a niobium silicide β-Nb5Si3 matrix, with a small quantity of Hf solid solution particles (HfSS). By sintering at 1773 K and 1873 K (1500 °C and 1600 °C), most of the NbSS phase in the samples prepared from 5 and 10 ball milled powder mixtures showed a narrow strip morphology. Upon increasing the milling time to 20 hours, the morphology changed to a near-equiaxed shape, which became finer with increased milling time. In contrast, the TiSS phase in all as-sintered samples more or less had a near-equiaxed shape. Spheroidizing tendency took place in both NbSS and TiSS phases during an annealing heat treatment at 1773 K (1500 °C) for 50 hours. Interestingly, when the milling time was extended from 5 to 20 hours, the volume fractions of the β-Nb5Si3 and TiSS phases increased, whereas that of the NbSS phase decreased. This resulted in the reduction of the fracture toughness KQ and an enhancement of the Vickers hardness Hv of the bulk as-sintered and heat-treated samples. A fractography analysis was also carried out to elucidate the fracture behavior of phases, with particular emphasis on the interaction between phases and cracks.

High-temperature Nb-Si composites. Part 1

The article presents an overview of the projects aimed at development of a new class of high-temperature Nb-Si composites consisting of a niobium matrix and a strengthener in the form of niobium silicides. Nb-Si binary diagrams and Nb-Cr-Si, Nb-Hf-Si, Nb-Ti-Si, and Nb-Si-Mo ternary diagrams, as well as Nb-Si-Ti-Cr and Nb-Si-Ti-Hf quaternary diagrams, are analyzed, on the basis of which the compositions of the composite materials are selected. Major methods of obtaining of the composites are considered, including directional solicitation. The main physicomechanical properties are analyzed: short- and long-term strength, destruction viscosity, elasticity modulus, thermal expansion, high-temperature strength, and protective coatings. The density of Nb-Si composites is 6.6–7.2 g/cm3, and the operating temperature is 1350°C, which is 200°C higher than temperature ability of monocrystals of nickel high-temperature alloys of the last generations.

High-temperature Nb-Si composites. Part 2

The article presents an overview of the projects aimed at development of a new class of high-temperature Nb-Si composites consisting of a niobium matrix and a strengthener in the form of niobium silicides. Nb-Si binary diagrams and Nb-Cr-Si, Nb-Hf-Si, Nb-Ti-Si, and Nb-Si-Mo ternary diagrams, as well as Nb-Si-Ti-Cr and Nb-Si-Ti-Hf quaternary diagrams, are analyzed, on the basis of which the compositions of the composite materials are selected. Major methods of obtaining of the composites are considered, including directional solicitation. The main physicomechanical properties are analyzed: short- and long-term strength, destruction viscosity, elasticity modulus, thermal expansion, high-temperature strength, and protective coatings. The density of Nb-Si composites is 6.6–7.2 g/cm3, and the operating temperature is 1350°C, which is 200°C higher than temperature ability of monocrystals of nickel high-temperature alloys of the last generations.

Silicidation of Niobium Deposited on Silicon by Physical Vapor Deposition

Niobium was deposited by physical vapor deposition (PVD) using e-beam evaporation on bare (100) silicon substrates and SiO2 surfaces. The formation of niobium silicide was investigated by annealing PVD Nb films in the temperatures range 400–1000°C. At all elevated annealing temperatures the resistivity of Nb silicide is substantially higher than that of Nb. The Nb silicidation as a function of temperature has been investigated and different NbXSiy compounds have been characterized. It has been observed that the annealing of the Nb film on Si is accompanied by a strong volume expansion of about 2.5 of the resulting reacted film. The films’ structural properties were studied using X-Ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM), which was not previously presented in the context of the extant NbSi literature. The X-Ray diffraction characterization of the Nb on Si sample annealed at 1000°C, showed the presence of hexagonal Nb5Si3 phases, with a dominant peak at the (200) plane, and NbSi2 phases. Fractal dimension calculations indicate a distinct transition from Stranski-Krastanov to Volmer-Weber film growth for NbSi formation at the annealing temperature of 600°C and above.

Microstructural Properties of Nb-Si Based Alloys Manufactured by Powder Metallurgy

An NbTiHfCrAlSi niobium silicide based atomized powder has been compacted by a conventional technique (hot extrusion) and by spark plasma sintering to nearly fully dense alloys. Both materials exhibit a metastable fine micrometer-sized microstructure that has been coarsened by a subsequent heat treatment. The densification of the SPS sample takes place between ca. 800°C and 1300°C.

Atomic Layer Deposition and Superconducting Properties of NbSi Films

Atomic layer deposition was used to synthesize niobium silicide (NbSi) films with a 1:1 stoichiometry, using NbF5 and Si2H6 as precursors. The growth mechanism at 200 °C was examined by in-situ qua...

Silicide characterization at alumina–niobium interfaces

Alumina–niobium interfaces formed by liquid-film-assisted joining with copper/niobium/copper interlayers exhibited microstructures that depend on the nature of the alumina components. Characterization of these interfaces in the transmission electron microscope provided insight on the relationship between interfacial microstructure and fracture performance. Interfaces between sapphire and niobium and those between high-purity (99.9%) polycrystalline alumina and niobium were free of secondary phases. However, niobium silicides were found at interfaces between lower-purity (99.5%) alumina and niobium, identified by electron diffraction analysis as the body-centered tetragonal α-Nb5Si3 phase. Spatially resolved compositional analysis was conducted on silicide particles at and away from the interface.

Deposition of aluminide and silicide based protective coatings on niobium

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl3 in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi2 in the inner layer and a ternary compound of Nb–Si–Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi2 exhibits much stronger covalent character as compared to NbAl3. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226mJ/m2 and 3545mJ/m2, respectively, indicating stronger Nb–Si bonding across the interface.

Microstructure and room temperature mechanical properties of hot-pressed Nb–Si–Ti–Fe alloys

Bulk alloys were fabricated by uniaxial hot pressing from ball milled Nb-Si–Ti–Fe powder mixtures with the fixed Si content (16 at.%) and various compositions of Ti and Fe, and the effects of Ti and/or Fe on the ball milling, microstructure and room temperature mechanical properties of the materials were investigated. It was revealed that the Ti addition inhibited the ball milling process with the same milling parameters. For the hot-pressed alloys, Nb 4FeSi phase was detected in the alloys containing Fe, and Fe mainly dissolved into niobium silicides from the result of elemental mapping. The solubility of Si in Nbss was enhanced by the addition of Ti, and Nb 3Si expected was not observed in the Ti-bearing alloys that may be due to the lower Ti content in the reaction zone between the particles. The Ti-added alloys hot pressed from a shorter time showed the coarser microstructure with Nbss in the shape of narrow strip distributed in river pattern, while near-equiaxed macrostructure was obtained in the Nb–Si–Fe alloys. The improvement in both room temperature fracture toughness and flexural strength was achieved in Ti-added alloys. The toughening and strengthening mechanisms were discussed from the ductile-phase toughening of crack bridging and solid solution strengthening, respectively.

Effect of mechanical alloying on microstructure and mechanical properties of hot-pressed Nb–16Si alloys

Nb–16 at.% Si bulk alloys were fabricated by hot pressing from compound powders milled for different times. The influences of mechanical alloying on microstructure and mechanical properties of the hot-pressed alloys were investigated. The results indicate that the particle size of powders was significantly refined and the addition of absolute ethyl alcohol suppressed the agglomeration of powders. When milling time increased from 6 to 24 h, the constituent phases (Nbss, Nb 5Si 3 and Nb 3Si) changed from large size trips to fine near-equiaxed shape, and the volume fraction of ductile Nbss decreased. In addition, Vickers hardness and flexural strength of bulk alloys increased gradually owing to the enhancement of the volume fraction of niobium silicides. Fracture toughness was in range of 10.98–13.23 MPa m 1/2, which was higher than that of cast materials. It was attributed to the positive role of ductile phase and fine-grain toughening.

Effects of Ti, Al and Hf on niobium silcides formation in the Nb-Si in situ composites

The effects of alloying elements Ti, Al and Hf on niobium silicides formation in the Nb-Si in situ composites have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The binary, ternary and multicomponent alloys have been fabricated by vacuum non-consumable arc melting method. The results show that Ti tends to stabilize Nb3Si phase, while Al promotes the direct formation of β-Nb5Si3 phase with a tetrahedral D8m structure. Exceptionally, it seems that Hf is beneficial to the formation of γ-Nb5Si3 phase with a hexangular D88 structure. For the multicomponent Nb-Si in situ composites, the cooperative effects of different elements on niobium silicides formation basically maintain the character of ternary system.

Solid-state reactions of silicon carbide and chemical vapor deposited niobium

Niobium films were deposited on silicon carbide by chemical vapor deposition using niobium chloride and hydrogen at a temperature range of 900–1300°C. The solid-state reactions between the deposited niobium and silicon carbide matrix were studied by examining the obtained films using X-ray diffraction and energy dispersion spectroscopy. The results indicated that niobium silicides could be formed at the beginning, which blocked further reactions between carbon and niobium to form niobium carbides. When the deposition temperature was increased, silicon would diffuse outward, which allowed the formation of niobium carbides. The reaction process and mechanism are discussed based on the thermodynamics and kinetics.

Effect of Ductile and Brittle Phases on Deformation and Fracture Behaviour of Molybdenum and Niobium Silicide Based Composites

A comparative study on the microstructure-mechanical property relationships in the molybdenum and niobium silicide based composites has been carried out with emphasis on the role of the ductile and brittle phase constituents at ambient and elevated temperatures. The MoSi2, MoSi2-20 vol.% SiC and -Mo-Mo3Si-Mo5SiB2 composites have been prepared by powder metallurgy processing. Furthermore, the niobium silicide based composites, having a eutectic of Nb solid solution (Nbss) and (Nb,Mo)5Si3, and either Nbss or (Nb,Mo)5Si3 as the primary phase in the hypoeutectic or hypereutectic compositions, respectively, have been processed by arc melting. The increase in fracture toughness with respect to that of MoSi2 is modest in the MoSi2-SiC composites, and more significant in the multiphase Mo-Mo3Si-Mo5SiB2 and Nbss-(Nb,Mo)5Si3 based in-situ composites with ductile reinforcements. The ductile phase, either Mo or Nbss aids in toughening chiefly through crack arrest and bridging, and undergoes plastic yielding under constraint during deformation, leading to a higher energy of fracture. In the MoSi2 and MoSi2-SiC composites, the matrix grain size has a significant role in high temperature strength retention and strain hardening behaviour. In the ductile phase reinforced composites, the hard silicide-based intermetallic phases contribute to elevated temperature strength, while the constrained plastic deformation of the -Mo or Nbss is responsible for much higher rate of strain hardening than in the MoSi2 and MoSi2-SiC composites.

Deformation Behavior of Niobium Silicides during High Temperature Compression

AbstractDeformation behavior of (Nb)/α-Nb5Si3 two-phase alloys is examined by high temperature compression tests. The alloys exhibit brittle fracture behavior at temperatures up to 1473 K, while reasonable compressive deformability at 1673 K. Upon high temperature compression of the alloys, the flow stress gradually decreases after the peak stress due to the recrystallization/recovery in the (Nb) phase, as well as the increase in the density of mobile dislocations within the α-Nb5Si3 phase. Two types of slip systems that operate in α-Nb5Si3 have been identified as {011)<11-1] and {001)<100] in the present study.

Microstructure and Mechanical Properties of Directionally Solidified Nb‐22Ti‐16Si‐7Cr‐3Al‐3Ta‐2Hf‐0.1Ho Alloy

The attractive combination of properties makes Niobium silicides one of the promising candidates of the next generation materials for high-temperature structural applications. However, the low ductility and toughness of Niobium silicides at room temperature have limited its practical applications. It was recently reported that directional solidification simultaneously improved the mechanical properties as does the addition of Ho. To further improve the strength and ductility of Nbss/Nb5Si3 composites, the authors combined the above-mentioned two benefits into one system.

Microstructural and mechanical characterization of Nb-based in situ composites from Nb–Si–Ti ternary system

This study deals with the Nb–niobium silicide-based composites developed by the hot-pressing of Nb–Si–Ti ternary powder mixtures with a fixed Ti addition (6 at.%) and Si content ranging from hypereutectic (11 at.%) to near-eutectic compositions (18 at.%). The effects of Si content, Ti addition and strain rates on the sample microstructural characterization, flexural strength, fracture toughness, quasi-static compressive deformation and failure processes were investigated. It was revealed that the volume fraction of silicides increased with increasing Si content, and most of the Ti atoms dissolved into the niobium silicides to form (Nb,Ti) 5Si 3 solid solutions instead of binary titanium silicides. The experimental evidence showed that a moderate improvement in the flexural strength, fracture toughness and compressive yield stress of the composites was achieved by the addition of Ti. Higher Si additions produced a much more remarkable enhancement in the compressive yield stress and bulk hardness, whereas both the flexural strength and fracture toughness decreased with increasing Si content owing to the existence of residual porosities in the samples. The composites showed remarkable superiority to the arc-melted Nb–Si alloys and monolithic niobium silicides in fracture toughness ( 8.3 – 13.0 MPa m vs. 4.5 MPa m ), where the toughening effect was attributed mainly to crack bridging and crack deflection by the remaining ductile Nb phase. Moreover, quasi-static uniaxial compression tests at strain rates between 10 –5 and 10 –3 s −1 indicated that the deformation behavior and failure processes were significantly affected by Si content and strain rates. The strain-rate-hardening behavior for all the strain rates was observed in the composite materials and the strain-rate sensitivity decreased with increasing Si content. At a lower strain rate, the composite materials with a hypoeutectic Si composition failed with a pseudoplastic response and at an angle of ∼45° off the compressive loading direction. However, in the case of higher strain rate and Si content, the brittle failure in the samples occurred through vertical splitting and spalling of fragments.

A comparative study on combustion synthesis of Nb–Si compounds

A comparative study on the preparation of niobium silicides (such as Nb 3Si, Nb 5Si 3, and NbSi 2) in the Nb–Si system was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of different stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Test specimens with five different compositions including Nb:Si = 3:1, 5:3, 3:2, 1:1, and 1:2 were employed in this study. Experimental observations indicated that except for the sample of Nb:Si = 3:1, upon ignition self-sustained combustion was well established and proceeded throughout the entire sample. Measured results showed that the reactant compact of Nb:Si = 5:3 had the highest flame-front propagation velocity, followed sequentially by the powder compacts with Nb:Si = 3:2, 1:1, and 1:2. Variation of the combustion temperature with sample initial stoichiometry is in a manner consistent with that of the flame-front velocity. On account of the close atomic ratios, the test specimens made up of either Nb:Si = 5:3 or 3:2 yielded a single-phase silicide Nb 5Si 3 in both α and β forms. As identified by the XRD analysis, it was found that α-Nb 5Si 3 was dominant in the final product from the Nb:Si = 5:3 compact, but β-Nb 5Si 3 was the major composition formed by the Nb:Si = 3:2 compact. Because of the nonexistence of a silicide phase with the atomic ratio Nb/Si = 1, a multiphase product consisting of α-Nb 5Si 3 and NbSi 2 was synthesized from the reactant compact of Nb:Si = 1:1. For the samples with Nb:Si = 1:2, the disilicide NbSi 2 was monolithically produced along with trivial amounts of unreacted Nb and Si. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of Nb 5Si 3 and NbSi 2 were determined to be 259.2 and 160.9 kJ/mol, respectively.

Effects of impurities on alumina–niobium interfacial microstructures

Light optical microscopy, scanning electron microscopy, and transmission electron microscopy were employed to examine the interfacial microstructural effects of impurities in alumina substrates used to fabricate alumina–niobium interfaces via liquid-film-assisted joining. Three types of alumina were used: undoped high-purity single-crystal sapphire; a high-purity, high-strength polycrystalline alumina; and a lower-purity, lower-strength polycrystalline alumina. Interfaces formed between niobium and both the sapphire and high-purity polycrystalline alumina were free of detectable levels of impurities. In the lower-purity alumina, niobium silicides were observed at the alumina–niobium interface and on alumina grain boundaries near the interface. These silicides formed in small-grained regions of the alumina and were found to grow from the interface into the alumina along grain boundaries. Smaller silicide precipitates found on grain boundaries are believed to form upon cooling from the bonding temperature.

Preparation of Nb5Si3 Intermetallic and Nb5Si3/Nb Composite by Self‐Propagating High‐Temperature Synthesis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation of Nb 5Si 3 intermetallic and Nb 5Si 3/Nb composite by self-propagating high-temperature synthesis

Production of niobium silicide Nb 5Si 3 and composite Nb 5Si 3/Nb was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of sample green density, preheating temperature, and initial stoichiometry on the combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained combustion front propagating in a spinning mode. Moreover, phase transformation continues during the cooling of the burned sample, leading to the emergence of an afterglow on the compact. As the sample density and preheating temperature increased, the combustion temperature was found to increase and the flame-front velocity was correspondingly enhanced. XRD analysis shows that the nature of end products is significantly influenced by the starting stoichiometry of sample compacts. It was found that the compact with an initial composition of Nb:Si = 5:3 yielded predominantly α-Nb 5Si 3, whereas the sample made up of Nb:Si = 3:2 produced largely β-Nb 5Si 3. Based upon the data measured from the samples of Nb:Si = 5:3, an activation energy of 259.2 kJ/mol was deduced for the formation of Nb 5Si 3 by SHS. In the synthesis of the Nb 5Si 3/Nb composite, the increase of elemental Nb content in the final composition lowered the combustion temperature and thereby reduced the flame-front velocity. The Nb 5Si 3/Nb composites with a concentration of Nb ranging from 5 to 15 mol.% were directly produced by SHS in this study. XRD analysis of the Nb 5Si 3/Nb composite identifies the formation of Nb 5Si 3 dominated by the α form, along with the existence of elemental Nb.