Thermodynamic characteristics of self-intercalated niobium diselenide of the limiting composition

Vaporization of niobium diselenide has been investigated by mass spectrometry, mass-loss effusion, chemical analysis, and x-ray analysis. We have established that the equilibrium vapor above niobium diselenide consists of mainly Se.₂ molecules at 1400–1600 K; as selenium is removed from the sample, self-intercalation of diselenide occurs up to Nb_1.28Se₂. We have determined the standard enthalpies of dissociation and formation of self-intercalated diselenide of limiting composition as 405.3 kJ/mole and −260.4 kJ/mole, respectively. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 61–64, July–August, 1998.     

Solid-gas reactions driven by mechanical alloying of niobium and tantalum in nitrogen

Solid-gas reactions of niobium and tantalum with molecular nitrogen driven by mechanical alloying (MA) have been investigated by X-ray diffraction, transmission electron microscopy, and differential thermal analysis. It was found that the phase transition followed a sequence of Nb₂N → Nb₃N₄ → NbN when Nb was milled with N₂. However, Ta₂N and an amorphous phase were formed when Ta was milled with N₂. The chemosorption of nitrogen onto the clean metal surfaces created by ball milling is believed to be the fundamental process governing solid-gas reactions, and the defects generated during MA can promote the diffusion of adsorbed nitrogen, and consequently the formation of metal nitrides. The difference in phase transition between the two systems is discussed.     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C.     

The role of oxygen and zirconium in the formation and growth of Nb3sn grains

One method of producing Nb₃Sn is to react a molten tin alloy with a solid niobium alloy. Using this process, the addition of zirconium and oxygen to the niobium foil has been found to dramatically reduce the Nb₃Sn grain size and affect the Nb₃Sn superconducting critical current properties. Nb₃Sn grains grow semicoherently on the niobium alloy foil. The initial grain size is about 50 nm. These initial Nb₃Sn grains coarsen rapidly to become equiaxed grains about 0.2 μm in diameter. The equiaxed Nb₃Sn grains away from the Nb/Nb₃Sn interface are completely surrounded by a tin alloy phase that would have been liquid at the reaction temperature. Based on transmission electron microscopy observation and electrical property characterization, it is concluded that ZrO₂ clusters, less than 10 Å in size, form in the niobium alloy foil during processing. These clusters combine at the Nb/Nb₃Sn interface to form ZrO₂ precipitates. The ZrO₂ precipitates are found in all of the Nb₃Sn grains that have formed from a reaction between the liquid tin and the solid niobium at the Nb/Nb₃Sn interface. The precipitates are coherent with their host Nb₃Sn grains. During Nb₃Sn grain growth, the ZrO₂ precipitates dissolve in shrinking grains and reprecipitate in growing grains, as the migrating grain boundary intersects the precipitate. This dissolution/reprecipitation process slows the growth of Nb₃Sn grains. Formerly with GE Corporate Research & Development, Schenectady, NY,     

Effect of oxygen and zirconium on the growth and superconducting properties of Nb3sn

The intermetallic compound Nb₃Sn is a type II superconductor of interest because of its high superconducting critical current density in high magnetic fields. One technique for forming Nb₃Sn is to react a molten tin alloy with a solid niobium-zirconium-oxygen alloy. It was found that the properties of Nb₃Sn are directly related to its microstructure, which is in turn directly related to the O: Zr atom ratio in the starting niobium foil. For a niobium alloy foil with an O:Zr atom ratio of 2, the resulting Nb₃Sn layer is fine grained and grows linearly with reaction anneal time until the entire Nb-Zr-O alloy core is consumed. This leads to a linear increase in critical current with time and a relatively constant critical current density. For a niobium foil without oxygen, the resulting Nb₃Sn grains are large and columnar and grow with a diffusion-limited layer growth rate. The resulting critical current density is low and decreases with reaction time. For a niobium alloy foil with an O:Zr ratio of >0 but <2, fine-grained Nb₃Sn is formed initially and grows with a linear layer growth rate, followed by a second layer of large, columnar-grained Nb₃Sn growing with a diffusion-limited rate. As a function of reaction anneal time and similar to the grain growth, the critical current initially increases linearly and then decreases. Formerly with GE Corporate Research & Development Schenectady NY     

Reactions of niobium and tungsten with phosphorus

X-ray diffraction methods have been applied to the component interaction in the Nb-W-P system in the region of 0–0.67 molar parts of P. An isothermal section of the phase diagram at 1070 K has been constructed. No ternary compounds are formed in the system. The phosphide Nb₃P (structure of Ti₃P type) dissolves tungsten to the limiting composition N_1.8W_1.2P (a = 1.0006(6) nm, c = 0.5073(2) nm). The solubility of tungsten in other niobium phosphides does not exceed 0.05 molar fraction. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 76–80, July–August, 2006.     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C. The transition in morphology of the carbide phase is discussed in terms of the relative volume fraction of the phases, growth rate, and orientation relationships. The carbide morphology is influenced by the growth rate and carbon content. For constant growth rate increasing the volume fraction of the carbide phase favors the lamellar morphology. At low growth rates the lamellar morphology is favored, and at high growth rates the rod-like morphology is favored. Growth crystallography has no direct influence on the transition in carbide morphology.     

Ni-Nb-Sn Bulk Metallic Glass Matrix Composites Fabricated by Microwave-Induced Sintering Process

Using a gas-atomized Ni_59.35Nb_34.45Sn_6.2 metallic glassy alloy powder blended with Sn powder of various contents, Ni-Nb-Sn bulk metallic glassy matrix composites were fabricated by a microwave (MW)–induced sintering process in a single-mode 2.45 GHz MW applicator in a separated magnetic field. The Ni_59.35Nb_34.45Sn_6.2 glassy alloy powder and its mixed powders containing Sn particles could be heated well in the magnetic field. The addition of Sn particles promoted densification of the sintered Ni_59.35Nb_34.45Sn_6.2 metallic glassy powder. Bulk samples without crystallization of the glassy matrix and with good bonding state among the particles were achieved at a sintering temperature of 833 K.     

Experimental tests of proposed relations between the critical field and dislocation cell structure of superconducting niobium

Experimental tests were made of the suggestion that the anomalously large magnitude and anisotropy of the resistive critical field of severely deformed niobium is due to the development of dense dislocation cell walls. The basis for the tests was the theoretical prediction that as the cell size approaches the coherence length ξ_Nb for superconductivity, the superconducting properties of the cell walls would be smeared out over the entire micro-structure, destroying the anisotropy and a good part of the parallel critical field. Resistive measurements were made on swaged niobium wire and powder composites with true strains ranging up to 10.61, at temperatures from 4.2‡K to T_c. Two approaches were used: either by raising the temperature above 4.2‡K, ξ_Nb could be enlarged, or by increasing strain the cell size could be reduced. The predicted effects were indeed observed; from the temperatures at which the anisotropy disappeared, values of ξ_Nb were obtained which agree well with the expected magnitudes and variation of the cell size with cold work in drawn or swaged niobium. Formerly a National Science Foundation Trainee, Department of Metallurgy and Materials Science, M.I.T., Cambridge, Mass.     

Ion-plasma Ti-Al-N coatings on a cutting hard-alloy tool operating under conditions of constant and alternating-sign loads

Structure and phase formation during the deposition of coatings of the Ti-Al-Ni system (Al ≈ 3.5 at %) by the ion-plasma method are investigated. The controlled process parameter was the bias potential (U _b) applied to the substrate made of the VK6 hard alloy. At U _b = 120 V (samples of group 1), titanium nitride close to the stoichiometric composition and solid solution of Al in α-Ti are formed, while at U _b = 120 V (group 2), nonstoichiometric titanium nitride and complex nitride (Ti, Al)N are formed. The hardness and elasticity modulus for coatings of group 1 were equal to 23.8 GPa and 462 GPa, and for group 2 they were 30.8 GPa and 565 GPa, respectively. The latter are characterized by a level of adhesion strength of 53–55 N as opposed to 39–40 N for coatings of group 1. Qualification tests for the durability of the cutting tool with developed coatings are performed. For example, upon turning steel 45, it increases by a factor of 6.3 and for gray cast iron it increases by a factor of 5; during end milling cut of the EI 698-VD alloy it increases by a factor of 2.5.     

Structural transformations occurring in flame-sprayed Ni60Nb40 alloy coatings during heating in the presence of oxygen

Conclusions During the heating of flame-sprayed Ni₆₀Nb₄₀ alloy coatings in the presence of oxygen the process of crystallization of their amorphous matrix occurs at T > 750°K through the formation of the niobium oxide Nb₂U₅ and precipitation of nickel. Decomposition of the amorphous matrix is accompanied by a decrease in length of the coatings. The oxidation of niobium at 750–890°K prevents the formation of the equilibrium phases of the Ni-Nb system during the crystallization of the amorphous structure. During heating in the 890–1110°K range the metastable Ni₈Nb phase is present in the coatings.Translated from Poroshkovaya Metallurgiya, No. 12(300), pp. 21–26, December, 1987.     

Age-hardening in the V-C and Nb-C systems

Age hardening measurements have been carried out for alloys of vanadium and niobium containing 0.3 at. pct carbon. For both systems, four distinct stages of hardening are observed as a function of time. For all stages, carbon diffusion in the metal is the rate controlling factor. The process is thought to involve formation of a sequence of subcarbides prior to formation of the stable V₂C or Nb₂C, according to the following scheme: solid solution → carbon-rich zones → series of subcarbides → Me₂C (hexagonal) Overaging occurs as the final coherent subcarbide begins to transform into the stable hexagonal phase with attendant loss of lattice coherency.     

Dynamic phase transformation during superplastic deformation of Nb/Nb<Subscript>3</Subscript>Al <Emphasis Type="Italic">in-situ</Emphasis> composite

Nb_ss/Nb₃Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nb_ss) and A15 ordered Nb₃Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained at 1573 K and at a strain rate of 1.6 × 10⁻⁴ s⁻¹ for as-forged specimens. Phase transformation from Nb_ss to Nb₃Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding and the rapid dynamic phase transformation to achieve maximum superplastic elongation.     

The generalized lewis acid-base titration of palladium and niobium

The high thermodynamic stability of alloys composed of platinum group metals and group IVB and VB metals has been explained by an electronic interaction analogous to the Lewis acid-base concept for nontransition elements. The analogy is further demonstrated by the titration of palladium by addition of niobium. The activity of niobium in solid palladium was measured as a function of concentration by solid-state galvanic cells and study of the ternary oxide phase diagram. The galvanic cells were of the type Pt/NbO₂,Nb₂O_4.8/YDTJNbO_y,Nb_pd/Pt where the solid electrolyte is yttria-doped thoria (YDT). Ternary phase diagrams for the Pd-Nb-0 and Rh-Nb-0 systems were obtained by characterizing samples equilibrated at 1000 °C. The phase relationships found in the ternary diagrams were also used to derive thermochemical data for the alloys. Thermochemical quantities for other acid-base stabilized alloys such as Nb-Rh, Ti-Pd, and Ti-Rh were also measured. The excess partial molar ΔG^xs/R of niobium at infinite dilution was determined to be -31 kilo-Kelvin at 1000 °C, and theAG°JR of formation of a mole of NbPd_3.55 is —21 kilo-Kelvin. These results and those for the other systems are used to assess the importance of valence electron configuration, nuclear charge, and crystal field effects in the context of generalized Lewis acid-base theory. It is concluded that both the nuclear charge of the atom and crystal field splitting of the valence orbitals significantly affect the basicity of the platinum group metals.     

Influence of the precursor composition and the reduction conditions on the characteristics of magnesium-thermic niobium powders

The influence of the composition and the conditions of the reduction of niobium oxide compounds with magnesium vapor on the specific surface area of the produced metallic powder is studied. When magnesium niobate Mg₄Nb₂O₉ is used as a precursor, the specific surface of the powder increases by several times compared to that of an Nb₂O₅ precursor. Niobium powders with the specific surface up to 150 m² g^–1 (calculated particle size 4.7 nm) and a bulk density of 0.8 g cm^–3 are formed by the reduction of Mg₄Nb₂O₉. The powders are characterized by a mesoporous structure, and the most part of the specific surface consists of pores with a diameter less than 4 nm.     

The Kinetics of the Nitriding of Ternary Fe-2 at pct Cr-2 at pct Ti Alloy

The mechanism and the kinetics of growth of the nitrided zone of ternary Fe-2 at pct Cr-2 at pct Ti alloy was investigated by performing gaseous nitriding experiments at temperatures of 833 K and 853 K (560 °C and 580 °C) and at nitriding potentials r _N = 0.004 atm^−1/2 and 0.054 atm^−1/2. The microstructure of the nitrided zone was investigated by transmission electron microscopy and the elemental compositional variation with depth was determined by employing electron probe microanalysis. Fine platelet-type mixed Cr_{1 – x} Ti _x N nitride precipitates developed in the nitrided zone. To describe the evolution of the nitrogen concentration depth profile, a numerical model was developed with the following parameters: the surface nitrogen content, the solubility product(s) of the alloying elements and dissolved nitrogen in the ferrite matrix, and a parameter defining the composition of the inner nitride precipitates. These parameters were determined by fitting model-calculated nitrogen depth profiles to the corresponding experimental data. The results obtained demonstrate that the type of nitride formation (i.e., whether Cr and Ti precipitate separately, as CrN and TiN, or jointly, as mixed Cr_{1 – x} Ti _x N) as well as the amounts of mobile and immobile excess nitrogen taken up by the specimen considerably influence the shape and extent of the nitrogen concentration profiles.     

Ni-Nb-Ta bulk metallic glasses designed by a cluster-plus-glue atom model

The formation of Ni-based ternary Ni-Nb-Ta bulk metallic glasses (BMGs) is explored in this work. Alloy compositions are designed by the cluster-plus-glue atom model based on a eutectic-related binary cluster Ni-Ni₆Nb₆, which is derived from the binary eutectic crystalline phase NiNb (Fe₇W₆ type). According to the cluster-plus-glue atom model, the well-known binary BMG-forming composition Ni₆₂Nb₃₈ can be described by a composition formula [Ni-Ni₆Nb₆]Ni₃ = Ni_62.5Nb_37.5. With an aim to further improve the glass-forming ability of the Ni-Nb alloy, Ta is selected as an alloying addition to partially replace Nb in the [Ni-Ni₆Nb₆]Ni₃ composition formula. The experimental results verified that BMGs with a critical size of 3 mm can be achieved at compositions [Ni-Ni₆Nb_6−xTa_x]Ni₃ (x = 0.9 ∼ 1.1). Thermal and mechanical properties of the obtained BMG alloys are also investigated.     

Microstructural properties of Nb-Si alloys investigated using EBSD at large and small scale

Texture and crystallographic orientation relationships in arc-melted hypoeutectic and hypereutectic binary Nb-Si alloys are investigated. Electron backscattered diffraction (EBSD) is used here in conventional conditions, i.e., at relatively high spatial resolution (<1 μm) for ∼400×400 μm fields, as well as on very large fields (1.1×1.1 mm), at lower resolution, to get a statistical overview of the microstructure. In as-cast Nb-16Si and Nb-22Si alloys (compositions are in at. pct), [001]_Nb3Si is found parallel to the local thermal gradient, with Nb₃Si + Nb eutectic cells, giving rise to a microstructure similar to that obtained by directional solidification. In Nb-22Si alloy, the following orientation relationships between poles of metallic and silicide phases have been found: (111)_Nb//(111)_Nb3Si (as cast), (011)_Nb//(011) _α-Nb5Si3, and (111)_Nb//(100) _α-Nb5Si3 (heat treated at 1500 °C, 75 hours). This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Effects of Zr on the eutectoid decomposition behavior of Nb<Subscript>3</Subscript>Si into (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

The effect of Zr on the formation of Nb/Nb₅Si₃ lamellar microstructure by eutectoid decomposition reaction of Nb₃Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb₃Si phase into Nb and Nb₅Si₃ phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation (TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs) and Zr distribution in the parent Nb₃Si phase during solidification. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.