A fundamental study on the preparation of niobium aluminide powders by calciothermic reduction

Fine niobium aluminide powders such as NbAl₃ were produced directly from mixtures of Nb₂O₅ and aluminum powder by calciothermic reduction. Prior to the reduction experiment, phase equilibria between Nb-Al and Ca-Al alloys were studied. Isothermal annealing of the specimens in the Nb-Al-Ca system at 1273 K showed that NbAl₃ is in equilibrium with CaAl₂ and Al-rich Ca-Al liquid alloys and that Nb₃Al and Nb₂Al equilibrate with Ca-Al alloys containing around 9 to 18 and 18 to 36 mol pct Al, respectively. Based on these experimental phase equilibria and on reported thermodynamic data of the Al-Ca system, the activities of Al in Nb-Al alloys were evaluated. This information is necessary in determining suitable compositions and conditions for the coreduction and ultimate production of single-phase niobium aluminide. The procedure for preparation of niobium aluminide powders by calciothermic reduction of Nb₂O₅ consists of three steps: (1) blending of starting materials (Nb₂O₅ + Al); (2) high-temperature reaction with calcium; and (3) acid leaching. After the reduction of Nb₂O₅ with calcium and aluminum to produce niobium aluminide powders, by-products of Ca-Al alloy, CaAl₂, and CaO were formed. These were removed by leaching in aqueous acid solution. The NbAl₃ particles obtained were a few micrometers in size and contained about 0.15 wt pct oxygen.     

Effect of Alloying Elements on Nb-Rich Portion of Nb-Si-X Ternary Systems and In Situ Crack Observation of Nb-Si-Based Alloys

To find a new route for microstructure control and to find additive elements beneficial for improving high-temperature strength, a systematic investigation is performed on hypoeutectic Nb-15 at. pct Si-X ternary alloys containing a transition element, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt, or Au. Information on phase equilibrium is classified in terms of phase stability of silicide phases, α Nb₅Si₃, Nb₄SiX, and Nb₃Si, and the relationship between microstructure and mechanical properties both at room temperature and high temperature is investigated. All the additive elements are found to stabilize either α Nb₅Si₃ or Nb₄SiX but destabilize Nb₃Si. A microstructure of Nb_ss/α Nb₅Si₃ alloy composed of spheroidized α Nb₅Si₃ phase embedded in the Nb_ss matrix is effective for toughening, regardless of the initial as-cast microstructure. Also the plastic deformation of Nb_ss dendrites may effectively suppress the propagation of longer cracks. High-temperature strength of alloys is governed by the deformation of Nb_ss phase and increases with higher melting point additives.     

Molybdenum in Nb-C alloys

The effects of molybdenum alloying additions to niobium on the carbide phases and their precipitation behavior were investigated. The experimental alloys included Nb-0.1C, Nb-15Mo-0.1C, and Nb-30Mo-0.1C. After selected heat treatments the microstructural changes were determined by metallography and the carbide phases were extracted and identified by X-ray diffraction and chemical analysis. The results are essentially in agreement with recent phase diagram determinations. Additions of 30 wt pct Mo appears to slightly increase the solubility of carbon in niobium at temperatures around 1650°C. The solubility of molybdenum in Nb₂C is very small. Discontinuous precipitation of β-Nb₂C was found to occur in the Nb-30Mo-0.1C alloy during annealing at 1200°C. The important, overall effect of molybdenum in Nb-C alloys is to decrease the rate of niobium carbide precipitation so that appreciable carbon supersaturation can be achieved even after comparatively slow furnace cooling.     

Structural states and electrical conductivity of oxidized niobium nanopowders

The structure of niobium nanopowders (particle size 0.03–0.07 μm) oxidized in air is studied by X-ray diffraction. The nanopowder particles have a significant fraction of an amorphous phase. The amorphous component is likely to block the well-known mechanism of niobium oxidation Nb → Nb_s.s → Nb₆O → NbO → NbO_x, which was proposed on the basis of the results of studying the oxidation of niobium powders at high temperatures. Here, Nb_s.s is the solid solution of oxygen in niobium and NbO_x are the higher niobium oxides NbO₂ and Nb₂O₅. The amorphization of the surface of niobium nanopowders oxidized at 20°C can be one of the main causes of a rather high electrical resistivity (ρ ≈ 10⁸ Θ cm) of the samples compacted from these powders.     

Phase relations and a15-phase diffusion layer formation in the system Ag-Nb-Ga

Nb₃Ga layers can be synthesized by diffusion from Ag-Ga alloys at 1100°C. This is con-sistent with the hypothesis that the orientation of two-phase tielines in a ternary system, such as Ag-Nb-Ga, play an important role in determining whether superconducting layer formation will occur. Although the superconducting transition onset was 12.3 K, the Nb₃Ga layer growth rates in this system are too slow for practical application. Chemical modification of the phase diagram however does appear to be a feasible approach for pro-moting the diffusion synthesis of A15 phase layers for multifilamentary conductors.     

Activity Measurements of Mg in the Ternary Cu-Mg-Si System Using Thermogravimetric Knudsen Effusion Method

The high vapor pressure of Mg in comparison with Cu and Si enables the use of thermogravimetric Knudsen effusion method (KEM) to determine the thermodynamic properties of binary Cu-Mg, Mg-Si, and ternary Cu-Mg-Si alloys. In the current study, the weight loss of solid Mg with time has been determined at different constant temperatures between 705 K and 788 K (432 °C and 515 °C) by using KEM, and from these diagrams, the sublimation rate of Mg was calculated. By introducing the sublimation rates into the equation derived from the kinetic gas theory, the enthalpy change of sublimation reaction of Mg at the experimental temperatures was calculated to be 147.5 ± 6.5 kJ mol^?1, which is close to the 143.8 ± 0.5 kJ mol^?1 calculated using the thermodynamic data available in the literature. Similar procedure was also applied to the binary Cu-Mg, Mg-Si, and ternary Cu-Mg-Si alloys where the activities of Mg with respect to the Mg wt pct with W _Cu:W _Si = 20:80 were calculated. The diversion points in the activity–composition diagrams gave the phase boundary compositions in the phase diagrams. The phase boundary compositions of Mg in the alloys determined using KEM were in good agreement with the known binary and the constructed ternary phase diagram using FactSage thermochemical software and databases.     

RECENT DEVELOPMENTS IN THE FIELD OF SILICIDES AND BORIDES OF THE HIGH-MELTING-POINT TRANSITION METALS

Abstract

The binary systems of silicides of the high-melting-point transition metals are now well understood, except for the hafnium-silicon system. Research since 1954 is reviewed, with particular reference to the compound Me₅Si₃ and its position in the silicide systems. Reference is also made to the pseudo-binary and pseudo-ternary silicide systems.

The structures of many of the intermetallic phases in the binary boride systems have now been determined, but complete equilibrium diagrams still remain to be established in some cases. New tentative diagrams are given for the systems vanadium–boron, niobium–boron, and tantalum–boron, and structures are suggested for the borides V₃B₂, Nb₃B₂, and Ta₃B₂ with the T2 structure (isostructural with U₃Si₂).

Ternary alloys of the systems Me–Si–B are of great interest, not only structurally but also for practical reasons. The complete systems Me–Si–B of Group VI (Mo–Si–B and W–Si–B) have accordingly been studied by X-ray, thermal-analysis, and micrographic methods. The system Cr–Si–B has been determined and attention is directed to the possible commercial applications of certain alloys containing additions of metals of the iron group, in particular nickel, for sprayed coatings resistant to liquid aluminium.

The question of cementing silicides and borides with metals and alloys is discussed theoretically. The character of the silicide or boride system in question and the behaviour of the intermediate phase in relation to the bonding material are of decisive importance for the selection of the latter. Only very limited data are to be found in the literature on the behaviour of silicides and borides in relation to metals and alloys. Alloys based on TiB₂, ZrB₂, MoSi₂, and WSi₂, impregnated with numerous metals and alloys, have been prepared. Their structures have been studied and the technical suitability of various combinations is discussed on the basis of their technological properties.     

Evolution of microstructures in the nickel modified titanium trialuminides near the L12 phase field

This study focuses upon the evolution of microstructures during solidification processing of several intermetallic alloys around the Ll₂ phase in the Al-rich corner of the Al-Ti-Ni ternary system. The alloys were produced by double induction melting and subsequent homogenization followed by furnace cooling. The microstructure was characterized by means of optical and scanning electron microscopy with energy-dispersive spectroscopy (EDS) analysis and X-ray diffraction. The microstructural evolution in homogenized alloys was dependent on both nickel and titanium content. Very fine precipitates of Al₂Ti were observed within the Ll₂ phase in alloys containing 62 to 65 at. pct Al and at least 25 at. pct Ti. The Al₂Ti precipitates are stable at least up to 1000 °C and undergo complete dissolution at 1200 °C. In alloys containing around 66 at. pct Al and 25 to 31 at. pct Ti, phases such as Al₃Ti, Al₅Ti₂, and Al₁₁Ti₅ were observed. A modified room temperature isotherm in the Al-Ti-Ni ternary system is proposed, taking into account the existence of Al₂Ti, Al₁₁Ti₅, Al₅Ti₂, and Al₃Ti in equilibrium with the Ll₂ phase. It seems that at room temperature, the Ll₂ phase field for homogenized alloys is extremely small. It will be practically impossible to obtain a single-phase microstructure at room temperature in the Al-Ti-Ni ternary alloys after homogenization at 1000 °C followed by furnace cooling. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo     

Phase Equilibria of Sn-Co-Cu Ternary System

Sn-Co-Cu ternary alloys are promising lead-free solders, and isothermal sections of Sn-Co-Cu phase equilibria are fundamentally important for the alloys?? development and applications. Sn-Co-Cu ternary alloys were prepared and equilibrated at 523?K, 1073?K, and 1273?K (250?°C, 800?°C, and 1000?°C), and the equilibrium phases were experimentally determined. In addition to the terminal solid solutions and binary intermetallic compounds, a new ternary compound, Sn₃Co₂Cu₈, was found. The solubilities of Cu in the ??-CoSn₃ and CoSn₂ phases at 523?K (250?°C) are 4.2 and 1.6?at. pct, respectively, while the Cu solubility in the ??-Co₃Sn₂ phase is as high as 20.0?at. pct. The Cu solubility increases with temperature and is around 30.0?at. pct in the ??-Co₃Sn_2?at 1073?K (800?°C). The Co solubility in the ??-Cu₆Sn₅ phase is also significant and is 15.5?at. pct at 523?K (250?°C).     

Thermodynamics and phase relations in refractory metal solid solutions containing carbon,nitrogen, and oxygen

The refractory metals Nb, Ta, Mo, and W dissolve C, N, and O by forming interstitial solid solutions. The concentration of these components can be increased or decreased by annealing treatments, depending on the partial pressure of gases such as N₂, O₂, H₂O, CH₄, or CO and on the temperature of the specimen. New results in binary and ternary systems combined with those obtained in the last few years now provide most of the data needed for the thermochemical analysis of the solid solution phases and for the establishment of p-T-c phase diagrams. The mechanisms of the gas-metal reactions and the general feature of the constitution diagrams are similar for all refractory metals. However, marked differences exist between group Va metals niobium and tantalum and group VIa metals molybdenum and tungsten in the absolute values and temperature dependence of the equilibrium pressure of gaseous species and therefore also in the amount of gas absorbed. The data now available for the estimation of the final content of interstitials in solid solution after annealing treatments are compiled and discussed. On leave from the Max-Planck-Institut fur Metallforschung, Institut fur Sondermetalle, Stuttgart, Germany     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

Transition metal alloys of extraordinary stability; An example of generalized Lewis-acid-base interactions in metallic systems

The Engel theory of metals predicts unusually high thermodynamic stability for certain classes of alloys of transition metals for which generalized Lewis-acid-base interactions are possible. To test these predictions, phase equilibria were studied for ternary systems of Zr, C, and the transition metals Re, Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au. Similar literature data for Nb, Ta, Hf, Th, Y, Ce, Er, and Pu with Re, Ru, Rh, Ir, Pt, and Au were evaluated. Thermodynamic data for the carbides of Zr, Hf, Th, Nb, Ta, U, and Y were critically evaluated, tabulated for 1200 to 2300 K, and used to fix the Gibbs energies of formation in kcal/g-atom of alloy, or their limits, for the binary phases of the above metals. In addition, for Zr, activity coefficients and excess Gibbs energies are tabulated. The predicted high stabilities for alloys of Rh, Ir, Pd, and Pt are confirmed with excess Gibbs energies ranging to —100 kcal/g-atom and activity coefficients as low as 10^-12 for zirconium or hafnium in dilute solutions of platinum at 1800 K. Some of the properties of these unusually stable compounds have been measured. An erratum to this article is available at .     

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,     

An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb₃Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.     

Phase Evolution in and Creep Properties of Nb-Rich Nb-Si-Cr Eutectics

In this work, the Nb-rich ternary eutectic in the Nb-Si-Cr system has been experimentally determined to be Nb-10.9Si-28.4Cr (in at. pct). The eutectic is composed of three main phases: Nb solid solution (Nb_ss), β-Cr₂Nb, and Nb₉(Si,Cr)₅. The ternary eutectic microstructure remains stable for several hundred hours at a temperature up to 1473 K (1200 °C). At 1573 K (1300 °C) and above, the silicide phase Nb₉(Si,Cr)₅ decomposes into α-Nb₅Si₃, Nb_ss, and β-Cr₂Nb. Under creep conditions at 1473 K (1200 °C), the alloy deforms by dislocation creep while the major creep resistance is provided by the silicide matrix. If the silicide phase is fragmented and, thus, its matrix character is destroyed by prior heat treatment [e.g., at 1773 K (1500 °C) for 100 hours], creep is mainly controlled by the Laves phase β-Cr₂Nb, resulting in increased minimum strain rates. Compared to state of the art Ni-based superalloys, the creep resistance of this three-phase eutectic alloy is significantly higher.     

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.     

沉金后液中金、铂、钯、碲的组态特征及其碲捕集稀贵金属机制

铜阳极泥沉金后液是回收铂族金属铂、钯的重要原料来源.根据铜阳极泥氯化浸出过程稀贵金属可能发生的电极反应,分析了沉金后液中金、铂、钯、碲的存在价态,通过热力学计算绘制了金、铂、钯、碲的多形态组分图,并研究了单一金、铂、钯体系碲捕贵金属机制.沉金后液中稀贵金属金、铂、钯和碲的价态分别为Au3+,Pt2+,Pt4+,Pd2+...     

The use of nitride intermediates in the preparation of metals. A study of the reduction of Nb2O5 with NH3

A kinetics study of the reduction of Nb₂O₅ with NH₃ was conducted at 600° to 1300°C, using vertical fixed-bed, flow-through reactors, with the goal of using the nitride as an interme-diate in the preparation of niobium (columbium) metal via a thermal decomposition step. The effects of reactor materials (stainless steel, nickel, molybdenum, graphite, alumina, and Vycor) upon ammonia reactivity toward Nb₂O₅ were investigated. At low temperatures, the metal reactor systems were more catalytically reactive, yielding faster rates of reac-tion and a greater degree of nitride conversion, whereas at high temperatures, the non-metal reactor systems performed better. In general, the initial reaction rate-temperature data exhibited a maximum, associated with oxynitride formation, near 700°C for the metal reactor systems and 800° to 900°C for the nonmetal reactor systems, followed by a mini-mum, associated with NbO₂ formation, at 800° to 850°C for the metal reactor systems and 950° to 1000°C for the nonmetal reactor systems where NbN formation commences. A sec-ond maximum, associated with the hexagonal NbN phase, occurred at 1200°C. The ranges of activation energies for these regions were from 15 to 30 kcal/mole for region I, 8 to 22 kcal/mole for region II, and 10 to 22 kcal/mole for region III.     

Beta decomposition of (HFxZr1_x)80Nb20 ternary alloys

The processes of beta decomposition have been examined in ternary alloys of the form (Hf_xZr_1-x)₈₀Nb₂₀ to determine the influence of Hf additions to a basic Zr₈₀Nb₂₀ composition. In the chill cast condition, Hf additions have been found to decrease the temperature coefficient of electrical resistivity from a value of-0.0015 pct/K for the binary Zr8oNb2o alloy to a value of-0.011 pct/K for a (Hf_xZr_1-x)₈₀Nb₂₀ ternary alloy. This change is explained in terms of the bcc lattice instability typical of Ti, Zr, and Hf alloys. The Hf additions enhance the kinetics of ω-phase precipitation during aging at 648 K. The aging of a (Hf₀₅Zr₉₅)₈₀Nb₂₀ alloy for 12 h results in the precipitation of a high volume fraction of cuboidal shaped co-phase particles. A phase separation which results in the formation of solute lean discs (β₁) occurs together with the precipitation of the ω-phase. These discs formed both randomly within the matrix and heterogeneously along dislocations and at grain boundaries.