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.     

Atomic diffusion and phase equilibria at the interfaces of the CoAl/Ir multilayer on Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>-base alloys

Atomic diffusion and phase equilibria have been investigated at the interfaces of Ir/CoAl and Ir/Nb₅Si₃ to evaluate the suitability of a diffusion-barrier layer of Ir between an oxidation-resistant layer of B2-CoAl and a base material Nb₅Si₃. Diffusion couples were prepared by hot pressing and annealed at 1573 K for up to 178 hours. Diffusion layers of (Ir, Co) solid solution and B2-(Ir, Co)Al were formed at the Ir/CoAl interface. The concentration of Al dramatically dropped at the interface, which indicates that the Ir layer effectively works as the diffusion barrier against the inward diffusion of Al. To quantitatively evaluate the potential of Ir as a diffusion barrier, the Boltzmann-Matano analysis was employed to determine the diffusion coefficient of Al using Ir-8 at. pct Al/Ir diffusion couples annealed at temperatures of 1573, 1673, and 1773 K. For instance, an extremely low value of 7.0×10⁻¹⁹ m²/s is evaluated for Ir-4 at. pct Al at 1573 K. At the Ir/Nb₅Si₃ interface, the intermetallic phases Ir₃Si and Ir₃Nb are formed on the Ir side and the Nb₅Si₃ side, respectively. The formation of Ir₃Si is controlled by the diffusion of Si through Ir₃Nb in which the solubility of Si is limited quite small. 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.     

The thermodynamic studies of liquid copper alloys by electromotive force method: Part I. The Cu−O,Cu−Fe−O,and Cu−Fe systems

Oxygen activities in liquid Cu−O and Cu−Fe−O alloys were measured in the temperature range 1100° to 1300°C by the solid oxide electrolyte emf method with mixtures of Ni−NiO and Co−CoO as reference electrodes. The Cu−O and Cu−Fe−O alloys were analyzed for iron and/or oxygen content. The activity coefficient of oxygen at infinite dilution in liquid copper was found to be 0.115, 0.195, and 0.286 at 1100°, 1200°, and 1300°C, respectively. The results are compared with previous investigations on the Cu−O system. Based on this comparison, the best equation for the free energy of solution has been suggested. The standard free energy of formation of CoO(s) has been calculated at the experimental temperatures. In the liquid Cu−Fe−O system at 1200°C, a minima in oxygen solubility is reached at 1.1 at. pct Fe in the alloy. The value of interaction coefficient, , is −565 at 1200°C. Iron activities in the liquid Cu−Fe alloys have been calculated at 1100° and 1200°C, and a strong positive deviation from ideality is observed. Results of this study were combined with literature data at 1550°C to obtain the values of and at infinite dilution in liquid copper. A. D. KULKARNI, formerly with Chase Brass and Copper Co., Cleveland, Ohio     

An ω phase in the Cu-16.5 at. pct Sn alloy

In many zirconium- and titanium-base alloys a precipitate of an ω phase is found after quenching from the high temperature β phase region. In a Cu-16.5 at. pct Sn alloy the same phase is present after quenching from the γ-phase region to room temperature. The quenched Cu−Sn alloy shows electron diffraction effects which are very similar to those of the zirconium- and titanium-base alloys,i.e., diffuse scattering and shift of the ω reflexions. In order to explain this shift a model, based on the presence of certain faults in the ω structure, is presented. After aging at 100° and 130°C the ω phase is replaced by a phase which closely resembles the stable Cu−Sn σ phase. Nuclei of this phase are thought to be at least partially responsible for the diffuse scattering observed. W. VANDERMEULENFORMERLY formaerly Assistant, Katholieke Universiteit Leuven, Leuven, Belgium     

High-temperature interactions of refractory metal matrices with selected ceramic reinforcements

Interdiffusion and reactions occurring at high temperatures between refractory metals (Nb and Ta) and ceramic materials (SiC and A1₂O₃) have been investigated. Diffusion couples were fabricated by depositing Nb and Ta films of ~l-μm thickness onto polished ceramic substrates. These diffusion couples were vacuum annealed at high temperatures for various times. Interfacial reactions were evaluated using optical metallography, Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Kinetic studies in the 800 °C to 1200 °C temperature range for the Nb/SiC system indicated that Nb₂C initially forms, followed by the more stable NbC_xSi_y phase. In some instances, layered structures containing the phases NbC, Nb₂C, and NbC_xSi_y, were observed. The activation energies of formation for the NbC_x and NbC_xSi_y, phases were determined from these measurements. Results from the Ta/SiC system were found to be similar to those from the Nb/SiC system. In both Nb/Al₂O₃ and Ta/Al₂O₃ diffusion couples, annealing for up to 4 hours in the 1100 °C to 1200 °C range did not result in any significant reactions. These results suggest that A1₂O₃ may be a promising diffusion barrier between Nb and Ta metal matrices and SiC ceramic reinforcements. formerly with Lockheed Research and Development Division, is Senior Member of Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185     

Continuous cooling transformation temperatures determined by compression tests in low carbon bainitic grades

The transformation behaviors of six steels containing microalloying additions of B, Nb, and Mo were investigated under continuous cooling conditions. Continuous cooling compression (CCC) tests were employed to study the effects of chemical composition (mainly, Nb, Mo, and B) and deformation parameters (reheat temperature, prestrain, and holding time) on the transformation temperatures (A _r3 and B _s). It was found that for the Mo−Nb−B, Mo−B, and B steels, the transformation temperatures are relatively stable, and vary in a range of about 20°C when the reheat temperature is changed from 900°C to 1200°C. Both the stress-temperature curves and the associated microstructures show that transformation in the Mo−Nb−B steel is basically of the γ-to-B type; i.e., the resulting microstructure is low carbon bainite. By contrast, for the Nb−B steels, the transformation temperatures vary significantly when the reheat temperature is changed. The concentration of boron in solution strongly affects the transformation behavior of this type of steel. In the Nb−48B steel, the latter is of the γ-to-B type, while in grades with either higher (Nb−64B) or lower (Nb−15B) boron concentrations, it is mainly of the γ-to-α type. Large Fe₂₃(C,B)₆ particles, which were found at low reheat temperatures and long holding times, are considered to be responsible for raising the transformation temperatures. T.M. MACCAGNO, formerly with the Department of Metallurgical Engineering, McGill University     

Diffusion paths and structures in multiphase Cu−Ni−Zn couples at 775°C

Multiphase diffusion was investigated in the Cu−Ni−Zn system at 775°C for the development of diffusion structures involving two interfaces. Selected series of diffusion couples characterized by a common γ (cubic) terminal alloy joined to a set of α (fcc) alloys developed an intermediate β (bcc) phase with two interfaces, α/β and β/γ. The α/β interface showed transitions from planar to nonplanar and back to planar morphology, as the copper concentration of the α terminal alloy was decreased from 100 to about 30 at. pct. Planar β/γ interfaces were observed for all but two of the couples. The compositions on either side of planar α/β interfaces were consistent with those based on equilibrium tielines, while the compositions at nonplanar α/β interfaces differed from those of equilibrium. Selected series of couples assembled with γ and β alloys were also investigated for the development of interface instability at the β/γ interface. The diffusion paths of γ/β couples were consistent with those of γ/α couples.     

High-rate sputter deposition and heat treatment of thick Nb-Al-Ge and Nb-Al superconductors

Crystal structures, microstructures and critical temperatures were determined for Nb-Al-Ge and Nb-Al sputter-deposits in order to evaluate their dependence on sputter-deposition conditions, heat treatment procedures and composition. High-rate sputter deposition techniques were used to make the deposits at rates up to 1 Μm/min. Compositions studied were Nb₃(Al_0.6Ge_0.4), Nb₃(Al_0.75Ge_0.25), Nb₃Al, Nb_2.52(Al_0.84)Ge_0.16), Nb_2.33Al, Nb_3.07(Al_0.75Ge_0.25), and Nb_4.15(Al_0.71Ge_0.29). The investigation indicated it is feasible to make practical A-15 phase superconductors by high-rate sputter deposition. Of all the deposits studied, Nb₃(Al_0.75Ge_0.25),i.e., Nb₂Al₃Ge, deposited at 15°C and heat treated at 750°C for 1 to 5 days had the highest critical temperature (18.5 K), and it had a very high critical current density;e.g., 4.4 × 10⁵ A/cm² at 100 kOe and 4.2 K. Deposits having the highest critical temperatures consisted only of undecomposed metastable A-15 phase. The high current density was attributed to the presence of very small A-15 phase grains, which were observed to be about 350å in diameter by transmission electron microscopy. The crystal structures for deposits made at 15°C were not always clearly defined, but probably were all body-centered-cubic. Body-centered-cubic phases were transformed by heat treatment for short times at 550°C to 850°C to an A-15 phase that was supersaturated with Al and Ge. If heat treatment temperatures were too high or heat treatment times were too long, however, minor phases formed as the A-15 phase decomposed. Nb₃(Al_0.75Ge_0.25) deposits made at elevated temperatures (485°C and 750°C) predominantly consisted of the A-15 phase, but the presence of minor phases even before heat treatment indicated Al and Ge were not completely retained in the A-15 phase solid solution during deposition. Deposits made with-20 V substrate-anode bias had nearly the same composition as the sputtering target, but deposits made with -50 V and -75 V bias had significantly lower Al and Ge contents than the target. The compactness of the A-15 crystal structure relative to the bcc structure was noted in a comparison of average atomic volumes for the two structures in Nb₃Al. The average atomic volume in the A-15 phase formed by heat treatment was 1.36 pct less than the average atomic volume in the bcc phase that existed before heat treatment in sputter-deposited Nb₃Al made at 15°C.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nb_ss and γ-Nb₅Si_3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb₃Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.     

INCOLOY 908, a low coefficient of expansion alloy for high-Strength cryogenic applications: Part I. Physical metallurgy

INCOLOY 908 is a low coefficient of thermal expansion (COE) iron-nickel base superalloy that was developed jointly by The Massachusetts Institute of Technology and the International Nickel Company for cryogenic service. The alloy is stable against phase transformation during prolonged thermal treatments and has a COE compatible with that of Nb₃Sn. These properties make the material ideal for use as a structural component in superconducting magnets using Nb₃Sn. The evolution of microstructure has been studied as a function of time at temperature over the temperature range of 650 °C to 900 °C for times between 50 and 200 hours. A detailed analysis of precipitated phases has been conducted using X-ray diffraction (XRD), transmission electron microscopy (TEM), and analytical scanning and scanning transmission electron microscopy (STEM) techniques. The primary strengthening phase has been found to be γ’, Ni₃(Al, Ti). INCOLOY 908 is stable against overaging, which is defined as the transformation of γ’ to η, Ni₃Ti, for times to 100 hours at temperatures up to 750 °C. Upon overaging, the strengthening phase transforms to η. A new phase,H _x, has been identified and characterized.     

Morphological stability of α-β phase interfaces in the Cu−Zn−Ni system at 775°C

An experimental investigation into the morphological stability of α-β phase interfaces in the Cu−Zn−Ni system at 775°C has been undertaken. As a preliminary to this study, it was necessary to also determine certain significant diffusion and equilibrium parameters for the composition range of interest (35 to 50 wt pct Zn and 0 to 10 wt pct Ni). Using two-phase infinite diffusion couples, all with the same α terminal composition but with various β terminal compositions, the transition from a stable to an unstable planar α-β interface was indexed. The time evolution of unstable phase interfaces was also examined. The experimentally observed transition is in good agreement, with the transition which is predicted on the basis of linear perturbation theory.     

Combined refinement of diffusion coefficients applied on the Nb-C and Nb-N systems

A novel technique was used for the calculation of diffusion coefficients in the niobium carbides and nitrides prepared by reaction diffusion. The temperature ranges investigated were 1500 °C to 2100 °C for the Nb-C system and 1400 °C to 1800 °C for the Nb-N system. Three independent theoretical approaches were applied and their results are compared. In the metalloid-rich phases, the concentration-dependent diffusion coefficients were calculated from the concentration profiles; two models of layer growth were used to obtain the concentration-independent diffusion coefficients in all phases. It was found that the diffusion coefficient of carbon in δ-NbC_1−x shows a decrease with increasing metalloid concentration, whereas the diffusivity of nitrogen in δ-NbN_1−x is nearly independent of the nonmetal concentration. The concentration dependence of the carbon diffusion coefficients in δ-NbC_1−x is a result of a lower activation energy of carbon diffusion in the substoichiometric δ-NbC_1−x than in the δ-NbC. On the contrary, the activation energy of nitrogen in δ-NbN_1−x does not change with the nitrogen concentration. This behavior could be explained by the different occupancies of metal sublattices, which remain constant in δ-NbC_1−x but decrease with increasing nonmetal concentration in δ-NbN_1−x .     

Structural characteristics of Nb3Sn produced by three P/M techniques

Powder metallurgical techniques have been applied to the problem of preparing monolithic samples of Nb₃Sn of a homogeneity, density and stability suitable for unambiguous plastic deformation studies. Cold pressing and reaction sintering, infiltration, and hot isostatic pressing (HIP) of Nb and Sn powders have been evaluated, with HIP processing producing a decidedly superior structure. The most homogeneous structure was produced by HIP processing at 1630°C for one h at a pressure of 160 MPa. A continuous matrix of Nb₃Sn was produced with a porosity of 2.6 pct and a secondary phase content of 3.3 pct. The principal secondary phase was NbO and no unreacted Nb remained. The Nb₃Sn matrix was quite homogeneous with microprobe analysis revealing an off-stoichiometric composition of 72.2 to 73.2 pct Nb. An equiaxed grain size of about 60 μm was developed and X-ray diffraction analysis revealed a high degree of long range order. HIP processing at 1200°C produced a finer grain size, increased porosity, and an incompletely reacted structure involving 3.5 pct unreacted Nb. The composition of the Nb₃Sn phase was nearly the same, regardless of processing technique. Considerable evidence of dislocations arrayed in low angle, sub-grain boundaries was observed in the 1630°C HIP processed material. Simple, isolated dislocations were predominant in the 1200°C HIP processed material.     

The Elastic Modulus and Flow Stress of Nb3Sn at Elevated Temperatures

The plastic deformation of Nb₃Sn has been the subject of a number of investigations, and the hot deformation of Nb₃Sn polycrystals has been extensively studied in the 1150 to 1650 °C range.^[1–4] The hot deformation stress-strain rate-temperature relationships are largely those of “power law creep”, with activation energies for creep roughly in the 400 to 500 kJ/mol range.^[2,3] Grain size refinement increases flow stress in the power law creep regime.^[3] Hot deformed Nb₃Sn displays polygonized dislocation structure.^[5]     

Plastic deformation in polycrystalline Nb3Sn

A study of the high temperature plastic deformation of polycrystalline Nb₃Sn has been undertaken on hot isostatically pressed material having grain sizes in the 12 to 60 (μm range. Through compression testing and load-relaxation testing deformation has been studied over a strain rate range from 10⁻⁶to 10⁻²s and a temperature range from 1150 to 1650 °C. Plastic deformation can be observed in compression at 1400 °C and above and extensive deformation is possible at 1650°C. Except for the lowest strain rates at 1650 °C, load-relaxation stress-strain rate relationships are consistent with “power law creep”. Analysis of stress-strain rate-temperature relationships projects an activation energy for creep of very roughly 500 kJ/mol. Observations on yield point behavior and fracture mode transition are presented. A comparison to monocrystalline V₃Si behavior is made, and the role of the sub-structure during testing is considered.     

Kinetics of the solid-state carbothermic reduction of wessel manganese ores

Reduction of manganese ores from the Wessel mine of South Africa has been investigated in the temperature range 1100 °C to 1350 °C with pure graphite as the reductant under argon atmosphere. The rate and degree of reduction were found to increase with increasing temperature and decreasing particle sizes of both the ore and the graphite. The reduction was found to occur in two stages: (1) The first stage includes the rapid reduction of higher oxides of manganese and iron to MnO and FeO. The rate control appears to be mixed, both inward diffusion of CO and outward diffusion of CO₂ across the porous product layer, and the reaction of carbon monoxide on the pore walls of the oxide phase play important roles. The values of effective CO-CO₂ diffusivities generated by the mathematical model are in the range from 2.15 x 10⁻⁵ to 6.17 X 10⁻⁵ cm².s⁻¹ for different ores at 1300 °C. Apparent activation energies range from 81. 3 to 94.6 kJ/kg/mol. (2) The second stage is slower during which MnO and FeO are reduced to mixed carbide of iron and manganese. The chemical reaction between the manganous oxide and carbon dissolved in the metal phase or metal carbide seems to be the rate-controlling process The rate constant of chemical reaction between MnO and carbide on the surface of the impervious core was found to lie in the range from 1.53 x 10⁻⁸ to 1.32 x 10⁻⁷ mol . s⁻¹ . cm⁻². Apparent activation energies calculated are in the range from 102.1 to 141.7 kJ/kg/mol. Formerly Doctoral Student, Department of Metallurgy and Materials Engineering, University of the Witwatersrand, Johannesburg,     

The diffusivity of oxygen in liquid copper-lead alloys

The oxygen diffusivity in liquid copper-lead alloys at 1403 K (1130° C) was measured using the electrochemical cell: Ni−NiO/ZrO₂(+CaO)/O in liquid Cu−Pb alloy(I)/ZrO₂(+CaO)/O in liquid Cu−Pb alloy (II). Oxygen in liquid Cu−Pb alloy (I) was transferred to the right by applying a preselected voltage between the two liquid Cu−Pb alloys. The oxygen diffusivity in liquid Cu−Pb alloy(I) was calculated from the emf change with time between the Ni−NiO and liquid Cu−Pb alloy (I) electrodes. The results were: It was found that the oxygen diffusivity in liquid copper-lead alloys did not change drastically over the entire composition range, in contrast with that reported by other investigators for liquid copper-nickel alloys. The oxygen diffusivity in pure liquid lead agreed with the results of our previous work using an FeO−Fe₃O₄ mixture as a sink for oxygen.     

Reaction diffusion and phase equilibria in the V-N system

The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 °C to 1700 °C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V₂N and δ-VN_1−x develop. The morphology of the β-V₂N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V₂N and δ-VN_1−x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V₂N) and 2.93 eV (δ-VN_1−x ). By using δ-VN_1−x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V₂N is formed on the surface of the couple as a result of out-diffusion of nitrogen.