Reactive hot pressing of nonstoichiometric uranium dioxide

High density UO_2+x pellets have been produced by reactive hot pressing uranyl oxalate at temperatures up to 700°C. Rapid densification occurred during the decomposition reactions resulting in densities in the range 90 to 92 pct of the theoretical. A density of 98 pct of the theoretical value was achieved by further hot-pressing at 650° to 700°C for 30 min. This densification behavior can be related to the nonstoichiometry and submicron sized particles of UO_2+x produced in the decomposition reactions. The kinetics of hot-pressing of powder compacts of this UO_2+x were studied in the temperature range 500° to 700°C. The results were analyzed utilizing models proposed by Fryer. The activation energy of 53 kcal per mole, obtained from this analysis is the same as that for creep of nonstoichiometric urania in the temperature range 975° to 1400°C, suggesting that the mechanism controlling the rate of the final stage of densification may be a creep process.     

Creep behavior of Ni-Cr lamellar eutectic alloy

The structural changes that occur during creep deformation at 625° and 760°C, with creep and creep rupture data of a directionally solidified Ni-Cr lamellar eutectic alloy are presented and discussed. It is shown that the characteristic features of stage I deformation are the formation of dislocation tangles in the nickel-rich phase and shearing of the cellular structure; these features are then carried into stage II without any additional changes. The onset of accelerated creep is associated with the fracture of the chromium-rich lamellae. During this stage well-defined dislocation cells are formed. More than an order of magnitude increase in lifetime over cast specimens is obtained in the lamellar material with intermediate results for partially dendritic specimens. The activation enthalpy for creep is strain dependent, increasing from about 40 kcal per mole at low strains to a constant value of 80 kcal per mole at about 2 pct plastic strain. Stress dependence of steady-state creep for both test temperatures conforms to the expression έ =Aσ ⁿ witha − 7 for the lamellar eutectic anda − 5 for cast specimens.     

The creep behavior of W-5 re

Polycrystalline W-5 wt pct Re was creep-tested in tension from 1500° to 1900°C at stresses from 2500 to 10,000 psi in a vacuum of 10^?8 torr. The steady-state strain rate was directly proportional to stress to the 5.5 power, and the apparent activation energy for creep was 104 kcal per mole. Dislocation substructure that developed during high-temperature deformation was studied by transmission electron microscopy. The total dislocation density was dependent on stress to the 2.1 power and was insensitive to temperature and strain. No subgrains were found in creep tested specimens. The rate-controlling deformation mechanism was ascribed to dislocation climb where the governing diffusion process was dislocation core diffusion. Comparison of creep data for tungsten, W-5 wt pct Re, and W-25 wt pct Re showed that W-5 wt pct Re alloy has significantly better creep properties than the other two materials.     

A PRELIMINARY STUDY OF THE REACTIONS BETWEEN TUNGSTEN CARBIDE AND TUNGSTEN CARBIDE - 6% COBALT POWDERS AND WET AND DRY HYDROGEN

Abstract

The apparent activation energy for the following reactions has been determined by measuring the amount of carbon removed from samples of powder heated to various temperatures in an atmosphere of either dry or wet hydrogen:

Dry Hydrogen

“As-carburized” tungsten carbide: no reaction up to 900°C.

Water–milled tungsten carbide: 8·38 kcal/mole over the range 600–900°C.

Water–milled tungsten carbide+ 6% cobalt: complex reaction over the range 600–900°C.

Wet Hydrogen

“As-carburized” tungsten carbide: 58·1 kcal/mole over the range 800–900°C.

Water–milled tungsten carbide: 34·8 kcal/mole over the range 700–900°C.

Water–milled tungsten carbide+ 6% cobalt: 38·4 kcal/mole over the range 825–900°C; 7·38 kcal/mole over the range 600–800°C.

It has been shown that ball-milling in water changes both the physical and chemical properties of tungsten carbide powder. These changes are discussed in relation to the marked differences in reaction rates. A number of possible reasons for these differences are given.     

Enhancement of the creep resistance of metals

It is shown that the presence of an unstable surface layer has a very marked effect on the creep behavior of metals. By eliminating this surface layer the creep rate can, for example, be reduced by a factor of 38 for 321 steel tested at 1400° F at 60 pet of the proportional limit. By eliminating the surface layer, activation energy for creep was increased by 13, 21, and 49 kcal/mole for Haynes 188, titanium (6Al-4V), and 321 steel, respectively. This change indicates that two mechanisms control the creep process:     

Extraordinary snoek damping in an Fe−Mn−N alloy

Extraordinary Snoek damping was examined in an Fe-2 pct Mn-0.01 pct N alloy over a temperature range of ?50° to 290°C. The complex damping spectrum is composed of a relatively small nitrogen Snoek peak and a series of at least four extraordinary Snoek peaks that lie between 7° and 135°C at 1 Hz. The low temperature peak (at 7°C for 1 Hz) was investigated in some detail by varying the frequency between 0.19 and 2.5 Hz and by analyses of the peak shape. The peak-shift analyses give, an activation energy of 17.1 kcal per mole, which is 2 to 3 kcal per mole more than that given by the peak-shape analyses. Therefore, the lowtemperature peak is not characterized by a single relaxation time. Furthermore, analyses of a series of synthetic curves indicated that the low-temperature peak can be characterized by two relaxation times with the strength of one relaxation being about 15 pct of the other. The activation energy of 17.1 kcal per mole is in excellent agreement with the previously proposed proportionality between peak temperature and activation energy of relaxation peaks. The present findings are in general consistent with the model for the low-temperature peak that envisions a nitrogen atom jumping about a pair of manganese atoms.     

Kinetics of the thermal dissociation of Co4S3 under reduced pressure

Kinetics of the thermal dissociation of Co₄S₃ were investigated under reduced pressures of 5 × 10^-2 and 1.5 × 10^-4 mm Hg in the temperature range 1120° to 1405°C. The initial 10 pct dissociation was found to take place in accordance with a linear rate law, with activation energies of 20 and 17 kcal per mole under low and high vacuums, respectively. Subsequent dissociation up to about 35 pct was observed to be parabolic in nature, with activation energies of 38 and 40 kcal per mole under the respective vacuums. Further dissociation up to about 90 pct was found to fit into another linear rate law, with activation energies of 47 and 49 kcal per mole under the reduced pressures of 5 × 10^-2 and 1.5 × 10^-4 mm Hg, respectively. Beyond 90 pct, the dissociation was found to be very sluggish. These results have been interpreted with the help of the Co-S phase diagram. It has also been possible to achieve more than 99 pct dissociation of Co₄S₃ in 2 hr at 1405°C under low vacuum or 1355°C under high vacuum.     

Effect of solutes in binary columbium (Nb) alloys on creep strength

The effect of seven different solutes in binary columbium (Nb) alloys on creep strength was determined from 1400 to 3400°F (760 to 1871°C) for solute concentrations to 20 at. pct using a new method of creep strength measurement. The technique permits rapid determination of approximate creep strength over a large temperature span. All of the elements were found to increase the creep strength of columbium except tantalum. This element did not strengthen columbium until the concentration exceeded 10 at. pct. Hafnium, zirconium, and vanadium strengthed columbium most at low temperatures and concentrations, whereas tungsten, molybdenum, and rhenium contributed more to creep strength at high temperatures and concentrations.     

Precipitation kinetics in Nb(Cb)-N alloys

Niobium wires were doped with 0.2 wt pct N and then directly aged at temperatures from 300° to 650°C for various lengths of time. Metallographic examination revealed a product phase at subgrain boundaries which exhibited linear growth for each reaction temperature. An activation energy of 5.6 kcal per mole for the process was obtained. Although the reaction was thought to be discontinuous precipitation, additional information from electron microscopy, internal friction, and hardness measurements indicated that both precipitation and a coarsening process were occurring simultaneously     

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.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

The activation energy for creep of columbium (Niobium)

The activation energy for creep of nominally pure columbium (niobium) was determined in the temperature range 0.4 to 0.757T_M by measuring strain rate changes induced by temperature shifts at constant stress. A peak in the activation energy vs temperature curve was found with a maximum value of 160 kcal/mole (672 kJ/mole). A pretest heat treatment of 3000F (1922 K) for 30 min (1800 s) resulted in even higher values of activation energy (>600 kcal/mole, 2520 kJ/mole) in this temperature range. The activation energy for the heat-treated columbium (Nb) could not be determined near 0.5T_M because of unusual creep curves involving negligible steady-state creep rates and failure at < 5 pct creep strain. It is suggested that the anomalous activation energy values and the unusual creep behavior in this temperature range are caused by dynamic strain aging involving substitutional atom impurities and that this type of strain aging may be in part responsible for the scatter in previously reported values of activation energy for creep of columbium (Nb) near 0.5TM.     

The compressive creep behavior of a Pu-1 wt pct Ga δ-stabilized alloy at elevated temperatures

Constant stress compression creep tests were performed in vacuum on a high-purity Pu-1 wt pct Ga ö-stabilized alloy over the temperature range from 252° to 382°C for stresses from 700 to 2500 psi. Although the primary creep behavior could not be correlated by established techniques, the creep rates developed after true creep strains of about 0.15 provided good agreement with the temperature and stress dependence of creep for pure metals and dilute alloys. A power stress law for steady-state creep of the alloy was found forδ/E values less than 5 x 10′4, with the stress exponent being 4.0, and it was concluded that the alloy exhibits Class I solid solution behavior. For higher stress, exponential stress dependence was observed. The true activation energy for creep was found to be 38,900 cal per mole which is in good agreement with the value for self-diffus ion of plutonium in the ô-stabilized alloy. The primary creep behavior could be divided into three types: 1) at low strain rates, the creep rate gradually increases to a nearly steady-state; 2) at intermediate strain rates, the creep rate first decreases and then increases to steady-state; and 3) at high strain rates, the creep rate decreases gradually to steady-state. It was concluded that the failure of established creep correlations for primary creep of Pu-1 wt pct Ga was the result of some temperature-dependent component of creep structure, possibly resulting from radiation damage byα particles.     

Kinetics of diffusion in the Nb-Al system

Diffusion kinetics were studied by varying the time and temperature (800° to 1300°C) of dipping solid niobium into molten aluminum. It was found by X-ray diffraction analysis that only the Al₃Nb phase forms at the surface of the niobium specimen and that the thickness of this layer for a given dip temperature varies parabolically with time. The activation energy and the preexponential factor of the diffusion parameter, (δD) were found to be 36.5 ∓ 0.45 kcal per mole and 2.0 ∓ 0.34 sq cm per sec, respectively.     

Creep resistance and high-temperature metallurgical stability of titanium alloys containing gallium

Tensile properties up to 1100°F and the creep resistance at 1000°F were correlated with composition for twelve complex developmental titanium alloys with additions of Al, Ga, Sn, Mo, Zr, and Si. Creep resistance for these alloys in the β heat-treated condition was found to be strongly dependent on the totalα stabilizer content and the silicon concentration. The creep activation energy for a Ti-4.5 Al-2 Sn-3 Zr-3 Ga-1 Mo-0.5 Si alloy, established over the 900° to 1100°F temperature range, was about 100 kcal per g-mole. This high creep activation energy is hypothesized to result from dispersion strengthening within theα matrix by the Ti₃ X (X = Al, Ga, Sn) phase and pinning of the interplatelet and priorβ grain boundaries by the Zr₅Si₃ phase. Both phases were identified by transmission electron microscopy in these respective locations. Metallurgical instability, as evidenced by decreased fracture toughness, is also shown to be relatable to the totalα stabilizer content. The activation energy for the embrittlement process is about 45 kcal per g-mole. which approximates that for interdiffusion of gallium inα titanium.     

Kinetics of and preparation of nickel by Ni3S2−NiO reaction under reduced pressure

The reaction between Ni₃S₂ (liquid) and NiO (solid) resulting in the formation of Ni and SO₂ was investigated in the temperature range 800° to 1200°C under a reduced pressure of <0.1 mm Hg. From the kinetic studies in the temperature range 950° to 1150°C, the reaction was found to proceed in three stages: i) Up to about 25 pct reduction, the rate of reaction was high and followed approximately a cubic rate law. During this stage, the reaction is thought to be under mixed control. Activation energy for the first 10 pct reduction was found to be approximately 45 kcal per mole. ii) From about 25 to 90 pct reduction, it obeyed the parabolic rate law, with an activation energy of 86±6 kcal per mole. This value is in agreement with the activation energy reported in the literature for the diffusion of sulfur in nickel. iii) Beyond 90 pct reduction, the reaction was very sluggish owing to the poor availability of the reactants. Optimum conditions for preparing nickel sponge by the above reaction and its processing into thin strips have been standardized. Some of the properties of the metal thus produced have also been incorporated.     

Copper diffusion in iron during high-temperature tensile creep

The diffusion of liquid copper in iron from a notched surface has been studied by metallographic, microanalysis, and sessile drop techniques. The diffusivity of copper was found to be 0.59×10^?6 sq cm per sec at 1100°C and 0.97×10^?6 sq cm per sec at 1130°C. The diffusion factor,D ₀ was 0.78×10^?3 sq cm per sec and the activation energy 19.0 kcal per mole. The predominant mode of copper penetration was along grain boundaries, but when larger volumes of copper at the iron surface were used, surface diffusion increased and grain boundary penetration remained constant. The most frequently occurring dihedral angle for liquid copper was 34 deg at 1100° and 1130°C. The liquid copper/austenite interfacial energy was found to be 444 ergs per sq cm between 1100 and 1130°C. From sessile drop measurements, the contact angle was determined as 35 deg at 1100°C and 28 deg at 1130°C, from which values the respective interfacial energies were calculated to be 387 ergs per sq cm and 301 ergs per sq cm.     

Grain-boundary penetration of type 316 stainless steel exposed to cesium or cesium and tellurium

When 20 pct cold-worked Type 316 stainless steel is exposed to Cs at 700°C under controlled oxygen-chemical potential environment, Cs penetration into the stainless steel grain boundaries occurs at oxygen potentials ΔG_o2 ≥ -96 kcal per mole. At lower oxygen potentials (~ΔG_o2 ≤ —110 kcal per mole), no corrosion occurs. Under the same experimental conditions, when the stainless steel is exposed to Cs:Te (2:1, atomic), corrosion occurs and penetration morphology appears to depend strongly on the oxygen-potential environment. The stainless steel suffers intergranular corrosion by Te (in the presence of Cs-Te) under conditions where chromium oxidation is not expected to occur. The kinetics of grain-boundary penetration by Te have been studied at temperatures between 550 and 700°C. The depth of the penetrated zone varies as (time)^1/2, and the process has an activation energy of 34 kcal per mole. The results are discussed, and the effects of stainless steel microstructure and externally applied stress on corrosion reactions are also described.     

Cellular precipitation in Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys

The phases present in directionally solidified Ni-51Cr lamellar eutectic and cast Ni-44Cr alloys are identified. These are the nickel-and chromium-rich lamellae, a Widmanstätten precipitation of nickel in the chromium-rich lamellae, grain boundary precipitation of chromium, and scattered discrete particles of chromium oxide. Upon annealing, the amount of the nickel precipitates increases drastically and a well defined cellular precipitation appears in the nickel-rich lamellae. The cellular precipitation conforms in almost every way to typical cellular reaction such as in the Pb-Sn system. Cellular (linear) growth rate, G, and interlamellar spacing, S, were measured on specimens annealed for times ranging from 1/2 to 100 hr at 625°, 700°, 760°, and 850°C. G increased from (average for both Ni-Cr alloys) 2.3 × 10^?8 cm per sec at 625°C to 7.7 × 10^?7 cm per sec at 760°C and decreased again at 850°C. S varied from 2 to 10 × 10^?5 cm as the annealing temperature was increased. The calculated grain boundary diffusivity, D^B, representing the diffusion of chromium in fcc Ni-Cr solid solution, increased from 6.7 × 10^?11 sq cm per sec at 625°C to 8.6 × 10^?8 sq cm per sec at 850°C. The activation energy 64 kcal per mole, is of the order of that obtained for self-diffusion.     

Microstructure and Properties of an Advanced Nickel-base PM Superalloy

The need for nickel-base powder metallurgy （PM） superalloy turbine discs is becoming increasingly evi dent. With the eventual aim of improving thrust-to-weight ratio of aeroengines for power generation, well integration of significantly high strength, high damage tolerance and high-temperature capability would be reasonably required. An advanced PM superalloy, which was designed for applications up to 815- 8 5 0 ℃, was experimentally investigated. Emphasis was primarily put on microstructure and mechanical properties. The results indicated the measured phases in the sample were composed of γ,γ＇, MC, and Ma B2. With uniform coarse grain microstruc ture （ASTM 5-6）, the sample appeared to exhibit overwhelming superiority over the prior art materials FGH95, FGH96, FGH97 and FGH98. The dominant embodiments consisted of high tensile strength （Rm = 1000 MPa and Rp0.2 800 MPa at 850℃）, strong creep resistance （ξp 0.12% at 815 ℃/400 MPa/50 h）, and considerable stressrupture life （τ=457.4 h at 815 ℃/450 MPa）. The technical practicability of applications up to 815-850 ℃ of this alloy was conclusively proved.