Phase composition of Al-Rich Al-Sc-Cr-Zr alloys at 640, 600, and 500°C

Metallographic and electron microprobe analyses and hardness and electrical resistivity measurements are used to study Al-rich Al-Sc-Cr-Zr alloys annealed and quenched from 640, 600, and 500°C. The boundaries of Al-based solid solution are determined; the Al-based solid solution is found to be in equilibrium with the CrAl₇ compound and ScAl₃- and ZrAl₃-based phases, in which zirconium and scandium are dissolved, respectively. Sections of isothermal tetrahedra at 640, 600, and 500°C are constructed.     

Evolution and coarsening of Al2O3 dispersoids at 500 °C to 600 °C in a mechanically alloyed Al-Ti-O based material

The formation and coarsening of Al₂O₃ dispersoids have been investigated at 500 °C, 550 °C, and 600 °C in a mechanically alloyed (MA) extrusion of composition Al-0.35wt pct Li-1wt pct Mg-0.25wt pct C-10vol pct TiO₂ for times up to 1500 hours. In the as-extruded condition, the dispersed phases included Al₃Ti, Al₄C₃, MgO, cubic TiO (C-TiO), monoclinic TiO (M-TiO), TiO₂, and a small amount of Al₂O₃. However, numerous Al₂O₃ dispersoids (various polymorphs: η, γ, α, and δ) with “block-shaped” morphology were formed after heat treatment due to reduction of C-TiO, M-TiO, and TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed conclusively the transformation of these phases to additional Al₂O₃ and Al₃Ti. High resolution TEM showed that the α-Al₂O₃ dispersoids exhibited some lattice matching with the α-Al matrix. Coalescence of the block-shaped Al₂O₃ dispersoids occurred after heat treatment, and Al₄C₃ also became attached to them. The length and width of the block-shaped Al₂O₃ dispersoids increased by a factor of ∼1.55 between 340 and 1500 hours at 600 °C.     

Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO

NiAl and NiAlSi alloys were internally oxidized at temperatures of 1073–1273 K by the Rhines Pack method. For the NiAl alloy, the oxidation process follows parabolic law and the oxidation front was flat with severe integranular oxidation occurring at 1073 K and extensive grain boundary sliding at 1273 K. As for NiAlSi alloys, the oxidation rate increased with increase of Si content at 1073 K but the rate decreased at higher temperatures due to total or partial continuous oxide layer formation at the internal oxidation front. The depth of intergranular oxidation was also greatly reduced. For all samples, nickel was found to be transported out to the surface with the amount proportional to the Si content. Lattice diffusion (Nabarro-Herring creep) was believed to be the main cause for nickel transport in the NiAl alloy while dislocation pipe diffusion is the mechanism for NiAlSi alloys.     

Thermal stability of invar FeNi alloys above 500°C

The high-temperature thermal stability of three FeNi alloys (with 35.0, 40.1 or 43.8 at. % Ni) was investigated by aging samples for 230 days at 625°C. Anomalous small-angle X-ray scattering, analytical transmission electron-microscopy and magneto-thermogravimetric Curie temperature measurements were used for this purpose. The wave-length dependence of the anomalous scattering effect was found to be opposite to what would be expected from a decomposed solid solution. The absence of concentration fluctuations was further confirmed by the other two techniques. Taking into account previous aging results, it is concluded that, in the investigated composition range, FeNi alloys are thermally stable above 500°C.     

Anomalous coarsening behavior of small volume fractions of Ni3Al precipitates in binary NiAl alloys

The kinetics of coarsening of γ′ precipitates in binary NiAl alloys containing nominally 5.72, 5.74 and 5.78 wt% Al and aged at 630°C were investigated by transmission electron microscopy and magnetic analysis. The alloys were each pre-aged at a temperature just below its solvus prior to re-aging at 630°C. The volume fractions of γ′ were low between 0.02 and 0.03. the pre-aging treatment produced microstructures containing precipitates that nucleated heterogeneously on dislocations and grain boundaries, leaving empty matrix areas in which coherent γ′ precipitates subsequently nucleated and coarsened. Measurements were made only on these precipitates. The coarsening kinetics are anomalous in that the rate constant decrease with increasing volume fraction, in contradiction with all the theories of this process. Furthermore, the increase is quite large, exceeding a factor of four over a range of volume fractions that increased by less than 40%. Additionally, the rate constants exceed the values expected from the literature by factors of 10–40. The distributions of particles sizes were measured, and with few exceptions were found to agree with those of earlier investigations. It is suggested that elastic interactions among the γ′ precipitates play a pivotal role in decelerating the kinetics of coarsening, overpowering the expected accelerating effect of increasing volume fraction.     

Deformation and strength behavior of two nickel-base turbine disk alloys at 650 °C

Two powder metallurgy nickel-base turbine disk alloys, RENE’95* and KM4, were studied for strength and deformation behavior at 650 °C. Two classes of microstructures were investigated: unimodal size distributions of γ′ precipitates with particle sizes ranging from 0.1 to 0.7 μm and commercially heat-treated structures with bimodal or trimodal size distributions of γ′ precipitates. The strength and deformation mechanisms were heavily influenced by the microstructure. In both alloys, deformation during compression tests consisted of a combination of a/2〈110〉 antiphase boundary (APB)-connected dislocation pairs and a/3〈112〉 partials creating superlattice intrinsic stacking faults (SISFs). In unimodal alloys, the fault density increased with decreasing particle size and decreasing strain rate. These trends, observed in compression testing, are consistent with earlier studies of similar alloys, which were tested in creep. As the γ′ size was reduced, the nature of the faults changed from being isolated within single precipitates to being extended across entire grains. Commercially heat-treated alloys, containing a bimodal distribution of γ′ particles, exhibited significantly more faulting than unimodal alloys at the same cooling γ′ size. This augmentation of the faulting in commercial alloys was apparently due to the presence of the fine, aging γ′ particles. The two typical commercial heat treatments (supersolvus and subsolvus) resulted in different deformation structures: the subsolvus behavior was similar to that of unimodal alloys with γ′ sizes between 0.2 and 0.35 μm, while the supersolvus deformation was similar to that of unimodal alloys with the 0.1 μm γ′ size. These differences were attributed to differences in the size of the fine, aging γ′ particles. Creep deformation in a commercially heat-treated material at 650 °C occurred solely by SISF-related mechanisms, resulting in a macroscopic slip vector of 〈112〉. The effects of alloy chemistry, APB energy, and microstructure on the deformation and mechanical behavior are discussed in detail, and possible effects of the faulting mechanisms on the mechanical behavior are explored. Finally, models for yield strength as a function of microstructure for bimodal alloys with large volume fractions of precipitates are found to be in need of development. RENE′95 is a trademark of General Electric Company, Fairfield, CT.     

Multiphase diffusion in Fe−Ni−Al system at 1000°C: II. Interdiffusion coefficients for β and γ alloys

Ternary interdiffusion coefficients were determined at 1000°C at several Fe−Ni−Al alloy compositions with multiphase β(bcc)vs γ (fcc) diffusion couples which developed planar β/γ-interfaces. The coefficients, (i,j=Al or Ni) were calculated at compositions corresponding to points of intersections of diffusion paths with Fe taken as the dependent component. These coefficients varied with composition by 1 to 2 orders of magnitude in the β-phase but relatively little in the γ-phase. Empirical relations were derived to describe the composition dependence of the main coefficients. and . Interdiffusion coefficients with either Al or Ni as the dependent component were also evaluated. The relative diffusivities of the elements increase in the order, Fe, Ni, Al for both β- and γ-alloys. The ternary diffusion data were consistent with binary interdiffusion coefficients for Fe−Al and Fe−Ni alloys. G. H. CHENG, formerly a Graduate Student at Purdue University     

Effect of silicon on the rate of de-sulphurization of liquid Fe-S alloys by argon-hydrogen at 1550°C

Abstract

The passivation of copper anodes due to precipitation of copper sulfate on the anode surface was investigated as a function of electrolyte composition and temperature, and of anode composition. The slime layer present on the anode surface was shown to be the primary factor in causing passivation by inhibiting the diffusion of copper ions. Factors such as temperature, free acid level, Ni²⁺ and Cu²⁺ ion levels were also important in so far as they affected the mass transfer characteristics of Cu²⁺ ions and the solubility of copper sulfate.

Résumé

La passivation des anodes de cuivre due à la précipitation du sulfate de cuivre sur la surface de l'anode a été étudiée en fonction de la composition et la température de l'électrolyte et aussi en fonction de la composition de l'anode. Nous avons montré que la couche d'impuretés présente sur la surface de l'anode est la cause première de la passivation car elle empêche la diffusion des ions cuivre. D'autres facteurs tels que la température, le niveau d'acide libre, le niveau d'ions Ni²⁺ et Cu² sont également importants en ce qu'ils affectent les caractéristiques de transport de matière des ions Cu²⁺ et la solubilitédu sulfate de cuivre.     

Hydrogen embrittlement in ductile Ni3Al—Effects of hydrogen content,strain rate and pre-deformation

Hydrogen embrittlement has been studied in continuous cast sheet of an Ni₃Al alloy (Ni_77.83Al_21.73Zr_0.34B_0.1). When tensile tests were performed at the initial strain rate of 5.8 × 10⁻⁵ s⁻, the elongation decreased from 32.7% for no charging to 1.9% for 330 min of cathodic charging with 50 mA/cm² current, but the yield stress did not change. The fracture mode changed partially from dimple to intergranular and cleavage modes. At a faster strain rate of 5.8 × 10⁻³ s⁻¹, hydrogen embrittlement was less pronounced, but the yield stress increased with hydrogen content and multiple cracks were generated. Plastically pre-deformed (2–26% elongations) and subsequently charged specimens failed after yielding, which occurred at the final pre-deformation stress. Our results suggest that the grain boundary or the interior of the grain was not weakened by hydrogen, rather hydrogen-enhanced localized plasticity caused the loss of ductility in B-doped Ni₃Al alloy.     

The order-disorder transformation in Ni3Al and Ni3AlFe alloys—II. Phase transformations and microstructures

From a combination of dilatometric and micrographie evidence, the precipitation of the β′ phase at temperatures above the metastable ordering transition, T_tm, is demonstrated for those NiAlFe alloys containing at least 19 at.% Al. Ni-AlFe alloys which have been heated above T_tm and then slow-cooled are found to contain a network of disordered γ phase which outlines the boundaries of antiphase domains in the ordered γ′ phase. The implication of these observations is that the formation of equilibrium γ phase from off-stoichiometric γ′ requires antiphase domain boundaries as nucleation sites: the T_tm determined by dilatometry represents an upper absolute instability limit at which the ordered phase is converted into disordered phase of the same composition, whereas the equilibrium disordered phase would have a different composition. The microstructure of a series of binary Ni-Al alloys are examined: at 22–23 at.%Al, the γ′ phase consists of grains which partly contain fine antiphase domains (with γ at the APD boundaries) and partly very coarse APD's; microanalysis established that this is due to microségregation, and that the coarse APD's are in regions with excess Al so that T_tm is locally above the freezing temperature. The structural distinction between directly and sequentially ordered alloys is thereby made clear: fine APD's form only in alloys which freeze in disordered form.     

Bioremediation Kinetics of Crude Oil at 5°C

This bench scale experiment investigated the mineralization of crude oil at cold temperatures. Biodegradation rates of Alberta Sweet Mix crude oil at 5°C and at ambient temperatures (21°C) were compared. The experimental design consisted of 14 soil columns. Off-gas samples for each column were analyzed every second day using a gas chromatograph to determine CO2 production rates. The run lasted for 219 days, at which time the soil columns were sacrificed for analysis. Oil mineralization values were modeled by a number of methods, including CO2 production determination from GC measurements and gravimetrically determined hydrocarbon loss using soxhlet extraction. The results of the experiment suggested that temperature only affected the biodegradation rates of crude oil in the initial phase of the biodegradation process. After approximately 3 months, the degradation rates of crude oil at 5°C and 21°C were similar at about 11 mg hydrocarbon∕kg dry soil∕day. The conclusion of this study was that significant mineralization of Alberta Sweet Mix crude oil can occur at cold temperatures.     

Oxidation behaviour at 1100°C in air of 25 wt-% Cr-containing cobalt-based alloys containing high quantities of hafnium carbides

Hafnium, usually added to improve the high temperature oxidation resistance of alloys, allowed obtaining HfC carbides which are very efficient for the creep-resistance. For that Hf must be added in particularly high quantities which may possibly influence the oxidation behaviour. Three HfC-strengthened cast cobalt alloys were studied all along thermogravimetry tests at 1100°C. They were compared to similar but Hf-free ternary alloys. The mass variation were plotted according to {m?×?dm/dt?=?f(–m)} to specify all kinetic oxidation constants, and versus temperature to study the oxidation beginning during heating and the scale spallation during cooling. The presence of many HfC carbides obviously influences the high temperature oxidation: mass gain occurring sooner during heating, faster isothermal mass gains but better behaviour in oxide scale spallation during cooling. This deterioration of oxidation behaviour must be corrected to hope benefiting from the high creep-resistance brought by this new type of strengthening.     

Reactions of Fe-Cr and Ni-Cr alloys in CO/CO2 gases at 850 and 950 °C

A series of Fe-Cr and Ni-Cr solid solution alloys was reacted at 850 and 950 °C in CO/CO₂ gas mixtures in which FeO and NiO were unstable. The compctitive tendencies toward the carburization and oxidation of the chromium solute, as compared to a graphical thermodynamic "metastability" criterion, were tested experimentally. Relatively good agreement was found between predictions and experiments for the occurrence of Cr carburization beneath Cr₂O₃ internal oxides or external scales. The chromium contents required for the transition from internal oxidation of Cr to the formation of Cr₂O₃ external scales in CO/CO₂ gas mixtures were established for Fe-Cr and Ni-Cr alloys. The Cr₂O₃ external scales formed on Fe-Cr alloys were found to be relatively impervious to carbon penetration for short (12-hour) experiments. No carburization was observed in the Ni-Cr alloys, but the only alloys that were predicted to carburize were the ones that formed external scales. Formerly Graduate Student, The Ohio State University