Oxidation mechanism of three Fe-Al alloys with and without addition of 0.1 at% yttrium at 800℃

The isothermal oxidation behavior of Fe-10 Al,Fe-15 Al,Fe-20 Al alloys with and without the addition of0.1 at% Y was studied at 800℃ under 1×10~5 Pa of flowing pure O_2 for 24 h.The oxidation of three Fe-Al alloys can be divided into transient state and steady state oxidation stages.The oxidation of each stage is approximately in accordance with the parabolic law.The addition of 0.1 at% Y changes the oxidation behavior obviously and leads to a significant increase of the weight gain of Fe-10 Al and Fe-15 Al.The scale grown on Fe-10 Al is much thicker and more complicated than that grown on Fe-20 Al,which is composed of an exclusive thin layer of Al_2 O_3 protective film.Due to the formation of a large number of nodules,the scales grown on Fe-15 AI cannot provide full protection for the alloy.Scale micro structure of the three Fe-Al-0.1 Y alloys is similar to their corresponding Fe-Al alloys.However,nodules with very small size still appear on the surface of Fe-20 Al-0.1 Y alloy.The critical Al concentration to form an exclusive Al_2 O_3 protective layer for Fe-Al binary alloy is on the borderline between 15 at%-20 at%.For Fe-Al-0.1 Y alloy,the presence of 20 at% Al is not enough to inhibit the growth of nodules.     

Mechanism and Kinetics of Stibnite Chloridizing with Calcium Chloride-Oxygen at 500 °C to 650 °C

Metallurgical and Materials Transactions B - Copper concentrates with high levels of toxic impurity elements such as As, Sb Bi, must be pretreated before processing in the current...     

Leaching of lanthanide and yttrium from a Central Appalachian coal and the ashes obtained at 550–950 °C

In this work, we investigated leaching of lanthanide and yttrium (REY) from a Central Appalachian coal and its ashes obtained at 550–950 °C with the main purpose of understanding the impact of ashing temperature on REY leachability in water, ammonium sulfate, and hydrochloric acid. It is found that the coal contains a negligible amount of water-soluble REY, less than 1% ion-exchangeable REY, and about 28% of HCl-soluble REY. Ashing leads to dramatic changes in REY leachability in both ammonium sulfate and hydrochloric acid solutions, which is believed to be related to transformation and redistribution of organically-associated REY in coal during the ashing process. Ashing temperature significantly affects REY leaching from coal ashes; higher ashing temperature results in lower REY leachability in both solutions. Clay minerals may play a significant role in changing the leachability of REY after ashing. In addition, the results also suggest that the organic matter in the coal is relatively enriched in heavy REY.     

Influence of the Co/Ni Ratio and Dendritic Segregations on the High-Temperature Oxidation Resistance of Multinary Co-Rich Superalloys at 850 °C and 1050 °C

Metallurgical and Materials Transactions A - Excellent inherent oxidation resistance is a prerequisite for the use of superalloys in many high-temperature applications. To achieve this goal,...     

Oxidation behavior of novel rail steels at 500°C

Thermo gravimetric analysis (TGA) was used to study isothermal oxidation behavior of four novel rail steels at 500∧C in static air condition. These steels contained different combinations of microalloying elements, Cu, Cr, Ni and Si. The results were compared with the oxidation behavior of three rail steels already in commercial application (C-Mn, Cr-Mn and Cu-Mo). The lowest parabolic rate constant was obtained in case of Cr-Cu-Ni-Si contained rail steel. Spallation of the oxide layer was not observed in any of the rail steels. C-Mn, Cu-Mo and Cu-Si rail steels showed higher values of parabolic rate constants k_p. In contrast to others Cu-Si exhibited linear rate law and larger weight gain compared to all other rail steels. X-ray diffraction (XRD) analysis confirmed that the major phase in the oxides was magnetite (Fe₃O₄). The surface morphology of the oxide scales were correlated with the oxidation rate.     

The effect of aluminum content on the corrosion behavior of Fe-Al alloys in reducing environments at 700 °C

The high-temperature corrosion behavior of monolithic Fe-Al alloys, with 0 to 20 wt pct Al, was investigated at 700 °C in a reducing atmosphere (p(S₂) = 10⁻⁴ atm, p(O₂) = 10⁻²⁵ atm) for up to 100 hours. Postexposure characterization of the corrosion reaction products consisted of surface and cross-sectional microscopy, in combination with energy dispersive spectroscopy, electron probe microanalysis, and quantitative image analysis. From the kinetic data, three stages of corrosion behavior (i.e., inhibition, breakdown, and steady state) were found with the observance and/or duration of each stage directly related to the aluminum content of the alloy. The first stage, labeled the inhibition stage, was characterized by low weight gains and the absence of rapid degradation of the alloy. Typically observed for compositions with 10 to 20 wt pct Al, protection was afforded due to the development of a thin, continuous alumina scale. For alloys with 7.5 wt pct A1, the ability to maintain the initially formed alumina scale was not observed, resulting in the breakdown stage. Localized corrosion product nodules, containing iron sulfide (Fe_1-x S) and the spinel-type tau phase (FeAl₂S₄), developed through the alumina scale due to sulfur short-circuit diffusion. These growths were accompanied by relatively high corrosion rates. Further decreasing the aluminum content to 5 wt pct and below lead to the formation of a continuous sulfide scale whose growth was controlled by iron and sulfur diffusion through the previously formed product. The alloy wastage rates in the steady-state stage were relatively high when compared to the previous two regions.     

Oxidation Behavior of Copper Concentrate,Gold Concentrate,and Their Mixtures Between 1173 K (900 °C) and 1373 K (1100 °C)

Metallurgical and Materials Transactions B - Gold is typically extracted from ores using a cyanidation process. However, environmental regulations and low productivity are the main drawbacks of...     

The nitriding behavior of Ti-Al alloys at 1000 °C

The nitriding behavior of a series of alloys in the binary Ti-Al system has been determined at 1000 °C, under a controlled atmosphere of pure nitrogen gas, for times ranging between 7 and 100 hours. The scales and subscales were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive and wavelength dispersive X-ray analysis, electron energy loss spectroscopy, and optical microscopy. Upon formation of a surface nitride scale, the subscale became enriched in Al and resulted in the formation of a series of Al-rich intermetallic phases. This enrichment has been linked to the transport processes in the scale and subscale and a shifting of the diffusion path toward the Al-rich corner of the ternary isotherm. The formation of Al-rich intermetallic phases in the subscale was shown to result in rapid “breakaway” nitriding of the TiAl and TiAl₂ alloys. The stoichiometry of the binary nitrides AlN and TiN was measured, as well as the composition of the ternary nitride “Ti₂AlN.”     

Distribution of Grain Boundary Carbides in Inconel 617 Subjected to Creep at 900 °C and 950 °C

Metallurgical and Materials Transactions A - A post-creep deformation analysis is carried out on the nickel-based superalloy Inconel 617 in order to identify the grain boundary carbide (GBC)...     

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.     

Experimental Investigation of the 1073 K (800 °C) Isothermal Section of the Al-V-Zr Ternary System

Metallurgical and Materials Transactions A - This work is focused on an experimental investigation of the phase equilibria of the Al-V-Zr system at 1073 K (800 °C). The phase...     

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.     

Phase Equilibria in the Fe-Mo-Ti Ternary System at 1173 K (900 °C) and 1023 K (750 °C)

Alloys with fine-scale eutectic microstructures comprising Ti-based A2 and TiFe B2 phases have been shown to have excellent mechanical properties. In this study, the potential of alloys with further refined A2-B2 microstructures formed through solid-state precipitation has been explored by analyzing a series of six alloys within the Fe-Mo-Ti ternary system. Partial isothermal sections of this system at 1173 K (900 °C) and 1023 K (750 °C) were constructed, from which the ternary solubility limits of the A2 (Ti, Mo), B2 TiFe, D8₅ Fe₇Mo₆, and C14 Fe₂Ti phases were determined. With these data, the change in solubility of Fe in the A2 phase with temperature, which provides the driving force for precipitation of B2 TiFe, was determined and used to predict the maximum potential volume fraction of B2 TiFe precipitates that may be formed in an A2 (Ti, Mo) matrix.

     

Phase equilibria in Al-rich Al-Mg-Sc-Zn alloys at 430 and 300°C

Optical microscopy and X-ray diffraction and electron microprobe analyses are used to study Al-Mg-Sc-Zn alloys annealed at 430 and 300°C. The Al-based solid solution is found to be in equilibrium only with binary and ternary phases of the corresponding systems; these are Al₃Sc, β(Al₃Mg₂), η(MgZn₂), and τ(Al₂Mg₃Zn₃). Sections are constructed for the isothermal tetrahedra of the Al-Mg-Sc-Zn phase diagram that correspond to a scandium content of 0.5% and magnesium and zinc contents of up to 20%.     

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.     

Dynamic embrittlement of boron-doped Ni3Al alloys at 600°C

Boron-doped Ni₃Al alloys, with and without 0.5at.%Hf, were tensile tested in vacuum and in oxidizing environments at 600°C. Tensile ductility was found to be strongly dependent on test environment, with much lower ductilities observed in air than in vacuum. The loss in ductility is accompanied by a change in fracture mode from transgranular to intergranular. The severity of this environmental effect on elevated-temperature ductility is also affected by preoxidation in air as well as by the aluminum content of the aluminide. Tests of preoxidized specimens indicate that the embrittlement is due to a dynamic effect simultaneously involving localized stress concentrations, elevated temperature, and gaseous oxygen. The oxygen embrittlement becomes less severe with a decrease in aluminum concentration from 24 to 21 at.%.     

Experimental Study on Electrical Conductivity of MnO-CaO-SiO2 Slags at 1723 K to 1823 K (1450 °C to 1550 °C) and Various Oxygen Potentials

Metallurgical and Materials Transactions B - The electrical conductivity of molten slag has many important and practical effects in modeling and operating the electric smelting furnace. In the...     

Reaction of Cerium Oxide with Zirconium and Yttrium Oxides at 1250°C

Phase relationships in the ternary system ZrO₂ Y₂O₃ CeO₂ were investigated by x-ray diffraction analysis and electron microscopy for the first time, and phase relationships in the binary systems ZrO₂ CeO₂ and CeO₂ Y₂O₃ defined more accurately at 1250°C over the entire range of concentrations. The 1250°C isothermal section of the phase diagram for ternary system was constructed. Specimens were prepared from nitrate solutions by evaporation, drying, grinding, and subsequent annealing at 1250°C. The systems were characterized by the formation of broad solid solution ranges based on C -Y₂O₃, F-CeO₂, F-ZrO₂, and narrow based on T-ZrO₂ and -phase. No new phases were found in the systems. The path of isoparametric lines in the F-ZrO₂ solid solution field suggests that yttrium and cerium ions can substitute for each other without appreciable substitution of zirconium ions.     

Interaction of sintered aluminum nitride with the KF-AlF3 melt at 700–800°C

Interaction kinetics of the aluminum nitride-based sintered composite material with the potassium cryolite melt with the cryolite ratio KO = 1.3 at 700–800°C is investigated by the continuous weighing method. It is established that the composite is almost unaffected by the melt at t = 700°C. The corrosion rate increases as the temperature increases. The material interacts with the molten salt through its oxidation stage by oxygen dissolved in the melt. This is also accompanied by the dissolution of oxygen-containing impurities that are initially present in the material. No interaction of sintered aluminum nitride with the melt under study was observed in the absence of the oxidant and with the concentration of alumina dissolved in cryolite close to saturation.     

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