The high temperature oxidation of Al-4.2 Wt Pct Mg alloy

The high temperature oxidation of Al-Mg alloys is characterized by the rapid formation of thick, micro-crystalline oxide films. The oxidation kinetics of an Al-4.2 wt pct Mg alloy under dry and moist 20 pct O₂/Ar have been measured, and oxide films grown on bulk specimens complementary to the weight gain curves have been characterized using electron optical techniques (TEM, SEM). Initial oxidation takes place by the nucleation and growth of primary crystalline oxides at the oxide/metal interface and by the formation of secondary oxides of MgO by the reduction of the original amorphous over-layer of γ-Al₂O₃ by Mg. Subsequent oxidation is dominated by the further nucleation and growth of primary oxides. The presence of water vapor in the oxidizing environment initially reduces oxidation rates through a modification of the mechanical properties of the amorphous overlayer but does not affect the overall oxidation mechanism. A microstructural model has been developed which describes oxidation of Al-Mg alloys in terms of fracture of the original air-formed film by primary MgO nucleation and growth and modification to this film by the presence of water vapor in the oxidizing environment.     

In situ observations of crystalline oxide formation during aluminum and aluminum alloy oxidation

Anin situ morphological study of the oxidation of electron transparent specimens of aluminum and aluminum alloys containing zinc and magnesium has been carried out in the temperature range 400 to 520°C using the hot stage of a 1 MeV transmission electron microscope. The structure and morphology of the crystalline oxide produced in each alloy has been carefully examined by selected area electron diffraction and stereomicroscopy. In pure aluminum, oxidation takes place after a temperature dependent induction period, by the nucleation of crystalline γ-Al₂O₃ at the amorphous oxide/metal interface. This process is delayed by additions of zinc which modify the structure of the oxide. In alloys containing magnesium, oxidation takes place by the rapid nucleation and growth of MgAl₂O₄ or MgO, with a secondary form of magnesia developing from the reduction of the amorphous γ-Al₂O₃ surface layer.

     

Some water vapor effects during the oxidation of alloys that are <Emphasis Type="Italic">α</Emphasis>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> formers

Oxidation studies were performed at 1100 °C in dry air and air containing fixed partial pressures of water vapor on a number of alloys and coatings that form α-Al₂O₃ scales under oxidizing conditions. The alloys investigated included RENé N5, PWA 1484, diffusion aluminide coatings (with and without Pt modification) on RENé N5, and a Ni-8 wt pct Cr-6 wt pct Al model alloy. The water vapor affected the oxidation of the alloys in three important ways: (1) The scales spalled more profusely during cyclic oxidation in wet air than in dry air, particularly for those alloys with alumina scales, which are only moderately adherent under dry conditions. The results were consistent with the mechanism previously proposed (Reference 1), whereby the water molecules decrease the fracture toughness of the alumina/alloy interface. (2) Thicker oxides are formed during oxidation in wet air than dry air. This effect comes primarily from accelerated transient oxidation during exposure in wet air. (3) Spinel was found to form on top of the alumina scales during long-term exposure. This phenomenon occurred in all atmospheres but was much more pronounced for exposures in wet atmospheres. Mechanisms for the preceding observations are proposed.     

Interface interactions during fabrication of aluminum alloy-alumina fiber composites

The feasibility of fabricating fiber-reinforced aluminum alloys by addition of discontinuous fibers to vigorously agitated, partially solid metal slurries was investigated. In the first phase of the program, reported herein, emphasis was placed on the study of interface interactions between polycrystalline A1₂O₃ fibers and Al-2 to 8 pct Mg, Al-4.5 pct Cu and Al-4.5 pct Cu-1 to 2 pct Mg alloys. In general, it was observed that the incorporation of fibers could be readily achieved by this technique, and that fibers appeared wetted after a few minutes of contact with the melt. The composites produced exhibited an intimate, void free bond between the constituents. In addition, a region of significantly altered microstructure resulted from accumulation of oxide and/or aluminate particles which either formed within the melt and were attached to the moving fibers, or used the fiber surface as a substrate to grow on. Microscopic examination of this interaction zone and thermodynamic considerations indicate that it consists of fine α-Al₂O₃, aluminates, oxides of the alloying elements, and probably some intermetallic compounds. For example, it is shown that a stable MgAl₂O₄ spinel forms at the interface of A1₂O₃ fibers and Al-Mg alloys. Examination of composite specimens fractured under tension indicated that the interfaces produced were strong enough to permit the transfer of loads at strengths in the order of 250 to 350 MPa.     

Reduction of CaO and MgO Slag Components by Al in Liquid Fe

This study documents laboratory-scale observations of reactions between Fe-Al alloys (0.1 to 2 wt pct Al) with slags and refractories. Al in steels is known to reduce oxide components in slag and refractory. With continued development of Al-containing Advanced High-Strength Steel (AHSS) grade, the effects of higher Al must be examined because reduction of components such as CaO and MgO could lead to uncontrolled modification of non-metallic inclusions. This may lead to castability or in-service performance problems. In this work, Fe-Al alloys and CaO-MgO-Al₂O₃ slags were melted in an MgO crucible and samples were taken at various times up to 60 minutes. Inclusions from these samples were characterized using an automated scanning electron microscope equipped with energy dispersive x-ray analysis (SEM/EDS). Initially Al₂O₃ inclusions were modified to MgAl₂O₄, then MgO, then MgO + CaO-Al₂O₃-MgO liquid inclusions. Modification of the inclusions was faster at higher Al levels. Very little Ca modification was observed except at 2 wt pct Al level. The thermodynamic feasibility of inclusion modification and some of the mass transfer considerations that may have led to the differences in the Mg and Ca modification behavior were discussed.     

Probing the initial stage of synthesis of Al2O3/Al composites by directed oxidation of Al-Mg alloys

In the directed oxidation of Al-Mg alloys, the amount of MgO that forms in the initial stage prior to the incubation period affects the rate of oxidation of Al to Al₂O₃ in the composite growth stage. The mechanism of formation of MgO and the duration of the initial stage were investigated experimentally and theoretically. The variables studied were total pressure in the reaction chamber, partial pressure of oxygen, and the nature of the diluent gas which affects the diffusion coefficients of magnesium vapor and oxygen in the gas phase. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and the diffusivity of oxygen. The duration of the initial stage decreased with the increase in oxygen pressure. To understand the role of magnesium evaporation in the oxidation behavior of the alloy, the velocity, temperature, and concentration fields in the gas phase were simulated numerically. The calculated concentration profiles of magnesium vapor and oxygen as a function of time were consistent with the experimentally measured oxidation rates and confirm reaction-enhanced gaseous diffusion-limited vaporization of magnesium in the initial stage of oxidation of Al-Mg alloys. The region where the magnesium vapor is oxidized in the gas phase moved progressively closer to the alloy surface during the initial stage of oxidation. The end of the initial stage and the start of the incubation period corresponded to the arrival of the oxygen front close to the surface when the spinel formation occurred.     

Surface oxidation behavior of Mg-Y-Ce alloys at high temperature

After the Mg-Y-Ce magnesium alloy was exposed to air at a temperature up to 1173 K for 0.5 hours, the dense and compact oxide film formed on the surface. Accordingly, oxidation and ignition of magnesium alloys at elevated temperature was successfully retarded by the Y and Ce additions. Thermogravimetric measurements in air revealed that the oxidation dynamics curves measured at 673 and 773 K followed the parabolic-line law and the curve investigated at 873 K followed the complicate quartic law. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis indicated that the oxide film on the surface of Mg-Y-Ce alloys exhibited a duplex structure, which agreed with the results of thermodynamic analysis. The oxidation film included two layers: the outer layer was a multiple structure of Ce_0.202Y_0.798O_1.601 and Y₂O₃, and the inner layer mainly consisted of metal Mg and MgO.     

Vaporization Studies from Slag Surfaces Using a Thin Film Technique

The investigations of vanadium vaporization from CaO-SiO₂-FeO-V₂O₅ thin film slags were conducted using the single hot thermocouple technique (SHTT) with air as the oxidizing atmosphere. The slag samples were analyzed after the experiments by SEM/EDX. The vanadium content was found to decrease as a function of time. The loss of vanadium from the slag film after 30 minutes of oxidation was approximately 18 pct and after 50 minutes, it was nearly 56 pct. The possible mechanism of vanadium loss would be the surface oxidation of vanadium oxide in the slag, VO _x to V⁵⁺, followed by surface evaporation of V₂O₅, which has a high vapor pressure at the experimental temperature.     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute     

Fabrication of Al-3 Wt pct Mg matrix composites reinforced with Al2O3 and SiC particulates by the pressureless infiltration technique

In Al-3 wt pct Mg/Al₂O_3p (or SiC _p ) composites fabricated by the pressureless infiltration method, the infiltration behavior of molten metal, the mechanical properties, and the interfacial reactions were investigated. The spontaneous infiltration of the molten Al-3 wt pct Mg alloy into the powder bed occurred at a relatively low temperature (700 °C for 1 hour under a nitrogen atmosphere). Spontaneous infiltration of the molten metal is related to the formation of Mg₃N₂ by the reaction of Mg and nitrogen. The tensile strength and 0.2 pct offset yield strength and elongation tend to decrease with increasing infiltration temperature and time, because of an increased interfacial reaction. In Al-3Mg/Al₂O₃ composites, MgAl₂O₄ was observed at interfaces between Al₂O₃ and the matrix, as well as at oxide films of the Al powder surface. In addition, MgO was observed at interfaces between Al₂O₃ and the matrix. On the other hand, Al₄C₃ was formed at interfaces between SiC and the matrix in Al-3Mg/SiC composites. In addition, MgAl₂O₄ was observed as a reaction product at the interfaces between oxide films of SiC and the matrix, as well as at oxide films of the Al powder surface. Since the Si released as a result of the interfacial reaction is combined with Mg, age hardening can occur by the precipitation of Mg₂Si via T6 treatment.     

Effect of oxide scale on carbon deposition on Fe-Ni alloys in carburizing gas

The behavior of carbon deposition on preoxidized Fe-Ni alloys containing 0 to 57.0 mass pct Ni in 10 pct CH₄-H₂ mixture at 1203 K was studied by metallography and thermogravimetry. Nickel retarded carburization and carbon deposition by lowering the solubility limit of graphite in austenite and by reducing catalytic activity for the pyrolytic reaction of CH₄. On oxidation in air, the addition of nickel to iron depressed the development of FeO and, thereby, caused a significant decrease in the thickness of the scale. The exposure of the alloys to 10 pct CH₄-H₂ mixture after the oxidation in air led to a sudden mass loss in the early stage and then a rapid mass gain. This mass change is primarily ascribed to mass loss by reduction of iron oxides and to mass gain by carbon deposition. The rapid mass gain by carbon deposition is probably due to the formation of active iron by reduction of iron oxides and to the increase in the reaction area by spallation of the scale; the active iron formed may promote filamentous carbon deposition through Fe₃C formation and decomposition. Carbon deposition on the alloys containing 27.2 mass pct Ni or more was considerably retarded because of the formation of a thin oxide scale which consists of α-Fe₂O₃ and spinel (Ni_xFe_3−xO₄) and the reduction of catalysis by enrichment of nickel in the subscale. However, the amounts of carbon deposition increased compared with those on the as-polished alloys, owing to the presence of reducible iron oxides.     

Interfacial reactions in Al-Mg (5083)/AI2O3p composites during fabrication and remelting

The interface reactions between a-Al₂O₃ particles and 5083 Al-Mg alloys during fabrication by compocasting and subsequent remelting at 800 °C for 30 minutes have been studied using analytical transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Experimental results show that MgO is the main reaction compound produced by the reaction 3Mg (1) + A1₂O₃ (s) = 3MgO (s) + 2A1 (1). The MgO crystals formed by interfacial reaction are very small (about 5 to 20 run in diameter), and the reaction zone is about 50 to 80-nm wide for as-cast materials and about 100 to 150 nm after remelting. The reaction kinetics is controlled by seepage of Mg and release of Al (1) along the newly formed MgAl₂O₄ or MgO grain boundaries. Because of the large volume expansion during the formation of MgO from the reaction between Mg (1) and A1₂O₃ and because of the low diffusivity in MgO crystals, the reaction rate is very low.     

Characterization of a diffusion-bonded Al-Mg alloy/SiC interface by high resolution and analytical electron microscopy

The interfacial structure of a diffusion-bonded Al-4.55 at. pct Mg/SiC interface was examined by conventional and high-resolution transmission electron microscopy. Formation of Mg₂Si, MgO, and Al₂MgO₄ was observed. The monoclinic Mg₂Si phase formed at the Al/SiC interface, while the oxides MgO and Al₂MgO₄ formed at the monoclinic Mg₂Si/Al interface. It is shown that the formation of these phases can be predicted using simple thermodynamic criteria such as the relative bond strengths between Al, Si, C, O, and Mg. In addition, precipitation of some equilibrium Al₈Mg₅ precipitate was also observed at the interface. The interfacial structure observed in the Al-Mg/SiC system is contrasted with that observed in the pure Al/SiC system.     

Viscosity of copper slags from chalcocite concentrate smelting

The viscosity of smelting slags from the Glogow copper plant in Poland was measured using a concentric cylinder viscometer. These slags contain typically 45 pct SiO₂, 16 pct CaO, 8 pct MgO, 11 pct Al₂O₃, and only 5 to 7 pct total iron. The viscosity was measured as a function of the CaO, MgO, SiO₂, Cu₂O, Cr₂O₃, and Fe₃O₄ contents in the temperature range from 1473 to 1623 K. Silica and chromium oxide additions increased the viscosity, while small additions of the other oxides decreased the viscosity. However, at large additions of CaO or MgO, cooling resulted in a rapid increase in the viscosity upon reaching the transition temperature. This critical transition temperature increased with increasing additions of CaO and MgO. This was explained by the precipitation of solid particles upon reaching the saturation limit. Depending on the slag composition, the activation energy for viscous flow was found to be in the range from 200 to 370 kJ/mol.     

Low-temperature oxidation of molybdenum disilicide

Cyclic oxidation rates of 95 to 97 pct dense, powder-source molybdenum disilicide (MoSi₂) in dry air, wet air, and oxygen have been measured between 400°C and 600°C. Dense MoSi₂ does not disintegrate catastrophically (pest) in these atmospheres for exposure times up to 688 hours. Between 400°C and 500°C, Mo and Si oxidize simultaneously to form amorphous SiO₂, monoclinic Mo₉O₂₆, and vapor-deposited MoO₃ plates, and the oxidation rate of MoSi₂ in air is influenced by its microstructure, composition, and surface defects. Rapid oxidation obeying a linear rate law occurs over a narrow temperature range near 500°C, where Mo vapor transport by (MoO₂) _n species is sufficiently rapid to produce large numbers of surface MoO₃ plates but simultaneously is slow enough to allow nucleation and growth of solid Mo oxides in conjunction with SiO₂. Addition of water vapor to the oxidant stream at 500°C retards nucleation and growth of solid Mo oxides by formation of MoO₃·H₂O (g), which has a high vapor pressure relative to those of (MoO₃) _n species. The transition from nonselective oxidation to high-temperature selective oxidation of Si to form a protective SiO₂ layer occurs between 500°C and 550°C. Preoxidation of MoSi₂ at 1200°C creates a SiO₂ barrier layer which prevents further oxidation upon subsequent exposure at 500°C. The oxidation kinetics and microstructural observations support the model of MoSi₂ pest in which oxidation in pores and cracks is required for disintegration. Based on these results, low-temperature oxidation phenomena are not expected to restrict the use of MoSi₂ as a high-temperature material. P.J. MESCHTER, formerly Senior Scientist, McDonnell Douglas Research Laboratories, St. Louis, MO, is Materials Scientist, General Electric Company, Corporate Research and Development, Schenectady, NY 12301.     

Oxidation of Aluminium Alloy Melts and Inoculation by Oxide Particles

One of the main concerns in recycling aluminium alloy scrap is the removal of oxide inclusions. Understanding the nature and behaviour of oxide films in the alloy melts is an important step for developing efficient recycling technologies. In this work, characterisation of oxides formed in pure Al and Al?CMg alloy melts was carried out. In commercially pure Al melt, ??-Al₂O₃ platelets and ??-Al₂O₃ particles were found to form at 750 and 920?°C, respectively. The oxides were in the form of liquid-like films consisting of numerous individual particles. The addition of 0.49 and 0.70?wt% Mg resulted in the formation of MgAl₂O₄, and the MgAl₂O₄ particles were {1 1 1} faceted and had a cube-on-cube orientation relationship with ??-Al. The MgAl₂O₄ films were also liquid-like in which large numbers of the particles were held by the melt. Grain refinement was achieved by intensive shearing of the melts prior to solidification. It is believed that intensive melt shearing broke up the oxide films and dispersed the potent oxide particles which in turn enhanced the heterogeneous nucleation, resulting in grain refinement. The potency of the oxide particles and the mechanism of the inoculation by the oxides were discussed on the basis of the TEM results and theoretical analysis of the lattice misfits at the interfaces along specific orientation relationships.     

Microstructural Analysis of Multi-phase Ultra-Thin Oxide Overgrowth on Al–Mg Alloy by High Resolution Transmission Electron Microscopy

High-resolution transmission electron microscopy analyses are carried out to understand the microstructure of the ultra-thin oxide-film grown on a (native) amorphous Al₂O₃-coated Al-0.8 at.% Mg alloy substrate at T = 600 K for t = 2 h and at pO₂ of 1 × 10^?2 Pa. This oxide-film is found to be non-uniformly thick with thicknesses varying from 1.50 to 4.60 nm. Occasionally, this oxide is found to diffuse into the Al–Mg alloy substrate, forming oxide thicknesses up to 10.5 nm. Overall, this oxide-film is found to consist of a mixed amorphous, (poly) crystalline and an intermediate amorphous-to-crystalline transition regions, with crystalline regions consisting mostly of MgO and the diffused oxide regions into the Al–Mg alloy substrate coated with γ-Al₂O₃. These observations are then compared with the experimental results obtained using angle-resolved X-ray Photoelectron Spectroscopy analysis and thermodynamic predictions for the growth of an ultra-thin oxide-film due to dry, thermal oxidation of Al–Mg alloy substrates.     

Mechanical properties and microstructures of Al-Mg-Sc alloys

The mechanical properties of Al-(Mg)-0.5Sc alloys have been investigated. Room-temperature tensile and toughness properties were found to reflect a superposition of the properties of Al-Mg and Al-0.5Sc alloys and are quite competitive with high-performance Al alloys. A combination of substructure refinement by Mg and stabilization by Al₃Sc precipitates produces exceptional superplasticity as exemplified by superplastic forming (SPF) elongations in excess of 1000 pct at a strain rate of 0.01 s^-1. Overall, these alloys demonstrate an extremely attractive combination of strength, toughness, density, and SPF fabricability.     

The effects of alloy microstructure refinement on the short-term thermal oxidation of NiCoCrAlY alloys

Three γ + β NiCoCrAlY alloys (a cast alloy, a laser-surface-melted (LSM) alloy, and a coating as deposited by electron beam-physical vapor deposition (EB-PVD)) with similar average composition (Ni-20Co-19Cr-24Al-0.2Y in at. pct), but with different microstructures prior to oxidation, were oxidized for 0.5 and 1 hours at 1373 K in an Ar 20 vol pct O₂ atmosphere (i.e., at a partial oxygen pressure of 20 kPa). It was found that on the alloy with β precipitates larger than 20 μm, the oxide layer was nonuniform in thickness, and had a laterally inhomogeneous composition and phase constitution. In this case, the oxide layer developed on top of the γ phase was thicker than that formed on top of the β phase and consisted of a NiCr₂O₄/Cr₂O₃ outer and an α-Al₂O₃ inner layer. For the thinner oxide formed on top of the β phase, the outer layer was constituted of a Cr and Co containing NiAl₂O₄ spinel and the inner layer also consisted of α-Al₂O₃. For the alloys with β precipitates smaller than 3 μm, a uniform and laterally homogeneous oxide formed, consisting of a Cr and Co containing NiAl₂O₄ outer layer on top of an α-Al₂O₃ inner layer. After oxidation, Y was distributed as numerous, small precipitates within the oxide layer for a homogeneous Y distribution prior to oxidation, or as a few, very large pegs along the γ/β phase boundaries of the alloy for an inhomogeneous Y distribution prior to oxidation. The performance of the alloys upon thermal cycling was improved for smaller β precipitates and for a more homogeneous Y distribution in the alloy prior to oxidation.