The behaviour of double oxide film defects in Al–4.5 wt% Mg melt

The possibility of bonding of the two layers of a double oxide film defect when held in a liquid Al–4.5 wt% Mg alloy was investigated. The defect was modelled experimentally by maintaining two aluminium oxide layers in contact with each other in an Al–4.5 wt% Mg liquid alloy at 750 °C from 2 min to 16 h. Any changes in the composition and morphology of these layers were studied by SEM, EDX and XRD. The results showed that in contrast to previous studies reported in the literature on Al–0.3 wt% Mg in which the two layers bonded to each other after a holding time of 5 h, no bonding took place between the two oxide layers even after a holding time of 16 h. Based on the comparison between the two studies, it was concluded that a transformation involving rearrangement of atoms at the interface between the two oxide layers is essential for the bonding to take place between the two oxide layers. This criterion could be used to predict the bonding behaviour of oxide film defects when held in different liquid aluminium alloys, or when subjected to a HIPping process.     

Modelling behaviour of oxide film during vibration diffusion bonding of SiCp/A356 composite in air

Abstract

The vibration liquid phase diffusion bonding of SiC_p/A356 composite in air has been investigated. The surface of specimens to be bonded was treated with and without vibration under the bonding condition. It was found by atomic force microscopy analysis that some of the oxide film could be broken down when ridges on the surface of the matrix were ground down. Dissolution of the base metal by the filler metal occurred with removal of the oxide film during vibration liquid phase bonding, and SiC particles in the base metal entered the bond region. A removal process model for vibration bonding has been established with and without filler metal. Results show that shearing and impacting actions are the two main breaking mechanisms during vibration; the oxide film bulk is generally broken down by shear, and dissolution of the base metal by the filler metal promotes particle segregation from the matrix and their entry into the bond region.     

Hydrogen,porosity and oxide film defects in liquid Al

This article reports results from an experiment where a bubble of air was held at a constant temperature in a liquid Al melt, with the volume of the bubble monitored continuously using real-time X-ray equipment. When the H content of the surrounding melt was low, the volume of the trapped air bubble reduced with time, as the O and N in the bubble atmosphere reacted with the Al to form Al₂O₃ and AlN. When the H content of the melt was increased to about 0.3 ml 100 g⁻¹ Al, the H in solution passed into the air bubble causing its expansion. In an Al casting the same effect would cause an entrained double oxide film defect to act as a site for the growth of H-driven gas porosity. The way in which the oxide film defects might behave in forming H porosity has been discussed.     

An impedance spectroscopy study of oxide films formed during high temperature oxidation of an austenitic stainless steel

Impedance spectroscopy has been used to study an oxide film formed on an AISI304 austentic stainless steel by oxidation at 800°C for 200 h. Impedance spectra of the oxide film clearly showed two semicircles, which correspond to two independent oxide layers present in the oxide film. The oxide film was also examined by using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Although it was difficult to identify the two phases in the oxide film using SEM, XRD showed the presence of Cr₂O₃ and MnCr₂O₄. In addition, surface analysis of the oxide film using XPS showed the presence of MnCr₂O₄. By comparing the relaxation frequencies of Cr₂O₃ and MnCr₂O₄ with those of the two semicircles, it is identified that the semicircles in the impedance spectra correspond to Cr₂O₃ and MnCr₂O₄. Furthermore, the electrical properties of both Cr₂O₃ and MnCr₂O₄ in the oxide film have been determined by impedance measurements, indicating that the chromia layer is apparently thicker than the spinel layer.     

Dense spinel MnCo2O4 film coating by aerosol deposition on ferritic steel alloy for protection of chromic evaporation and low-conductivity scale formation

Conducting ceramic layers with a spinel structure of MnCo₂O₄ and a thickness of ~3 μm were deposited on ferritic stainless steel (SS) by aerosol deposition (AD), for use as an oxidation-resistant coating layer on the metallic interconnects of a solid oxide fuel cell (SOFC). The microstructural changes in the interface between the MnCo₂O₄ and SS were analyzed, as were the subsequent electrical conductivity changes at an SOFC operating temperature of 800 °C in air. The coated spinel layers were dense without pores or cracks, and maintained good adhesion even after oxidation at 800 °C for 1,000 h in air atmosphere. Close observation of the interface between the coated spinel oxide and SS substrate indicated the presence of ~1-μm thick, Cr-rich scale formation; however no MnCrCoO₄ or MnCr₂O₄ spinel phase was detected. The area specific resistance (ASR) of the MnCo₂O₄-coated alloy after heat treatment at 800 °C for 1,000 h was 13.4 mΩ cm².     

Microstructural development in the directed melt-oxidized (DIMOX) Al-Mg-Si alloys

Metal-ceramic composites produced via directed melt oxidation (DIMOX) of aluminium alloys are of recent interest. Thein situ composite forming method is based on the reaction of a molten alloy with a gaseous oxidant. In the present study, Al-Mg-Si alloys were subjected to directed melt oxidation and the progressive microstructural evolution was examined by interrupted growth experiments. In the early stages, liquid alloy oxidizes to form a duplex oxide layer (MgO+MgAl₂O₄) on the surface. The openings in these oxide layers allow the liquid alloy to wick through to form small nodules on the surface. When further wicking occurs through these nodules, a cauliflower type of colonies is formed. During the early part of the second stage, spinel growth dominates to form a multi-layered structure. In the final stage, as the magnesium reaches low levels, Al₂O₃ formation dominates the growth, and alumina crystals grow continuously for several tens of micrometres. The oxygen required for alumina formation is expected to come from two sources: (i) from the ingress of oxygen through microcracked oxide layers, and (ii) demixing of magnesium-containing oxides in the underneath layers.     

Interfacial reactions and wetting in Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique

The present paper considers the microstructures of Al–Mg/oxide ceramic interpenetrating composites made by a pressureless infiltration technique. The composites were produced using an Al–10 wt.% Mg alloy with two oxide ceramic foams, spinel (MgAl₂O₄) and mullite (Al₆Si₂O₁₃), at 915 °C in a flowing N₂ atmosphere. Full infiltration of the aluminium alloy into the ceramic preform has been achieved with good bonding between the metal and ceramic phases. The composites were characterised by a range of techniques and compared with those for alumina from the literature. It has been found that the metal–ceramic interface of the composite consisted of an oxide layer near the ceramic phase and a nitride layer from Mg₃N₂ to AlN near the metal phase. The improvement of Al wetting and adhesion on the oxide ceramics by the addition of Mg and in the presence of N₂ was studied by a sessile drop technique to clarify which compound that formed at the interface contributed to the spontaneous infiltration.     

Structure and composition of oxide film on 5083 alloy at brazing temperature

The structure and composition of the surface oxide film on the 5083 aluminium alloy at a brazing temperature of 500°C were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy. The results showed that the original γ-Al₂O₃ film on the surface of the cold rolled 5083 aluminium alloy was transformed into a complex oxide film consisting of MgO, MgAl₂O₄, and free Al atoms after heating. The thickness of this oxide film, which could be divided into two distinct layers, was approximately 130 nm. The outer layer was mainly composed of an amorphous MgO phase, while the inner layer was MgO based, with a few free Al atoms and a small number of nanocrystalline MgAl₂O₄ particles distributed in it.     

Effect of vibration on behaviour of bifilms in A356 and A357 melts

The effect of applying vibration to a melt on the behaviour of bifilm defects in A356 and A357 melts was studied using a reduced pressure test technique. The results showed that vibrating a melt can have a dual effect on bifilms. This effect depends on the rate of phase transformations that occur in the oxide films. If the transformation occurs fast enough then the vibration would facilitate the formation of bonding between the layers of bifilm defects by causing the atmosphere of the defects to be consumed faster. Otherwise, the vibration might facilitate the diffusion of hydrogen into the atmosphere of the defects, and hence prevent or delay the formation of bonding between the oxide layers.     

Preparation and characterization of MnFe2O4 by a microwave-assisted oxidative roasting process

An innovative and effective technological process is proposed to synthetize spinel manganese ferrite (MnFe₂O₄) by a microwave-assisted oxidization roasting process. In the current paper, the effect of operating variables on the formation mechanism of Fe-MnO systems for synthesizing spinel MnFe₂O₄ is initially investigated to reveal the oxidation mechanism and phase transformation during the microwave–assisted oxidative roasting process. The synthetic MnFe₂O₄ microparticles with a saturation magnetization (Ms) of 70.5 emu/g, a coercivity (Hc) of 4.65 Oe and a ratio of the saturation magnetization to remanent magnetization (Ms/Mr) of 0.055 were successfully prepared by heating for 30 min at 1000 °C in an air atmosphere. The MnFe₂O₄ samples exhibited normal and inverse spinel crystal structures, inevitably coexisting and co-transforming simultaneously at high temperature (≥950 °C). Furthermore, this innovative and effective technological process can be extended to fabricate other spinel ferrite particles of interest.     

Ultrasonic brazing of high fraction volume of SiC particulate reinforced aluminium matrix composites

Abstract

The ultrasonic brazing of 55 vol.-%SiC_p/A356 composites in air has been investigated. When the ultrasonic vibration is applied for 0·5 s, the oxide layer is still continuous at most places between the filler metal and the composites. As the ultrasonic action time increases, the oxide film sufficiently disappears in the bond region, resulting in the complete wetting of Zn–Al alloy with the composites and the significant mass transfer between the SiC/A356 and the Zn–Al alloy is observed. As the dissolving of the composites on the surface, the SiC particles in the base materials get into the bond metals in which reinforcements are distributed uniformly. The shear strength of bonds increases with the ultrasonic acting time.     

The effects of oxide film and annealing treatment on the bond strength of Al–Cu strips in cold roll bonding process

In this study the influence of Al₂O₃ coating and post-rolling annealing on the bond strength of dissimilar Al–Cu strips was investigated. For this purpose different degrees of thickness of Al₂O₃ film on Al strips were coated using anodizing process. Anodized aluminum and copper strips were then cold-rolled at different reduction levels. To investigate the effect of annealing treatment on bond strength after cold rolling, selected strips were annealed. Peeling test was used to investigate the effect of ceramic-based oxide film on bonding strength of Al–Cu strips. It was found that bond strength was improved after applying higher reductions and was decreased dramatically by providing oxide film. However, by increasing the thickness of oxide film up to a certain value (20 μm), bond strength was increased after which it was decreased. A decrease in bond strength was observed by post-rolling annealing.     

Keimbildung und kristallwachstum des reaktions-produktes bei festkörperreaktionen: bildung von MgCr2O4 aus den oxiden

If MgO single crystals react with gaseous Cr₂O₃ at temperatures much lower than the eutectic temperature in the system MgOMgCr₂O₄ the epitaxial formation of the spinel MgCr₂O₄ on {001} MgO proceeds by nucleation and crystal growth within a quasi-liquid phase which can be described as a supersaturated solution of spinel in MgO. On a thin coherent spinel layer containing sufficient pores and gaps a quasi-liquid film is formed; this is the matrix for the spinel crystal growth. On thick spinel layers the existence of a Cr₂O₃ film, e.g. an adsorption film, which is a solution of spinel and from which the spinel crystal growth occurs, is discussed.     

Effect of admixtures of oxygen on the oxidation of iron and Fe−Cr alloys in lead melts

We study the process of oxidation of Armco iron and Fe−16Cr and Fe−16Cr−1Al model alloys held in lead melts with different concentrations of oxygen for 1000 h at 650°C. It was discovered that the intensity of oxidation, the structure, and phase composition of oxide layers are determined by the activity of oxygen in the liquid metal. By the methods of layer-by-layer X-ray diffraction analysis and microscopic X-ray diffraction analysis, it was shown that, for low concentrations of oxygen in lead (C ₀≤10⁻⁶ wt.%), a thin (1–5μm) oxide [magnetite (Fe₃O₄] film is formed on the surface of iron. If alloys are held under the same conditions, then we also observe an increase in the concentration of chromium in the subsurface layers. For higher concentrations of oxygen (up to 10⁻⁵ wt.%), a film of magnetite (with inclusions of pure lead) is formed on the surface of unalloyed iron. In alloys, under the layer of magnetite, we detect the formation of oxide layers with the same composition as a solid solution of Fe₃O₄ and FeCr₂O₄ and the structure of spinel. These layers efficiently suppress the process of penetration of lead but do not completely terminate the process of diffusion of oxygen into the bulk of the material, which eventually leads to the internal oxidation of alloy. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 97–101, May–June, 1997.     

Study of air oxidation of amorphous Zr<Subscript>65</Subscript>Cu<Subscript>17.5</Subscript>Ni<Subscript>10</Subscript>Al<Subscript>7.5</Subscript> by X-ray photoelectron spectroscopy (XPS)

The oxidation of the bulk amorphous alloy Zr₆₅Cu_17.5Ni₁₀Al_7.5 in air in its amorphous and the supercooled liquid states was studied in the temperature range 573–663 K using X-ray photoelectron spectroscopy (XPS). The oxide film mainly consisted of the oxides of Zr (as ZrO₂) and Al (as Al₂O₃). No Cu or Ni was found in the oxide film formed on the amorphous state of the alloy while significant Cu (as CuO) was present in the oxide film formed on the alloy in its supercooled liquid state. The role of the various alloying elements during oxidation at high temperatures in air is discussed in the paper. The XPS data from oxide film support the previously suggested mechanism for oxidation of this alloy, i.e. the rate controlling process during oxidation of the alloy at low temperatures (in the amorphous state) is the back-diffusion of Ni and Cu, while the oxidation at high temperatures (in the supercooled liquid state) is dominated by the inward diffusion of oxygen.     

Effect of NH3?O2 gas mixtures on the Protective Oxide Film on high chromium alloy steels

High Cr steels are hardly nitrided in on ammonia gas atmosphere at usual nitriding temperatures, whereas they are easily nitrided when a small amount of oxygen is added to the ammonia atmosphere (called an oxynitriding atmosphere). Hence the protective oxide film of the steels is reduced by adding oxygen to the ammonia. This paradoxical phenomenon has not been explained. With respect to the reducing behavior of the NH₃? O₂ gas mixture on the oxide film, the role of nitrogen oxide (NO) formed by a reaction between the NH₃ and the O₂ was noted. In the oxynitriding operation, the NO was detected in the exhaust gases. Based on this fact, Type 304 stainless steel was nitrided in NH₃? NO gas mixtures at elevated temperatures, and the action of the NO in NH₃ as a reductant for the oxide film of the steel was discovered. Furthermore, nitriding in the NH₃? NO gas mixture resulted in a very high surface hardness (as high as Hv 1300-1700) for the steel. On the other hand, the oxynitrided surface was always of lower hardness than the case-hardened layers formed in the steel. It is believed that NH₃? NO atmosphere nitriding will become a new nitriding method for steels.     

Joining molybdenum to aluminium by diffusion bonding

The joining of molybdenum to aluminium and aluminium-copper alloy using diffusion bonding has been investigated. Bond strengths have been measured by means of a simple shear jig and the joint microstructures characterized by electron microscopy and electron-probe microanalysis. Successful joints were produced by using a copper foil interlayer to form a eutectic liquid during the bonding process which helped disrupt the oxide film on aluminium and promote metal diffusion across the joint interface. When bonding commercial-purity aluminium to molybdenum, the iron present as an impurity caused a ternary eutectic liquid to form and, after solidification of the liquid phase, a thin film of Al₇Cu₂Fe was left behind on the aluminium. Failure of this joint occurred at a shear stress of 75 MPa, with the fracture path contained within the aluminium. With super-purity aluminium, a binary eutectic liquid was produced and the ensuing interface reaction resulted in a multi-layered structure of molybdenum-containing phases. The bond failed at the molybdenum interface at a stress of 40 MPa. When bonding aluminium-copper alloy to molybdenum without a copper interlayer, general melting at the interface via eutectic phase formation did not occur and the interface showed only localized reaction. The joint failed by separation from the molybdenum, at a stress of 25 MPa. When, however, a copper interlayer was used, fairly thick regions of multi-layered molybdenum intermetallics formed and the remaining surface was covered by a layer of Al₇Cu₂Mo phase. Failure of this joint occurred at a stress of 70 MPa, mainly by separation at the molybdenum interface.     

Spallation and transient oxide growth on PWA 1484 superalloy

A multi-layered oxide forms on single crystal PWA 1484 alloy when oxidized at high temperatures, consisting of a spinel ((Ni(Cr,Al,Co)₂O₄) at the air/oxide interface, interspersed with tantalum rich oxide particles (CrTaO₄/NiTa₂O₆), and an inner-most layer of α-alumina (α-Al₂O₃) in contact with the alloy. Observations on rapid cooling after oxidation show that the spallation behavior of this multi-layer oxide depends on time in room temperature laboratory air, the sulfur content of the alloy, and the alloy surface preparation. Time-lapse optical microscopy reveals that spallation of the multi-layer oxide occurs differently on polished surfaces in comparison to ground surfaces. On ground surfaces spallation occurs solely at the metal/alumina interface in both alloys, and follows the grinding scratches’ long axes. The higher S alloy (1.7 ppmw S) almost completely spalls to bare metal (95% oxide spalled), whereas very little spallation (5%) occurs on the lower S alloy (0.2 ppmw S). On polished surfaces spallation occurs along both the alumina/spinel and alumina/alloy interfaces of the higher sulfur containing alloy but only along the alumina/spinel interface of the oxide on the low-sulfur alloy, leaving the alumina in contact with the metal. The lack of significant difference in measured residual stress in the α-alumina combined with the fact that spallation occurred over periods of several hours at room temperature suggests that failure is a time-dependent process likely associated with the presence of moisture, once a critical oxide thickness has been exceeded. The role of sulfur is likely associated with a reduction in fracture toughness of the oxide and oxide/metal interfaces through formation of voids or reduction in bond energy.Re-heating of the alumina-covered, low-sulfur alloy (0.2 ppmw) indicates that outward grain boundary diffusion of aluminum and tantalum occurs through the α-alumina but not nickel. Also, no new Ni-rich oxides reform confirming that they are indeed transient oxides formed during the initial stages of oxidation of the bare alloy.     

Early stages of oxidation of yttrium-implanted Ni-20% Cr

The effect of ion-implanted yttrium on oxidation at elevated temperatures (500 to 1000° C) has been studied in Ni-Cr alloys using high-voltage electron microscopy (HVEM) ofin situ oxidized specimens, electron microscopy of scales oxidizedex situ, and Rutherford back scattering. The presence of yttrium enhances nucleation and growth of Cr₂O₃ at the alloy surface. Although the radiation damage induced by the ion implantation accelerates oxide nucleation and the initial rate of oxidation at 500° C, the growth of a mature oxide scale is slowed down, and the main influences seem to come from the chemical characteristics of the implanted yttrium. A Cr₂O₃ layer forms first on the yttrium-implanted alloys. Outward diffusion of Ni²⁺ cations through this layer forms an outer NiO scale. The initial growth process on unimplanted alloys is opposite. Here, NiO is the predominant initial oxide. As it thickens, a porous spinel and Cr₂O₃-enriched layer is formed between alloy and NiO. The inner oxide layer on yttrium-implanted alloys is fully dense and contains more than 50% Cr₂O₃. On the unimplanted alloy, the inner spinel layer is porous and contains a lesser enrichment in Cr₂O₃. The porous spinel delays formation of a protective Cr₂O₃ layer and gives poor scale adherence. The oxide growth mechanisms are discussed in the light of TEM results.     

Surface crystallization of amorphous Fe-B-Si alloy annealed in a gas atmosphere containing organic vapour

The relationship between the annealing atmosphere and the magnetic properties of Fe_78.5B₁₃Si_8.5 amorphous alloy has been studied, showing that annealing in an inert gas atmosphere containing a borosiloxane resin significantly improves the core loss of the amorphous ribbon, and that annealing in an inert gas without any organic resins, gives much better results than annealing in an inert gas atmosphere containing cellulose and vinyl acetate resins. A boron-depletion zone was detected by Auger electron spectroscopy under the oxide film formed during annealing in the inert gas atmosphere containing cellulose and vinyl acetate resins. Annealing of the amorphous ribbon in the cellulose and vinyl acetate resins selectively oxidizes boron in the alloy to form a B₂O₃ film and a boron depletion zone; the alloy in this zone is then crystallized into -Fe. This surface crystalline layer induces out-of-plane magnetic anisotropy in the amorphous ribbon and thus deterioration of the core loss. On the other hand, annealing in the borosiloxane resin results in neither selective oxidation of boron nor surface crystallization of the amorphous ribbon