Papers by Keyword: Oxide Films

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Authors: Mahmoud El-Sayed, Hanadi G. Salem, Abdel-Razik Kandeil, William D. Griffiths
Abstract: The mechanical properties of Al castings are reduced by inclusions, particularly double oxide films, or bifilms, which are formed due to surface turbulence of the liquid metal during handling and/or pouring. These defects have been reported not only to decrease the tensile and fatigue properties of Al alloy castings, but also to increase their scatter. Recent research has suggested that the nature of oxide film defects may change with time, as the air inside the bifilm would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of Al alloy castings. In order to follow changes in the composition of the internal atmosphere of a double oxide film defect within an Al melt, a series of analogue experiments were carried out to determine the changes in gas composition of an air bubble trapped in a melt of commercial purity Al, subjected to stirring. The bubble contents were analysed using a mass spectrometer to determine their change in composition with time. Also, the solid species inside the bubbles solidified in the melt were analysed. The results suggested that first oxygen and then nitrogen inside the bubble were consumed, with consumption rates of 2.5x10-6 and 1.3x10-6 mol m-2s-1, respectively. Also, hydrogen diffused into the bubble from the melt at an average rate of 3.4x10-7 mol m-2s-1, although the rate of H diffusion increased significantly after the consumption of most of the oxygen inside the bubble. Based upon these reaction rates the time required for a typical alumina bifilm to lose all its oxygen and nitrogen was determined to be just under 10 minutes.
Authors: Yan Hui Li, Shu Zhong Wang, Xue Dong Li, Meng Meng Ren
Abstract: In this paper, by means of Scanning Electronic Microscope (SEM), X-ray diffraction analyzer (XRD) and Cross hatch scanning analysis, the Structure and mechanism of cracking and spalling for Super304H steel oxide films in high temperature steam were investigated. With the oxidation proceeding, the surface of Super304H steel specimens is covered by a large amount of tiny holes, which causes the occurrence of an outer oxide layer. The Super304H oxide film generally has a double-layer structure: the outer layer mainly for iron oxide Fe3O4, also being covered by a small amount of Fe2O3, and the inner layer with the FeCr2O4 spinel phase as primary component. Because of the inhomogeneity of oxide distribution, large amounts of smaller gaps appear in the Super304H steel oxide films. Due to the differences among the outer layer, the inner layer and steel matrix in thermal expansion coefficient, when the steam temperature around the specimen changes periodically, the stress variation arise in the Super304H steel and oxide films, which causes the cracking and spalling of oxide films.
Authors: A.V. Sidashov, A.T. Kozakov, S.I. Yaresko
Abstract: The samples of the tool steel P6M5 were modified by means of laser irradiation (hereinafter - LO). The chemical composition of the sample surface before and after the LO was studied using the Auger and X-ray photoelectron spectroscopy (hereinafter - AES and XPS respectively). It was found that while the steel is exposed to LO, the thick oxide layer consisting mainly of the Fe2O3 oxide is formed. It was established that the modification with LO leads to increasing of wear resistance and durability of the R6M5 steel because of a double reduction of the friction coefficient.
Authors: Yun Wang, Hu Tian Li, Zhong Yun Fan, Geoff Scamans
Abstract: Oxide films in Al-0.7Mg and Al-9.4Si-2.3Cu-1.0Zn-0.49Mg (in wt%) alloy melts were characterised using advanced analytical electron microscopy. The oxides were collected by pressurised melt filtration for direct examination by SEM and TEM. The results showed that the oxide films consisted of numerous sub-micron sized oxide particles, rather than continuous solid films. The oxide particles formed in the two Mg-containing alloys were identified as MgAl2O4 spinel by selected area electron diffraction (SAED) and high resolution TEM combined with EDS analysis. The low level of Mg in the melt resulted in the change of the oxide from alumina to MgAl2O4 spinel. The MgAl2O4 crystals were typically faceted with their {1 1 1} crystal planes and were about 0.2-1.2 μm in size. High resolution TEM examination of the MgAl2O4 / a-Al interfaces revealed that there was a cube-on-cube orientation relationship between the two crystals. The possibility of MgAl2O4 particles to act as nucleation sites for α-Al grains during solidification is discussed in terms of the lattice matching at the MgAl2O4 / α-Al interfaces along the specific crystallographic orientation relationship.
Authors: Guo Wu, Keyna O'Reilly, Marina Galano
Abstract: In this paper, typical defects in aluminium alloys cast by conventional pouring of liquid metal into a cylindrical alumina crucible and an induction melting process are characterized by using macroetching, optical microscopy and scanning electron microscopy. Oxide film defects are generated in both processing methods. Oxide films formed during casting are a major type of defect observed in the microstructures of aluminium alloys products. It has been found that pouring of molten metal into a mould and induction stirring lead to different forms of oxide defects. Under induction stirring, such low-density entrained defects are found to be carried to near the surface of the liquid aluminium alloys and submerged surface oxide films become entangled.
Authors: Guo Qiang Xie, Osamu Ohashi, Norio Yamaguchi, Ming Hui Song, Kazuo Furuya, Tetsuji Noda
Authors: Anne Marie Huntz, M. Andrieux, Gael Sattonnay, Marc Condat
Authors: J. Pan, C. Leygraf, T. Otsuka, R.F.A. Jargelius-Pettersson, Bo Ivarsson, J. Lindén
Authors: Ming Wei Chen, Hai Peng Qiu, Jian Jiao, Yu Wang, Wei Jie Xie
Abstract: Oxidation thermodynamics of silicon carbide (SiC)ceramic was studied by means of HSC Chemistry code, and the weight change, morphology and phase of oxidation products were analyzed by thermogravimetric analysis(TG), scanning electron microscopy(SEM ) and X-ray diffraction (XRD). The results showed that SiC ceramic could be oxidized to silicon dioxide(SiO2) with release of small molecular gases under oxidizing atmosphere at 800°C, and the formed SiO2 film with appropriate fluidity and low oxygen diffusion coefficient could prevent the spread of oxygen with the oxidation temperature increasing up to 1200°C, which favored the anti-oxidation of SiC ceramic matrix composite.
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