Healing of Double-Oxide Film Defects in Commercial Purity Aluminum Melt
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TRODUCTION
CAMPBELL [1] described the concept of an entrained double-oxide film and its deleterious effects on the properties of an aluminum casting. Each time the surface of the metal folds on itself, the surface oxide film becomes entrained in the bulk liquid. This occurs as a doubled-over oxide film in which the internal surfaces are not bonded but have a layer of gas from the local atmosphere (presumably composed predominantly of air) trapped between them. Consequently, it leads to a crack in the solidified casting. Therefore, the defect necessarily resembles and acts as a crack that not only deteriorates the mechanical properties of the solidified casting but also could act as an initiation site for the formation of other defects (e.g., hydrogen pores[2–5] and Fe-rich phases[6]) before the solidification. The first oxide to form on the liquid of commercial purity Al alloy is reported to be an amorphous alumina layer.[7,8] The two amorphous layers of a newly formed and submerged double-oxide film transform to a crystalline c-Al2O3 and then to a-Al2O3 crystals after incubation times of approximately 5–10 minutes[9] and 5 hours[10] (at 1023 K [750 °C]), respectively. The transformation to a-Al2O3 is accompanied by a 24 pct decrease in volume of the oxide, and the tensile stresses induced by this volume change could fracture the oxide layers.[10,11] FATEMEH NAJAFZADEH BAKHTIARANI, Post Graduate Student, and RAMIN RAISZADEH, Assistant Professor, are with the Department of Metallurgy and Materials Science and Mineral Industries Research Center, Shahid Bahonar University of Kerman, 76169 Kerman, Iran. Contact e-mail: [email protected] Manuscript submitted October 30, 2010. Article published online February 1, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
Nyahumwa et al.[12,13] suggested that a double-oxide film defect could consume its internal atmosphere of oxygen and nitrogen after the incubation time associated with the transformation of c- to a-Al2O3, when the fracture of the oxide layers bring the internal atmosphere of the defect and melt into contact. The consumption of the internal atmosphere has been verified recently by Raiszadeh and Griffiths,[14] who monitored the change with time of the volume of a trapped air bubble in different Al alloy melts using real-time X-ray radiography. Their results showed that, first, the oxygen of the trapped air bubble reacted to form Al2O3, and second, the nitrogen reacted to form AlN. These reaction processes were continuous, with no incubation time required. They suggested that the cracks that formed on the oxide layer around the air bubble during its movement in the liquid metal provided the necessary paths for the contact of the internal atmosphere of the bubble and the surrounding melt. It was speculated[12] that after the consumption of the oxygen and nitrogen, the sides of the film defect would be forced into contact, at least at some points, and the films might bond together because of the changes that might occur in the nature of the oxide layers with time. The defect might t
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