The nucleation of Fe-Rich phases on oxide films in Al-11.5Si-0.4Mg cast alloys
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20/6/03
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The Nucleation of Fe-Rich Phases on Oxide Films in Al-11.5Si-0.4Mg Cast Alloys X. CAO and J. CAMPBELL The microstructures of Al-11.5Si-0.4Mg alloys with various Fe and Mn contents have been studied to investigate the potential influence of oxide films on the precipitation of Fe-rich phases from the liquid metal. Oxide films are incorporated into melts by an entrainment process. This is an enfolding mechanism of incorporation. Folded oxide films in melts have two sides: the dry unbonded inner surfaces and their wetted exterior surfaces. The wetted outer interfaces of oxide films appear to act as preferred substrates for the nucleation and growth of some Fe-rich phases. It is expected that there will be good atomic contact between the wetted side of the film and Fe-rich phases or aluminum matrix. However, the gap between the two dry sides of a double film constitutes a crack, thus explaining the commonly observed cracks in and around such intermetallics and aluminum matrix. The calculation of planar disregistries between some typical oxides and a-Fe phase (primary or script type) indicated that MgO, MgO Al2O3, g-Al2O3, and a-Al2O3, might all be good substrates for the nucleation of a-Fe phase.
I. INTRODUCTION
ALUMINUM alloy castings have achieved wide use in automobile, aerospace, and other applications because of their high strength to weight ratio. Unfortunately, however, castings have a well-known reputation for unreliability due to the considerable scatter in their mechanical properties. This unreliability of castings has restricted their wider application. Recent research has also shown that current cast aluminum alloys are, at best, only achieving and using 1 pct of their potential fatigue lives.[1] An increase in fatigue life of two orders of magnitude would be greeted with mixed acclaim and disbelief. However, the evidence suggests that this is entirely possible. Until now, it seems that the mechanical properties of cast aluminum alloys and their reliability have mainly been limited by the large and uncontrolled number of double oxide films, which act as cracks in the liquid metal and the castings. A good example of a large oxide film, folded dry side to dry side to form a crack, but opened by the tensile fracture, is shown in Figure 1.[2,3] Clearly revealed are the opposing faces of the oxide films on the opposing fracture surfaces of the tensile test casting. By carefully controlling the quality of the melt prior to casting and using casting processes that will prevent the formation and entrainment of oxide film defects, castings may become as reliable as or, possibly, even more reliable than forgings. Over recent years, the removal of oxide films from aluminum melts has been one of the main development efforts in the aluminum industry. Success, however, using conventional methods such as fluxing, degassing, filtering, etc. has been mixed.[3] Thus, the removal of oxide films from X. CAO, formerly PhD Student with the Department of Metallurgy and Materials, University of Birm
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