Modification in the aluminum silicon system
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INTRODUCTION
THISpaper is concerned with the well-known modification of a faceted phase during solidification, specifically that of silicon in the AI-Si eutectic system (e.g., References 1 through 5). In this system, as in other faceted/nonfaceted combinations, there are microstructural changes with growth conditions (rate and temperature gradient) and, of particular interest, with minor additions of a third component. Probably the most effective modifying addition to this system is sodium, at concentration levels ~0.01 wt pct, although other alkali and alkaline earth metals and elements of Group VB of the Periodic Table may also have various effects upon the structure. In the case of sodium the structural change, under certain growth conditions, is from one of interconnected silicon flakes in the metal matrix to an apparently finer distribution of irregular fibers 4'6'7 with attendant improvement in mechanical properties. One can imagine that such a structural change might be caused by one, or a combination of four possible mechanisms: (a) The effect of a soluble addition must involve some change in the equilibrium phase diagram. While it does not seem probable that this influence will be very important at concentrations as low as, e.g., 0.01 wt pct of sodium, it is conceivable that solute build-up at a growth front might involve some ternary phase reaction. The A1-Na system has been determined with some care at low sodium levels 8,9 and involves a monotectic reaction at -~0.014 wt pct Na and at about 1 K below the freezing point of the pure metal. The A1-K system is thought to be of similar form but has been less carefully studied. It is possible (e.g., Reference 4) that ternary additions of these metals to the binary A1-Si system also involve a monotectic type reaction at similar low concentrations. M.D. HANNA, formerly Postdoctoral Research Associate, Michigan Technological University, is now a Research Scientist, General Motors Research Center, Warren, MI 48090. SHU-ZU LU, Visiting Scholar, Peoples Republic of China, and A. HELLAWELL, Professor, are with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted August 5, 1983.
METALLURGICALTRANSACTIONSA
A recent report by Kobayashi et aI.1~ based on thermal analysis of normal and modified alloys claimed that one effect of sodium was to elevate the aluminum liquidus temperature by 1 to 2 K and so displace the apparent binary A1-Si eutectic to higher silicon concentrations. This was not confirmed in the present work. The A1-Si-Li and A1-Si-Sr liquidus surfaces have recently been determined, u'l: and each shows ternary eutectic reactions at 1.4 wt pct Li and 1.8 pct Sr, respectively, some 2 to 3 K below the binary A1-Si eutectic. One may note that complete modification by sodium is effected at a level close to that of the binary A1-Na monotectic composition, while the morphologically similar modification by strontium J3a4 occurs at much lower concentrations than that of the ternary eutectic poi
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