A study of the influence of mischmetal additions to ai-7si-0.3mg ( lm 25/356 ) alloy
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I.
INTRODUCTION
HYPOEUTECTIC-cast A1-7Si-0.3Mg alloy used with the designation LM 25 or 356 has widespread applications in the general engineering, automotive, and aerospace industries. This alloy exhibits excellent castability, good mechanical properties, corrosion resistance, weldability, and low thermal expansion.t~l The mechanical properties of the casting are affected significantly by the morphology of the eutectic silicon. At low cooling rates, the eutectic Si is present as coarse, acicular needles which act as stress raisers, and consequently, the material fractures in a brittle mode. [2] The addition of small amounts of sodium, strontium, or antimony modifies the morphology of the Si from acicular to fibrous or lamellar shape, [s,4,s]thereby improving the mechanical properties. Modification by Na also improves the feeding characteristics of the melt in both sand and metal molds, t61 Dissolution of Na is instantaneous at the processing temperatures, but because of the very high vapor pressure (0.2 arm at 1003 K), a large fraction of Na added boils off almost immediately, leading to the so-called "fading" and poor recovery,is-81 Strontium fades slower than Na and does not overmodify the Si when present in excess, unlike Na. However, if present in larger amounts, Sr can result in the formation of undesirable intermetallic compounds such as SrSi2 and SrA12Si2. Besides, Sr necessitates longer incubation periods for effective modification.tg~ Rare earth (RE) elements such as La, Ce, Nd, Y, and mischmetal (MM) (a mixture of RE elements) are reported to be capable of modifying the eutectic structure in AI-Si alloys.[3-5,7,1~19] Modification with RE elements was explainedt~u using the critical growth temperature hypothesis,ta] in which the modifying element should exhibit a tendency to form compounds with the precipitating phase (Si) at a temperature below the normal eutectic temperature and also should exhibit little compound-forming tendency with the solvent phase (a-A1). Lanthanum meets these requirements, but Ce and Nd satisfy them only partially. In addition to
M. RAVI and U.T.S. PILLAI, Scientists, B.C. PAI, Head, Metal Processing Division, and A.D. DAMODARAN, Director, are with the Regional Research Laboratory (CSIR), Trivandrum 695 019, India. E.S. DWARAKADASA, Professor, is with the Structure Property Correlation Group, Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted June 1, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
the preceding, RE elements do satisfy the minimum criteria such as atomic radius, melting point, vapor pressure, and free energies of oxide formation,t2oJ thus exhibiting the capability to modify the eutectic Si. Rare earth elements have been added to A1-Si alloys as MM and also in the form of specific RE metals and as fluorides. Lanthanum addition to this alloy reduced the interdendritic porosity and secondary porosity considerablyY 1~ Eutectic undercooling in A1-7Si0.3 Mg alloy is increased by as much as 25 K, with an increase in
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