Modification of Cast Al-Mg 2 Si Metal Matrix Composite by Li
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- and Mg-based composites, reinforced with particulates of Mg2Si, have been lately introduced as a new group of particulate metal matrix composites (PMMCs) that offer attractive advantages such as low density, good wear resistance, and good castability.[1,2] Al-Mg2Si hypereutectic alloys seem to be potential candidates to replace Al-Si alloys used in aerospace and engine applications. In-situ preparation of Al-Mg2Si composites seems to be the best way of fabricating such PMMCs, because advantages such as an even distribution of the reinforcing phase, good particle wetting, and low costs of production are guaranteed. There have been reports of Mg-Mg2Si composites made by mechanical alloying and hot extrusion methods in which the alloys show good tensile properties, but these manufacturing routes are expensive. Therefore, over the past few years, efforts have been made to produce such composites by a casting process. Despite the fact that Mg2Si intermetallic particles exhibit a high melting temperature, low density, high hardness, low thermal-expansion coefficient, and equilibrium interface, their coarse and rough morphology in the Al matrix has been thought to lead to the low ductility observed in these materials.[3]
R. HADIAN, Doctoral Student, is with McMaster University, Hamilton L8S 4L8, Canada. Contact e-mail: [email protected] M. EMAMY, Professor, is with the School of Metallurgy and Materials, University of Tehran, Tehran, Iran. J. CAMPBELL, Emeritus Professor, is with the Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, United Kingdom. This article is based on a presentation given in the ‘‘3rd Shape Casting Symposium,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online June 24, 2009. 822—VOLUME 40B, DECEMBER 2009
Thus, investigations have been carried out to modify the primary and eutectic Mg2Si structure in an effort to improve the properties of these composites. Other than expensive methods of rapid solidification and mechanical alloying,[4] high cooling rates and impurity modifications are well-known techniques that have been used so far. Zhang et al.[5] used a wedge copper mold with a range of cooling rates, and Ourfali et al.[6] applied Bridgman solidification with different withdrawal velocities. Both methods were found to be effective in refining the coarse Mg2Si structure and producing a final dendritic structure. Dendrites, however, can result in anisotropic properties, and in order to improve tensile properties, a more isotropic microstructure is preferred. Other efforts have been focused on the modification of the structure with addition of different alloying elements such as Sr, Ce, sodium salt (assumed to introduce Na), K2TiF6, and extra silicon content.[7–11] Rare earth elements have also been reported to be capable of modifying the primary Mg2Si particles.[12] Recently, P wa
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