Influence of TiB 2 Addition on the Precipitation Kinetics in Al-7Si-0.3Mg In Situ TiB 2 Composites
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Cast A356 alloys are Al-Si-Mg alloys which have excellent castability, corrosion resistance, and high strength to weight ratio in the heat-treated condition. They have widespread applications in automotive and aerospace industries.[1] Metal matrix composites (MMC) of aluminum alloys in general gained lot of importance due to their improved strength and stiffness. In this respect, in situ metal matrix composites have gained significance for the finer size, better wettability, and uniform distribution of second phase particles. The second phase particles in these composites are found to significantly affect the precipitation behavior of the intermediate Mg2Si precipitates. The accepted precipitation sequence in Al-Si-Mg alloys is the following[2]: Super saturated solid solution ðSSSÞ ! ðMg þ SiÞ cluster ! GP zones plate like=spherical ! b00 needles ! b0 rods þ Si þ others ! b plates þ Si:
S.H. NANDAM, Graduate Student, formerly with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India, is now with Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein Leopoldshafen, Germany. Contact e-mail: [email protected] B.S. MURTY, Professor, and S. SANKARAN, Associate Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Manuscript submitted November 18, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Precipitation sequence in excess Si alloys is found to be almost the same compared to the balanced alloy, although additional overaged metastable phases such as B¢ laths, type A, type B, type C precipitates, and Si were identified.[3–5] Si precipitation is clearly observed in 339 aluminum alloys (nominal composition of Si—12 wt pct) immediately after b¢ precipitation.[5] The compositions of GP zones and b¢¢ needles are still under considerable debate. Andersen et al.[3] found that the composition of b¢¢ is Mg5Si6 in a Al-0.2Fe-0.5Mg0.53Si-0.01Mn (wt pct) alloy. The Mg/Si ratio of the b¢ rods is established to be 1.7 and that of the b¢¢ precipitate is close to 1 in an excess Si alloy.[6] Final b precipitate will have a composition of Mg2Si. It is already well established that addition of second phase particles accelerates the precipitation in Al-Si-Mg alloys.[7,8] Dutta et al.[8] carried out investigation on 6061Al-based composites and concluded that the aging of the composites is accelerated with second phase particle addition. This was attributed to the increase in dislocation density because of the thermal coefficient mismatch between the matrix and second phase particles. However, the influence of second phase particles on precipitation sequence is not very well documented in excess Si (A356 type) alloys.[9–11] Mandal et al.[9] found that the precipitation kinetics of the Mg2Si precipitation accelerated with the addition of TiB2 in A356/TiB2 (A356 is an excess Si alloy) in situ composites but the overall peak hardness was found to be lower than the unreinf
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