Microsegregation during Solidification of Graphitic Fiber-Reinforced Aluminum Alloys under External Heat Sinks
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the microscale level, also known as microsegregation, is an important phenomenon, which occurs during the evolution of the solidification structure. Microsegregation can lead to nonhomogeneous properties, particularly when secondary processing is not involved. Hence, extensive efforts have been made aimed at measuring and predicting solute microsegregation effects in Al alloys during solidification. Martorano et al.[1] found that the extent of solute segregation in aluminum alloys is strongly influenced by the cooling rates, with severe microsegregation effects expected at reduced cooling rates. Moreover, the role of solute segregation on the secondary precipitation reactions has been investigated in some detail by several authors.[2–4] In particular, it has been found that as the extent of microsegregation increases, secondary precipitation is effectively promoted. Nevertheless, the effect of processing parameters on the extent of microsegregation during the solidification of fiber-reinforced Al-composites has not been investigated in detail. From the available literature on fiber-reinforced Al alloys,[5-7] it is apparent that the extent of solute H.G. SEONG, H.F. LOPEZ, and P.K. ROHATGI are with the Department of Materials Engineering, University of Wisconsin–Milwaukee, Milwaukee, WI 53211, USA. Contact e-mail: [email protected] Manuscript Submitted May 20, 2005. 138—VOLUME 38A, JANUARY 2007
segregation is strongly influenced by the type, volume fraction, and morphology of fibers, as well as by the melt cooling rates and alloy chemistry. Gungor et al.[5] found that in an Al2O3(F)-reinforced Al-Cu alloy, a dendritic structure with normal solute coring develops within the fiber interstices when the interfiber spacing, kF, exceeds the secondary dendrite arm spacing, kDAS. In particular, the exhibited minimum Cu contents were found to increase linearly with 1/kF.[6] Moreover, the minimum Cu segregation levels in the local matrix surrounding the fibers tended to increase as the mean kF was reduced, or the solidification times were increased. Mortensen et al.[7] investigated the microsegregation effects in directionally solidified Al-4.5 wt pct Cu alloys reinforced with Al2O3(F). In their work, the minimum Cu contents were found to increase with increasing local solidification times (above 750 seconds). These times were significantly large when compared with those found in unreinforced Al-4.5 wt pct Cu alloys under similar cooling conditions. In addition, no eutectic phases were detected on the alumina fiber interfaces, nor in the local matrix surrounding the fibers. In contrast, when the reinforced alloy was solidified within 1 second, a eutectic constituent was found to form in the reinforced matrix, preferentially along the dendrite boundaries. In other related works, Petitcorps et al.[8] found that in short Al2O3 fiber-reinforced Al-7Si-0.3Mg and Al-7Si-2.2Mg composites, the level of Mg segregation in high Mg composites was roughly 5 wt pct higher at the fiber/ METALLURGICAL AND MATERIALS TRANSACTIONS A
matrix interfaces than that in the bul
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