Heat flow parameters affecting dendrite spacings during unsteady-state solidification of Sn-Pb and Al-Cu alloys
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DURING solidification of alloys, the observed microstructures are diverse, but in general can be classified into two basic groups: cells/dendrites and eutectic morphologies. Dendrite growth is the common mechanism of crystallization from metallic melts, and the morphology, which is formed, consists of an array of dendrites with a sidebranch configuration. The solute, which is redistributed due to the solubility difference between the solid and liquid phases, provokes an important consequence of such a structure, i.e., the occurrence of microsegregation between the dendrites branches. The dendritic array characterized by primary and secondary spacings and the segregated products greatly affect the mechanical properties and homogenization kinetics of solidified alloys.[1,2] Therefore, in order to control the properties of casting materials, it is important to understand the solidification parameters that affect the growth of dendritic spacings during solidification. Numerous directional solidification studies have been reported with a view to characterizing primary (l1) and secondary (l2) dendrite arm spacings as a function of alloy solute concentration (C0), tip growth rate (VL), and temperature gradient ahead of the macroscopic solidification front (GL).[1–54] A recent article by Bouchard and Kirkaldy[29] has summarized these studies grouped into two categories: those involving solidification in steady-state heat flow and those in unsteady-state regime. In the former category, solidification is controlled and the significant controllable variables, GL and VL, are maintained constant and are practically independent of each other. In the latter, which characterizes, OTÁVIO L. ROCHA and CLÁUDIO A. SIQUEIRA, Doctorate Students and Research Assistants, and AMAURI GARCIA, Professor, are with the Department of Materials Engineering, State University of Campinas— Unicamp, 13083-970 Campinas-SP, Brazil. Contact e-mail: amaurig @fem.unicamp.br Manuscript submitted July 12, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
for instance, the solidification conditions of a body of irregular shape, these variables are interdependent, cannot be controlled, and vary freely with time. The analysis of dendritic structures in the unsteady-state regime is very important, since it encompasses the majority of industrial solidification processes. The measurements of primary and secondary dendrite arm spacings involve looking at the microstructure after complete solidification. Primary spacings do not coarsen with time and can be accurately measured from the microstructure and compared with growth models. On the other hand, secondary spacings are seen to rapidly coarsen during solidification, and the effect of coarsening has to be taken into account by the predictive growth models.[29] Most of the results in the literature, concerning steady and unsteady regimes, pertaining to l2 in hypoeutectic alloys, indicate a decrease in spacing with increasing cooling rate for a given alloy composition and with increasing solute content for
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