Cellular/Dendritic Transition and Microstructure Evolution during Transient Directional Solidification of Pb-Sb Alloys
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SINCE the early 1980s, a number of metallurgists, physicists, and mathematicians have investigated the solid/liquid interface morphologies during solidification in which the cellular/dendritic growth is one of the most complicated structural patterns and is also the most prevalent form of crystallization.[1–7] The cellular and dendritic spacings are important microstructural parameters resulting from the solidification process, because it is well known that these spacings exercise a significant influence on the properties of castings. They affect the microscopic segregation existing between the cellular or dendritic ramifications, and consequently, the mechanical behavior. Some recent studies have pointed out the effect of microstructure and particularly of dendrite spacings on mechanical properties of as-cast alloys.[8–10] It has also been recently reported in the literature that the microstructural morphologies have a strong influence on the corrosion resistance of binary alloys. The cooling rate imposed during solidification affects cellular and dendritic spacings, and the solute redistribution, which connected with the electrochemical behavior of DANIEL M. ROSA, Postdoctoral Fellow, is with the Faculty of Technology, University of Brası´ lia, 70910-90, Brası´ lia, DF, Brazil. JOSE´ E. SPINELLI and IVALDO L. FERREIRA, Postdoctoral Fellows, 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 November 22, 2007. Article published online May 7, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
solute and solvent will affect the resulting corrosion resistance.[5,10–14] The correct determination of thermal parameters such as the temperature gradient (GL), solidification velocity _ acting during solidification is (VL), and cooling rate ðTÞ very important, because the growth of regular cells occurs in low growth rate conditions, is perpendicular to the solid/liquid interface, in the direction of heat flow extraction, and practically independent of the crystallographic orientation. If GL is reduced and VL increased, the region constitutionally supercooled is extended and the cells begin to change their behavior to configuration of the malt cross type, and the crystallographic direction starts to exercise a strong effect. With the gradual increase of VL, the cells begin to present side perturbations that are denominated ramifications or secondary arms defining the dendritic structure.[1,15,16] During unidirectional growth under steady-state conditions, GL and VL are independently controlled and held constant with time, while in the non-steady-state regime, the temperature gradient and growth rate vary freely in time. It is well known that the solidification microstructure of a single phase alloy undergoes a transition from a cellular to dendritic interface as the velocity increases. Figure 1 shows a schematic representation of such influence on microstructure formation. Further incre
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