Captura-de-pantalla-2018-10-08-a-la_s_-12.36.50.ocr.docx

CONSTITUTIONAL SUPERCOOLING AND CELL FORMATION In the preceding chapter numerous examples have been given of how a solute-rich boundary layer builds up in front of a solidifying planar interface. Several early workers, among them Papapetrou,1 understood qualitatively how such a solute buildup could lead to instability of the plane front. Not until the important studies of Chalmers and coworkers,2’3 however, were the ideas quantified and applied to metal crystal growth from the melt. Figure 3-1 shows qualitatively how the driving force for instability of the plane front develops. A solute-rich layer is present in front of a growing interface, in which liquid composition is a maximum Cl at the interface and decreases with increasing distance from the interface. Now, with the aid of the phase diagram, it is a simple task to plot the equilibrium liquidus temperature of the liquid as a function of distance from the interface; this is done in Fig. 3-lc and d. The equilibrium liquidus temperature increases with distance from the interface because the lower the solute content, the higher the liquidus temperature. Next, the actual temperature in the growing crystal is superimposed on the same graph. Since equilibrium is assumed at the solid-