On a possibility of avoiding axial segregation in growing pseudobinary alloys from a liquid zone

There is considerable interest in macroscopically homogeneous crystals in a great variety of pseudobinary systems A1 travelling heater method-THM), are known to provide with crystalline ingots being fairly uniform in composition, except for the initial part of the crystal. For a binary system the solute distribution is described by the well-known zone melting function (PFANN) : l) Besides a complete mixing of the melt and other prerequisites a homogeneous composition xo of the source material is demanded which is identical with the zone composition a t the very beginning xL(0) = xo. Just this original concentration is the cause of the initial part, of the distribution function providing an inhomogeneous first-to-grow region until reaching steady-state conditions (xL = xo/kl) within the liquid zone. DIBENEDETTO, CRONAN mentioned the possibility of avoiding the initial course in a zone crystallization distribution by selecting the suitable zone composition by means of trial and error. Nevertheless, most workers dealing with problenis of THM crystal growth of solid alloys use a pure solvent zone a t the very beginning of the growth run to dissolve solid material at the interfaces. It is the intention of this note to give the expression for an appropriate zone composition that avoids the initial course of the distribution function, which usually takes a considerable portion of the grown crystal. I n principle, it is possible to start with a separately prepared zone material of just the steady-state coniposition xL(0) = xo/kl. This idea seems to become even more important if a method is used, which (i) necessitates in any case a separately prepared zone material, and (ii) provides crystals being not much longer than some few zone lengths (LIZ lo), i.e., which not much exceed the initial part of PFANN'S zone melting distribution. The travelling heater method must be considered such a technique. Besides the mentioned distribution coefficient k, = xs/xL in the pseudobinary system describing the A/B ratio of the alloy, another coefficient k, = ys/yL = 1/2yL a t the section x = constant within the ternary system becomes interesting. It is responsible for the THM principle (as well as for solution growth, in general) and describes the solubility with respect to an either (A, B) or C excess in the liquid zone l) For the purpose of this s t u d y compositions are described i n terms of.niole fractions.