SUBSTRUCTURES IN DILUTE ALLOYS

Abstract

i\/Iany observatiolls have been made of the substructures occurring in crystals grown from the melt. Recent summaries (Elbaum 1959; I-Iurle 1962) suggest that, in general, interest has been concentrated on either the cellular substructure (Smialowslti 1937; Rutter and Chalmers 1953), or on the striation substructure described by Teghtsooilian and Chalmers (1931). Very few attempts (Xtwater and Chalmers 1957; Sekerlta, Bolling, and Tiller 1960; Doherty and Davis 1961; I-Iunt and Smith 1962) have been made to examine the interrelationships of these two. The heretofore published photographic evidence showing the two substructures coexisting on a decanted solid-liquid interface suggests that, whilst the presence of the cellular interface affects the configuration of a striation boundary (Hunt and Smith 1962), the cellular morphology is not modified by the boundary region itself (Blaha 1953). Where adjoining grains have appreciable lnisorientation such that the average cell size and shape of one differs markedly from those of its neighbor, the cellular morphology of each is unmodified right up to the grain boundary with no apparent transitional region. (See Fig. 18 of Tiller and Rutter (1956).) The purpose of the present communication is to show that this is not always observed. During current solute-redistribution studies, certain binary alloy crystals were grown under conditions which gave rise to the Smialowslti structure and were decanted from their melts using a lnodification of the decanting apparatus described by Elbauin and Chalmers (1955). When the growth conditions were such a s to render a cellular interface just stable with respect to a planar solid-liquid interface (LValton et al. 1055) a marked change in the cellular nlorphology was observed on the decanted interface in the neighborhood of grain and subgrain boundaries. Whereas in a fully developed cellular interface sub-boundaries are often difficult to see, under the above conditions, these boundary regions gave rise to a iiladder-like strl~cture. This is seen in Fig. 1, which shows the decanted solid-liquid interface of a tin alloy containing 0.025 at.yo Pb. (Here the distribution coefficient, k , is approximately 0.1 and the crystal was grown with G/R = 2.5X103 C sec.c~n-~, where G defines the temperature gradient in the liquid and R is the axial rate of solidification.) A typical sub-boundary region is shown in Fig. 2. For the experiments the zone-refined tin was kindly supplied by the Department of Metallurgical Engineering, University of Toronto. High-purity antimony, bismuth, and thallium were obtained from Johnson, Matthey and Co. Ltd., England. The lead used was Tadanac, of a purity such that a planar interface was stable a t (G/R) values considerably snlaller than those used in these experiments. A detail of Fig. 1 involving both highand lowangle boundaries is shown more clearly in Fig. 3. I t is seen that some boundary migration has occurred from the grooves a t the centers of the ladders.