Stability of microstructures in chill-cast aluminum alloys containing twinned columnar growth structures
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INTRODUCTION
COMMERCIAL aluminum ingots are made by the directchill (DC) technique, which is a semicontinuous casting process. This process results in the establishment of large, unidirectional thermal gradients at the surface of an ingot and can stimulate the growth of a long columnar region which grows from the surface into the center of the ingot. In some instances, the columnar grains are twinned. The large, unidirectional thermal gradient in these regions is a necessary condition for the formation of this growth structure, which has been labeled "twinned columnar growth" (TCG) structure. [1,2,3j As seen from the micrograph (Figure 1), the boundaries between the lamellae alternate as smooth and rough. The smooth boundary is a fully coherent twin which lies along the centerline of the twinned dendrite. 11,2,4] This smootl/boundary solidifies first, while the rough boundary, called an "intertwin channel," solidifies last. Microsegregation within the TCG structure occurs consistent with the solute distribution coefficient, k. For alloying elements with k < 1, the twin plane is depleted in solute, whereas the rough boundaries separating the twins are enriched in solute. This enrichment promotes the formation of equilibrium as well as nonequilibrium eutectic phases. The nonequilibrium eutectic can be homogenized by proper postcasting thermal treatment and is termed "soluble," whereas the equilibrium eutectic is stable upon thermal treatment and is termed "insoluble." For systems solidifying with k > 1, the first regions to solidify are enriched in solute, and no eutectic is formed. Some elements which promote TCG include iron, nickel, chromium, manganese, and zirconium.tS,6~For a specific heat transfer condition, a critical solute content, characteristic of each element, appears to promote TCG.[2.6.7[
H.R. LAST, Graduate Research Assistant, and T.H. SANDERS, Jr., Professor, are with the School of Materials Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. J.M. GONSALVES, Graduate Assistant, is with the School of Materials Engineering, Purdue University, West Lafayette, IN 47907. Manuscript submitted July 5, 1989. METALLURGICAL TRANSACTIONS A
Since alloying elements with values of k which are either less than 1 or greater than 1 develop the TCG structure, one would expect any solute element to stimulate TCG formation. When colonies of the TCG structure are present, macroetching reveals the characteristic fanning out of the individual twinned dendrites in a structure which is often characterized as "feathery" (Figure 2). These feathery twinned dendrites seem to exhibit a growth advantage over regular columnar dendrites. 0,4,71 It has been suggested that the twinned dendrites form with the solid/ liquid interface advancing into the melt with a grooved tip at the leading edge of the twin plane. [2] As in columnar regions containing a regular dendritic structure, the TCG structure is crystallographically oriented with respect to the thermal gradient. Although numerous observations have been made re
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