Primary dendrite spacing and the effect of off-axis heat flow
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I.
INTRODUCTION
P R I M A R Y dendrite arm spacings, &, in cast or directionally solidified alloys have long been a subject of interest to researchers. The primary and associated secondary arms serve to effectively isolate potentially detrimental interdendritic phases. Furthermore, the dendrite trunk is essentially a continuous fiber which contributes to the product's ductility and toughness. Many theoretical~ 4] and experimental~4-2~ studies have shown the primary spacing to be a function of growth velocity, V, composition, Co, and temperature gradient, G. These controlling parameters usually are related to the primary spacing in the following form: A~ = KV -1/4 C~o/4 G 1/2, where K is a materials constant. The intent of this work is to show that the primary dendrite spacing is also a function of the growth direction with respect to orientation of the thermal gradient. II.
EXPERIMENTAL
PROCEDURE
A temperature gradient stage, in conjunction with thin glass cells containing succinonitrile-water alloys, was used in this study. Succinonitrile with its low entropy of fusion has long been used to simulate the growth phenomena of metals, I17'18m'221with its transparent nature allowing for direct observations and documentation (via a microscope employing transmitted light) of the solidification process. Distilled water was added to previously distilled and zone-refined liquid succinonitrile. The well-mixed alloy later was introduced in air at a small opening on one end of the glass cell which rapidly filled by capillary action. The alloy was solidified quickly by placing the cell on a cold plate, and the ports were sealed with an epoxy resin. The cell was constructed from thin glass plates and spacers which were bonded either by an epoxy resin or a glass powder, the latter fusing the pieces together after a heat treatment. The dimensions of the cell containing R.N. GRUGEL, formerly Visiting Researcher at Northwestern Polytechnical University, Xian, China, is Research Assistant Professor at the Center for the Space Processing of Engineering Materials, Vanderbilt University, Nashville, TN 37235. Y. ZHOU is Professor of Metallurgical Engineering, Northwestern Polytechnical University, Xian, Shaanxi Province, China. Manuscript submitted September 4, 1987. METALLURGICAL TRANSACTIONS A
the alloy were 70 mm in length, 15 mm in width, and either 250 or 325 /xm in thickness. Temperature gradients were measured ahead of the dendrite front by a chromel-alumel thermocouple (25 /xm diameter wires, - 5 0 /xm bead diameter), which had been incorporated in the cell parallel to the isotherms prior to filling. The cell was placed on the hot block of a temperature gradient stage and then pulled across the gap and onto the cold block by a thread attached to a traction device, the cell, or growth, velocity being regulated by a variable speed motor. By reversing the growth (cell) direction after each run, the alloy structure inside the cell can be made to be composed of two or three grains. These grains consist of a framework of aligned
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