Interdendritic Spacing: Part I. Experimental Studies
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INTRODUCTION
DURING solidification of alloys, dendritic and cellular structures are very commonly observed. The dendritic structure, under given directional experimental growth conditions, consists of a regular array of dendrites with each dendrite having an initially periodic sidebranch configuration. One of the important consequences of such a structure is the occurrence of microsegregation between the dendrites. This microsegregation is characterized by primary and secondary dendrite spacings which greatly influence the mechanical properties of solidified alloys.l Consequently, in order to control the properties of cast or welded alloys, it is necessary to understand the mechanism and the characterization of primary and secondary dendrite spacings during the solidification of alloys. Since dendrite characteristics depend both on the solidification rate and the temperature gradient in the liquid, directional solidification studies are needed to isolate the effects of these two variables. Dendritic morphology also occurs when a pure liquid is undercooled. Our understanding of dendritic structures in such a case is now well understood due to the careful experimental studies by Glicksman and co-workers 2'3'4 on pure succinonitrile and the detailed theoretical work by Langer and Miiller-Krumbhaar (LM-K). 5 The agreement between theory and experiment is quite precise when convection effects are negligible. Furthermore, experimental studies by Huang and Glicksmanz have now well established that the dendrite tip assumes the value predicted by the marginal stability criteria of LM-K. 5 Furthermore, they have shown that a scaling law exists between the initial secondary dendrite spacing, h2, and the dendrite tip radius, p. They found h2/p ~- 3.0, which is close to the ratio of 2.1 predicted by LM-K. Although our understanding of dendrite characteristics is quite complete for a pure undercooled melt, the agreement between theory and experiment is not as complete for dendrite growth in alloys under directional solidification conditions. 6-1~Since many practical problems of casting and welding have positive temperature gradients in the liquid ahead of the interface, it is important to carry out experiK. SOMBOONSUK is a Graduate Assistant and J.T. MASON is an Associate Metallurgist at Ames Laboratory-USDOE. R. TRIVEDI is a Senior Scientist and Group Leader at Ames Laboratory-USDOE and Professor with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. This paper is based on a presentation made at the symposium "Establishment of Microstructural Spacing during Dendritic and Cooperative Growth" held at the annual meeting of the AIME in Atlanta, Georgia on March 7, 1983 under the joint sponsorship of the ASM-MSD Phase Transformations Committee and the TMS-AIME Solidification Committee. METALLURGICALTRANSACTIONS A
mental work on a well-characterized system so that one can precisely measure various dendrite morphological features as a function of imposed growth rate and temperature gradient
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