Simulation of freckles during vertical solidification of binary alloys
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INTRODUCTION
AN increasing number of technological applications require the operation of critical mechanical components under severe conditions of high temperatures and stresses. If improperly manufactured, these components are subject to creep fracture and thermal fatigue failures, which are almost always associated with grain boundaries that are transverse to the applied stress. The modern day directional solidification technique provides an effective means of controlling the grain shape, producing a columnar microstructure with all of the grain boundaries running in the longitudinal direction of the casting. This greatly improves material performance at elevated temperatures. Still better properties can be obtained by casting a single crystal (i.e., a dendritic monocrystal), in which only one columnar grain is allowed to grow and form the main body of the casting. I~l Without proper control, directionally solidified (DS) castings are not free of defects. Particularly troublesome are localized segregates at the macroscopic scale, which are found in many DS alloys, including some superalloys. These segregates are observed as long and narrow trails, aligned parallel to the direction of gravity in DS castings, and are enriched in the normally segregating elements and depleted of the inversely segregating elements; i.e., their composition is shifted toward the eutectic composition. In horizontally solidified ingots of steel, similar defects are known as ~A ~ segregates, while in DS castings they have a more pronounced channel shape and are termed "freckles." This nonuniformity of composition is highly undesirable, because the resulting variation in physical properties within the casting can lead to inferior performance of the components manufactured from the casting. In ingot production, an excessive number of defects can require a large amount of cropping at a considerable cost of energy and material. S.D. FELICELLI, formerly Graduate Student, University of Arizona, is Research Engineer with the Argentine Atomic Energy Commission, Centro Atomico Bariloche, 8400 San Carlos de Bariloche, Argentina. J.C. HEINRICH, Professor, Department of Aerospace and Mechanical Engineering, and D.R. POIRIER, Professor, Department of Materials Science and Engineering, are with the University of Arizona, Tucson, AZ 85721. Manuscript submitted April 15, 1991. METALLURGICAL TRANSACTIONS B
Many analytical and experimental works have been done in recent years in order to observe, explain, and prevent the formation of both "A" segregates t2,3"41 and freckles tS-13J in DS alloys. This article addresses the latter type of defects, i.e., freckles, when an alloy is cooled from below. The opacity of metals prevents direct observation of the nucleation and growth of channels during solidification. Observations are usually done by quenching the ingot at different stages of solidification and analyzing the solidified macrostructure. Much of what is known about channel dynamics has been learned from the transparent analog NH4C1-H20 system, tS'
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