Motion and remelting of dendrite fragments during directional solidification of a nickel-base superalloy
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I.
INTRODUCTION
THE avoidance of grain defects is of critical importance in the directional solidification of single-crystal castings. Two types of defects commonly observed in Ni-base superalloy components are so-called freckle chains and isolated ‘‘spurious’’ grains, both of which can feature highly misoriented, equiaxed grains. Recent examples of such grain defects, including micrographs, can be found in the experimental work of Pollock and Murphy,[1] who noted that the onset of freckle formation occurs under the same conditions as for the spurious grains. The formation of freckle chains or channel segregates due to thermosolutal convective instabilities is relatively well understood since the original work of Giamei and Kear.[2] In a companion article,[3] we presented a detailed solidification and melt convection model, coupled with a phase equilibrium subroutine for Ni-base superalloys, that allows for a realistic prediction of channel segregates in directional solidification of superalloys. The reader is referred to Reference 3 for a review of related work in this area. Figure 1 shows an example of model calculations for a convectively unstable case corresponding to the CMSX2 alloy. Noteworthy is the long, open channel in the mush, through which highly segregated liquid flows upward to feed a low-density plume, or finger, above the channel in the single-phase liquid region. In the solidified casting, the channels persist as regions of strong macrosegregation containing a chain of equiaxed grains. The equiaxed grains inside the channels originate from separation of secondary or tertiary dendrite arms from the main trunks in the channels during solidification.[4,5,6] The separation is generally thought to occur at the joints (or ‘‘necks’’) between an arm and its trunk and is caused by localized remelting and coarsening. Hellawell[7] and others J.P. GU, Postdoctoral Researcher, and C. BECKERMANN, Professor, are with the Department of Mechanical Engineering, University of Iowa, Iowa City, IA 52242-1527. A.F. GIAMEI, Principal Scientist, is with the United Technologies Research Center, East Hartford, CT 06108. Manuscript submitted October 28, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
(references therein) have proposed mechanisms for the detachment process, but predictive models are not available. The separated dendrite fragments are subject to drag forces from the relatively strong melt flow in the channels, to their own weight, and to blocking by other dendrite arms. Sometimes, a fragment can rotate only slightly but stays otherwise fixed, leading to the presence of a minimally misoriented equiaxed grain in the channel (micrographs in Reference 1). The presence of highly misoriented grains in the channels indicates that some fragments undergo more extensive rotational and translational motion. Experiments with transparent model alloys have shown that the low-density plumes associated with the channels sweep some of the dendrite fragments into the bulk liquid.[4–9] If the fragments survive in the melt ab
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