The breakdown of single-crystal solidification in high refractory nickel-base alloys
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INTRODUCTION
TRANSITIONS in modes of solidification, from plane front to cellular, cellular to columnar dendritic, or columnar dendritic to equiaxed, have long been of interest because of the sensitive dependence of mechanical properties on the grain structures of cast materials. The transition to equiaxed solidification is of particular importance for nickel-base single crystals, since the mechanical integrity of single-crystal components such as turbine blades is dependent on the elimination of high-angle grain boundaries. Unfortunately, this transition is the most difficult to quantitatively predict, due to the complex geometry of the dendritic array and the accompanying redistribution of solute during single-crystal solidification. Furthermore, since the transition involves nucleation of new grains, it is not only essential to understand the multiple mechanisms by which equiaxed grains may nucleate, but also the conditions under which grains may subsequently grow in a process where positive thermal gradients are maintained at the solidification front. Several mechanisms for the columnar to equiaxed transition (CET) have been suggested. ChalmersIll proposed that the CET develops due to blockage of columnar growth by equiaxed grains that nucleate as a result of the initial contact of the melt with the cooler container walls. Winegard and Chalmers t21 proposed and Hunt I3I analytically treated the alternate situation of blockage of columnar growth by nucleation of equiaxed grains in front of the advancing dendritic front, due to constitutional supercool-
T.M. POLLOCK, Assistant Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. W.H. MURPHY, Research Engineer, is with the Engineering Materials and Technologies Laboratories, General Electric Aircraft Engines, Cincinnati, OH 45215. Manuscript submitted November 14, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing. Jackson et al. t4I observed remelting and fragmentation of secondary dendrite arms in transparent compounds and postulated that the equiaxed zone in casting was primarily due to detachment of dendrite arms and subsequent relocation to the central supercooled region of the casting. This CET transition has recently been examined in more detail via numerical modeling,I5,6~but accurate predictions are still not possible due to an inadequate understanding of the nucleation events, the difficulty of incorporating convection into numerical models, and the uncertainty regarding the correct thermal and sotutal boundary conditions. Also, although the underlying mechanisms of the transition to equiaxed solidification in large ingots are likely to be similar to those encountered during directional solidification of large single crystals, it is important to consider the differences in local conditions, i.e., the nearly constant positive, unidirectional thermal gradient imposed during directional solidification, and the orientation of the mushy zone. In addition to suppressing a transition
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