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FRECKLES are a common grain defect found in directionally solidified superalloy components. They generally appear as long and narrow trails of equiaxed grains roughly parallel to the direction of gravity. It is agreed that freckles arise due to channels associated with thermosolutal convection in the mushy zone and that they are driven by a variation in density originating from interdendritic segregation.[1,2] Some porosity and feeding shrinkage may also be present in and adjacent to the freckle line. During the vertically directional solidification of superalloy components in the Bridgman furnace, dendrites will grow upward against the heat flux. The heavier elements like W and Re segregate preferentially into the solid phase, while the lighter elements like Al and Ti will be rejected into the interdendritic liquid. The resultant buoyancy force associated with the density difference between the interdendritic liquid and the bulk liquid ahead of the dendrite tips can produce thermosolutal convection. Low-density liquid emanates from the mushy zone in the form of solutal plumes, which are fed by a flow of segregated liquid through the surroundings, leading to the formation of a narrow open channel which finally solidifies and then appears as freckle chains.[1–6] Freckles are highly undesirable in critical applications such as turbine blades because of their deleterious effect DEXIN MA, Senior Scientist, is with the Foundry Institute, RWTH Aachen University, Aachen, Germany, and also Professor with the School of Material Science and Technology, Jiangsu University, Zhenjiang 212013, P.R. China. Contact e-mail: d.ma@gi. rwth-aachen.de ANDREAS BU¨HRIG-POLACZEK, Professor, Leader, is with the Foundry Institute, RWTH Aachen University. Manuscript submitted December 17, 2012. Article published online November 7, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A

on the material’s mechanical performance. Thus, castings with freckles must be scrapped, causing considerable economic loss because they cannot be removed by thermomechanical treatments. Developments in large aircraft engines and large land-based gas turbines for power generation require a considerable scale-up in size. This leads to the main problem that extensive freckling is observed in these large castings. Freckles tend to occur more frequently at significant cross-sectional areas,[1,3,7,8] because the wide mushy zone can provide a sufficient reservoir to support the interdendritic convection and hence the onset of freckling. Besides the size effect, the occurrence of freckles has also been known to be mainly dependent on the alloy chemistry and process parameters. The probability of freckle formation is strongly dependent upon the composition of the alloys. Studies designed to investigate the influence of alloy composition[1–3,8,9] have found out that superalloys with more Re, W, Ti, and Al are more prone to freckling during directional solidification. However, the superalloy’s composition latitude is relatively small due to its property requirements.[8,10] Another factor