Single-crystal solidification of new Co-Al-W-base alloys

Solidification processing of new high-temperature Co-Al-W-base single-crystals has been investigated. Single-crystal bars with compositions of Co-9.4Al-10.7W, Co-8.8Al-9.8W-2Ta and Co-7.8Al-7.8W -1.5Ta-4.5Cr (at%) were successfully grown with a conventional Bridgman process. Helical grain selectors resulted in [001] dendritic growth with primary dendrite arm spacings in the range of 278 -364 mu m. Segregation of constituent elements in the dendritic structure was very limited, compared to Ni-base single-crystals. Concentration profiles obtained by electron microprobe and analyzed via the Scheil equation indicate distribution coefficients for Co, Al, W and Cr that are close to 1. The distribution coefficient of Ta was approximately 0.6, indicating preferential segregation to the liquid during solidification. These observations collectively suggest that convective instabilities and freckle formation are unlikely to occur during solidification of single-crystals of this new class of high strength Co-Al-W-base alloys. (C) 2011 Elsevier Ltd. All rights reserved.