The Role of Carbon in Solidification of Nickel-Base Single Crystals

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9807648 Pollock Carbon additions have a beneficial effect on reducing the incidence of grain defects that develop during solidification of superalloy single crystals. This research examines the fundamental basis for this effect. The program is a collaboration between Carnegie Mellon University and General Electric Corporate Research and Development and General Electric Power Systems, and falls under the GOALI program funded by the DMR Metals Research Program and the MPS Office of Multidisciplinary Activities. The specific goals of this research on superalloys are twofold: (1) to determine the mechanisms by which carbon additions influence grain defect formation during directional solidification, and (2) to obtain an improved understanding of the range of alloy composition over which carbon serves as a beneficial addition. This research aims to develop a better overall understanding of the processes which result in the breakdown of single crystal solidification in nickel-base alloys. This is accomplished with an experimental program that examines a number of different aspects of alloy structure and chemistry that influence the solidification process. First since refractory-rich carbides precipitate in the vicinity of the liquidus temperature, carbide structure and compositions are examined as a function of alloy chemistry. Differential thermal analysis is employed to measure solidus, liquidus, and carbide solution temperatures. To examine the distribution of alloying elements in the mushy zone and to obtain measurements of fraction solid as a function of temperature, directional solidification experiments interrupted by quenching are conducted. Electron microprobe analyses are used to characterize segregation in these samples. To examine the influence of alloy chemistry on grain defect formation, single crystal directional solidification experiments are conducted with a number of compositions that have distinctly different characteristics in terms of se gregation, carbide density, and carbide precipitation temperatures. %%% This research examines defect formation in high temperature superalloy castings and relates to applications in turbine engines. Educational benefits include student interaction with General Electric Corporation laboratories. ***