Solidification Behavior and Microstructure Formation in a Cast Nickel Based Superalloy: Experiment and Modeling

Abstract

The solidification behavior of a nickel based superalloy (GTD111 l/) has been determined by using a modified DTA technique which allowed us to measure the liquidus, solidus and eutectic temperatures on heating and the maximum undercooling and the local solidification time, on cooling, obtained in each sample. The concentrations of alloy elements at the center and edge of dendrite arms and in the carbide and matrix phases have been determined by WDS microanalysis, which helped us to identify the phases formed during solidification. The partitioning of each alloy element has been studied by the average ratio of concentration at the center of dendrite arm against that at the dendrite edge. The grain sizes and dendrite arm spacings at different cooling rates were measured on lightly etched sections of the samples which had been solidified in the modified DTA cooling runs while the eutectic size was measured on the polished section of the samples. An empirical relationship between the maximum undercooling and the grain size has allowed us to establish a quantitative nucleation law based on a previous analysis used in cast iron, steel, cast aluminum and aluminum aerospace alloys. A general solidification microstructure model initially developed for the binary system has been modified for a multi-component system such as nickel based superalloys. This model predicts the microstructural parameters in the castings: grain size and its distribution, and the primary and secondary dendrite arm spacings, all as functions of the cooling rate. The predictions of this model compare well with the measured results. This model should be applicable to other superalloys. t/ GTD111 is a patented nickel-based superalloy by General Electric Co. (see Ref. [ 11). Superalloys 1992 Edited by S.D. Antolovich, R.W. Stusrud, R.A. MacKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom The Minerals, Metals & Materials Society, 1992