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The influence of tin particles on the nucleation of spurious grains during single crystal casting of superalloys Wang, Tao

Abstract

Spurious grains are defects that arise in single crystal turbine blades of superalloys. Because they can act as favorable locations for crack initiation and in-service failure, they often lead to blades being rejected. Although spurious grains in single crystal castings may nucleate by many mechanisms, strong but indirect evidence exists that the presence of nitride and oxide inclusions can reduce the amount of undercooling necessary for grain nucleation and hence increase the formation of spurious grains. In this project, this hypothesis, in regard to nitride inclusions, was tested directly by experiments. In the experiments, the nucleation of spurious grains was "turned on" by providing nitride inclusions at the liquidus temperature, in alloys where the oxide content is constant and known. Solidification front undercooling was varied by changing the solidification velocity in order to estimate the critical undercooling for nucleation. A thermal model of the single crystal casting process was developed to analyze the thermal history of the casting process. Equiaxed grains and columnar to equiaxed transitions (CET) were observed in alloys with a nitrogen content much higher than the saturation solubility, in those cases where the solidification front undercooling was in excess of the critical undercooling. Experiments were done on samples with a simple shape. Dendrite tip undercoolings were estimated by using the well-known KGT model using the pseudo binary approximation. The influence of nitrogen content on the nucleation kinetics was quantitatively studied based on theories given in the literature.

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