Study on the mechanism of ' phase rafting in a 4th generation nickel-based single crystal superalloy during thermal exposure at high temperatures

Non-directional rafting of gamma' phases in high generation nickel-based single crystal superalloys happens when the alloys are thermally exposed at high temperatures, but so far the reason remains unclear. The microstructures and internal stress fields of a 4th generation single crystal superalloy during thermal exposure at 1100( degrees)C and 950(degrees)C are investigated using transmission electron microscopy and finite element analysis. It is found that the non-directional rafting of gamma' phases occurs after 1100 C-degrees/200 h and 950 C-degrees/500 h thermal exposures. The gamma' phases are mainly rodlike shape in the space after thermal exposures and a small quantity of gamma' rafts exist as well. The elastic strain energy gradient in gamma matrix caused by the inhomogeneity of matrix channel width in the alloy is most likely the driving force for the non-directional rafting. Additionally, dislocations are observed only after the thermal exposure process at 1100 C-degrees. The effect of dislocations on non-directional rafting during thermal exposure is discussed in detail in the paper.