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

Non-directional rafting of γ' 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℃ and 950℃ are investigated using transmission electron microscopy and finite element analysis. It is found that the non-directional rafting of γ' phases occurs after 1100℃/200h and 950℃/500h thermal exposures. The γ' phases are mainly rodlike shape in the space after thermal exposures and a small quantity of γ' rafts exist as well. The elastic strain energy gradient in γ 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℃. The effect of dislocations on non-directional rafting during thermal exposure is discussed in detail in the paper.