Investigations into the surface strain/stress state in a single-crystal superalloy via XRD characterization

The present study was aimed at determining the surface strain/stress state in an Ni-based single-crystal (SC) superalloy that was subjected to two different cooling rates from solid solution temperature through using the X-ray diffraction (XRD) method. The normal stresses σ^s₁₁ and σ^s₂₂ were determined, then the Von Mises stresses (σ^s VM) were derived from them. Field emission gun scanning electron microscope (FEG-SEM) and transmission electron microscope (TEM) micrographs were taken to illustrate the strain/stress state change. The precipitation of the secondary γ 0 phases in the γ phase and the formation of the dislocation in the interphase upon a slower cooling rate caused the γ phase lattice distortion to increase, so a larger σ^s_VM of the γ phase was realized in comparison to the faster cooling sample. For both of the two cooling modes, we found that the σ^s_VM of the γ 0 phase increased due to the growth of the γ 0 phase during the aging process. Also, the aging process led to pronouncedly anisotropic lattice mismatches in the {331} and {004} planes. In addition, the surface strain/stress states of a cylinder sample and a tetragonal sample were also studied using a faster cooling rate, and σ^s₁₁ and σ^s₂₂ were analyzed to explain the influence of the shape factor on the stress anisotropy in the [001] and 110 orientations. The strain in the [001] orientation of the γ phase is more sensitive to the shape change.