Recoating and Refurbishment of Single crystal Ni- Based Superalloys for Advanced Gas Turbines

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This work has been conducted to make clear the formation of the abnormal microstructure, or cellular transformation, resulting from the local plastic deformation in single crystal Ni-base superalloys, CMSX-2 and CMSX-4, in order to get fundamental understanding to establish the refurbishment technology for advanced gas turbine components. During the study special attention was paid to get understandings on the nucleation and growth behavior from viewpoints of crystallography and thermo dynamics. The relationship between the transformation and the actual failure damages was also explored. Furthermore, the effect of the local cellular transformation on high temperature fatigue strength was investigated.The conclusions obtained are summarized as follows :(1) When CMSX-2 and CMSX-4 were subjected to damages associated with local plastic deformation, followed by the re-heat treatments, the cellular transformation associated with γ/γ' microstructure coarsened in lamellar or equiaxed array wa … More s originated. This was not a special phenomenon, but it was actually reproduced in the material subjected to fatigue and thermo-mechanical fatigue damages.(2) The transformation was originated and developed with high anisotropy, being influenced by the following many factors : i.e., the strain field by the local plastic deformation, the crystallographic orientation, the re-heat treatment temperature and time, and the microsegregation in the material. The γ'-solvus in the reheating process drastically accelerated the local recrystallization.(3) The shearing of γ' precipitates in the straining process may play an intrinsic role in the cellular transformation : when the {111}<110> systems were activated, the formation was significant. When the {100}<110> slip systems were activated such as in creep failure at higher temperature, on the other hand, it was negligible.(4) The growth of the transformation was successfully represented by the Johnson-Mehl type equation. The growth of the transformation must be controlled by diffusion process of A1 and Ni, where activation energy was determined by about 36 kcal/mol.(5) The small fatigue crack propagation rate was remarkably enhanced at the transformed region. Less