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In order to get fundamental understanding to establish a refurbishment technology for advanced gas turbine components, the cellular formation associated with a y/y microstructure coarsened in lamellar or equiaxed arrays in single crystal Ni-base superalloys, CMSX-2 and CMSX-4, have been studied, supposing the case in which they were previously subjected to a damage associated with local plastic deformation, followed by a re-heat treatment. During this study special attention was paid to understand the nucleation and the growth of the transformation from viewpoints of crystallo-plasticity and therm0 dynamics. The experimental evidence indicated that the transformation originated and developed with high anisotropy, being influenced by the following factors: the strain field produced by local plastic deformation, the crystallographic orientation, the re-heat treatment temperature and time, and the microsegregation in the material. It was shown that the transformation was reproduced in material previously subjected to fatigue and thermo-mechanical fatigue damage, in which the shearing of y precipitates resulting from the activation of { 11 l}4fO> slip systems was significant. It was shown that diffusion was controlling the growth rate of the transformation, accompanied with an activation energy of 36 kcal/mol. Furthermore, the effect of the local cellular transformation on the high temperature small fatigue crack propagation was quantified experimentally. supeml10ys 2oc0 Edited byT

The microstructural changes in a single-crystal Ni-base superalloy, CMSX-4, that might occur during the processes of repair and recoating of hot section components for advanced gas turbines were studied. It is shown that the cellular ␥/␥Ј microstructure is formed when the material is subjected to local plastic straining, followed by the reheat treatments during the course of damage recovery. The formation of cellular microstructure in the material led to the remarkably reduced fatigue strength. In order to reduce or prevent the preceding undesirable effect resulting from cellular microstructure, a new method based on applying overlay coating technique was developed. The method is based on an idea that the alloying elements that are depleted in base alloys could be supplemented via the overlay coating. An X alloy, which contains grain boundary strengthening elements, was selected and coated on the CMSX-4 with the cellular microstructure by low-pressure plasma spraying. The fatigue tests on the coated CMSX-4 specimens demonstrated the effectiveness of the method. The observations of the crack initiation site, the fatigue fracture mode, the crack density in the cellular transformed area, and the crack propagation morphologies near the prior interface strongly supported the validity of this approach. The method is expected to build a road to a so-called damage cure (or recovery) coating.

Recently considerable attention has been paid to large utility gas turbines, because of their high ef®ciency and many other advantages. This accelerates the introduction of advanced superalloys and coatings. While these technologies are traditionally from aero turbines, many novel challenges have arisen in their application, e.g. long operating time, weight, cyclic duty, environment using cheaper fuel, and size. This paper describes the mechanical properties and failures of the superalloys and the coatings used at the hot sections of utility gas turbines. Special focus is put on thermo-mechanical fatigue failure, and the collaborative test results by the Subcommittee of ªSuperalloys and Coatingsº in The