Undesirable Effect of Local Cellular Transformation in Microstructurally-Controlled Ni-base Superalloys Subjected to Previous Damage on High Temperature Fatigue Strength and the Prevention : For Recoating and Refurbishment Technology

The microstructural change in a single crystal Ni-base superalloy, CMSX-4, was studied, in order to get basic understandings on the problems that might be serious on the process of repair and recoating of hot section components for advanced gas turbines. It was shown that a cellular γ/γ' microstructure was nucleated, when the material was subjected to damage associated with local plastic straining, followed by the re-heat treatments for damage recovery. Once the cellular microstructure was nucleated inside the material, the fatigue strength was remarkably reduced, hence the cellular γ/γ' microstructure might be a very serious hurdle to be solved. Special efforts were made to explore the method to reduce and prevent the unfavorable effect due to the cellular microstructure and to detect it nondestructively. The experiments indicated that the application of the surface modification technique, or protective metallic coating, on the superalloys, was found to be a possible method. It was also indicated indirectly that the control of parent alloy composition could be important to reduce the harmful effects.