A New Concept Coating to Prevent the Fatigue Life Reduction due to Cellular Microstructure in CMSX-4 Subjected to Previous Damage -For Refurbishment Technology of Single Crystal Ni-Base Superalloys-
Masakazu OKAZAKI, Issei OHTERA and Yoshio HARADA
Abstract:Cellular microstructure, or colony, in a single crystal Ni-base superalloy, CMSX-4, that might be formed relating to refurbishment and recoating process has been studied. At first it was shown that this microstructure was frequently originated, when the material experiences a local plastic strain, followed by the re-heat treatments for damage recovery. This phenomenon is not special, but often the case induced by many kinds of processes: e.g. during the in-service period and the fabrication, refurbishment and recoating processes. Once this kind of cellular transformation was formed, the fatigue strength was found to be remarkably reduced. In order to prevent the above problem, two methods were explored: one was a simple preheat treatment that was aimed to release strain energy stored due to the local plastic straining; the second was a new method employing coating technique, based on the new concept that the alloy elements that might be too lacking in base alloys to endure the undesirable effect should be supplemented from the overlay coating layer. It was shown by the fatigue tests that the former was not successful, but the latter might be very effective. The observations of the fatigue crack initiation site, the fracture mode, the crack density in the cellular transformed area, and the crack propagation morphologies near the prior interface, strongly supported the validity of the latter method. The method is expected not only to be a countermeasure against the present problem but also to open a new road as a damage cure coating. Key Words:Refurbishment, Single crystal Ni-base superalloys, Damage, Cellular microstructure, Reduction of fatigue strength, Prevention, Stress shielding, Damage cure coating, Grain roundary strengthening elements