In-service degradation of metallurgical and mechanical properties of aluminized coatings and substrates in gas turbine blades

Abstract

In-service degradation of metallurgical and mechanical properties of aluminized CoCrAlY coatings and Ni-base superalloy substrates in advanced gas turbine blades has been studied. The aluminized coatings of the unexposed and in-service exposed blades consisted of four layers with different microstructure and chemical composition. In-service environmental attack led to the deposition of Fe oxides on the top aluminized coating and formation of a thin-layered Al2O3. While in-service, Ni diffused extensively from the substrate into the near-surface coating region. The interdiffusion of Co/Ni resulted in the formation of Al/Ni rich precipitates in all the coating regions, except a near-surface coating region indicating Cr rich precipitates. A number of Cr rich precipitates were found in the substrate near the interdiffusion zone. The near-interface coating region and substrate softened at room and elevated temperatures. The ductility and low cycle fatigue life of the internal coating region at room temperature was not degraded. However, the ductility of the internal and near-interface coating regions and substrate at elevated temperatures was substantially degraded. In-service mechanical degradation of the aluminized CoCrAlY coating is discussed in light of the metallurgical evolution.