Degradation of Diffusive Aluminide Coating after Service in ZhS6K Superalloy

Abstract

We considered the degradation of hollow turbine blades made of ZhS6K nickel-based superalloy after service in an aircraft engine. The blades were coated with a diffusive aluminide coating (Al-Si) to improve resistance to oxidation and hot corrosion. Turbine blades work under extreme conditions and a complex state of stress. During service, creep and fatigue occur. The interaction among hot combustion gases causes oxidation of the surface layer, hot corrosion, and micro-cracking of the coating. Moreover, changes occur in the morphology of the γ’ phase just under the coating, and transformations of the primary carbides take place. The factors limiting the lifetime of a turbine blade are the quality of the aluminide coating and the microstructure of the superalloy, depending on the service parameters (the temperature and the duration service). We found that exposure to high temperatures above the critical value for several seconds substantially decreased the engine power and its durability. We analysed the microstructure, chemical composition, and phase composition of turbine blades after service. An evaluation of the extent of degradation was performed using scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS), and electron backscatter diffraction (EBSD). The EBSD technique was used to analyse the phase composition in micro-areas, especially to identify carbides before and after transformations.