Degradation of Microstructure after Service in ZhS6k Superalloy with Diffusive Aluminide Coating

Abstract

This paper presents the degradation of hollow turbine blades made of ZhS6K nickel-based superalloy after service in DV2 aero engines. The blades were coated with a diffusive aluminide coating (Al-Si) to improve the oxidation and hot corrosion resistance. Turbine blades work under extreme conditions and a complex state of stress. During service, creep and fatigue of various natures take place. The interaction among hot combustion gases causes oxidation of the surface layer and hot corrosion and micro cracking of the coating. Moreover, changes in morphology of the γ’ phase just under the coating and transformations of the primary carbides take place. The factors limiting the lifetime of turbine blades are the quality of the aluminide coating andmicrostructure of the superalloy, depending on the service parameters—the temperature and the duration of service. It was found that exposure to high temperatures above the critical value for several seconds substantially decreased the engine power and its durability. In this paper, analysis of the microstructure, chemical composition and phase composition of the turbine blades after service were carried out. Evaluation of the extent of degradation was performed using the scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS) and electron backscatter diffraction (EBSD) techniques. The EBSD technique was used to analyse the phase composition in micro areas, especially to identify carbides before and after transformations.