Study of the oxide scale integrity on Ni‐base alloy CMSX‐4 during isothermal and thermal‐cycling exposure

Abstract

AbstractThe excellent isothermal high‐temperature corrosion resistance of the single crystalline Ni‐base superalloy CMSX‐4, which is due to external alumina formation, is substantially reduced under thermal‐cycling conditions. Particularly, tapered edges exhibit a high cracking and spallation rate leading to premature onset of breakaway oxidation in combination with strong internal corrosion attack. To characterize these effects, specimens with different geometries were prepared, i.e., rectangular‐shaped and wedge‐shaped specimens. Measurements under isothermal and thermal‐cycling conditions at a temperature of 1100°C in air were carried out using a self‐designed thermobalance. By using wedge‐shaped specimens the conditions for spallation, Al depletion and eventually, breakaway oxidation in combination with internal corrosion (Al2O3, TiN and AlN) could be correlated with the change in thickness of the specimen and the duration of the high‐temperature dwell time. As compared to rectangular‐shaped specimens, wedge geometries revealed a much higher susceptibility to oxide spallation and consequently, a higher rate of aluminium depletion. An evaluation of the surface fraction affected by oxide spallation and an analysis of the spalled oxidation products were carried out in order to determine the role of dwell time duration as well as the effect of the number of thermal cycles.