Influence of Long Term Exposure in Air on Microstructure, Surface Stability and Mechanical Properties of UDIMET 720LI

Abstract

Alloy Udimet 720LI was designed for high temperature application of aircraft-engine disks up to about 730°C. Operating at such high temperatures for long times raises the question of thermodynamical stability of the alloy (bulk stability) and surface integrity. Subject of the present paper was to investigate the influence of long time exposure up to 1000 hours at high temperatures up to 845°C in air on microstructure and phase stability as well as on surface integrity. It could be shown by appropriate microscopy (LM, SEM, TEM) that coarsening of the y’-phase occurred with increasing time of exposure at temperatures of 760°C and 845°C. Additional formation of topological closed packed (TCP) phases (0) could be observed. It was found that this lack of stability exhibited an impact on mechanical properties such as tensile strength, creep resistance, and low cycle fatigue strength, especially when a dwell time was superimposed. Furthermore, the reaction of the alloy in the near-surface area of the specimen exposed to air at high temperatures with superimposed stresses was investigated by light microscopy (LM) and SEM. A quantitative analysis by EDX investigation in a SEM of the thus formed surface oxide layer and the subsurface area with a depletion of alloying elements was performed.