IV - Influence de l’environnement sur la propagation des fissures en fatigue dans un superalliage base nickel

Effect of environment on creep-fatigue crack propagation in nickel-base superalloys

Abstract

The development of high temperature resistent nickel base alloys for turbine discs necessitates a better understanding of creep-fatigue-environment interactions which govern their durability.

Up to 650°C, it is well established that the environment plays a predominent role in the acceleration of fatigue crack growth, oxidation being the main factor at the origin of time-dependent effects. In contrast, creep-fatigue behaviour at higher temperatures still remains poorly documented. The work reported here provides some results on the effects inducted by environment on crack growth established on a PM Astroloy cycled in air and in vacuum at 650°C and 750°C. Results obtained in air and in vacuum at 650°C show a strong influence of environment in accordance with previous published data concerning Astroloy tested in similar conditions. At the opposite, complex mechanical and environmental interactions have been releaved at 750°C temperature at which the da/dN-ΔK relationship obtained in air appears to be highly dependent of the initial ΔK values applied at the beginning of the creep-fatigue test.