Mechanisms of Fatigue Failure of Nickel-Base Alloysat Room and Elevated Temperatures in the Very High Cycle Regime

摘要

Fatigue failure at very high number of cycles (N > 10~7) does not follow the same principles of damage mechanism as is observed in the range of conventional fatigue limit. With the applied global strain amplitude being well below the elastic limit, life is dominated by heterogeneously distributed and localized plastic deformation in the microstructure. Hence, the dislocation/particle interaction in precipitation hardened alloys, such as Nimonic 80A, plays a major role during VHCF cyclic deformation. The nickel-base superalloy Nimonic 80A was tested in the peak-aged and overaged condition with spherical agglomerated precipitates and cuboidal regularly distributed precipitates, respectively. Under room temperature conditions planar single slip and at elevated temperatures partially wavy dislocation arrangements were found in failed and in run-out samples. The overaged Nimonic 80A showed a slightly superior fatigue behaviour compared to the peak-aged condition. Isothermal cyclic deformation at elevated temperatures revealed a pronounced decrease of cyclic life at 800°C for the overaged condition.