Evolutionary stress cycle behaviour and damage mechanisms in nickel based superalloy under thermomechanical fatigue

Both in phase and out of phase thermomechanical fatigue (TMF) tests were conducted using a computer controlled, servohydraulic uniaxial fatigue system. The TMF behaviour of nickel based superalloy K417 was investigated. The experimental results show that, in comparison with isothermal low cycle fatigue, both in phase and out of phase TMF lead to much reduced fatigue lives, although there is a difference between the in phase and out of phase cases. Creep and oxidation are the main concerns in the lower fatigue life of in phase TMF. The cyclic hardening at initial cycles results from the obstacle of gamma' precipitates to the slipping dislocations and the increase in dislocation density owing to deformation. Shearing of gamma' precipitates and loss of coherency of gamma' precipitates with the matrix at high temperatures are the reasons for cyclic softening at later stages of fatigue life.