Lifing the thermo-mechanical fatigue (TMF) behaviour of the polycrystalline nickel-based superalloy RR1000

Microstructural damage and subsequent failures resulting from thermo-mechanical fatigue (TMF) loading within the temperature range 300-700 degrees C are investigated for the polycrystalline nickel superalloy, RR1000. Strain controlled TMF experiments were conducted over various mechanical strain ranges, encompassing assorted phase angles, using hollow cylindrical test pieces. The paper explores two scenarios; the first where the mechanical strain range is held constant and comparisons of the fatigue life are made for different phase angle tests, and secondly, the difference between the behaviour of In-phase (IP) and -180 degrees Out-Of-Phase (OOP) tests over a variety of applied strain ranges. It is shown that different lifing approaches are currently required for the two scenarios, with a mean stress based approach being more applicable in the first case, whereas a Basquin-type model proves more appropriate in the second.