Thermo-mechanical fatigue of a polycrystalline superalloy: The effect of phase angle on TMF life and failure

As part of the project thermo-mechanical fatigue: the route to standardisation, the behaviour of the Ni-based superalloy, Nimonic 90, has been investigated over a range of phase angles. This refers to the phase angle between non-complex strain and temperature cycles during TMF. In accordance with the other investigations an ‘out of phase’ (OP, 180°) test with a strain range of ±0.4% and a temperature range of 400–850 °C was used as the baseline. Further tests at 180 ± 10° and 180 ± 20° together with in phase (IP, 0°) tests and an intermediate counter clockwise diamond shape test (CCD, 270°) were also performed. All tests were done according to the draft code of practice developed within the project. The TMF life of the tests with phase deviations of 180 ± 10° lie within the one standard deviation of those of the reference specimens. Further increasing the phase shift to 180 ± 20° had no significant effect on the TMF life of this superalloy. The tests did, however, show significant asymmetry with the 180 + 20° phase test showing increased high temperature inelastic deformation compared to the 180 − 20° phase test. All tests investigating the effect of phase angle were performed at a strain range of ±0.4%. At this strain, the TMF lives of the OP specimens were, on average, four times longer than those of the IP specimens. Decreasing the mechanical strain range to ±0.3% increased the TMF life of the IP specimens by a factor of 4.