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 degrees) test with a strain range of +/- 0.4% and a temperature range of 400-850 degrees C was used as the baseline. Further tests at 180 +/- 10 degrees and 180 +/- 20 degrees together with in phase (IP, 0 degrees) tests and an intermediate counter clockwise diamond shape test (CCD, 270 degrees) 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 degrees lie within the one standard deviation of those of the reference specimens. Further increasing the phase shift to 180 +/- 20 degrees had no significant effect on the TMF life of this superalloy. The tests did, however, show significant asymmetry with the 180 + 20 degrees phase test showing increased high temperature inelastic deformation compared to the 180 - 20 degrees 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. (C) 2007 Elsevier Ltd. All rights reserved.