Thermo-mechanical fatigue of the nickel-base superalloy nimonic 90

The then-no-mechanical fatigue (TMF) behaviour of the Nimonic 90 Nickel base superalloy has been investigated within two laboratories. In-phase-tests (IP) where the maximum mechanical strain occurs at the maximum temperature (850 degrees C), and 180 degrees-out-of-phase-tests (180 degrees OP) where the maximum mechanical strain coincides with the minimum temperature (400 degrees C) have been applied. All tests were carried out at varying mechanical strain ranges with a constant strain ratio of R epsilon = center dot 1. A temperature rate of 5 K/s was used throughout the whole cycle without any additional cooling system during decreasing temperature. The fatigue life of 180 degrees OP tests is longer compared to identical IP tests. The stress/mechanical strain hysteresis loops are completely different and some characteristic values are compared to each other. The fracture surfaces observed show that fatigue crack (or cracks) starts on the external surface and propagates inwards. The fractures of 180 degrees OP tests are transgranular showing the presence of fatigue striations, while the fractures of IP tests are mixed transgranular and intergranular with no fatigue striations.