Aspects of thermomechanical fatigue of two single crystal nickel based superalloys

Abstract

Thermomechanical fatigue (TMF) behaviour of two third generation single crystal superalloys, TMS-75 and TMS-113, has been investigated. The two alloys had similar chemical composition with a γ ′ volume fraction of 60% (TMS-75) and 70% (TMS-113). It was shown that the lifetime of TMF for specimens with a hold period in the compression phase decreased severely by an order of magnitude compared with those without a hold period. Compared with TMS-75, TMS-113 superalloy showed superior creep behaviour at 1173 K, but inferior TMF behaviour between 673 Kand 1173 Kunder out of phase conditions. Microstructural studies by electron microscopy were performed to reveal the deformation mechanisms. Microstructures developed during TMF differed for the two superalloys, probably due to the difference in γ ′ volume fraction. Compressive stress relaxation behaviours also differed, which playeda keyrole in the different maximum stresses and rupture lives. Results indicate that a balance between creep and TMF properties must be made by changing the volume fraction of γ′ phase when designing new superalloys.