Enhancement of thermomechanical fatigue resistance of a monocrystalline nickel-base superalloy by pre-rafting

In gamma'-hardened monocrystalline nickel-base superalloys having a negative gamma/gamma' lattice misfit, the prior introduction of so-called gamma/gamma' rafts aligned parallel to the stress axis (by a small creep pre-deformation in compression) has been shown to enhance both the high-temperature isothermal fatigue strength and the tensile creep resistance. It was hence of interest to perform a systematic study of the thermo-mechanical fatigue (TMF) behavior of specimens of a monocrystalline nickel-base superalloy with the main aim to test whether an initial gamma/gamma' raft microstructure can also enhance the TMF resistance. The experiments were performed on [001]orientated monocrystalline specimens of the superalloy SRR 99 which contained either the initial gamma/gamma'-microstructure with cuboidal gamma' precipitates or a gamma/gamma' raft microstructure aligned roughly parallel to the stress axis. The latter was introduced by a small compressive creep deformation of less than 0.4% at a temperature of 1050degreesC and a stress of 120 MPa. Different strain-controlled TMF cycle forms were employed. However, in most tests a counter-clockwise-diamond (CCD) cycle (temperature interval DeltaT = 600degreesC-1100degreesC) was used with a total strain rate of 6.67(.)10(-5) s(-1). This CCD cycle is considered to be close to service conditions. The mechanical strain range Deltaepsilon(m), was varied in the range of Deltaepsilon(mech) = 1%, leading to fatigue lives in the order of some 1000 cycles. It could be shown that pre-rafting in compression enhances the TMF fatigue life significantly for all cases of CCD-cycles investigated.