ASPECTS OF HIGH-TEMPERATURE LOW-CYCLE THERMOMECHANICAL FATIGUE OF A SINGLE CRYSTAL NICKEL-BASE SUPERALLOY

High-temperature thermomechanical fatigue (TMF) tests were performed on monocrystalline specimens of the γ′-precipitate-strengthened nickel-base superalloy CMSX-6. The specimens had axial orientations near 001]. In-phase, out-of-phase, clockwise-diamond and counter-clockwise-diamond TMF tests were conducted in total strain control in symmetric push-pull. The temperature interval extended from 873 to 1373 K, the total strain amplitude was 0.5% with a cycle time of 300 s. The stress response, the plastic strain amplitudes and the fatigue lives were measured. In addition, fatigue-induced micro-structural changes were investigated. The shapes of the hysteresis loops and the mechanical TMF behaviour varied markedly with the shape of the strain-temperature cycles. The TMF life of the alloy was found to correlate with the stress amplitude in the tensile half-cycle. The evolution of the microstructure shows the formation of dislocation networks around the precipitates and in particular, severe changes in the size and the morphology of the γ'-precipitate structure. These changes vary with the strain-temperature cycle shape. Failure occurred exclusively by fatigue (and not creep) in the form of localized shear.