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

Abstract

AbstractHigh‐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.