Characterisation and modelling of crystal rotations during multiaxial creep in single crystal superalloys

Life prediction procedures for single crystal superalloys are largely based on the interpretation of uniaxial creep and low cycle fatigue data although turbine blades experience significant multiaxial stresses in service. Several models that have been proposed to represent the anisotropic creep of single crystal superalloys have the potential of being extended to multiaxial loading. It is appreciated that uniaxial stresses in non-symmetric orientations cause crystal rotations during creep. It is less well known that multiaxial stressing of symmetric single crystals can also lead to large crystal rotations that can be spatially heterogeneous. In this study, creep of the single crystal nickel-based superalloy CMSX-4 with <001>, <111> and <011> nominal orientations has been studied at 850degreesC on cylindrical specimens with circumferential notches and net-section stresses between 600 and 850 MPa. The predictions of crystal rotation resulting from creep deformation are compared,with the experimental results as a contribution to validating the model.