Neutron diffraction study of strain and stress induced by thermomechanical fatigue in a single crystal superalloy

The relationship between internal stress and thermomechanical fatigue (TMF) in a Ni-based single crystal superalloy grown along the [001] axis was studied by neutron diffraction. The macroscopic internal stress along the loading axis generally increases with the TMF deformation. However, the internal stress in the precipitate gamma' phase increases more significantly than that in the matrix. phase and contributes more to the stress growth. During the TMF, the stress in the gamma' phase is larger than that of the gamma phase, thus the gamma' phase is stretched whereas the gamma phase is compressed along the loading axis. The microstrains in the gamma and gamma' phases also increase during TMF, but the trend does not follow that of the stresses. The evolutions of the internal stresses and microstrains, and the associated microstructure changes, appear to indicate that the failure by TMF is mostly caused by the residual stress concentration in the hardened gamma' phase.