Cyclic-Loading Induced Lattice-Strain Asymmetry in Loading and Transverse Directions

Cyclic-loading effects on a nickel-based superalloy are investigated with in-situ neutron-diffraction measurements. The temperature evolution subjected to cyclic loading is estimated based on the lattice-strain evolution. The calculated thermoelastic responses are compared with the measured bulk temperature evolution. Two transitions in the temperature-evolution are observed. The first transition, observed with the neutron-measurement results, is associated with the cyclic hardening/softening-structural transformation. The second transition is observed at a larger number of fatigue cycles. It has a distinct origin and is related to the start of irreversible structural transformations during fatigue. A lattice-strain asymmetry behavior is observed. The lattice-strain asymmetry is quantified as a grain-orientation-dependent transverse/loading parameter. This strain-asymmetry evolution reveals the irreversible plastic deformation subjected to fatigue. The irreversible fatigue phenomena might relate to the formation of the microcracks. At elevated temperatures, the cyclic hardening/softening transition starts at lower fatigue cycles as compared to room temperature. A comparison between the room-temperature and the elevated-temperature fatigue experiments is performed. The asymmetry-parameter evolutions show the same irreversible trends at both the room and elevated temperatures.