Crystallographic evolution and cyclic softening behavior of reduced activation ferritic-martensitic steel under different fatigue modes

The cyclic softening behavior of reduced activation ferritic-martensitic (RAFM) steel was investigated by low-cycle fatigue tests with and without dwell times at 550 °C, and a mechanistic understanding of the softening was achieved by analyzing the grain boundaries and crystal orientation in the grains using the electron backscatter diffraction (EBSD) technique. The cyclic stress response of the steel was divided into two distinct rate stages with the number of cycles, namely, the initial rapid stage and the uniform softening stage. The former stage was dependent on and the latter was independent of the input strain amplitudes and fatigue modes. The softening in the two stages was led by both the disappearance of lath and packet boundaries and the release of lattice distortion by dislocations in the initial RAFM steel; their relative contributions in each stage were different depending on the input strain ranges and fatigue modes. A new equation was proposed as a measure of the lattice distortion in grains, which showed a strong correlation with the amount of cyclic softening under different fatigue conditions.