Fatigue behavior, microstructural evolution, and fatigue life model based on dislocation annihilation of an Fe-Ni-Cr alloy at 700 °C

Dislocation annihilation behavior of Sanicro 25 alloy was investigated based on various total strain amplitudes (Delta epsilon(t)/2). The slopes of cyclic stress response curves during stage II were compared and showed that obvious stable cyclic behavior occurred at Delta epsilon(t)/2 = 0.3%. Microstructure analysis reveals that the number of slip plane increased with increasing Delta epsilon(t)/2. When Delta epsilon(t)/2 = 0.3%, dislocations can slip on the slip plane (11 (1) over bar). When Delta epsilon(t)/2 increased to 0.4%, the slip planes included ((1) over bar 11) and (11 (1) over bar). The fraction of substructured grains under Delta epsilon(t)/2 = 0.3% was obviously larger than that under (Delta epsilon(t)/2) = 0.4%, which was attributed to the obvious dislocation annihilation. The deformed grains were dominant at Delta epsilon(t)/2 = 0.4%. The annihilation mechanisms of dislocations were analyzed during tensile and compressive loading based on the cyclic stress-time curves. The results showed that when Delta epsilon(t)/2 = 0.3%, during stage II, dislocation annihilation mainly occurred during compressive loading. Dislocation annihilation included annihilation caused by cross-slip of screw dislocations and annihilation of adjacent edge dislocation dipoles. Furthermore, considering the critical annihilation distance between two adjacent edge dislocation dipoles, a modified dislocation annihilation model was presented. The dislocation annihilation model was verified based on the actual annihilation rate of dislocations. Further, the relationship of microstructure and fatigue properties was discussed. Based on the dislocation annihilation mechanisms, the fatigue life model considering dislocation annihilation was established. When the ratio of (rho) over bar (-) to (rho) over bar increased, the fatigue life increased. Therefore, for cyclic hardening materials, obvious dislocation annihilation can increase fatigue life.