Mechanistic Modeling of Cyclic Softening and Slip Localization in Ni-Based Superalloys

Ni-based superalloys are typically strengthened with ordered fcc \( \gamma^{\prime} \) precipitates (L1₂ structure) which impart a number of hardening mechanisms including anomalous hardening due to cross slip from {111} to {010} planes. On the other hand, the presence of shearable \( \gamma^{\prime} \) precipitates promotes cyclic softening and slip localization. The onset of cyclic softening and slip localization processes in \( \gamma^{\prime} \)-strengthened Ni-based superalloys is still poorly understood and has not been modeled. In this investigation, the salient mechanisms responsible for cyclic softening and slip localization in Ni-alloys containing \( \gamma^{\prime} \) precipitates are reviewed and the pertinent information is utilized to develop mechanistic models for predicting the cyclic response, slip localization, and fatigue life of this class of engineering alloys. Mechanistic modeling reveals that cyclic softening and slip localization can be attributed to three processes: (1) stable expansion of superkinks with subcritical kink heights, (2) shearing, and (3) bowing of \( \gamma^{\prime} \) precipitates. The onset of cyclic softening commences in the microplastic regime and occurs when the accumulated shear strain along the operative {111} plane exceeds the average \( \gamma^{\prime} \) size. The roles of cyclic softening and slip localization in reducing high-cycle fatigue strength and fatigue life are elucidated and discussed.