On the plastic deformation of a CoCrFeNiW-C alloy at elevated temperatures: Part II. Grain boundary sliding and damage mechanisms

To complement the dislocation-driven serration mechanisms studied in Part I, the present work aims to deepen the fundamental understandings of grain boundary-mediated deformation phenomena, emphasizing their con-tributions to damage development in the CoCrFeNiW-C alloy at elevated temperatures. In situ scanning electron microscope-based tensile tests were carried out in the 650-750 degrees C temperature range using nominal strain rates of 10-3 and 10-4 S-1. Evident intergranular fracture took place at 700 and 750 degrees C under a strain rate of 10-4 S-1, followed by a serrated-to-stable plastic flow transition. Grain boundary damage evolution processes were quantified in detail based on the in situ observations. By combining theoretical calculations and in situ interrupted test using surface micro-grid lines, a Rachinger-type of grain boundary sliding mechanism is confirmed at 750 degrees C under a 10-4 S-1 strain rate, which enables plastic strain accommodation but plays the critical role in acceler-ating grain boundary damage nucleation. Deformation substructural studies evidence the involvement of dislocation cross-slip in accommodating grain boundary sliding. Based on the testing conditions and the experimental observations, a series of deformation micro-events are identified and their roles in damage nucleation are explored.