Creep-induced microstructures in Zircaloy-4: Comparing thermal and irradiation creep

Irradiation creep is one of the most critical deformation mechanisms observed in nuclear reactor components, which results in a significant shape change during the service period. In irradiation creep, the magnitude of the shape change is typically significantly higher than under thermal creep under comparable stresses and temperatures. Hence, it is essential to consider the effect of irradiation creep for the safe and economical operation of nuclear reactors. Although numerous works have been conducted to understand the irradiation creep behaviour, many aspects are still not fully understood. The present study explores the irradiation creep behaviour using a multiscale characterization approach by combining high-resolution electron backscatter diffraction and transmission electron microscopy analysis. The microstructure that has evolved during irradiation creep is compared with two thermal creep examples, with our analysis considering the residual elastic strains, GND densities, and dislocation (also irradiation defect) microstructures. It is observed that the irradiation creep is driven by both diffusion- and dislocation-based mechanisms, with a significant diffusion creep component, owing to the radiation-enhanced diffusion rate and the abundant number of point defects produced during irradiation. On pyramidal traces, numerous localized concentrations are observed in GND density and residual strain maps, resulting from the cross-slip of dislocations and the remanent debris accumulated through the process. 3D tomography analysis further reveals the dislocation creep mechanism in Zr alloys, where dislocations emerge as small dislocation loops on pyramidal planes and multiply through cross-slip.