Evolution of fatigue-induced cracks in Cr-coated Zr alloy cladding in high-temperature steam environment

This study investigates the crack evolution and corresponding mechanical degradation of Cr-coated Zr alloy subjected to sequential high-temperature fatigue (350 °C) and steam oxidation (1200 °C). The results demonstrate that fatigue cracks penetrate the Cr coating and extend into the Zr substrate, displaying distinct width-dependent evolution during subsequent oxidation. Specifically, wide cracks (600–1000 nm) cannot be timely filled by Cr₂O₃ scales within a short duration (600 s). The residual gaps severe as short-circuit diffusion paths for oxidizing media, triggering localized oxidation of the Zr alloy at the crack tips. Conversely, narrow cracks (200–600 nm) exhibit an autonomous “self-healing” behavior governed by coupled passive and active functional responses, where gaps are rapidly sealed by Cr₂O₃ and subsequently reduced by Zr or ZrCr₂ at the interface (active healing). The healed crack region ultimately transformed into a composite zone of Cr₂O₃ oxide, reduced Cr, and sporadically ZrO₂ stringers, effectively impeding ingress of steam. Nanoindentation tests further reveal a strong correlation between crack healing and local mechanical integrity. Unhealed regions, characterized by ZrO₂ and α-Zr(O) islands, exhibited localized hardening and reduced fracture toughness, which may cause premature fracture and failure of the fuel cladding. In contrast, healed regions maintain a more homogeneous microstructure with relatively uniform hardness, preserving mechanical durability. These findings elucidate the fatigue-oxidation coupling mechanism and underscore the governing role of crack aperture in modulating the healing efficiency. By identifying the critical width threshold for effective chemical sealing, this study provides new insights into maintaining the safety margin and structural integrity of Cr-coated claddings under extreme conditions.