An understanding of irradiation-assisted stress corrosion cracking of heavy ion-irradiated 321 stainless steel in high temperature and high pressure water

The mechanism of irradiation-assisted stress corrosion cracking (IASCC) in 321 stainless steel, an important reactor core material, remains unclear. IASCCs from slow strain rate tensile test in high-temperature and high-pressure water after heavy-ion irradiation were analysed. The irradiation damages are consistent with those from neutron irradiation at similar dose, confirming heavy ion is effective for IASCC study. IASCC susceptibility increases with dose, and local-deformation remains the primary driving factor. Radiation-induced segregation and oxidation under tensile stress synergistically initiate IASCCs. Pits induced by Si effect and the corrosion of γ-phase by Ni-Cr effect are important for the initiation of transgranular cracks. • Heavy-ion irradiation induces similar damage defects (dislocation loop and RIS) in SS as neutron irradiation at similar doses. • Local deformation, oxidation (including grain boundary oxidation and matrix oxidation) and radiation-induced segregation interacted synergistically to initiate stress corrosion cracking in heavy ion irradiated 321 SS. • Brittle Fe-Ni spinels and defect structures at grain boundaries directly contributes to the initiation of IGSCC when 321 SS is subjected to tensile stress, attributing to the radiation-induced segregation, oxidation and dissolution of Ni and Si at grain boundaries. • Intergranular stress corrosion cracks propagate in a periodic process of the migration of oxide tip along grain boundaries, the formation of new oxide forefront and cracking under tensile stress. • At the end of the heavy ion irradiation region, the irradiation can still promote the initiation of IASCC, but the effect diminish as the distance from the peak damage area increases. When IASCCs propagate from the irradiated to unirradiated region, wherein Ni oxide is transformed into Cr oxide at the crack tip, accompanied by a double-layer structure consisting of Cr oxide and (Ni, Cr) oxide as the transition zone. • Irradiation induces local enrichment of Si within intragranular. The defect structures formed by the dissolution of SiO x create pits under tensile stress and irradiation-electrochemical effect, with stress concentration at the bottom. In combination with the corrosion effect of CrO x on the γ-phase, TGSCC is initiated.