Synergetic Effect of Irradiation and Helium on Microstructure and Mechanical Property of Eurofer97 Steel

Eurofer97 reduced-activation ferritic/martensitic (RAFM) steel stands as a primary structural candidate for future fusion reactors. However, ensuring its long-term structural integrity under the synergistic effects of high-energy neutron bombardment, substantial displacement damage (dpa), and high rates of helium (He) generation remains a formidable challenge. This study presented a systematic investigation into the microstructural evolution and mechanical property degradation of Eurofer97 steel irradiated within the Swiss Spallation Neutron Source (SINQ) STIP-Ⅲ program. Specimens were subjected to a broad irradiation temperature window ranging from 150 ℃ to 530 ℃, achieving accumulated doses up to 20 dpa and helium concentrations reaching 1 740 appm. Detailed transmission electron microscopy (TEM) characterization revealed distinct, temperature-dependent regimes of defect evolution. In the low-temperature regime (＜300 ℃), the microstructure is dominated by a high density of black dot clusters and small dislocation loops, which act as primary obstacles to slip. Conversely, at elevated temperatures (＞400 ℃), these conventional displacement defects are largely annealed out. Instead, helium bubbles emerge as the predominant microstructural feature, driven by the high transmutation rates. Quantitative analysis indicates that the mean bubble size increased from 1.17 nm to 2.49 nm as irradiation temperatures rose from 235 ℃ to 511 ℃, while bubble density exhibits a non-monotonic trend of initial increase followed by coarsening-induced reduction. Notably, at 511 ℃, significant bubble accumulation at grain boundaries and adjacent denuded zones (approximately 20-30 nm) is observed, highlighting substantial helium migration. Tensile testing demonstrates significant irradiation hardening across all conditions, yet the underlying physical mechanisms shift with temperature. Low-temperature hardening is governed by defect clusters, leading to dislocation channeling, strain localization, and ductility exhaustion. In contrast, high-temperature hardening is controlled by the dispersed barrier effect of high-density helium bubbles. The recovery of uniform strain hardening behavior in high-temperature specimens suggests that bubbles larger than 0.8 nm function as strong, thermally stable obstacles to dislocation motion. Based on the dispersed barrier hardening (DBH) model, the strength parameter \alpha for helium bubbles in the 1-2 nm size range was calculated to be approximately 0.14-0.18. Furthermore, fractographic analysis reveals a critical shift from ductile dimple rupture to brittle intergranular fracture in specimens containing high helium concentrations (＞1 000 appm) irradiated above 400 ℃. This transition confirms that helium segregation severely compromises grain boundary cohesion, inducing high-temperature helium embrittlement. These findings establish a direct physical correlation between microstructural helium evolution and macroscopic failure modes, providing essential experimental data for validating lifetime prediction models for fusion reactor blanket components.