Hefei Huang

Pressure-boundary components in molten salt reactor systems experience repeated thermal-mechanical transients during start-up, shutdown, load changes, and local temperature-gradient events. For these components, low-cycle creep-fatigue interaction can reduce structural integrity through the combined effects of cyclic strain, tensile dwell, stress relaxation, and time at temperature. In this study, strain-controlled low-cycle fatigue and creep-fatigue interaction tests were conducted in air at 700 °C using strain amplitudes of 0.2%-0.5% and tensile hold times of 60-1800 s, together with complementary creep and oxidation-kinetics tests. The fatigue life of GH3535 alloy decreased systematically with increasing strain amplitude and hold time. Higher strain amplitudes promoted multiple crack-initiation sites through intensified slip-band activity, whereas longer hold times accelerated crack propagation and favored a mixed intergranular-transgranular fracture path. For engineering assessment, a modified strain energy density exhaustion model (M-SEDE-O) was developed by combining a revised stress-relaxation equation with an oxidation damage term derived from oxide-growth kinetics and oxygen-concentration profiles. Within the present dataset, the proposed model placed most predictions within a 1.5-fold error band and reduced the root mean square error to 187.48, compared with 312.93 for M-SEDE and 440.15 for W-SEDE. This closer agreement can support more consistent preliminary screening of pressure-boundary thermal-transient histories and life-management priorities within the calibrated domain. The proposed framework provides an engineering approach for creep-fatigue life assessment of GH3535 alloy under high-temperature service conditions for pressure vessels and piping within the molten salt reactor system.

This study reveals that localized recrystallization occurs during creep of solution-annealed polycrystalline GH3535 Ni-based alloy at 650 ℃ and is consistently observed over the investigated stress range of 250–330 MPa. Electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and scanning transmission electron microscopy (STEM) were combined to systematically characterize the stage-dependent evolution of the local microstructure during creep. The local microstructure progressively evolved from dislocation accumulation and continuous lattice-orientation variation to the formation of dislocation walls and subgrain boundaries, followed by the development of high-angle grain boundaries (HAGBs) and recrystallized grains. These observations identify continuous dynamic recrystallization (CDRX) as the predominant mechanism of localized recrystallization. Localized CDRX introduces new HAGBs into the interiors of the original grains, thereby extending the interfacial network beyond the pre-existing grain-boundary network. The newly formed HAGBs show clear spatial associations with microvoids, microcracks, and local crack-path deflection, demonstrating that CDRX-generated HAGBs directly participate in late-stage creep damage evolution.

Silicon carbide (SiC) coatings are essential for nuclear graphite in molten salt reactors due to their ability to suppress molten salt infiltration. However, the role of substrate roughness in regulating interfacial adhesion remains unclear. In this study, SiC coatings were deposited via chemical vapor deposition on nuclear graphite substrates with controlled roughness (Ra = 6.2, 1.0, and 0.4 μm), and the effects of roughness on microstructure and adhesion were systematically evaluated. A nonlinear dependence of coating performance on surface roughness was identified. Moderate roughness (Ra = 1.0 μm) yielded dense and well-crystallized coatings with smaller and more uniform grain aggregates (~9.45 μm) and minimal interfacial defects, resulting in the highest adhesion strength (Lc₁ = 62.2 N) and superior thermal shock resistance. In contrast, excessive roughness induced irregular aggregates and infiltration-related defects, leading to premature interfacial failure, whereas overly smooth surfaces weakened mechanical interlocking and physical adsorption, thereby reducing adhesion strength. A coupled mechanism involving mechanical interlocking, chemical diffusion, and stress dispersion was elucidated, demonstrating that optimal roughness governs interfacial integrity. These findings provide critical guidance for substrate pretreatment of SiC coatings in nuclear applications.

Reliable creep-fatigue life assessment of GH3535 superalloy for molten salt reactor applications remains challenging because conventional linear damage summation models do not fully capture oxidation-assisted degradation and nonlinear stress relaxation during tensile hold periods. In this study, strain-controlled low-cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests were conducted in air at 700 °C using strain amplitudes of 0.2%-0.5% and tensile hold times of 60-1800 s, together with complementary creep and oxidation-kinetics tests. The fatigue life of GH3535 decreased systematically with increasing strain amplitude and hold time. Higher strain amplitudes promoted multiple crack-initiation sites through intensified slip-band activity, whereas longer hold times accelerated crack propagation and favored a mixed intergranular-transgranular fracture path. Based on these observations, a modified strain energy density exhaustion model (M-SEDE-O) was developed by combining a revised stress-relaxation equation with an oxidation damage term derived from oxide-growth kinetics and oxygen-concentration profiles. Within the present dataset, the proposed model placed most predictions within a 1.5-fold error band and reduced the root mean square error to 187.48, compared with 312.93 for M-SEDE and 440.15 for W-SEDE. The proposed framework provides an engineering approach for creep-fatigue life assessment of GH3535 under high-temperature service conditions relevant to molten salt reactors.