Microstructural evolution and mechanical behavior of 800H alloy for control rods in HTGRs under long-term thermal exposure

The 800H alloy, a candidate material for control rods in High-temperature gas-cooled reactors, requires exceptional mechanical stability under prolonged service (650 °C) and transient accident conditions (900 °C). However, systematic studies on its long-term thermal exposure behavior remain limited. This study investigates the microstructural evolution and mechanical responses of 800H alloy subjected to thermal exposure at 650 °C for up to 10000 h, followed by transient exposure at 900 °C for 1000 h. The microstructural characteristics were examined using a range of techniques, while mechanical behaviors were analyzed through tensile, impact, and hardness tests. After 650 °C/1000 h, yield strength and hardness increase by 147.89 % ± 3.5 % and 61.25 % ± 2.5 %, respectively, compared to the solution-annealed state, while elongation and impact energy decrease by 48.44 % ± 4.0 % and 77.98 % ± 5.2 %. Mechanical properties remained stable during 1000∼10000 h of exposure. The precipitation of granular M 23 C 6 carbides at grain boundaries and within grains enhanced strength and hardness, while the transition of grain boundary carbides from discrete particles (≤300 h) to continuous films (1000–10000 h) accelerated embrittlement. After the aging of 650 °C/10000 h + 900 °C/1000 h, partial dissolution of M 23 C 6 carbides occurred, leading to 60.18 % ± 3.8 % reductions in yield strength, while impact toughness increased by 60.54 % ± 5.0 %, restoring the overall mechanical properties to levels close to the solution-annealed state. This study elucidates the correlation between carbide stability and mechanical performance during thermal exposure, providing theoretical guidance for optimizing the application of 800H alloy in High-temperature gas-cooled reactor control rods. • Intragranular carbides pin dislocations enhancing strength. • Continuous carbide films accelerate intergranular fracture. • Carbide dissolution restores ductility at high temperature. • Stable properties achieved beyond 1000h at 650 °C.