Tailoring microstructure and mechanical performance of the graphite-Ni based superalloy brazed combination used for molten salt reactors through thermal exposure

The work investigated the effect of high- and medium-temperature thermal exposure on the microstructure and mechanical performance of the graphite/Au/Hastelloy N alloy brazed joint. The microstructure in the joint was studied with the aid of scanning electron microscope coupled with transmission electron microscope analysis. The nano-indentation was used to probe the nano-scale elastic modulus and hardness of reaction phases in the joint. The joint bond strength was evaluated by a shear test. Results indicate that: three zones (zone 1, 2 and 3) were observed in the joint with the zone 1 close to graphite while the zone 3 adjacent to Hastelloy N alloy. The precipitates in each zone were characterized and their formation mechanism was proposed. High-temperature thermal exposure was performed at 1333 K for 1-90 min. A longer soaking time facilitated the carbon diffusion and then produced a hierarchical structure with a more smooth transition of coefficient of thermal expansion, being favorable for the enhancement of the joint strength. Among the soaking time under investigation, the highest joint bond strength obtained was 34.1 MPa. Medium-temperature thermal exposure was carried out at 873-1073 K for 240 h. A high-quality joint was maintained when the joint was aged below 973 K. Over 973 K, a valid bonding was not achieved at the graphite/filler interface due to the precipitation of extensive carbon segregation in the zone 1 of the joint. The work performed will provide basic design principle for the graphite/Hastelloy N alloy brazed combinations used in molten salt reactors.