Formation of nano-sized MC carbides in Si-free GH3535 alloy.

GH3535 alloy is one of the most promising structural materials for molten salt reactors (MSRs). Its microstructure is characterized by equiaxed grains and coarser primary MC carbide strings. In this study, stable nano-sized MC carbides were obtained in GH3535 alloy by the removal of Si and thermal exposure at 650 °C. Nano-sized MC carbide particles precipitate preferentially at grain boundaries during the initial stage of thermal exposure and then spread all over the grain interior in two forms, namely, arrays along the {1 1 1} planes and randomly distributed particles. The precipitate-free zones (PFZs) and the precipitate-enriched zones (PEZs) of the MC carbides were found to coexist in the vicinity of the grain boundaries. All MC carbides possess one certain orientation relationship (OR) with the matrix. Based on microstructural characterizations, the formation process of MC carbides with different morphologies was discussed. The results suggested that the more-stable morphology and OR of MC carbides in the Si-free alloy provide higher hardness and better post-irradiation properties, as reported previously. Our results indicate the preferential application of Si-free GH3535 alloy for the low-temperature components in MSRs.