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 M₆C carbide strings. In this study, stable nano-sized M₂C carbides were obtained in GH3535 alloy by the removal of Si and thermal exposure at 650 °C. Nano-sized M₂C 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 M₂C carbides were found to coexist in the vicinity of the grain boundaries. All M₂C carbides possess one certain orientation relationship (OR) with the matrix. Based on microstructural characterizations, the formation process of M₂C carbides with different morphologies was discussed. The results suggested that the more-stable morphology and OR of M₂C 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.

The effect of long-term thermal exposure on the carbide evolution in a Ni-16Mo-7Cr base superalloy was investigated. The results show that M₁₂C carbides are mainly precipitated on the grain boundaries during thermal exposure, and the primary massive M₆C carbides can be completely transformed to M₁₂C carbides in situ at temperatures above 750 °C for long-term thermal exposure. The transformation from M₆C carbides to M₁₂C carbides is attributed to the release of C atoms from M₆C, which results in the morphology changes of massive carbides, and stabilization of the sizes of M₁₂C carbides precipitated on the grain boundaries.