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During long-term thermal exposure, the degeneration of primary MC carbide is a diffusion-controlled process. With the element exchange between the primary MC carbide and the γ matrix, three MC degeneration reactions, that is, γ + MC → γ' + M23C6 (Reaction I), MC + γ →η+ M23C6 (Reaction II) and MC + γ →η+ M6C (Reaction III), sequentially operates. Of them Reaction III has never been reported in the previous references and is kinetically most difficulty. It is also shown that the grain boundary MC decomposes much more rapidly than the grain interior MC and that Reaction III is not as often seen at the grain boundary as in the grain interior.

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