The microstructure evolution and its effect on the mechanical properties of a hot-corrosion resistant Ni-based superalloy during long-term thermal exposure

The microstructure evolution and its influence on the mechanical properties are investigated in a hot-corrosion resistant Ni-based superalloy during long-term thermal exposure. It is found that the tertiary γ' phase disappears and the secondary γ' phase coarsens and coalesces gradually, which acts as the main reason for the decreasing of strength at both room temperature and 900°C. During exposure, the grain boundary coarsens from discontinuous to half-continuous and finally to continuous structure. The optimum half-continuous grain boundary structure composed of discrete M₂₃C₆ and M₃B₂ wrapped by γ' film leads to the elongation peak at room temperature in the thermally exposed specimens. At 900°C, the increase in the elongation is attributed to the much softer matrix and the formation of microvoids. The behavior of primary MC decomposition is a diffusion-controlled process. During exposure, various derivative phases including M₂₃C₆, γ', η, M₆C and σ sequentially form in the decomposed region. Primary MC decomposition and the precipitation of σ phase have little effect on the mechanical properties due to their low volume fractions.