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

摘要

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 Y phase disappears and the secondary y' phase coarsens and coalesces gradually, which acts as the main reason for the decreasing of strength at both room temperature and 900℃. 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 M_(23)C_6 and M_3B_2 wrapped by γ' film leads to the elongation peak at room temperature in the thermally exposed specimens. At 900℃, 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 M_(23)C_6,γ', η, M_6C and σ sequentially form in the decomposed region. Primary MC decomposition and the precipitation of cr phase have little effect on the mechanical properties due to their low volume fractions.