Microstructural changes and their effect on tensile properties of a Ni-Fe based alloy during long-term thermal exposure

Microstructural changes and their effect on tensile properties of a Ni-Fe based alloy, considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, were investigated during exposure at 700-800 °C for up to 30000 h. The results show that the major precipitates are still spherical γ׳, blocky MC and discrete particle-like M23C6 after long-term thermal exposure. No η or σ and other detrimental phases were observed. The main changes in microstructure are γ′ coarsening and the precipitation and growth of M23C6 carbide. The coarsening behavior of γ′ precipitates is in good accordance with the prediction of the Lifshitz-Slozov-Wagner (LSW) theory during exposure at 700 °C. However, it deviates from the prediction of LSW theory at 750 and 800 °C and an anomalous coarsening behavior of γ′ precipitates was observed. For M23C6 carbide, the size and amount first increase, and then keep almost constant with increasing exposure time at 700 and 750 °C, but the dissolution of M23C6 carbide appears at 800 °C. The yield strength first increases and then slightly decreases with increasing exposure time at 700 °C, while it decreases gradually at 750 °C and decreases drastically and then keeps almost constant at 800 °C. However, the yield strength almost keeps the same level as that of the samples after standard heat treatment even if after exposure at 700 °C for 30000 h. The elongation has no obvious change with increasing exposure time at 700 and 750 °C, but it increases gradually at 800 °C. The changes in yield strength and ductility can be attributed to the γ′ coarsening and the evolution of M23C6 carbide.