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 degrees C advanced ultra-supercritical (A-USC) coal-fired power plants, were investigated during exposure at 700-800 degrees C for up to 30000 h. The results show that the major precipitates are still spherical gamma ', blocky MC and discrete particle-like M23C6 after long-term thermal exposure. No eta or sigma and other detrimental phases were observed. The main changes in microstructure are gamma ' coarsening and the precipitation and growth of M23C6 carbide. The coarsening behavior of gamma ' precipitates is in good accordance with the prediction of the Lifshitz-Slozov-Wagner (LSW) theory during exposure at 700 degrees C. However, it deviates from the prediction of LSW theory at 750 and 800 degrees C and an anomalous coarsening behavior of gamma ' 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 degrees C, but the dissolution of M23C6 carbide appears at 800 degrees C. The yield strength first increases and then slightly decreases with increasing exposure time at 700 degrees C, while it decreases gradually at 750 degrees C and decreases drastically and then keeps almost constant at 800 degrees 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 degrees C for 30000 h. The elongation has no obvious change with increasing exposure time at 700 and 750 degrees C, but it increases gradually at 800 degrees C. The changes in yield strength and ductility can be attributed to the gamma ' coarsening and the evolution of M23C6 carbide. (C) 2016 Elsevier B.V. All rights reserved.