Thermal Stability of a New Ni-Fe-Cr Base Alloy with Different Ti/Al Ratios

Influence of Ti/Al ratios on the thermal stability of a new low cost Ni-Fe-Cr base wrought alloy, designed for application at 700 degrees C in advanced ultra-supercritical coal-fired power plants (700 degrees C A-USC), was investigated both experimentally and thermodynamically. After standard heat treatment, the alloys with different Ti/Al ratios had the same microstructural characteristics. However, compared with the alloys with high Ti/Al ratio, the low Ti/Al ratio can increase the gamma' -solvus temperature, decrease gamma' coarsening rate and reduce the temperature range of eta phase precipitation. For the alloys with low Ti/Al ratio, the yield strength has no obvious decrease during long-term thermal exposure at 700 and 750 degrees C, but after thermal exposure at 750 degrees C for 5000 h, the yield strength of the alloys with high Ti/Al ratio obviously decreases due to the eta phase precipitation. The influence of eta phase on mechanical properties is related with its size. When the eta phase is small, it has no obvious influence on mechanical properties, but eta phase becomes the crack initiation site with the further growth of eta phase. It can be concluded that the decrease in Ti/Al ratio can improve the thermal stability to meet the requirement of 700 degrees C A-USC coal-fired power plants.