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 °C in advanced ultra-supercritical coal-fired power plants (700 °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 γ′-solvus temperature, decrease γ′ coarsening rate and reduce the temperature range of η 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 °C, but after thermal exposure at 750 °C for 5000 h, the yield strength of the alloys with high Ti/Al ratio obviously decreases due to the η phase precipitation. The influence of η phase on mechanical properties is related with its size. When the η phase is small, it has no obvious influence on mechanical properties, but η phase becomes the crack initiation site with the further growth of η phase. It can be concluded that the decrease in Ti/Al ratio can improve the thermal stability to meet the requirement of 700 °C A-USC coal-fired power plants.