The precipitation of η phase in an Fe-Ni-based superalloy with different Ti/Al ratios

Abstract

Abstract In this research, the formation and transformation mechanisms of η (Ni3Ti) phase in an Fe-Ni-based superalloy at different Ti/Al ratios were investigated. In addition to Ti content, Ti/Al ratio also affects the η phase. So alloys with different Ti/Al ratios were prepared and the microstructures were analysed using optical and scanning electron microscopy and X-ray diffraction. The alloy with lower Ti but higher Ti/Al ratio has a higher η volume fraction. Time-temperature-precipitation diagrams of η phase in the superalloys with different Ti/Al ratios are also presented. Based on the experimental results, when the Ti/Al ratio was decreased from 20 to 3, the time and temperature of η precipitation shifted to longer and higher values, respectively, and the η volume fraction decreased. Ti/Al of 3 could significantly retard the formation of η phase. Depending on the Ti/Al ratio, and the time and temperature of ageing, η was precipitated at twin and grain boundaries or inside the grains, either by γ′ → η transformation or directly by formation of austenite. It was also shown that a high Ti/Al ratio in the alloy would reduce the fraction of twin boundaries after solution annealing which affects the η phase nucleation.