Microstructure evolution and mechanical properties of a new cast Ni-base superalloy with various Ti contents

Microstructure evolution of a new cast Ni-base superalloy with various Ti contents is studied by thermodynamic calculations and experimental observations. According to thermodynamic calculations, when Ti content increases from 2.0 wt% to 4.0 wt%, the γ′ weight fraction firstly increases, and then tends to be relatively stable because of the precipitation of η phase. Two alloys containing 2.4 wt% and 3.0 wt% Ti are prepared to verify the accuracy of calculation results. After full heat treatment, γ′ phase, MC carbide and M23C6 carbide are precipitated from two alloys, but no η phase is detected. The quantity of γ′ phase and MC carbide in 3.0Ti alloy is higher than that in 2.4Ti alloy because Ti acts as one of their main forming elements. The higher amounts of γ′ phase and MC carbide obviously improve the mechanical properties of 3.0Ti alloy. To verify whether there is η phase in two alloys or not, long-term aging treatments are carried out at 700 °C, 750 °C and 800 °C for 3000 h. After long-term aging, needle-like η phase is precipitated from 3.0Ti alloy but not from 2.4Ti alloy. The quantity of η phase in 3.0Ti alloy gradually increases with the increase of aging temperature and time. Especially, the precipitation of η phase preferentially takes place beside grain boundaries and near MC carbides. The growth of η phase consumes γ′ phase and causes the appearance of γ′ depleted zones.