Effect of hafnium content on the microstructural evolution and stress rupture properties of K4750 nickel-based superalloy

Abstract

The microstructural evolution of K4750 nickel-based superalloy with different hafnium (Hf) contents was investigated by thermodynamic calculations (TC) and experiments. According to TC, with the increases of Hf content from 0 wt% to 1.5 wt%, the redissolving temperature of MC carbide first decreased rapidly and then tended to be relatively stable, the precipitated temperature of M23C6 carbide decreased gradually, while the γ′ weight fraction increased first and then stabilized. Three alloys containing 0 wt%, 0.11 wt%, and 0.28 wt% Hf were then prepared to further study the evolution of the microstructure. The experimental results showed that the increase of Hf content changed the morphology of MC carbide from long strip to rodlike and blocky, which is caused by the decrease of Ti and Nb contents and the increase of Hf content in MC carbides. The addition of Hf also changed the morphology of M23C6 carbide from continuous to granular due to the different MC distribution at grain boundaries (GBs). In addition, a small amount of Hf segregated into the γ′ phase, reducing the Al and Nb contents of the γ′ phase, resulting in an increased γ/γ′ lattice mismatch. As the Hf content increased from 0 wt% to 0.11 wt% and 0.28 wt%, the mean stress rupture life at 750 °C and 430 MPa increased from 99.54 h to 105.73 h and 170.96 h, which was closely related to the evolution of MC, M23C6 and γ′ precipitates. Therefore, the relationship between microstructure and stress rupture properties was discussed in detail.