Effects of sub-solvus ageing on the tensile and creep properties of a new cast nickel-based superalloy

Abstract

For nickel-based superalloys with medium volume-fraction γʹ phase (20 %–40 %), dual or multi-stage aging treatments are usually conducted to generate a microstructure containing the multimodal distribution of γʹ for a balance of strength and plasticity. In the present study, the microstructure and high-temperature properties of a novel cast nickel-based superalloy K4800 were investigated after being subjected to three heat treatments (HT) procedures, namely HT1: 1180 °C/4 h + 1090 °C/2 h + 800 °C/16 h, HT2: 1180 °C/4 h + 1060 °C/2 h + 800 °C/16 h and HT3: 1180 °C/4 h + 800 °C/16 h. It was found that the sub-solvus aging treatments at 1090 and 1060 °C precipitated sub-micron-sized (∼300 nm) primary γʹ phase which enhanced the ductility during 800 °C tensile (the total elongation of T1, T2, and T3 samples were 6.75 %, 7.3 %, and 3.25 %, respectively) without evidently impairing the strength. After careful microstructure observation and deformation mechanism analysis, the enhancement of elongation was rationalized that the precipitation of the sub-micron-sized primary γʹ phase decreased the volume-fraction and size of the nanometer-sized γʹ phase which was precipitated at 800 °C, and simultaneously, promoted the dislocation movement by suppressing the non-planar slip. However, an excessive amount of the sub-micron-sized primary γʹ phase led to a faster ripening process of the nanometer-sized γʹ during creep, which decreased the creep life at 800 °C/430 MPa (T1: 125 h, T2: 199 h, and T3: 198 h). Based on this, we monitored the number density of nanometer-sized γʹ phase coexisting with different amounts of large γʹ during creep. An area fraction less than 7 % of the sub-micron-sized γʹ phase was considered to have little detrimental effect on the creep life of K4800 alloy, which corresponded to a sub-solvus temperature range about 1080–1090 °C.