Heat treatment design of Inconel 740H superalloy for microstructure stability and enhanced creep properties

In this study, the influence of heat treatment on the microstructure and high-temperature mechanical properties of Inconel 740H superalloy was investigated. After solution treatment, various heat treatments were conducted: STD (standard): 800 °C/16 h/AC, S1 (intended for a higher γ′ fraction): 650 °C/16 h + 800 °C/16 h/AC, and S2 (intended for carbide stabilization): 1060 °C/2 h/AC + 800 °C/16 h/AC. All samples exhibited a γ-matrix with an average grain size of ∼90 µm, wherein spherical γ' particles with sizes of ∼30 nm were uniformly distributed with a similar fraction. However, the fractions of fine (Ti,Nb)C carbides at the grain boundaries were in the order of S2 (33%)> STD (14%)> S1 (10%), and the rest of the grain boundaries were decorated with M23C6. All samples exhibited similar microhardness and tensile properties at 750 °C. During thermal exposure at 750 °C for 5000 h, the sizes of γ' in the γ-matrix gradually increased with similar growth rates in all samples. The gradual growth of γ' led to a gradual decrease in microhardness and tensile strength. Meanwhile, the results of the creep tests at 750 °C/270 MPa showed a noticeable difference. The S2 sample exhibited the longest creep life, while entangled dislocations at the γ' particles were dominant in all samples during creep. All samples showed intergranular fractures, and the M23C6 carbides provided preferential sites for cavity nucleation. It was found that the higher fraction of MC carbides in the S2 sample accommodated the stress exerted on the grain boundaries by its decomposition to M23C6.