Intergranular creep resistance affected by grain boundary carbides to suppress lamellae γʹ precipitation in a specially designed inconel 740H variant

The effects of heat treatment on the microstructure and creep properties of a modified variant of Inconel 740H alloy were studied. This variant was tuned to have a refined composition, reducing C, Cr, and Co contents while increasing the Ti/Al ratio and Mo content to enhance creep resistance. Two heat treatments were compared: 1) single-aged (SAG: 800 °C/16 h) and 2) stabilization prior to SAG (STBG: 1060 °C/2 h + SAG). Both treatments resulted in a γ-matrix with uniformly distributed γʹ particles (∼35 nm) and a grain size of 90–100 μm. However, the SAG sample showed densely distributed fine granular carbides (M23C6 and MC) at the grain boundaries, whereas the STBG sample had fewer carbides, leaving ∼38% of grain boundaries undecorated. After thermal exposure at 750 °C for 5000 h, both samples exhibited γʹ coarsening and similar degradation in microhardness and tensile properties. However, the SAG sample demonstrated superior creep resistance. During creep, both samples showed Orowan bypassing dislocations around γʹ particles, but only the STBG sample exhibited coarse, nodule-like γʹ precipitation at carbide-free grain boundaries, leading to easier intergranular cracking. The higher fraction of grain boundary carbides in the SAG sample effectively suppressed nodule-like γʹ formation and reduced creep damage by strengthening the grain boundaries, preventing dislocation impingement. Consequently, the SAG treatment offered better protection against creep degradation.