Abstract:Using techniques such as SEM, TEM, and phase analysis, the precipitation behavior during long-term aging at 800°C and its effect on the mechanical properties of a Ni-Cr-W-Mo alloy were investigated. The results indicate that during aging, M23C6 carbides precipitated sequentially at grain boundaries, twin boundaries, and within grains in different morphologies. The intergranular lamellar M23C6 was a product of a discontinuous reaction, while the granular M23C6 at twin boundaries grew along the {111} twin planes. The intragranular nanoscale M23C6 contributed to pinning strengthening. M6C carbides underwent degeneration from the exterior to the interior between 1000h and 5000h of aging, decomposing into M23C6, α-(W,Mo), and W- and Mo-poor matrix. Due to the equilibrium segregation of W and Mo, α-(W,Mo) phase precipitated at grain boundary M23C6 after 5000h of aging. Changes in mechanical properties were mainly concentrated in the early aging stage. The increase in strength within the first 200h of aging was caused by a sharp rise in carbides, while the deterioration of ductility and toughness was attributed to the brittleness of intergranular M23C6 and its reduction of grain boundary cohesion. From 200h to 5000h of aging, the properties degraded gradually. The strength reduction in this stage was related to the weakening of solid solution strength by α phase and the coarsening of nanoscale M23C6, while the significant degradation of ductility and toughness was associated with the coarsening of intergranular M23C6 and the decomposition of M6C. The fracture mode transitioned from transgranular ductile fracture to a mixed mode after aging.