Microstructure evolution and mechanical properties of Ni–12Mo–7Cr–2Nb based superalloys pre-aged at high temperatures

The effects of short-term pre-aging treatment (800 °C–900 °C) on the microstructure and tensile properties of one Ni–12Mo–7Cr–2Nb based alloy have been systematically investigated. In the early stage of pre-aging, granular MC and M 2 C carbides form at grain boundaries, while flake-shaped MC carbides form in the matrix (extremely dense near grain boundaries). With pre-aging time increasing, these secondary carbides tend to become coarser, and the flake-shaped MC carbides near grain boundaries start to decompose and disappear due to the coarsening of grain boundary carbides. Such evolution processes behave more rapidly at higher temperatures. The tensile tests at room temperature (RT) and 650 °C show that the fine flake-shaped MC carbides can bring about the obvious dispersion strengthening effect, but such effect would lose due to the coarsening of carbides after pre-aged at 900 °C. At RT, the secondary carbides act as the crack nucleation sites in the transgranular fracture process, thus the pre-aging specimens show the lower ductility than solution-treated ones. At 650 °C, the ST specimens show the intergranular fracture tendency. After the pre-aging treatment, the dense grain boundary carbides can relieve strain concentration along grain boundaries and thus eliminate the intergranular fracture tendency in the pre-aged specimens, which lead to the higher elongation. Our results can provide key references for the design of heat-treating processes for Ni–12Mo–7Cr–2Nb based superalloys.