Effect of carbon content on the microstructure and tensile properties of IN718 superalloy by modified thermally controlled solidification process

The effects of carbon content (0.036 wt%, 0.065 wt%, 0.096 wt%, 0.12 wt%) on the microstructure and tensile properties of IN718 superalloy processed by Modified Thermally Controlled Solidification (MTCS) were systematically investigated. Differential scanning calorimetry (DSC) and in-situ observations revealed that increasing carbon content from 0.036 to 0.12 wt% increased the precipitation temperature of MC carbides while narrowing the solidification range. Under the MTCS process, increasing carbon content from 0.036 wt% to 0.12 wt% resulted in a refinement of average grain size from 114 μm to 78 μm. Concurrently, the variation in carbon content promoted the growth and proliferation of blocky MC carbides (predominantly NbC), while the depletion of Nb content in the residual liquid effectively suppressed the formation of brittle Laves phase. At 650 °C, the 0.036C alloy exhibited superior tensile strength (940.1 MPa) but limited ductility (9.8 % elongation) due to island-shaped laves. Notably, compared with the 0.096C alloy, the 0.12C alloy achieved an 11.4 % increase in strength and an 11.8 % improvement in plasticity. This improvement is attributed to the reduction of brittle Laves phase at grain boundaries, along with the decrease in the content of elongated carbides and needle-like δ phases.