M₇C₃ and MC-Type Carbide Formation during Sintering of Inconel 718/TiCN/GO Hybrid Alloy: Insights from Electrolytic Extraction

Inconel 718 (IN718) superalloy was mechanically alloyed with TiCN and graphene oxide and then sintered using the spark plasma sintering technique. The hybrid powder mixture was sintered at 1000°C for 10 minutes. Complex carbides of the type M₇C₃ and MC were formed during sintering while the composition and the lattice parameter of TiCN were shifted. Detailed analysis of the precipitates was conducted after eliminating the effect of the Ni-based matrix by electrochemically dissolving the matrix (IN718) in an electrolyte of methanol and 10% AA (10% acetylacetone-1% tetramethyl-ammonium bromide) at 3.8 V, keeping the current at 2.2 mA, for a period of 10 h. Inconel matrix was dissolved from the near-surface region, leaving behind a thick network enriched by chemically resistant precipitates, such as carbides and carbonitrides. The surface was then subjected to x-ray diffraction studies to determine the precipitate phases that formed, and a detailed chemical analysis of individual particles was conducted using energy-dispersive x-ray analysis in a scanning electron microscope. The major phase was TiCN, which experienced changes in its chemical composition and, thus, in lattice parameters. After sintering, the lattice parameter shifted from 4.28215 to 4.2836 nm, indicating a net increase in the lattice parameter of 0.0019 nm. The Ti/(C, N) ratio in the as-received powder ranged from 0.3 to 0.7, while after sintering, the ratio increased to 0.4-1.0 in more than 84% of the particles, according to the EDX analyses. A eutectic-like structure of M₇C₃ and MC carbide phases is probably formed during sintering. The mole fraction of MC: M₇C₃-type carbides varied from 51 to 97% in the eutectic. (Cr, Fe)₇C₃ metastable carbide is formed due to the carbon-rich environment with graphene oxide in the vicinity.