Structural evolution of nanocrystalline Inconel 718 metal matrix composites reinforced with solid lubricants: An in-situ high-temperature XRD study

Nanocrystalline Inconel 718 (IN718) metal matrix composites reinforced with solid lubricant (SL) materials were fabricated using colloidal mixing and high-pressure torsion, a severe plastic deformation technique enabling exceptional grain refinement. The effect of SL type and concentration on phase evolution and microstructural stability was investigated using in situ high-temperature X-ray diffraction (HT-XRD) and complementary ex situ techniques (differential scanning calorimetry and electron microscopy). HT-XRD revealed several temperature-driven processes in the as-deformed IN718 during annealing, including γ lattice changes, γ′/γ″ precipitation and dissolution, δ-phase formation, recovery, and recrystallization. All SL-reinforcements resulted in contracted γ-lattices, and up to 1 wt%, they followed phase evolution trends similar to the reference alloy. Transition metal dichalcogenides (MoS 2 , WS 2 ) showed consistent behavior across all concentrations, whereas nanocarbon reinforcements (graphite, graphene nanopowder, carbon nanotubes) suppressed δ-phase formation through carbide precipitation, with 5 wt% further inhibiting thermal expansion at elevated temperatures and delaying recrystallization. Remarkably, the nanocrystalline microstructure of as-deformed IN718 was retained up to 700 °C after short-term annealing, and even after long-term exposure at 900 °C, the γ-grain size remained below 500 nm, with nanocarbon materials being most effective in stabilizing the microstructure. Microhardness measurements revealed hardening behavior, dependent on solid lubricant type and concentration. Temperatures below 400 °C are considered most promising for future testing of, e.g., mechanical and tribological properties, as precipitation and recovery are suppressed while maintaining strain hardening capability and nanocrystallinity. This work offers a pathway for microstructure tailoring of SL-reinforced IN718 composites for potential self-lubrication applications at elevated temperatures.