Ultra-high strength of nanocrystalline Inconel 718 composites by high-pressure torsion deformation and graphite reinforcements

A combination of colloidal mixing and high-pressure torsion (HPT) deformation was used to fabricate bulk graphite-reinforced Inconel 718 composites from powder materials. This study provides insight into the development of microstructure and mechanical properties of these composites in dependency on applied strain, deformation temperature, and graphite content. Scanning electron microscopy reveals significant grain and phase refinement down to the nanocrystalline regime of the graphite-reinforced Inconel 718 as well as the pure deformed Inconel 718 reference material. Furthermore, HPT deformation represents an effective method to fragment and dissolve the undesired Laves phase at room temperature and to obtain bulk nanocrystalline Inconel 718 in a single process step. Moreover, the graphite reinforcement phase was able to be finely dispersed within the matrix. Deformation temperature and reinforcement phase fraction appear to be the key factors in controlling the Laves phase fragmentation. Cross-sectional microhardness gradients were observed as a result of the process parameter-dependent microstructural evolution, leading to a maximum hardness of ∼7.5 GPa. The nanocrystalline composites show ultra-high tensile strength values above 2100 MPa, retaining ductile deformation until fracture and improved thermomechanical properties. The functional properties of graphite and the high strength of the processed Inconel 718 composites might extend the material's lifespan and potentially open new application areas.