Graphene reinforced nickel-based superalloy composites fabricated by additive manufacturing

The graphene nanoplatelets (GNPs) reinforced K418 nickel-based superalloy composites were successfully fabricated by laser powder bed fusion (L-PBF). Plasma-assisted ball milling (P-milling) was used to prepare K418 composite powder with homogenous dispersion of GNPs on the surface. The effects of the addition of GNPs on microstructure and phase composition of L-PBF-processed GNPs/K418 samples were studied by comparing with their counterparts without GNPs fabricated under the same conditions. The microstructures of as-built K418 and GNPs/K418 composite sample were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. The possible chemical reactions between carbon and matrix elements were given by Materials Studio software based on density functional theory (DFT). The results show that the majority of GNPs were uniformly dispersed into the grain interior of the γ matrix, however, few of them with defects reacting with Nb and Mo to form (Mo,Nb)C carbides precipitating along the grain boundaries. The columnar grain of as-built K418 sample transformed into approximately equiaxed grain by the addition of GNPs. The yield strength and maximum tensile strength of L-PBF-processed K418 sample increased to from 912 MPa to 1078.17 MPa–1018 MPa and 1200 MPa as an addition of 1 0.1 wt% GNPs, respectively. Meanwhile, the tensile strain increased from 7.13% to 10.3%. The grain refinement, load transfer from matrix to GNPs, forest dislocation strengthening, and Orowan looping strengthening were the mainly strengthening mechanisms for GNPs reinforced K418 superalloy. Furthermore, the key to the improved ductility of GNPs/K418 composite was attributed to the reduction of cracking susceptibility and the homogeneous dispersion of GNPs in γ matrix.