Precipitation, transformation, and coarsening of carbides in a high-carbon Ni-based superalloy during selective laser melting and hot isostatic pressing processes

Carbides are vital phases in determining the mechanical properties of solid solution nickel-based superalloys. The precipitation, transformation, and coarsening of carbides in a high-carbon Ni-based superalloy during selective laser melting and the HIP process were investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and thermodynamic modeling. A high carbon content (0.2 wt%) resulted in complicated MxCy-type carbides enriched in W and Cr due to the uneven segregation of Cr and W in the as-built GH3230G. The complicated carbides were transformed into M6C-type carbides, and coarsening of carbides occurred after HIP treatment. Thermodynamic calculation results explained the precipitation and coarsening of carbides. In addition, larger carbide particles with a size of 1.1 µm were always distributed along the epitaxial grain boundaries, while smaller carbide particles at a scale of 0.3 µm were apt to form at the subgrain boundaries. The cross-section micrographs below the fractures showed that some microvoids were always initiated at coarse carbide/matrix interfaces, which induced intergranular fracture at elevated temperatures. This work can be extended to elucidate carbide formation, transformation, and evolution for additively manufactured superalloys and predict their impact on the mechanical properties of this kind of alloy.