Solid-State Phase Transformations Involving (Nb,Mo)₂CrB₂ Borides and (Nb,Ti)₂CS Carbosulfides at the Grain Boundaries of Superalloy Inconel 718

The present paper studied the solid-state phase transformations occurring at the grain boundaries of a conventional cast-and-wrought alloy within the chemical specifications of superalloy Inconel 718. These phase transformations involved the precipitation of (Nb,Mo)₂CrB₂ borides and (Nb,Ti)₂CS carbosulfides, which are usually phases rather associated to solidification phenomena than to solid-state phase precipitation in superalloy Inconel 718. The study started from a material solution-treated above the δ solvus. When this material was heated subsolvus δ from room temperature, (Nb,Mo)₂CrB₂ borides was found to precipitate at the grain boundaries, in addition to the δ phase. Then, when the material was reheated supersolvus δ after the subsolvus treatment, the complete dissolution of the δ phase and the partial dissolution of the (Nb,Mo)₂CrB₂ phase released niobium and titanium atoms in the grain boundaries. Both elements have a strong affinity with carbon and sulfur, and this favored the precipitation of secondary (Nb,Ti)C carbides and (Nb,Ti)₂CS carbosulfides at grain boundaries. An analysis of the interactions between the segregating species C, B and S evidenced that the precipitation of (Nb,Mo)₂CrB₂ borides at grain boundaries was very likely made possible by a decreased carbon activity during the subsolvus δ treatment.