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

Abstract

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.