Solid-State Phase Transformations Involving (Nb,Mo)2CrB2 Borides and (Nb,Ti)2CS Carbosulfides at the Grain Boundaries of Superalloy Inconel 718
Résumé
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 d solvus. When this
material was heated subsolvus d from room temperature, (Nb,Mo)2CrB2 borides was found to
precipitate at the grain boundaries, in addition to the d phase. Then, when the material was
reheated supersolvus d after the subsolvus treatment, the complete dissolution of the d 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 d treatment.
Domaines
Matériaux
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