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) 2 CrB 2 borides and (Nb,Ti) 2 CS 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) 2 CrB 2 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) 2 CrB 2 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) 2 CS carbosulfides at grain boundaries. An analysis of the interactions between the segregating species C, B and S evidenced that the precipitation of (Nb,Mo) 2 CrB 2 borides at grain boundaries was very likely made possible by a decreased carbon activity during the subsolvus d treatment.

Flash oxidation tests to evidence the segregation of boron at the grain boundaries of superalloy Inconel 718

“…Be fore oxidation the material had been solution treated at 1050 °C (thus above the solvus) and that for several durations. A previous work [14] had shown that this material exhibited several phases at grain boundaries: (Nb,Mo) 2 CrB 2 borides (which had precipitated during a prior subsolvus treatment), eventually some residual precipitates, and secondary (Nb,Ti)C carbides. But because the material had been pre viously solution treated at high temperature, large amounts of chemical species were still in solution in the microstructure.…”

“…More precisely, four types of samples corresponding to four durations of solution treatment were oxidized: ST 10 (solution treated for 10 min), ST 30 (solution treated for 30 min), ST 60 (solution treated for 60 min) and ST 120 (solution treated for 120 min). According to the study presented in [14] , the ST 10 type samples mainly exhibited precipitates and (Nb,Mo) 2 CrB 2 borides at grain boundaries ( Fig. 1.…”

“…The alloy studied here was the same cast and wrought alloy as that studied in [14] . Its chemical composition, which is reminded in Table 1 , fell within the AMS specifications for superalloy Inconel 718.…”

“…The flash oxidation experiments of the present study were performed on samples which had been solution treated at 1050 °C (air cooling) af ter a prior subsolvus treatment [14] . More precisely, four types of samples corresponding to four durations of solution treatment were oxidized: ST 10 (solution treated for 10 min), ST 30 (solution treated for 30 min), ST 60 (solution treated for 60 min) and ST 120 (solution treated for 120 min).…”

“…b c). In addition to these intergran ular phases, primary (Nb,Ti)C carbides, primary (Ti,Nb)(C,N) carboni trides and intragranular secondary (Nb,Ti)C carbides could be also no ticed [14] .…”