A Mechanism Leading to γ′ Precipitates with {111} Facets and Unusual Orientation Relationships to the Matrix in γ–γ′ Nickel-Based Superalloys

Abstract

Cast-and-wrought heavily alloyed γ–γ′ nickel-based superalloys may exhibit large recovered grains inherited from the ingot conversion and characterized by a high density of close-to-coherent micrometric γ′ precipitates. In the AD730™ nickel-based superalloy, a previous work (Vernier et al. Scr Mater 153:10–13, 2018) highlighted a new interaction between such precipitates and a recrystallization front passing through. This interaction resulted in γ′ precipitates with a close-to-twin orientation relationship to their recrystallized host grain. Called T-type precipitates, they were revealed to be {111} bounded plate-like particles. The present paper aims to clarify the mechanism whereby such precipitates form. The formation of T-type precipitates actually is part of a more global mechanism which also produces γ′ precipitates slightly misoriented from their surrounding matrix (C-type precipitates) and of same size and morphology as T-type precipitates. Both T- and C-type precipitates display {111} facets and are evidenced in the AD730™, René65, and PER72 alloys, supporting the idea that the mechanism can more generally occur in all low-lattice-mismatch γ–γ′ nickel-based superalloys. Finally, a scenario is proposed: T/C-type precipitates form at the recrystallization front of grains sharing a 〈111〉 axis with the recovered grain they consume, and develop {111} facets and specific orientations which minimize the interfacial energy on both recrystallized and recovered sides.