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 TM nickel-based superalloy, a previous work [18] 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 platelike 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 TM , 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.