Nucleation mechanism of hetero-epitaxial recrystallization in wrought nickel-based superalloys

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050°C and cooled at 5°C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.