Directional coarsening behavior of primary γ′ phase in Ni3Al-based superalloy during aging heat treatment

Polycrystalline Ni3Al-based superalloy with up to 70% gamma' phase owns excellent strength at around 1100 degrees C. It has been successfully used as a substitute for turbine blade crown, which is one of the aircraft components working in the severest environment (i.e., sometimes nearly overheating). However, few studies concentrate on the coarsening resistance and phase thermal stability in this newly designed polycrystalline superalloy at the working temperature, hence, directional coarsening behavior of primary gamma'(gamma(I)')Ihas remained restricted. This paper investigated directional coarsening behavior of gamma(I)' during aging treatment (1100 degrees C) after overheating at 1270 degrees C with furnace cooling (SFA), air cooling (SAA) and water cooling (SWA) respectively. It clearly showed that gamma(I)' grew into lamellar and gradually became irregular during the directional coarsening process. Moreover, its morphological combination mode was illustrated. Since parallel short lamellar gamma(I)' connected each other, long lamellar formed. When perpendicular long lamellar gamma(I)' connected each other, it became L-type or T-type, i.e., precursor of irregular gamma(I)'. The kinetics of directional coarsening regime of gamma(I)' was in line with the matrix-diffusion-controlled (MDC) theory according to the evaluation of gamma(I)' size. This was caused by higher migration of gamma(I)' interfaces than the diffusion of solute elements. It was also confirmed that driving force of directional coarsening mainly depended on the competition of elemental diffusion, lattice misfit distribution and gamma channel distance. (C) 2021 Elsevier B.V. All rights reserved.