Temporal Evolution, Coarsening Behavior and Oxidation Resistance of Ni-15al Superalloy

Abstract

The temporal evolution including size, amount, morphology and distribution of gamma' precipitates in gamma matrix in conjunction with coarsening kinetics and oxidation behavior of a model Ni-15Al alloy aged at 800 degrees C for 1, 4, 16, 64 and 256 h is studied in detail. The microstructure of the investigated samples consists of L1(2)-type ordered gamma' (Ni3Al) precipitates and a disordered Ni-rich gamma (FCC) matrix phase. Spheroidal-to-cuboidal-to parallelpipedic morphological transition is observed as the mean precipitate radius increases with increasing aging time. The temporal exponent p is calculated as 0.35 which agrees well with the diffusion-controlled LSW theory (p = 0.33) for coarsening of a binary alloy. However, the power law dependence of the number density (N-V) departs from the theoretical predictions which suggests a quasi-stationary coarsening. The cyclic oxidation behavior of as-cast and aged (800 degrees C for 4, 16, 64 and 256 h) Ni-15Al superalloys at 1000 degrees C in laboratory air were investigated in detail. The results showed that 4 and 16 h aged samples exhibited considerably better oxidation resistance compared to as-cast and 64 and 256 h aged samples. The oxide product is identified as NiO by X-ray diffraction (XRD) analyses. Uniform, even and adherent oxide scale formed without any void formation and important scale spallation. (C) 2019 Elsevier B.V. All rights reserved.