γ′-(L12) precipitate evolution during isothermal aging of a Co-Al-W-Ni superalloy

The coarsening kinetics and elemental partitioning behavior of gamma'-(L1(2)) precipitates in a gamma(f.c.c.)-matrix for a model quaternary Co-8.8Al-8.9W-9.9Ni at.% superalloy are investigated utilizing isothermal aging conditions at 650, 800 and 900 degrees C. The gamma'-precipitate mean radius, number density, and volume fraction, at 800 and 900 degrees C, were studied using scanning electron microscopy; the calculated temporal exponents associated with coarsening of gamma'-precipitates display good agreement with model predictions for quasi stationary coarsening. An atom probe tomographic (APT) investigation of the aged gamma/gamma' microstructure at 650 degrees C demonstrates that the compositions and volume fractions of both phases vary continuously up to 4096 h. The aged microstructure at 650 degrees C consists of interconnected nanoscale gamma'-precipitates, corroborated utilizing SEM for the 4096 h aged-specimen. The activation energy for coarsening is estimated for the temperature range 650-900 degrees C to be 283 kJ mol(-1), in reasonable agreement with activation energies for diffusion of Al, W, and Ni in Co, suggesting that coarsening of gamma'-precipitates is limited by bulk-diffusion. APT measurements of specimens aged for 1024 hat 800 and 900 degrees C demonstrate that the isothermal aging temperature has a significant effect on the compositions and partitioning behavior of Co, Al, W and Ni between the gamma- and gamma'-phases. The partitioning ratio of the concentrations between the gamma'- and gamma-phases is largest for W, decreasing linearly from 5.3 +/- 0.1 at 650 degrees C to 2.1 +/- 1.2 at 900 degrees C, and smallest for Co, decreasing from 0.86 +/- 0.01 at 650 degrees C to 0.73 +/- 0.01 at 900 degrees C. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.