The effects of refractory elements on Ni-excesses and Ni-depletions at γ(f.c.c.)/γ'(L12) interfaces in model Ni-based superalloys: Atom-probe tomographic experiments and first-principles calculations

The effects of refractory (R) elements (R = Re, Ru, W, or Ta) on a base Ni-Al-Cr alloy are studied, for an aging temperature of 1073 K (800 degrees C) and an aging time of 256 h, employing atom-probe tomography (APT) and first-principles calculations. We find that there are strong attractive chemical binding energies between R-elements and solute (S) atoms (S = Al, Cr) in Ni-Al-Cr based alloys utilizing experimental partial radial distribution function (RDF) results, and first-principles calculations performed at 0 K. We demonstrate that correlated R-S binding energies play a key role in the observed Ni retention-excesses at gamma(f.c.c.)/gamma'(L1(2)-structure) interfaces at aging times as long as 256 h. The total reduction of the gamma(f.c.c.)/gamma'(L1(2)) interfacial energy, as a result of Ni interfacial-excesses in both gamma(f.c.c.)-matrix and gamma'(L1(2))-precipitates, lies between -0.16 +/- 0.06 mJ m(-2) and -0.05 +/- 0.02 mJ m(-2). The R-S binding energies cause changes in the compositional diffusion flux-vectors in and out of gamma(L1(2))-precipitates, which result in larger solvent Ni retention-excesses and wider interfacial compositional widths at 256 h, when compared with the base Ni-Al-Cr alloy. Refractory elements are slow diffusers in nickel and the attractive R-Cr binding energies decelerate the solute diffusional fluxes, which results in a decrease of the Ni diffusivity, which in turn hinders the flux of Ni atoms away from the gamma(f.c.c.)/gamma'(L1(2)) interfaces. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.