The effects of diffusional couplings on compositional trajectories and interfacial free energies during phase separation in a quaternary Ni-Al-Cr-Re model superalloy

Abstract: The temporal evolution of ordered gamma-prime(L12)-precipitates and the compositional trajectories during phase-separation of the gamma(face-centered-cubic(f.c.c.)) and gamma-prime(L12)-phases are studied in a Ni-0.10Al-0.085Cr-0.02Re(mole-fraction) superalloy, utilizing atom-probe tomography, transmission electron microscopy, and the Philippe-Voorhees (PV) coarsening model. As the gamma-prime(L12)-precipitates grow, the excesses of Ni, Cr and Re, and depletion of Al in the gamma(f.c.c.)-matrix develop as a result of diffusional fluxes crossing gamma(f.c.c.)/gamma-prime(L12) heterophase interfaces. The coupling effects on diffusional fluxes was introduced (PV coarsening model) in terms of the diffusion tensor, D, and the second-derivative tensor of the molar Gibbs free energies, G", obtained employing Thermo-Calc and DICTRA calculations. The Gibbs interfacial free energies are (6.9 +- 1.4) mJ/m2 with all terms in D and G", which changes to (18.9 +- 2.1) mJ/m2, (37.7 +- 3.3) mJ/m2, and (-7.5 +- 1.2) mJ/m2 without including the off-diagonal terms in D, G", and both D and G", respectively. The experimental APT compositional trajectories are displayed and compared with the PV model in a partial quaternary phase-diagram, employing a tetrahedron. The compositional trajectories measured by APT exhibit curvilinear behavior in the nucleation and growth regimes, which become vectors, moving simultaneously toward the gamma(f.c.c.) and gamma-prime(L12) conjugate solvus-surfaces, for the quasi-stationary coarsening regime. The compositional trajectories are compared to the PV model with and without the off-diagonal terms in D and G". The directions including the off-diagonal terms in D and G" tensors are consistent with the APT experimental data.