| The phase separation of model Ni-Al-Cr-X-Y-Z (X=Re, Y=Ru, Z=W) alloys on the nanometer scale is studied using atom-probe tomography (APT), transmission electron microscopy (TEM) and first-principles calculations.;The effects of varying the pulse energy of a picosecond laser used in pulsed-laser atomprobe (PLAP) tomography of an as-quenched Ni-6.5 Al-9.5 Cr at.% alloy are assessed based on the quality of the mass spectra and the compositional accuracy of the technique. Compared to pulsed-voltage APT, PLAP tomography improves mass resolving power, increases the signal-to-noise ratio, and improves compositional accuracy. An analysis of the detected noise levels reveals that for properly chosen instrument parameters, there is no significant steady-state heating of the Ni-6.5 Al-9.5 Cr at.% tips during PLAP tomography.;The phase separation of the Ni-6.5 Al-9.5 Cr at.% alloy aged at 873 K is studied by APT and compared to the predictions of classical precipitation models. Phase separation in this alloy occurs in four distinct regimes: (i) quasi-stationary-state gamma' (L12)-precipitate nucleation; (ii) concomitant precipitate nucleation, growth, and coagulation and coalescence; (iii) concurrent growth and coarsening, wherein coarsening occurs via both gamma'-precipitate coagulation and coalescence and by the classical evaporation-condensation mechanism; and (iv) quasi-stationary-state coarsening of gamma'-precipitates, once the equilibrium volume fraction of precipitates is achieved. The predictions of classical nucleation and growth models are not validated in detail experimentally, likely due to the complexity of the atomistic kinetic pathways involved in precipitation. During coarsening, the temporal evolution of the gamma'-precipitate average radius, number density, and both the gamma(f.c.c.)-matrix and gamma'-precipitate supersaturations follow the predictions of classical models.;The temporal evolution of Ni-10.0 Al-8.5 Cr-2.0 Ru at.% aged at 1073 K was investigated using TEM and APT. The gamma'-precipitate morphology is spheroidal through 256 h of aging as a result of adding Ru, which decreases the lattice parameter misfit to 0.0014 according to JMat Pro calculations. The addition of Ru accelerates the compositional evolution of the gamma'- and gamma- phases, which achieve their equilibrium compositions after 0.25 h. Initially, Ru accelerates the partitioning of Ni and Cr to the gamma-phase, and the partitioning of Al to the gamma'-phase, but after 0.25 h, Ru, which partitions to the gamma-phase, decreases the partitioning of Ni and increases the partitioning of Al and Cr.;The temporal evolution of a Ni-10.0 Al-8.5 Cr-1.0 Re-1.0 Ru at.% alloy aged at 1073 K is investigated using TEM and APT. The gamma'-precipitate morphology is spheroidal through 256 h of aging as a result of Re and Ru additions, which decrease the lattice parameter misfit between the gamma- and gamma'-phases to 0.0005 from 0.0026 for a base Ni-10.0 Al-8.5 Cr at.% alloy. The addition of Re + Ru to the base Ni-Al-Cr alloy decelerates the coarsening kinetics more than the addition of Re or Ru alone, and accelerates the compositional evolution of the gamma- and gamma'- phases, which achieve their equilibrium concentrations after 1 h.;For Ni-10.0 Al-8.5 Cr-1.0 W-0.5 Re-0.5 Ru at.%, the addition of W to form increases the lattice parameter misfit, and hence decreases the time and precipitate radius at which a spheroidal-to-cuboidal morphological transition occurs. The additions of Re, Ru or W accelerate the evolution of volume fraction, o(t), while the simultaneous additions of Re, Ru and W decelerate the evolution of f(t). The time exponents and coarsening rate constants of the evolution of average radius, <R(t)>, are dependent on refractory additions. Different additions change the absolute values of precipitate number density, Nv(t), while the coarsening time exponents for Nv(t) do not vary among the alloys studied.;The partitioning behavior and sublattice site preference of Re or Ru in Ni3Al (L12) gamma'-precipitates of model Ni-Al-Cr alloys are investigated utilizing APT and first-principles calculations. Rhenium and Ru are experimentally observed to partition to the gamma(fcc)-phase, which is consistent with the smaller values of the gamma-matrix Re and Ru substitutional energies determined by first-principles calculations. APT measurements of the gamma'-precipitate composition indicate that Re and Ru occupy the Al sublattice-sites of the Ni3Al (L1 2) phase. The preferential site substitution of Re and Ru at Al sublattice sites is confirmed by first-principles calculations. |
