  The temporal evolution of ordered gamma prime (L12)-precipitates
precipitating in a disordered gamma(f.c.c.) matrix is studied in extensive
detail for a Ni-12.5 Al at.% alloy aged at 823 K (550 oC), for times ranging
from 0.08 to 4096 h. Three-dimensional atom-probe tomography (3-D APT) results
are compared to monovacancy-mediated lattice-kinetic Monte Carlo (LKMC1)
simulations, which include monovacancy-solute binding energies through 4th
nearest-neighbor distances, for the same mean composition and aging
temperature. The temporal evolution of the measured values of the mean radius,
<R(t)>, number density, aluminum supersaturations, and volume fraction of the
gamma prime(L12)-precipitates are compared to the predictions of a modified
version of the Lifshitz-Slyozov-Wagner coarsening model due to Calderon,
Voorhees et al. The resulting experimental rate constants are used to calculate
the Gibbs interfacial free-energy between the gamma(f.c.c.)- and gamma
prime(L12)-phases using data from two thermodynamic databases, and its value is
compared to all extant values dating from 1966. The diffusion coefficient for
coarsening is calculated utilizing the same rate constants and compared to all
extant archival diffusivities, not determined from coarsening experiments, and
is demonstrated to be the inter-diffusivity, , of Ni and Al. The
monovacancy-mediated LKMC1 simulation results are in good agreement with our
3-D APT data. It is demonstrated that the compositional interfacial width, for
the {100} interface, between the gamma(f.c.c.)- and gamma prime (L12)-phases,
decreases continuously with increasing aging time and <R(t)>, for the 3-D APT
results and monovacancy-mediated LKMC1 simulations, in disagreement with an
ansatz intrinsic to the trans-interface diffusion-controlled coarsening model,
which predicts the opposite trend for binary alloys.
