  The details of phase separation of a Ni-6.5Al-9.9Mo aged at 978 K for aging
times ranging from 0.125 to 1024 h are investigated by atom-probe tomography
and lattice-kinetic Monte Carlo (LKMC) simulations. On the basis of the
temporal evolution of the nanostructure, three experimental regimes are
identified: (1) concomitant precipitate nucleation and growth (t less than 0.25
h); (2) concurrent coagulation and coalescence (t 0.25 to 16 h); and (3)
quasi-stationary coarsening of gamma prime(L12)- precipitates (t 16 to 1024 h).
The temporal dependencies of the mean precipitate radius and precipitate number
density, Nv(t), are determined experimentally, 0.344 (0.012) and -0.95(0.02),
respectively, following the predictions of quasi-stationary coarsening models.
In this alloy aged at 978 K, Al partitions strongly to the gamma
prime(L12)-phase with a partitioning coefficient 4.06(0.04), whereas Mo and Ni
partition to the gamma(f.c.c.)-matrix with values of 0.61(0.01) and 0.90(0.01),
respectively. In the quasi-stationary regime(t larger than 16h), the temporal
exponents of the Al, Mo, and Ni supersaturations in both the
gamma(f.c.c.)-matrix and gamma prime(L12)-precipitates are in reasonable
agreement with a multi-component coarsening model's prediction of -0.33.
Quantitative analyses of the edge-to-edge inter-precipitate distances
demonstrate that coagulation and coalescence are consequences of the overlap of
the diffusion fields surrounding the gamma prime(L12)-precipitates. Both 3-D
APT and LKMC results demonstrate that the interfacial compositional width
decrease with increasing values. And the interfacial compositional width at
infinite ageing time are estimated to be 1.89(0.22) nm, 2.09(0.12) nm and
2.64(0.03) nm for Ni, Al and Mo, respectively.
