Ordering and precipitation of gamma' phase in low supersaturated Ni-Cr-Al model alloy: an atomic scale investigation

Both three-dimensional (3D) atom probe (AP) and transmission electron microscopy (TEM) techniques have been used to study the early stages of precipitation of the Ni3Al-type ordered gamma' phase in a model Ni-based superalloy Ni-Al-Cr. Investigations were conducted on a low supersaturated alloy (5.2 at.% Al and 14.7 at.% Cr) aged at 600 degrees C. Atom probe and TEM data indicate that clustering and ordering already occur after ageing for 15 min on a very fine scale (about three unit cells). 3D images reveal that small ordered gamma' particles, 2 nm in diameter, are present after 1 h. Their composition is close to the equilibrium composition of the gamma' phase and does not evolve as ageing proceeds. The time evolution of particle size and composition of the matrix suggests that a growth mechanism is involved before 4 h, followed by a coarsening regime up to 64 h. The comparison of the coarsening rate constant with data available in the literature for binary Ni-Al and ternary Ni-Cr-Al alloys leads to the conclusion that coarsening is essentially controlled by volume diffusion of Al. Interpretation of these results in the framework of Lifshitz-Slyozov-Wagner coarsening theory allowed the gamma-gamma' interfacial energy to be estimated as 0.011 J m(-2). 3D AP data for early stages of unmixing are thus interpreted within classical nucleation theory. The driving force for nucleation is calculated for the ternary Ni-Al-Cr system. On the basis of the thermodynamic data and phase diagrams available in the literature, the critical radius of nucleation is estimated to be 0.5 nm. This value is in good agreement with experimental data.