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 o f precipitation of the Ni3Al-type ordered gamma' phase in a model Ni-b ased superalloy Ni-Al-Cr. Investigations were conducted on a low super saturated 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 c ells). 3D images reveal that small ordered gamma' particles, 2 nm in d iameter, are present after 1 h. Their composition is close to the equi librium composition of the gamma' phase and does not evolve as ageing proceeds. The time evolution of particle size and composition of the m atrix suggests that a growth mechanism is involved before 4 h, followe d 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 a nd ternary Ni-Cr-Al alloys leads to the conclusion that coarsening is essentially controlled by volume diffusion of Al. Interpretation of th ese results in the framework of Lifshitz-Slyozov-Wagner coarsening the ory allowed the gamma-gamma' interfacial energy to be estimated as 0.0 11 J m(-2). 3D AP data for early stages of unmixing are thus interpret ed within classical nucleation theory. The driving force for nucleatio n is calculated for the ternary Ni-Al-Cr system. On the basis of the t hermodynamic 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.