On the nanometer scale phase separation of a low-supersaturation Ni-Al-Cr alloy

The phase separation of a Ni-6.5 Al-9.5 Cr at. % alloy aged at 873 K was studied by atom-probe tomography and compared to the predictions of classical precipitation models. Phase separation in this alloy occurs in four distinct regimes: (i) quasi-stationary-state gamma'(L1(2))-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 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 the gamma(fcc)-matrix and gamma'-precipitate supersaturations follow the predictions of classical models.