Atomic scale investigation of ordering and precipitation processes in a model NiCrAl alloy

The tomographic atom probe has been applied to the study of early stages of precipitation of the Ni3Al-type ordered γ′ phase in a model nickel base superalloy NiCrAl. The investigation of a low-supersaturated alloy (15 at% Cr, 5 at% Al) aged at 600°C shows that small γ′ particles 2 nm in diameter form already after 1 h. Even for such very small sizes, particles are well ordered. Their composition is close to the equilibrium composition of the γ′ phase and does not evolve as ageing proceeds. The time evolution of the particle size and of the composition of the γ matrix suggests that a growth mechanism is involved before 4 h, followed by a coarsening regime.