ATOMIC-SCALE INVESTIGATION OF ORDERING AND PRECIPITATION PROCESSES INA MODEL NI-CR-AL ALLOY

The tomographic atom probe has been applied to the study of early stag es of precipitation of the Ni3Al-type ordered gamma' phase in a model nickel base superalloy Ni-Cr-Al. The investigation of a low-supersatur ated alloy (15 at% Cr, 5 at% Al) aged at 600 degrees C shows that smal l gamma' particles 2 nm in diameter form already after 1 h. Even for s uch very small sizes, particles are well ordered. Their composition is close to the equilibrium composition of the gamma' phase and does not evolve as ageing proceeds. The time evolution of the particle size an d of the composition of the gamma matrix suggests that a growth mechan ism is involved before 4 h, followed by a coarsening regime.