TEMPORAL EVOLUTION OF NANOSTRUCTURES IN A MODEL NICKEL-BASE SUPERALLOY: EXPERIMENTS AND SIMULATIONS

摘要

The temporal evolution of the nanostructure of a model Ni-base superalloy (Ni-5.2 at.% Al-14.2 at.% Cr) is studied experimentally employing three-dimensional atom-probe (3DAP) microscopy in conjunction with kinetic Monte Carlo (KMC) simulations at 600℃. It is demonstrated that not only can the mean compositions of individual γ' (Ni_3Al with the L1_2 structure) precipitates be measured but the Ni, Al, and Cr concentration profiles within the precipitates can also be determined for precipitates with a mean radius () as small as 0.85 nm. The three asymptotic time dependencies of the Lifshitz-Slyzov-Wagner (LSW) theory of coarsening (Ostwald ripening) are measured and found to deviate from its theoretical predictions; possible explanations for these discrepancies are discussed. At 0.25 hr. There is 3DAP microscope evidence for the presence of precipitates of another nickel-rich phase, ≈"Ni_3Cr" (Ni_3Cr_(1-x)Al_x), which exhibits short-range order (SRO) and that is metastable with respect to Ni_3Al. This metastable phase is also found by KMC simulations and has the composition Ni_3Cr_(1-x)Al_x, which is Ni-2.91 at.% Al-21.98 at.% Cr at 16 hours. Our results demonstrate that the decomposition of the primary γ (FCC) phase results in the concurrent formation of an ordered phase and a disordered phase by 0.25 hours.