Atom probe tomography investigation of the microstructure of superalloys N18 Original Research Article

Microstructural features of nickel base superalloys N18 have been investigated by transmission electron microscopy (TEM), field emission gun-scanning electron microscopy (FEG-SEM), field ion microscopy (FIM), one-dimensional atom probe (APFIM) and three-dimensional atom probe (3DAP). Heat treatments conducted at 700 and 800°C lead to the precipitation of a large volume fraction of γ′ precipitates (57%) that are Al and Ti enriched. Cr, Co and Mo preferentially partition to the γ solid solution. Small Cr-enriched particles (θ) were discovered in the core of largest γ′ particles. These contain more Cr and less Mo than the γ solid solution but are thought to be tiny metastable γ particles that were formed in the inner region of γ′ particles during cooling from 800°C. Intergranular Ni-depleted borides arranged in “clusters” of particles very close to each other were observed using 3DAP. Their stoichiometry is M2B with M=Cr, Mo. One nanometer-thick Ni and Co enriched films (compared to borides) separate adjoining boride particles. Grain boundary (GB) segregation of B, Mo and to some extent Cr has been observed. The thickness of the segregated layer wetting GB is close to 1 nm.