Atom probe tomography investigation of the microstructure of superalloys N18

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 gamma precipitates (57%) that are Al and Ti enriched. Cr, Co and Mo preferentially partition to the gamma solid solution. Small Cr-enriched particles (0) were discovered in the core of largest gamma particles. These contain more Cr and less Mo than the gamma solid solution but are thought to be tiny metastable gamma particles that were formed in the inner region of gamma particles during cooling from 800 degreesC. 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. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.