3-DIMENSIONAL INVESTIGATION OF PRECIPITATION AND INTERFACIAL SEGREGATION PROCESSES ON AN ATOMIC-SCALE

The Tomographic Atom-Probe is a new generation of instrument which pro vides three-dimensional maps of chemical heterogeneities in a metallic material on a subnanometric scale. This technique was applied to the investigation of precipitation and grain-boundary (GB) segregation phe nomena in Astroloy nickel base superalloys. 3D images as provided by t he tomographic atom-probe reveal the serrated geometry of GB's on a ve ry fine scale and the presence of a strong segregation of both boron a nd molybdenum at grain-boundaries. Slight carbon enrichment is also de tected. Considerable chromium segregation is exhibited at gamma'-gamma ' grain-boundaries. Ail these segregants are distributed in a continuo us manner along the boundary over a width close to 0.5 nm. Long range order in the gamma'ordered phase is shown to be preserved up to the gr ain boundary.