ATOMIC-SCALE APFIM AND TEM INVESTIGATION OF GRAIN-BOUNDARY MICROCHEMISTRY IN ASTROLOY NICKEL-BASE SUPERALLOYS

Electron microscopy and atom-probes techniques, including 3D (three-di mensional) atom-probe, were applied to the investigation of grain-boun dary (GB) microchemistry and interfacial segregation phenomena in nick el base superalloys, Astroloy. Diffraction patterns and EDX microanaly ses exhibit the presence of intergranular M(23)C(6) chromium enriched carbides as well as intragranular TiC precipitates. Complex phases con taining Zr. C and S were also observed. Three-dimensional images as pr ovided by the tomographic atom-probe revealed the presence of a strong segregation of both boron and molybdenum at grain boundaries. Conside rable chromium enrichment at gamma'-gamma' grain boundaries and slight carbon segregation to GBs, whatever their chemical nature, were also detected. All these segregants are distributed in a continuous manner along the boundary over a width close to 0.5 nm. Experiments show that segregation occurs during cooling and more probably between 1200 and 800 degrees C. Boron, chromium and molybdenum GB enrichments art thoug ht to be mainly controlled by an equilibrium segregation process. No s egregation of zirconium was detected. Copyright (C) 1996 Acta Metallur gica Inc.