Elemental Sub-Lattice Occupation and Microstructural Evolution in gamma/gamma ' Co-12Ti-4Mo-Cr Alloys

Abstract

We report on comparative atom probe tomography investigations of gamma/gamma'-forming Co-12Ti-4Mo-Cr alloys. Moderate additions of Cr (2 and 4 at%) reduced the gamma/gamma' lattice misfit and increased the gamma' volume fraction of a Co-12Ti-4Mo alloy significantly. These microstructural changes were accompanied by changes in the elemental partitioning between gamma and gamma' and site-occupancy in gamma'. Spatial distribution maps revealed that Mo occupied both Co and Ti sub-lattice sites in gamma'. In agreement with the experimental data, thermodynamic calculations predicted a stronger tendency for Mo to occupy the Co-sites than for Cr and an increase in Cr fraction on the Ti-sites with increasing Cr content.