Site occupancy of alloying elements in the L12 structure determined by channeling enhanced microanalysis in γ/γ' Co-9Al-9W-2X alloys

Knowledge about the sublattice site preference of alloying elements in the L1₂-γ′ phase of novel Co-base superalloys is a necessary pre-requisite to understand their influence on the properties of the alloys in general and the γ′ phase in particular. In the present study, the atomic site occupancy of the alloying elements in the L1₂-γ′ structure in Co-9Al-9W-2X quaternary alloys after long-term annealing at 900 °C for 5000 h was determined using the atom location by channeling enhanced microanalysis (ALCHEMI) technique in combination with energy-dispersive X-ray spectroscopy (EDX) composition analysis in a transmission electron microscope (TEM). The experimental ALCHEMI data were evaluated by comparing them with those calculated by the program 'Inelastic Cross Section Calculator' (ICSC). The results show that Co mainly occupies one sublattice site and Al/W are located at the other sublattice site in the L1₂ unit cell in the ternary alloy. The additional elements Ti, V, Mo and Ta which partition strongly to the γ′ phase tend to occupy the Al/W sublattice site, and Cr which partitions more to the γ phase also favors the Al/W sublattice site, while Ni weakly partitions into the γ′ phase and favors the Co sublattice site. The results of this study can provide evidence to the predictions on the site preference in literature based on the phase composition or on theoretical studies.