Metastable intermetallic phase precipitation from supersaturated Co solid solution and the consequences on microstructure evolution
The precipitation of intermetallic phases from a supersaturated Co(Nb) solid solution is studied in a cooperation with the Hokkaido University of Science, Sapporo.
These investigations show that the formation of metastable, coherent L12 NbCo3 strongly affects the microstructural evolution and growth of the equilibrium phase Nb2Co7. While these metastable fine L12 precipitates always form on directly cooling from the single-phase solid solution range to room temperature, their clearly visible formation reaction can be circumvented by an intermediate annealing step below the solvus temperature. This annealing results in a Widmannstätten-type microstructure with fine plates of the equilibrium Nb2Co7 phase in a Co matrix as shown in Fig. 1.
Fig. 1: Microstructure of a Co-4 at.% Nb alloy that was kept for 100 h at 900 °C subsequent to a solid-solution heat-treatment at 1240 °C. The Widmanstätten-like morphology resulting from precipitation of the needle-shaped intermetallic phase Nb2Co7 from supersaturated Co(Nb) solid solution is nicely visible. The picture was taken by K. Angenendt with a Zeiss Merlin SEM in FSD mode. Different colours indicate different crystallographic orientations of the Co matrix.
