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Re-investigation of the binary Co-Ti system – Coexistence of the hexagonal and cubic polytype of the Laves phase

The Ni- and Co-based γ/γ’ superalloys are famous for their excellent high-temperature mechanical properties that result from their fine-scaled coherent microstructure of L12-ordered precipitates (γ’ phase) in an fcc solid solution matrix (γ phase). The only binary Co-based system showing this special type of microstructure is the Co-Ti system, where the Co solid solution is the γ phase and TiCo3 the L12-ordered γ’ phase.

Surprisingly, the phase relations in this binary system are not well established and there are fundamental inconsistencies between different versions of the phase diagram. The system is not only of high scientific interest because of the presence of the fine-scaled coherent γ/γ’ microstructure, but also due to the occurrence of two different, stable polytypes of the Laves phase TiCo2 (hexagonal C36 and cubic C15 type). The phase fields of these two polytypes should be separated by a two-phase field the existence of which however has never been clearly proven. Therefore, a re-investigation of the phase diagram is performed by not only making use of equilibrated alloys but also applying diffusion couples (see Fig. 1).

Fig. 1: Diffusion couple made of pure Co and the intermetallic phase TiCo after annealing at 1100°C for 6 days. Two diffusion zones are clearly visible. From measuring the composition profile perpendicular to the vertical boundaries, the diffusion zones can be identified as the Laves phase TiCo2 and the L12-ordered γ’ phase TiCo3.

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