The effects of alloying elements (Ta, Hf) on the thermodynamic stability of γ′-Co3(Al,W) phase

This paper investigated the effects of alloying elements (Ta, Hf) on the thermodynamic stability of γ′-Co3(Al,W) phase with an L12 structure at 900 °C, based on microstructure observations and electron microprobe analyses on bulk alloy samples heat-treated for periods up to 2000 h. An addition of up to 2.2 at. % Ta was suggested not to change the phase equilibria in the Co–Al–W ternary system: The γ′ phase is metastable and the three phases of γ-Co, CoAl and Co3W are in equilibrium. On the other hand, a small addition of Hf was found to stabilize the γ′ phase, which is in equilibrium with γ, CoAl, Co3W and a Hf-rich phase at a composition around Co-10.2Al-12.3W-0.3Hf in at. %.

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► We report alloying effects on the stability of γ′γ′ phase in Co–Al–W based alloys.
► 2.2 at. % Ta addition does not stabilize the γ′γ′ phase at 900 °C.
►  A small addition of Hf stabilizes the γ′γ′ phase at 900 °C.
► Thermodynamically stable γ/γ′γ/γ′ phases can be formed in Co–Al–W–Hf alloys.