Effects of quaternary alloying elements on the γ′ solvus temperature of Co–Al–W based alloys with fcc/L12 two-phase microstructures

The effects of quaternary elements on the γ′ solvus temperature of Co–Al–W based alloys with fcc/L12 two-phase microstructures have been investigated both experimentally and theoretically. The γ′ solvus temperature and partitioning behavior predicted by thermodynamic calculation agree well with those experimentally determined, indicating the usefulness of thermodynamic calculation in predicting the variation of the γ′ solvus temperature of Co–Al–W based alloys with quaternary elements. All investigated quaternary elements, except for Re, increase the γ′ solvus temperatures of Co–Al–W based alloys with varying efficiencies depending on quaternary element. Of the investigated quaternary elements, Nb, Ta and Ti are found to be the most effective in increasing the γ′ solvus temperature of Co–Al–W based alloys, which is consistent with the prediction from thermodynamic calculation. They have common thermodynamic characteristics that the ordering tendency with Co is high (but not significantly) and the phase-separation tendency with W is not so high.

Research highlights▶ Basic thermodynamic calculation is useful in predicting the variation of the γ′ solvus temperature of Co–Al–W based alloys. ▶ Nb, Ta and Ti are predicted to significantly increase the γ′ solvus temperature of Co–Al–W based alloys. ▶ Nb, Ta and Ti have a common characteristic that they are γ′ forming elements.