Evolution and Stability of the Two-Phase γ-γ' Microstructure in Co-Al-W Alloys

The discovery of a two-phase γ (FCC)-γ' (L1₂) field in the ternary Co-Al-W phase diagram has sparked significant research interest into possible Co-based superalloys analogous to traditional Ni-based ones used in turbine engine applications.  This work experimentally investigates microstructural in these ternary Co-Al-W alloys.  The evolution pathway in forming the familiar cuboidal γ-γ' is different than that found in Ni-based alloys.  Nucleation of ordered γ' precipitates 2 nm to 10 nm in diameter cannot be suppressed even under rapid solidification conditions.  Upon subsequent annealing, growth of these nuclei into irregular, non-cube shaped precipitates is very rapid, resulting in a volume fraction of γ' roughly equal to that in metastable equilibrium with the γ phase after only a few minutes.  The microstructure then slowly coarsens to reveal the familiar cuboidal γ-γ' microstructure similar to that found in Ni-based superalloys.  The two-phase microstructure is ultimately destroyed by the nucleation and growth of the additional phases Co₃W (D0₁₉) indicating γ' is metastable in ternary Co-Al-W.  The γ' is, however, very competitive with the equilibrium phases, and small alloying additions, readily stabilize it.