Phase transformations and γ′ phase stability in model CoAlW superalloys

The present work aims at understanding the transformation mechanisms of γ′-L1₂ precipitates in the ternary Co-Al-W system from quenched state to thermodynamic equilibrium. Three alloys with compositions Co-9Al-7W, Co-10Al-12W and Co-7Al-9W (at.%) were aged at 900 °C for 10 h, 200 h and 1000 h. The microstructure has been characterized by transmission and scanning electron microscopies, and the phase composition was determined using atom probe tomography. Results show that irrespective of the initial supersaturation, the quenched state is already decomposed into γ+γ'. This indicates that even in the Co-7Al-9W alloy that lies in the γ+Co₃W domain of the phase diagram, γ′ phase forms at first, indicating that γ′ is a metastable phase. In the two other alloys, during ageing at 900 °C γ′ phase dissolves and the equilibrium state is composed of γ, CoAl and Co₃W phases. The γ′ phase in the Co-7Al-9W alloy dissolves more rapidly compared to the two other alloys that are in the γ+γ′ metastable domain. The volume fraction of Co₃W phase increases rapidly with ageing time in the Co-7Al-9W alloy. Whereas γ′ phase is predominant at short ageing times, it becomes a minor phase after 1000 h of ageing, and is replaced mostly by the Co₃W phase. As already observed, the transformation from γ′-L1₂ to Co₃W-D0₁₉ phase is taking place through a stacking fault. This mechanism occurs in the three alloys. In addition, a new dissolution mechanism interpreted as destabilization of the γ′ phase is evidenced in the Co-9Al-7W. The γ′ precipitates tend to split along {111} planes. Transmission electron microscopy investigations show that the dissolution and splitting mechanism of the γ′ phase is initiated by stacking faults, that are subsequently wetted by the γ phase.