Enhanced microstructural stability of γ/γ′-strengthened Co-Ti-Mo-based alloys through Al additions

We studied the microstructural stability and high-temperature mechanical properties of γ/γ′ Co-Ti-Mo-based alloys of varying compositions (Co-12Ti-4Mo, Co-12Ti-4Mo-2Al, Co-12Ti-4Mo-4Al, Co-10Ti-4Mo-2Al, and Co-8Ti-4Mo-4Al (at.%)) using electron microscopy and density functional theory. The Co-12Ti-4Mo alloy underwent discontinuous precipitation into FCC (γ) and D019 (χ) phases after aging at 800°C for 500 h while the Co-12Ti-4Mo-4Al alloy maintained the γ/γ′ microstructure. Al additions improved the stability of the γ/γ′ microstructure by forming L21 (β′) precipitates along the grain boundaries, reducing the supersaturation of the γ matrix and the FCC/L12 interface energy. The Co-12Ti-4Mo-4Al alloy showed lower yield strength compared to the Al-free alloy, however, the difference in yield strength between these two alloys decreased with increasing temperature. This decrease is counterbalanced by a substantially enhanced specific yield strength at elevated temperatures as compared to other well-known Co-based alloys such as MarM302 and Co-9Al-9W.