Investigation on γ′ stability in CoNi-based superalloys during long-term aging at 900 °C.

γ′-strengthened CoNi-based superalloys containing Ti and Ta are studied to investigate the γ′ stability after long-term aging at 900 °C. Both thermodynamic predictions and experimental observations suggest that there is no formation of CoAl or Co 3 W phases, indicating that the γ′ phase remains stable in the investigated CoNi-based superalloys. Typical γ/γ′ structure with a relatively high amount of γ′ phase remains in the microstructures of explored CoNi-based superalloys containing Ti and Ta after aging for 3000 h. The secondary MC carbides separate out from γ matrix during aging and different orientation relationships between MC and matrix exist for the explored CoNi-based superalloys: (100) γ // (100) MC , [ 001 ] γ // [ 001 ] MC for the alloy with Ti; (11 1 ¯ ) γ // ( 1 ¯ 1 1 ¯ ) MC , [ 011 ] γ // [ 011 ] MC for the alloy with Ta. Nanoindentation was performed to evaluate the mechanical properties of individual γ channel and γ′ precipitate. It is found that indentation modulus of γ′ phase in the explored CoNi-based superalloys locate between those of Ni 3 Al and Co 3 (Al, W) alloys, and Ta exhibits stronger strengthening effects as compared to that of Ti. • γ′ phases are stable in CoNi-base alloys after long-term aging at 900 °C. • Young's modulus of γ′ in CoNi-base alloys are between those of Ni 3 Al and Co 3 (Al,W). • Ta has a stronger strengthening effect on γ′ than Ti. [ABSTRACT FROM AUTHOR]