Fig. 1. Calculated vertical sections based on Co-20Ni-7Al-8W-1Ta-4Ti: (a) 1:1 substituting Ni for Co; (b) increasing Ni and W from 20 at.% to 40 at.% and 0 at.% to 8 at.%, respectively; (c) 1:1 substituting Cr for Co; and (d) increasing Cr and decreasing W from 0 at.% to 15 at.% and 8 at.% to 0 at.%, respectively.

Fig. 2. (a) Cross-sectional schematic of the overall diffusion multiple, the diffusion couples investigated in this study are marked with yellow circles; and (b) schematic of the pure Ni cover that can prevent the oxidation during heat treatment.

Fig. 3. Schematic of the EPMA beam circles for the measurement of concentration profiles of the diffusion-couples.

Fig. 4. SEM-BSE images of typical microstructures of the initial alloys in the investigated diffusion-multiple after being aged at 1000 °C for 1000 h: (a) Base; (b) 40Ni; (c) 0W; and (d) 15Cr0W.

Fig. 5. SEM-BSE images of typical microstructures of the diffusion-couples in the investigated diffusion-multiple after being aged at 1000 °C for 1000 h: (a) Base-40Ni (including the SADP of the χ phase); (b) 0W-40Ni; (c) Base-0W; and (d) Base-15Cr0W.

Fig. 6. SEM-SE images with high magnification of typical microstructures of Base-15Cr0W diffusion-couple: (a) the base alloy side; and (b) the 15Cr0W side.

Fig. 7. The experimental concentration profiles for the investigated diffusion-couples after heat treatment at 1250 °C for 24 h and 1000 °C for 1000 h: (a) Base-40Ni; (b) 0W-40Ni; (c) Base-0W; and (d) Base-15Cr0W. All the compositions were measured using EPMA.

Fig. 8. The γ′ volume fraction profiles for three diffusion couples: (a) Base-40Ni; and (b) Base-0W and Base-15Cr0W.

Fig. 9. Elemental partitioning behavior of Base-40Ni diffusion couple at 1000 °C, analyzed by high-throughput thermodynamic calculation using the compositional data of this diffusion-couple. (a) and (b) are the elemental concentration profiles of γ and γ′ phases, respectively; and (c) and (d) are the elemental partitioning coefficients as a function of diffusion distance and Ni content, respectively.

Fig. 10. Partitioning coefficients at 1000 °C as a function of W and Cr calculated using the compositional data for (a) Base-0W and (b) Base-15Cr0W diffusion-couples, respectively.

Fig. 11. The comparison of the calculated and experimental concentration profiles for the Base-40Ni diffusion-couple after heat treatment at 1250 °C for 24 h and 1000 °C for 1000 h. Calculated and experimental profiles are represented by lines and symbols, respectively.

Fig. 12. The changes of the γ′ solvus temperature in Co-20Ni-7Al- xW-1Ta-4Ti alloys with different Cr additions (calculated results).

Fig. 13. Calculated isothermal section at 1000 °C through pseudo-ternary Co-W-Ni at constant 7Al-1Ta-4Ti superimposed with the measured compositions and corresponding phase constituents of the Base-40Ni, Base-0W and 0W-40Ni diffusion couples.