The contributions of multiple strengthening factors to the 0.2% flow stress have been studied in a cast and wrought Ni-Co base disk superalloy with varying grain size, twin fraction and bimodal or trimodal ′ size distributions. The alloy was heat-treated within the ′ sub-and super-solvus temperature ranges between 1100 o C to 1180 o C, followed by two step aging heat-treatment. The contribution of each strengthening factor are analyzed by measuring its Vickers hardness at over a structural scale ranging from nano to micro levels and then by converting the hardness differences to the contribution to the 0.2% flow stress on the basis of the empirical relation: 0.2 =2.46Hv. The results clearly show that the contribution of grain boundary strengthening decreased and reached zero with increasing solution temperature. The behavior followed Hall-Petch effect and Zener's pinning model, and strongly depended on the size and volume fraction of pinning primary ′ that is un-dissolved during solution treatment. Meanwhile, the contributions of secondary and tertiary ′ precipitation strengthening increased with increasing temperature. The behaviors also strongly depend on the volume fraction of the primary ′ since the designed total volume fraction of ′ of the alloy is approximately 0.5. As a result, the combinations of these strengthening factors lead to the maximum flow stress at solution temperature of 1135 o C. This would suggest that the primary ′ plays an important role to optimize the microstructure combination through the solution heat-treatment.
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