Microstructure optimization for higher strength of a new Fe–Ni-based superalloy

Tensile properties and deformation mechanisms of a Fe–Ni-based superalloy with different size of γ′ precipitates obtained through varied aging time are investigated at 700 °C. The γ′ phase size increases with aging time, the yield strength of the alloy increases first and then decreases, which is consistent with the change trend of critical resolved stress of deformation mechanism. The dominant deformation mode of the alloy with fine γ′ phase is weakly-coupled dislocation pairs with slip bands, and then changes to Orowan looping at a γ′ size of approximately 30 nm, resulting in the occurrence of peak value of strength. As the γ′ size is above 40 nm, the deformation mechanism is Orowan bowing along with stacking fault shearing, decreasing the yield strength of the alloy. When γ′ size is between 30 nm and 40 nm, the dominant deformation mechanism is Orowan looping with strongly-coupled dislocation pairs and the fracture mode is intergranular. Intermediate temperature brittleness occurs in the secondary aging treated alloy with a γ′ phase size of approximately 30 nm, which is attributed to the increase in critical resolved stress and strain localization caused by the transformation of tensile deformation mechanism, and stress concentration due to dislocation entanglement at grain boundary. The sub-aging state of the alloy used for advanced ultra-supercritical plants may be a good choice for obtaining simultaneously favorable strength and ductility.