Development of fine-grained structure and the mechanical properties of nickel-based Superalloy 718

Highlights: ► Grain refinement of Superalloy 718 can be achieved by good combination of solution treatment, cold forming and recrystallization treatment. ► A niobium-oversaturated matrix in addition to sufficient plastic deformation is essential for subsequent recrystallization. ► Formation of uniformly fine dispersed precipitates caused pinning effect for recrystallized grain to grow. ► A fine-grained structure with grain sizes less than 1 μm can be achieved. - Abstract: Grain refinement is the essential concern in producing Superalloy 718 components. Thermomechanical process as a means of producing fine grain structure has attracted significant attention. However, hot forging with deviations on forging temperature results in significant changes in the microstructure. High volume fractions of δ phase thus produced reduce the amount of niobium available for γ″ formation and consequently lower the strength. In this work, the combination of heavy plastic deformation with subsequent recrystallization was utilized as an effective method to achieve a fine-grained structure. Specimens in this process were first solution treated at high temperatures and then followed by plastic deformation. Subsequent recrystallization caused an immediate precipitation of a fine, uniformly dispersed δ phase from the niobium oversaturated matrix that prohibited grain growth. A fine-grained structure with grain sizes <1 μm was obtained, smaller than 2.9 μm that has ever been reported in literature. Tensile test results demonstrate a significant improvement in strength and ductility tested at temperatures of 25 °C and 650 °C, respectively.