Microstructural Evolution and Mechanical Behavior during Isothermal Multiaxial Forging of Nickel-Based Superalloy SUPERNI 718

Ni-Fe superalloy SUPERNI 718 with superior mechanical properties is processed through conventional forming methods for producing critical rotating aeroengine components, however for realizing large sized components, alternate industrial scale processing methods like incremental superplastic rolling forming (SPRF) method is explored to counteract the requirement of large tonnage forging facilities. In order to meet the essential prerequisite of superplasticity for SPRF, establishing viable severe plastic deformation methods for processing fine grained structure in bulk materials is challenging. This work is focused on achieving fine grained structure by severe plastically deforming IN718 superalloy through elevated temperature multiaxial forging (MAF) which involves repetitive imposition of compressive deformation. Solution-treated SUPERNI 718 superalloy samples are subjected to isothermal MAF up to 3 cycles thereby imparting total effective strain of 3.3. Microstructural evolution of deformed samples characterized employing multiple techniques like light microcopy, scanning electron microscopy, and electron back scattered diffraction revealed the formation of fine grains (similar to 0.9 mu m) after 3 cycles of MAF aided by discontinuous dynamic recrystallization along with deformation induced delta precipitates. The room temperature tensile behavior indicated significant improvement in yield strength and tensile strength by twice and 1.3 times, respectively, driven by continuous grain refinement. The Hall-Petch relationship is elucidated and the yield strength of the deformed SUPERNI 718 is modeled and benchmarked.