EBSD investigation and modeling of the microstructural evolutions of superalloy 718 during hot deformation

Abstract

Hot deformation tests of superalloy 718 were carried out both in torsion and uniaxial compression in order to investigate the microstructural evolutions of this alloy. Post-deformation holding times were also performed. The obtained dynamic and metadynamic - most often partially - recrystallized microstructures were characterized using the EBSD technique. Particular attention was paid to the local crystallographic rotation of specific subgrains as it leads to the nucleation of most of the first new grains. Twin boundary generation, and consecutive disorientation, for producing the second and further generations of recrystallized grains, were also studied. Thanks to these data, an empirical model, based on Avrami formulation, was developed. Furthermore, as δ phase (Ni3Nb) precipitation can modify significantly the recrystallization and subsequent grain growth kinetics of alloy 718, a mechanism-based model was designed to take into account the coupling effects of recrystallization and δ phase precipitation.