Role of Deformation Parameters, Delta Phase and Twin Boundaries on the Microstructural Evolution of Hot Deformed Nickel Superalloy

Hot deformation behaviour of solution treated wrought nickel-based superalloy over wide temperature (800 °C, 850 °C, 900 °C and 950 °C) and strain rate (0.0003 s⁻¹, 0.001 s⁻¹, 0.01 s⁻¹, 0.1 s⁻¹ and 1 s⁻¹) regimes revealed continuous flow softening characteristics at higher strain rates and mixed steady state and flow softening behaviour at lower strain rates. The role of temperature, strain rate and strain on the hot deformation induced microstructural evolution evaluated employing detailed electron back scattered diffraction studies divulged complex dependence of dynamic recrystallization (DRX) kinetics. Enhancement in DRX kinetics is observed with increasing temperature and strain driven by improved grain boundary mobility and increased deformation stored energy respectively. Meanwhile, DRX is maximized at intermediate strain rate (0.001 s⁻¹) and decreased DRX is revealed at higher (0.01 s⁻¹) and lower (0.0003 s⁻¹) strain rates. The revelation of necklace type microstructure along with serrated grain boundaries substantiated the dominance of discontinuous dynamic recrystallization and the absence of continuous dynamic recrystallization is manifested through the stabilized medium angle grain boundaries evolution. The role of twin boundaries in accelerating DRX is manifested through the gradual increase in twin boundary evolution with increasing DRX fraction. Moreover, the twin boundaries concomitantly evolved with increasing DRX fraction exhibited gradual decline in average misorientation deviation from the ideal twin boundary misorientation corroborating the role of twin boundaries driven DRX. The δ phase assisted particle stimulated DRX nucleation is manifested through the copious DRX nuclei formation in the vicinity of δ phase, meanwhile the δ phase aided grain boundary pinning is also corroborated.