Work hardening characteristics and microstructural evolution during hot deformation of a nickel superalloy at moderate strain rates

Abstract

Hot deformation behaviour of hot isostatically processed (HIPed) experimental nickel superalloy in the moderate strain rate regimes (0.01 s−1 & 0.1 s−1) is investigated by carrying out isothermal constant true strain rate compression experiments over wide temperature ranges (1000 °C, 1050 °C, 1100 °C and 1150 °C). From the experimentally obtained flow curves, the effect of deformation temperature on the work hardening behaviour is analyzed. Three stage behaviour exhibited by the work hardening characteristics is considerably influenced by deformation temperature and strain rate. The critical stress (σc) and critical strain (ec) for triggering dynamic recrystallization determined from the work hardening curves is related to Zener-Hollomon parameter through a power law relationship. Deformation microstructures investigated by Electron Backscattered Diffraction (EBSD) studies highlighted the dominance of discontinuous dynamic recrystallization (DDRX) evidenced by serrated/bulged grain boundaries and necklace type microstructure, although DRX volume fraction is strongly sensitive to deformation conditions. Dislocation accumulation studied by grain reference orientation deviation (GROD) maps corroborates to the observed deformation microstructures. Modest DRX is observed at the lowest deformation temperatures owing to the insufficient thermal energy; however, DDRX kinetics is enhanced at higher temperatures. Additionally, the grain boundary characteristics and misorientation angle distribution are studied and correlated with the microstructural evolution.