Hot Deformation Behavior and Microstructure Evolution of GTD222 Nickel-Based Superalloy

Hot workability of GTD222 nickel-based superalloy was examined by isothermal compression over 1010–1090 °C and 0.01–10 s−1 to a true strain of 1.2. The flow resistance increased as the temperature decreased or the strain rate increased. The measured curves were corrected for interfacial friction and adiabatic heating before constitutive analysis. A peak-stress relation and a strain-compensated Arrhenius model were then established; for the calibration dataset, the latter gave a correlation coefficient (R) of 0.978 and an average absolute relative error (AARE) of 6.89%. Processing maps derived from the dynamic materials model (DMM) and Prasad’s instability criterion at true strains of 0.3, 0.6, 0.9, and 1.2, together with microstructural evidence, delineated 1070–1090 °C and 0.01–0.1 s−1 as a favorable hot-working domain. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed that the dynamic recrystallization (DRX) fraction did not change monotonically with either temperature or strain rate, although the recrystallized grains became coarser with increasing temperature. At 1090 °C, the DRX fraction fell to 46.13% at 1 s−1 and recovered to 76.39% at 10 s−1, plausibly reflecting the competing influences of deformation time, stored deformation energy, and adiabatic heating. Grain-boundary bulging, serration, and necklace-like recrystallized grains indicate dominant discontinuous dynamic recrystallization (DDRX), whereas intragranular orientation gradients, cumulative misorientation, medium-angle grain-boundary development, and fine intragranular grains support a supplementary contribution from continuous dynamic recrystallization (CDRX).