Dynamic recrystallization of a β(B2)-Stabilized γ-TiAl based Ti–45Al–8Nb–2Cr-0.2B alloy: The contributions of constituent phases and Zener-Hollomon parameter modulated recrystallization mechanisms

Dynamic recrystallization (DRX) behavior of a high Nb-containing cast γ-TiAl based Ti–45Al–8Nb–2Cr-0.2B (at.%) alloy has been studied during hot compressive deformation in the temperature range of 1000–1200 °C and the strain rate range of 0.5–0.005 s−1. Detailed microstructural evolution in conjunction with flow curve analysis suggests that flow softening is associated with differential DRX behavior of the constituent phases as well as deformation instability. Such differential DRX of constituent phases leads to a considerable departure from the power-law dependence of the critical stress (σc) and strain (εc) parameters derived through the Poliak-Jonas analytical methodology. Further analysis of the kinetics of flow softening by the modified Avrami type Arrhenius equation indicates that the strain rate strongly influences the Avrami constants. The resultant curves of DRX kinetics predict a characteristic “C”-shape instead of the sigmoidal-shape presumably due to a flow instability facilitated by the differential contribution of constituent phases. Detailed characterization of microstructural evolution as a function of the Zener-Hollomon parameter (Z) is further augmented through phase-resolved sub-grain structures, misorientation (KAM) maps, and recrystallized grains evaluation by EBSD technique. The analyses in corroboration with transmission electron microscopy suggest that discontinuous dynamic recrystallization (DDRX) dominates at the high Z condition leading a fine and homogeneous grain structure. At lower Z conditions, on the other hand, a broad spectrum of recrystallized grain size distribution is developed as a result of a combination of DDRX and CDRX processes.