Strain rate dependent microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy

Isothermal constant true strain rate compression experiments coupled with electron back scattered diffraction (EBSD) analysis have been employed to understand the role of strain rate on the microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy. The flow behaviour of the alloy deformed at constant deformation temperature (1150 °C) and varying strain rates (0.001 s−1, 0.01 s−1, 0.1 s−1, 1 s−1) revealed two distinct strain rate dependent deformation characteristics. Microstructural observations revealed prevalence of superplastic flow behaviour preceded by dynamic recrystallization (DRX) with dynamic grain growth at the lowest strain rate studied. Sluggish degree of DRX is observed at intermediate strain rates characterized by low volume fraction of DRX grains and dominant presence of deformation substructure bounded with diffuse grain boundaries. Accelerated DRX is noticed at the highest strain rate owing to the higher accumulated stored energy driven large misorientation gradient and adiabatic temperature rise. Discontinuous DRX is found to be the prevalent DRX mechanism in the investigated conditions, which is substantiated by the evidence of necklace type microstructure, serrated grain boundaries, misorientation gradient analysis and relative fraction of grain boundaries. Towards the end, the dependence of strain rate on the evolution of primary twin boundaries (Σ3) and higher order twin boundaries (Σ9 & Σ27) are studied.