Role of stress state on dynamic recrystallization behaviour of Ni during hot deformation: Analysis of uniaxial compression and plane strain compression

In the present work, the role of deformation stress state (uniaxial compression (UC) and plane strain compression (PSC)) on work hardening, softening and dynamic recrystallization (DRX) behaviour of Ni during hot deformation is investigated. Hot deformation was performed through UC and PSC to a von Mises equivalent true strain of 0.7 at 973 K, 1023 K and 1073 K at strain rates of 0.01 s(-1), 0.1 s(-1) and 1 s(-1) in a thermomechanical simulator. Analysis of flow curves revealed rapid work hardening followed by softening in PSC samples, In UC samples, continuous work hardening at a slower rate was observed and at 1123 K, steady state was achieved. Analysis of the work hardening parameter 'h' and dynamic recovery parameter 'r' from the flow curve data showed that the h and r values of PSC deformed samples are higher than UC deformed samples. The DRX behaviour is dependent on temperature and strain rate and significant difference in DRX fraction was observed between UC and PSC samples deformed at strain rate of 0.1 s(-1). Evaluation of microstructure from electron back scatter diffraction maps showed that DRX fraction is significantly higher in PSC deformed samples. Samples deformed through PSC have higher annealing twin density and number of twins per grain. It is reasoned that annealing twin formation is activated by the presence of shear stress component during PSC mode of deformation. Annealing twin formation and growth in turn facilitate DRX and explain the observed differences between UC and PSC modes of deformation.