Influence of processing parameters on dynamic recrystallization and the associated annealing twin boundary evolution in a nickel base superalloy

Dynamic recrystallization (DRX) kinetics and its association with annealing twins in alloy 617 has been studied in a range of strain rates (0.001 s−1 to 10 s−1) and temperatures (1173–1473 K). In both low (< 0.1 s−1) and high (≥ 1 s−1) strain rate domains, DRX fraction is high as compared to intermediate strain rate (~ 0.1 s−1). The lower critical strain for DRX initiation with higher growth of DRX grains is responsible for large DRX fraction with coarse grains microstructure in the low strain rate domain. In contrast, DRX at higher strain rates is mainly controlled by the large stored energy along with adiabatic temperature rise which leads to higher DRX nucleation. It is also noticed that the evolution of twins is greatly influenced by deformation temperature at all imposed strain rates. Here, the evolution of twins is quantified by measuring twin boundary length fraction and length density. The twin-length density increases with increase in DRX fraction at low temperatures (< 1423 K) but decreases with increasing DRX fraction at high temperatures (≥ 1423 K). An exponential rise in recrystallized grain size with DRX fraction is observed at a temperature ≥ 1423 K. This suggests that rapid migration of high-angle boundaries happened at ≥ 1423 K which may not be favorable for nucleation of twins during DRX.