Processing maps for hot deformation of a high-Mn TWIP steel: A comparative study of various criteria based on dynamic materials model

The hot deformation behavior of a high-Mn twinning-induced plasticity (TWIP) steel was studied using processing maps over the temperature range of 900–1150 °C and strain rate range of 0.001–20 s−1. Both the strain rate sensitivity map and power dissipation map exhibited a peak domain in the temperature range of 950–1150 °C and strain rate range of 0.01–1 s−1, where dynamic recrystallization (DRX) was the primary restoration mechanism. DRX was also found to be dominant at higher strain rates where the values of strain rate sensitivity (m) and efficiency of power dissipation (η) were relatively low. Although the flow curves at higher strain rates did not show any well-defined peak stresses, both the work hardening analysis and microstructural observations indicated the occurrence of DRX. Two types of criteria based on the dynamic materials model (DMM), i.e., Gegelʹs and Alexander–Malasʹs stability criteria based on Lyapunov functions and Kumar–Prasadʹs, Murty–Raoʹs and Babuʹs instability criteria based on Zieglerʹs flow theory, were used to develop the instability maps. It was found that the instability maps based on Gegelʹs and Alexander–Malasʹs stability criteria (or based on Kumar–Prasadʹs, Murty–Raoʹs and Babuʹs instability criterion) were essentially similar. However, comparisons between the two types of DMM criteria suggested that the instability maps were quite different from each other. The instability criteria based on Zieglerʹs flow theory were found to be more reliable in predicting the occurrence of flow instabilities, which manifested as inhomogeneous flow and localized deformation bands.