Investigation of Serrated Flow in Intercritically Annealed Medium Manganese Steel by In Situ Synchrotron X‐Ray Diffraction

In situ X‐ray diffraction tensile experiments with high data collection frequency were conducted to investigate the occurrence of serrations in an intercritically annealed medium manganese steel. The tensile tests were conducted at two different constant strain rates of 2.5 × 10 −4 and 1 × 10 −1 s −1 , as well as with strain rate jump tests. The results show that varying the intercritical annealing temperature leads to differences in microstructures and mechanical properties of the test material. Serrations were observed in the stress–strain curve of samples annealed at 740°C tested at the lower strain rate, while samples annealed at 780°C did not show such serrations. The results suggest that serrations originate as a consequence of the dynamic strain aging (DSA) phenomenon. However, the precise origin of DSA could not be specified. Strain‐induced stepwise phase transformation of austenite, i.e., a burst of transformation followed by a pause, was observed at the lower strain rate in the samples annealed at 740°C. The stepwise transformation was induced by the continuously moving Portevin–Le Chatelier (PLC) bands that nucleate due to DSA. The bands exhibit a higher strain than the bulk of the sample, and therefore repeated passage of PLC bands through the sample triggered stepwise transformation.