Stacking Fault Energy and Microstructural Insight into the Dynamic Deformation of High-Manganese TRIP and TWIP Steels

The dynamic behavior of three high manganese steels with very different stacking fault
energy (SFE) values (4-30 mJ/m2) were studied using high strain rate torsional tests. The hotrolled microstructure of the steel with the lowest SFE of 4 mJ/m2 consisted of a duplex mixture of austenite and ε-martensite, but those of the other two steels were fully austenitic. The deformed microstructures were studied by optical and electron microscopy. The quasi-static deformation of the low-SFE steel was accompanied with profuse martensitic transformation. However, when this steel was deformed at high strain rates (> 500 /s), martensite formation was reduced due to the adiabatic temperature rise and the increased SFE of the steel. The deformation of the steel with
moderate SFE of 18 mJ/m2 at all the tested strain rates was mainly controlled by the formation of mechanical twins that was leading to an excellent ductility of about 55% even at the highest strain rate of ~1700 /s. In contrast, dynamic deformation of the steel with the highest SFE of 30 mJ/m2 led to the appearance of some shear bands. This was ascribed to the decreased twinning and work hardening rate in this steel. Finally, the topographic studies showed that the fracture surface of the low-SFE steel contained relatively larger cleavage areas and smaller dimples suggesting a relatively more brittle fracture. This was related to the presence of brittle ε and α` martensite phases in this steel.