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 faultenergy (SFE) values (4-30 mJ/m2) were studied using high strain rate torsional tests. The hotrolledmicrostructure of the steel with the lowest SFE of 4 mJ/m2 consisted of a duplex mixture ofaustenite and ε-martensite, but those of the other two steels were fully austenitic. The deformedmicrostructures were studied by optical and electron microscopy. The quasi-static deformation ofthe low-SFE steel was accompanied with profuse martensitic transformation. However, when thissteel was deformed at high strain rates (> 500 /s), martensite formation was reduced due to theadiabatic temperature rise and the increased SFE of the steel. The deformation of the steel withmoderate SFE of 18 mJ/m2 at all the tested strain rates was mainly controlled by the formation ofmechanical twins that was leading to an excellent ductility of about 55% even at the highest strainrate of ~1700 /s. In contrast, dynamic deformation of the steel with the highest SFE of 30 mJ/m2led to the appearance of some shear bands. This was ascribed to the decreased twinning and workhardening rate in this steel. Finally, the topographic studies showed that the fracture surface of thelow-SFE steel contained relatively larger cleavage areas and smaller dimples suggesting arelatively more brittle fracture. This was related to the presence of brittle ε and α` martensitephases in this steel.