The Influence of Manganese Content and Temperature on the Relative FCC/HCP Phase Stability and Strain-Hardening Behavior of High-Manganese TRIP/TWIP Steels

Abstract

New automotive structural steels require improved strength and ductility in order to affordably reduce vehicle weight and improve passive safety. Austenitic high-manganese transformation- and twinning-induced plasticity steels (TRIP/TWIP) meet these requirements, exhibiting superior strain-hardening, strength, ductility and at lower costs than alternative light-weight aluminum alloys. The stacking fault energy (SFE) of these steels plays a major role in the deformation mechanisms. In the current research, analysis of Shockley partial-dislocation configurations in the three Fe-22/25/28Mn-3Al-3Si alloys using weak-beam dark-field transmission electron microscopy yielded stacking-fault energy (SFE) values of 15 ±3, 21 ±3 and 39 ±5 mJ m², respectively. These SFE values were used in conjunction with a thermodynamic model to calculate the free energy difference of the FCC and HCP phases, and determine a probable range for the FCC/HCP interfacial energy in TRIP/TWIP steels of 8-12 mJ m². The proposed interfacial energy range is narrower than the previous range (5 to 27 mJ m²) employed in thermodynamic SFE calculations and will improve the accuracy of future SFE models. As the SFE was increased from 15 to 21 to 39 mJ m², the secondary deformation mechanisms changed from of α/ε-martensite, to mechanical twinning/ε-martensite, to mechanically twinning, respectively. The formation of ε-martensite in the Fe-22Mn-3Al-3Si caused grain refinement from the onset of plastic deformation and resulted in increased work-hardening at low strains. In contrast, mechanical twinning began influencing the work-hardening of the Fe-25/28Mn-3Al-3Si after 0.1 true strain, resulting in slightly reduced work-hardening at low strains but larger maximum uniform elongations. The true ultimate tensile strengths and maximum uniform elongations of the Fe-22/25/28Mn-3Al-3Si alloys at RT were 1172±19/1136±9/1104±14 MPa and 72±2/77±2/75±1%, respectively. Remarkably, only small changes in the mechanical properties are observed despite the large differences in SFEs of the three alloys. This research reveals that excellent mechanical propertied can be obtained in TRIP/TWIP steels over a wide range of SFEs (15 to 39 mJ m²).