A significant increase in the research activity dedicated to high manganese TWIP steels has occurred during the past five years, motivated by the breakthrough combination of strength and ductility possessed by these alloys. Here a review of the relations between microstructure and mechanical properties is presented focusing on plasticity mechanisms, strain-hardening, yield stress, texture, fracture and fatigue. This summarized knowledge explains why TWIP steel metallurgy is currently a topic of great practical interest and fundamental importance. Finally, this publication indicates some of the main avenues for future investigations required in order to sustain the quality and the dynamism in this field.

High manganese austenitic twinning induced plasticity steels: A review of the microstructure properties relationships

A model is proposed for the evaluation of the stacking fault energy (SFE) in Fe–Mn–C austenitic alloys, at different temperatures. It accounts for the variation of the Gibbs energy of each element during the austenite to e martensite transformation, plus their interactions. The Gibbs energy due to the antiferromagnetic to paramagnetic transition is also taken into account. The required data have been obtained from the literature. The result shows a decrease of the SFE with temperature, with a saturation below the austenite Neel temperature. The result agrees with the mechanical and thermal martensitic transformation limits proposed by Schumann. The plasticity mechanisms depend on the SFE. The mechanical martensitic transformation occurs below 18 mJ/m 2 , and twinning between 12 and 35 mJ/m 2 , in agreement with the tensile tests and the deformation microstructures observed in an Fe–22 wt.% Mn–0.6 wt.% C alloy at 77, 293 and 693 K.

Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

We present a thermochemical model of the stacking-fault energy (SFE) in the Fe–Mn–C system with few percent of Cu, Cr, Al and Si in addition. Aluminium strongly increases the SFE, contrary to chromium, while the effect of silicon is more complex. Copper also increases the SFE, but strongly decreases the Neel temperature. The SFE is the relevant parameter that controls mechanical twinning, which is known to be at the origin of the excellent mechanical properties of these steels. Using this model, copper containing Fe–Mn–C grades were elaborated with SFE below 18 mJ/m 2 , in the range where ɛ-martensite platelets form instead of microtwins during plastic deformation. This substitution of deformation modes, confirmed by X-ray diffraction, does not significantly damage the mechanical properties, as long as the SFE is greater than 12 mJ/m 2 , which avoids the formation of α′-martensite.

Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe–Mn–C steel

A subregular solution thermodynamic model was used to calculate the stacking fault energies (SFEs) of high-manganese (10 to 35 wt pct) steels with carbon contents of 0 to 1.2 wt pct. Based on these calculations, composition-dependent diagrams were developed showing the regions of different SFE values for the mentioned composition range. These diagrams were called SFE maps. In addition, variations in the SFE maps were observed through increasing the temperature, aluminum content, and austenite grain size. These changes were seen either as an increasing trend of SFE caused by raising the temperature and aluminum content, or as a decreasing behavior caused by increasing the grain size. The SFE value of 20 mJ/m2 within these diagrams was introduced as the upper limit for the strain-induced martensite formation. The variations in this limit caused by increasing the temperature and aluminum content were mathematically evaluated to find out the minimum amount of manganese that was required to avoid the martensitic transformation. By introducing the isocarbon and isomanganese diagrams of the SFE, it was seen that both temperature and aluminum had a greater effect on the SFE when added to the steels with the lower manganese contents. Moreover, by adding more aluminum to the composition of the high-manganese steels, its influence on the SFE decreased continuously.

Derivation and Variation in Composition-Dependent Stacking Fault Energy Maps Based on Subregular Solution Model in High-Manganese Steels

Room temperature tensile behavior of a high Mn–Al–C steel in the solid solution state was correlated to the microstructures developed during plastic deformation in order to clarify the dominant deformation mechanisms. The steel was fully austenitic with a fairly high stacking fault energy of ∼85 mJ/m 2 . The tensile behavior of the steel was manifested by an excellent combination of strength and ductility over 80,000 MPa% in association with continuous strain hardening to the high strain. In addition, the austenite phase was very stable during deformation. The high stacking fault energy and firm stability of austenite were attributed to the high Al content. In spite of the high stacking fault energy, deformed microstructures exhibited the planar glide characteristics, seemingly due to the glide plane softening effect. In the process of straining, the formation of crystallographic microbands and their intersections dominantly occurred. Microbands consisting of geometrically necessary dislocations led to the high total dislocation density state during deformation, resulting in continuous strain hardening. This microband-induced plasticity is to be the origin of the enhanced mechanical properties of the steel.

Microband-induced plasticity in a high Mn–Al–C light steel

The Gibbs free energy as a function of temperature of the fcc(γ) and hcp(e) phases in the Fe-Mn-Si system is evaluated by the application of the general model for predicting thermodynamic properties for ternary systems from binary ones, suggested by Chou and by the utilization of the available data from binary Fe-Mn, Fe-Si and Mn-Si systems and the SGTE DATA given by Dinsdale. The calculated result seems reasonable as compared with the experimental data.

Gibbs free energy evaluation of the FCC(γ) and HCP(ε) phases in Fe-MN-Si alloys

Multi-phase transformation-induced plasticity (TRIP) steels exhibit a combination of good ductility and high strength, and can be good candidates for automotive applications in improving crashworthiness of car bodies. Understanding of the mechanical properties of the TRIP steels under high strain rate is essential. In this paper, the tensile deformation behavior of cold-rolled TRIP-aided DP steels over the large range of the strain rates (400–1600 s −1 ) was studied using a pneumatic indirect bar–bar tensile impact tester based on one-dimensional elastic stress wave principle. The volume fraction of transformed retained austenite was analyzed by XRD. The strain rate effect on the amount of transformed retained austenite was also analyzed and discussed. The void initialization and morphology of fracture surfaces at different strain rates were observed by SEM. The results indicate that with increasing strain rates, both the yield strength and tensile strength of TRIP-aided steels increase, but the elongations decrease. The stability of retained austenite is strain rate dependent because the adiabatic temperature rises under high deformation rate. Increased strain rate results in the decrease of the amount of transformed retained austenite. The site of initialized void is located in the ferrite matrix resulting from TRIP effect. Furthermore, the morphology of fracture surface at high strain rate is different from that at the static or quasi-static tensile conditions.

Tensile deformation behavior of cold-rolled TRIP-aided steels over large range of strain rates

Effect of strain rate on deformation behavior of TRIP steels

Effects of intercritical annealing temperature on the microstructure and mechanical properties in low-carbon low-silicon TRIP (transformation-induced plasticity) steel containing aluminum and vanadium were investigated. The highest volume fraction of retained austenite of samples annealed at 1053 K was approximately 18%, while the product of strength and elongation of samples annealed at 1033 K was up to 22,000 MPa%. In addition, the retained austenite transformation kinetics were measured by tensile test and fitted by the function y = 1 − e−Ax. The correlation between the experimental and fitting results was good. A in the function might be a more effective factor to measure the mechanical properties of TRIP steel than the volume fraction of retained austenite.

Strain-induced transformation of retained austenite in low-carbon low-silicon TRIP steel containing aluminum and vanadium

The Al-C-Fe system was assessed in the framework of the CALPHAD approach Through the critical review of thermochemical properties of liquid Al-Fe alloys, the binary Al-Fe description was revised t 

Lin Li

Papers

Gibbs free energy evaluation of the FCC(γ) and HCP(ε) phases in Fe-MN-Si alloys

Tensile deformation behavior of cold-rolled TRIP-aided steels over large range of strain rates

Effect of strain rate on deformation behavior of TRIP steels

Strain-induced transformation of retained austenite in low-carbon low-silicon TRIP steel containing aluminum and vanadium

Thermodynamic assessment of the Al-C-Fe system