Most steels in this alloy category are in the System Fe-Mn-Al-C-Si. Specifically high-Mn steels attract much attention as lightweight steels due to their outstanding combination of strength and ductility caused by their high strain hardening capacity. Weight reduced variants of these high manganese austenitic steels are typically characterized by high manganese content (18 to 30 wt.%) and the addition of aluminum (< 12 wt.%) and silicon (< 3 wt.%) along with a relatively high content carbon (0.6-1.8 wt.%). As the increase in the aluminum content yields a reduction in the alloy's mass density by 1.5 % per 1 wt.% Al, high aluminum contents render these alloys lightweight.

Al-containing, high-Mn steels are characterized by an excellent combination of strength and ductility enabled by their high work hardening capacity. The solute addition of aluminum reduces the specific weight of the alloy, hence supporting the notion of lightweight and weight reduced high-Mn steels. Increasing the aluminum content by 1 wt.% yields a reduction in specific weight by about 1.5%. High-Mn lightweight steels usually contain a high content of manganese (18e30 wt.%), aluminum (<12 wt.%) and silicon (<3 wt.%) along with additions of
carbon (0.6e1.8 wt.%). The precipitation characteristics in the Fe-Mn-Al-C system have been investigated in detail in the last decades for instance by using atom probe tomography and correlative TEM studies. Due to the strong increase in yield strength in conjunction with a well-maintained tensile ductility upon annealing, the high-Mn lightweight steel systems are promising lightweight steel candidates for the manufacturing of high performance components.

A variety of strengthening and strain hardening mechanisms have been reported in high-Mn steels.

These are:

transformation-induced plasticity (TRIP)

twinning-induced plasticity (TWIP)

microband-induced plasticity (MBIP)

dynamical slip band refinemennt

precipitation hardening by kappa carbides

The activation of the prevalent deformation mechanism is mainly determined by the stacking fault energy (SFE). The TRIP effect is found to operate predominantly in low-SFE steels (< 20 mJ m-2). The deformation behavior of medium-SFE steels (20-40 mJ m-2) is characterized by the formation of nanometer thin deformation twins in the deformed microstructure, referred to as TWIP effect. The recently discussed MBIP hardening mechanism, reported in high-SFE alloys (~90 mJ m-2), is described by the formation of thin planar shear zones that are confined by a dislocation wall on either side. These features are called microbands. All the above mentioned hardening mechanisms result in strong refinement of the respective microstructures upon straining enabling high strain hardening rates.

Yes, twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) can occur in the same alloy depending on the stacking fault energy, on the substructure, on and the thermo-mechanical loading situation (stress, strain, strain rate, temperature, dissiptation rate) and on the transformation free energy difference associated with the transformation between the face centred cubic austenitic phase on the one hand and the hexagonal epsilon phase on the other hand. 

The SFE not only has a strong influence on the deformation mechanism, but it also controls the glide mode of the dislocations. A high SFE usually promotes cross-slip of dislocations leading to “wavy glide” in pure fcc metals. However, in concentrated solid solutions with high SFEs planar glide instead of wavy glide is frequently observed. The reasons for planar glide in such massively alloyed high-SFE materials have been controversially discussed. In the quaternary FeMnAlC system aluminum increases the SFE on the one hand but yet promotes planar dislocation glide on the other hand. Many authors attributed this seemingly contradicting finding to the material’s tendency to form clusters of short-range order (SRO), which leads to enhanced planar glide due to the glide plane softening phenomenon. As the addition of aluminum in these alloys promotes the formation of ordered precipitates (κ-carbides), SRO indeed seems to be a plausible explanation. However, to the authors’ best knowledge no studies providing experimental proof for SRO in high-Mn lightweight steels have been published so far. Some works though indicate the presence of SRO in the FeMnAlC system and similar alloy systems by using density functional theory (DFT) simulations.

The κ-carbide is a non oxidic perowskite type compound, i.e. a L’12-ordered phase with the stoichiometric composition (Fe,Mn)3AlC. However, κ-carbides exist at a broad compositional range and coherency strains have an additional influence on their composition. Depending on the aluminum concentration these ordered precipitates either grow during annealing at temperatures between 550°C and 700°C (<10 wt.% Al) or even during quenching from solution heat treatment temperature (>10 wt.% Al). Due to the strong influence of these precipitates on the mechanical properties the precipitation kinetics have been subject of several studies. 
A detailed quantitative characterization of the underlying kinetics of the substructure evolution in the FeMnAlC system has been performed in the case of Fe-30.5Mn-2.1Al-1.2C. With a SFE of about 63 mJ m-2, this alloy, however, falls in the regime between TWIP and MBIP alloys. Yoo et al. investigated the plastic deformation of Fe-28.2Mn-9.95Al-0.98C (wt.%) with a SFE of 120 mJ m-2  and Fe-27.8Mn-9.1Al-0.79C (wt.%) with a SFE of 85mJ m-2  rendering both alloys pure MBIP steels. They attribute the high strain hardening capacity to the formation of Taylor-lattices and microbands upon straining. However no quantitative results were presented. Furthermore the formation of microbands have been doubted of causing the observed strain hardening rate in the literature.  

The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high
ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the
formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands.We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size < 2 nm). The influence of these zones on planar slip is discussed.

We developed a novel Ti-bearing lightweight steel (8% lower mass density than general steels), which exhibits an excellent combination of strength (491 MPa ultimate tensile strength) and tensile ductility (31%) at elevated temperature (600 °C). The developed steel is suitable for parts subjected to high temperature at reduced  dynamical load. The composition of the developed steel (Fe–20Mn–6Ti–3Al–0.06C–NbNi (wt%)) lends the alloy  a multiphase structure with austenite matrix, partially ordered ferrite, Fe2Ti Laves phase, and fine MC carbides.  At elevated temperature (600°C), the ductility of the new material is at least 2.5 times higher than that of  conventional lightweight steels based on the Fe–Mn–Al system, which become brittle at elevated temperatures  due to the inter/intragranular precipitation of κ-carbides. This is achieved by the high thermal stability of its  microstructure and the avoidance of brittle κ-carbides in this temperature range.

We study the microstructures in such alloys by means of electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The combination of these two techniques constitutes a powerful method for giving insight into the underlying mechanisms of plastic deformation at different length scales. The TEM investigation focuses on the initial precipitation state, whereas the large field of view provided by ECCI provides statistical and quantitative data on the structural evolution of deformation features. High-magnification ECCI was used to provide details on the observed dislocation arrangements on the finer scale. In this study the applied ECCI technique exhibits an advantage over the commonly used TEM imaging in terms of sample dimensions, field of view, preparation artifacts and image forces. As TEM samples have a thickness of about 100 nm, dislocations in the sample can change their position due to image forces originating from the two surfaces of the thin foil or from bending. In contrast, ECCI is applied to bulk samples with only one polished surface. Therefore, deformation microstructures that are characteristic of the actual bulk material are investigated rather than relaxed dislocation structures influenced by stress relaxation. In the case of ECCI the image forces from the surface reach less than 20 nm into the sample, while the depth of observation is of the order of 100 nm. Dislocations will therefore not rearrange as strongly as in TEM thin foils, which enables investigation of the undistorted dislocation structure of bulk samples.

Reducing energy consumption in conjunction with improving safety standards is an essential target in modern mobility concepts. Hence, the development of strong, tough and ductile steels for automotive applications is an essential topic in steel research. In this context TWIP (twinning induced plasticity) steels with up to 30 wt.% Mn and >0.4 wt.% C content have shown an excellent combination of ductility and strength. Increasingly, the reduction in mass density of TWIP steels becomes an additional challenge.
Two effects enable such efforts:
The first one is that Mn increases the fcc lattice parameter. The second one is that very high Mn and C alloying stabilizes the austenite, so that it can tolerate Al additions up to about 10 wt.% without becoming instable, i.e. transforming into bcc-ferrite.
Such an alloy concept sustains many advantages associated with TWIP steels, e.g. mechanical twinning and very high strain hardening; yet, it enables density reductions of up to 18%. Hence, alloys based on the quartenary system Fe-Mn-Al-C are specifically promising for the design of low density TWIP steels.
Regarding the excellent mechanical properties of TWIP steels, which are characterized by the transition from dislocation and cell hardening to massive mechanical twinning, it has to be considered that Al increases the stacking fault energy (SFE). This means that the overall strain hardening behavior and the onset of mechanical twinning in density reduced TWIP grades may differ from those observed in conventional TWIP steels.
However, alloys based on the Fe-Mn-Al-C system offer an even larger variety in deformation and strain hardening mechanisms than those associated with the TWIP effect alone. This is due to the characteristic dislocation substructures and the higher number of phases present in the Fe-Mn-Al-C system, namely, fcc-austenite, bcc-ferrite, and ordered structures such as DO3 and L’12-type carbides. Depending on composition, low density steels can assume austenitic structure for the composition regime Mn: 15-30 wt.%, Al: 2-12 wt.%, and C: 0.5-1.2 wt.%. In order to combine the advantages of the TWIP mechanisms with the reduction in specific weight this alloy range is hence the most promising one. When increasing Al content to a range >6-8.wt%, strain hardening in these steels is less dominated by the TWIP effect but instead by the formation of nano-sized L’12-type carbides, so-called κ-carbides.
Density reduced steels with ferritic structure have compositions in the range Mn <8 wt.%, Al: 5-8 wt.%, and C <0.3 wt.%. Corresponding complex grades, consisting of austenite and ferrite can be synthesized by using compositions Mn: 5-30 wt.%, Al: 3-10 wt.%, and C: 0.1-0.7 wt.%. Besides these compositions, ordered D03 structures, i.e. near-ferritic Fe-Al-Cr alloys without Mn have also been addressed in the past in the context of density reduced alloy design.
When comparing the synthesis and properties among the different classes of weight reduced steels, alloys based on the austenitic Fe-Mn-Al-C system are most attractive due to their superior strain hardening, high energy absorption, high density reduction and robust response to minor changes in composition and processing. Even thin strip casting with associated in-line hot rolling has been successfully conducted in our group as a pathway for efficient small-scale manufacturing of such grades.
Recent publications on austenitic Fe-Mn-Al-C alloys have reported yield strength values of 0.5-1.0 GPa, elongations to fracture in the range 30–80%, and ultimate tensile strength in the range of 1.0–1.5 GPa.
When blended with an Al content below 5 wt.% a single austenite phase prevails at room temperature, showing excellent strain hardening which was attributed to the hierarchical evolution of the deformation substructure. Al also promotes formation of nano-precipitates upon aging with L’12 structure and approximate stochiometry of (Fe, Mn)3AlC. These phases are referred to as κ-carbides. They belong to the group of non-oxide perovskites. Due to their ordered fcc structure, κ-carbides have a lattice mismatch below 3% with respect to an austenitic Fe-Mn-C matrix phase and can hence form cuboidal nano-precipitates. When embedded in a ferritic matrix the lattice mismatch can be as large as ~6% which leads to semi-coherent interfaces and, hence, different precipitate morphologies.
This webpage provides a concise introduction to some recent developments in the field of low density Fe-Mn-Al-C TWIP steels placing attention on alloy design, synthesis routes, and microstructure-property relations. We also provide a brief outlook on pending questions associated with the role of κ-carbides on strain hardening and hydrogen embrittlement.