Acta Materialia 116 (2016) 188-199
Acta Materialia 116 (2016) 188 Dynamic S[...]
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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.
JOM, Vol. 66, No. 9, 2014:
Recent developments in the field of austenitic steels with up to 18% reduced mass density. The alloys are based on the Fe-Mn-Al-C system. Here, two steel types are addressed. The first one is a class of low-density twinning-induced plasticity or single phase austenitic TWIP (SIMPLEX) steels with 25–30 wt.% Mn and<4–5 wt.% Al or even<8 wt.% Al when naturally aged. The second one is a class of j-carbide strengthened austenitic steels with even higher Al content. Here, j-carbides form either at 500–600°C or even during quenching for >10 wt.% Al. Three topics are addressed in more detail, namely, the combinatorial bulk high-throughput design of a wide range of corresponding alloy variants, the development of microstructure–property relations for such steels,
JOM, Vol. 66, No. 9, 2014 page 1845 Low-[...]
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Low density Fe-Mn-Al-C austenitic steels / Weight reduced austenitic steels
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.
Here we introduce a new approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural weight reduced steels. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques.
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Hydrogen embrittlement of a precipitation-hardened Fe-26Mn-11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%.
2014 Int J Hydrogen Energy Hydrogen embr[...]
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We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe–30.5Mn–2.1Al–1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction.
PDF-Dokument [1.8 MB]
Scripta-Mater-68 (2013) 343–weight-reduc[...]
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Here we introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely,Simplex and alloys strengthened by nanoscale ordered kappa-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature.
Mater Sc Techn 2014 VOL 30 1099 weight r[...]
PDF-Dokument [390.1 KB]