Fig. 10. Fe-55Mn-3Al-3Si mass% TWIP-steel: Yield stress R p0.2 , tensile strength R m , uniform elongation e un and total elongation e f as functions of test temperature.

High-strength TRIPLEX light-weight steels of the generic composition Fe-xMn-yAI-zC contain 18 -28 % manganese, 9 -12 % aluminium, and 0.7 -1.2 % C (in mass %). The microstructure is composed of an austenitic y-Fe(Mn, AI, C) solid solution matrix possessing a fine dispersion of nano size x-carbldes (Fe,Mnh AIC 1x and u-Fe(AI, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation Ylcc -+ Ehcp amounts to AGY~' = 1757 J/mol and the stacking fault energy was determined to rSF = 110 mJ/m 2 • This indicates that the austenite is very stable and no strain induced a-martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm 3 and superior mechanical properties, such as high strength of 700 to 1100 MPa and total elongations up to 60 % and more. The specific energy absorption achieved at high strain rates of 10 3 s' is about 0.43 J/mm 3 . TEM investigations revealed clearly that homogeneous shear band formation accompanied by dislocation glide occurred in deformed tensile samples. The dominant deformation mechanism of these steels is shear band induced plasticity -SIP effect-sustained by the uniform arrangement of nano size K-carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.

Iron manganese steels with Mn mass contents of 15 to 30 % exhibit microstructural related superior ductility and extraordinary strengthening behaviour during plastic deformation, which strongly depends on the Mn content. This influences the austenite stability and stacking fault energy γfcc and shows a great impact on the microstructure to be developed under certain stress state or during severe plastic deformation. At medium Mn mass contents (15 to 20 %) the martensitic γ‐ε‐ά phase transformation plays an important role in the deformation mechanisms of the TRIP effect in addition to dislocation glide. With Increasing Mn mass content large elongation is favoured by intensive twinning formation. The mechanical properties of plain iron manganese alloys are strongly influenced by the alloying elements, Al and Si. Alloying with Al Increases the stacking fault energy and therefore strongly suppresses the martensitic γ‐ε transformation, while Si sustains the γ‐ε transformation by decreasing the stacking fault energy γfcc. The γ‐ε phase transformation takes place in Fe‐Mn‐X alloys with γfcc ≤ 20 mJm−2. The developed light weight high manganese TRIP and TWIP (twinning induced plasticity) steels exhibit high ultimate tensile strength (600 to 1100 MPa) and extremely large elongation of 60 to 95 % even at high strain rates of έ = 103 s−1. Particularly due to the advanced specific energy absorption of TRIP and TWIP steels compared to conventional deep drawing steels high dynamic tensile and compression tests were carried out in order to investigate the change in the microstructure under near crash conditions. Tensile and compression tests of iron manganese alloys with varying Mn content were performed at different temperatures and strain rates. The resulting formation of γ twins, ά‐ and ε martensite by plastic deformation was analysed by optical microscopy and X‐ray diffraction. The deep drawing and stretch forming behaviour at varying deformation rates were determined by performing cupping tests and digitalised stress‐strain‐analysis.

The influence of the micro alloying elements B, Ti and Nb on the recrystallization texture and mechanical properties of iron aluminium light‐weight steels, particularly with reference to their improved deep drawing properties was investigated. Depending on the combination of the alloying elements the microstructures of the investigated micro alloyed Fe‐6Al steels are influenced by grain refinement. Likewise, variable combinations of micro alloying elements differently affect the texture. Generally, the mechanical properties are improved. However, small amounts of B, Ti and Nb cause superior deep drawing and stretch forming properties of these iron aluminium light‐weight steels The microstructures of various micro alloyed cold rolled Fe‐6Al steel sheets were evaluated by optical microscopy, scanning electron microscopy (SEM) inclusively EDAX and X‐ray diffraction. Texture measurements were performed using a goniometer with a closed Eulerian cradle and analysed by ODF calculations. Tensile tests were carried out at room temperature and 200 °C, respectively. The deep drawing behaviour was determined by performing cupping tests and digitalised strain analysis.

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