Microstructures and mechanical properties of high-strength Fe-Mn-Al-C light-weight TRIPLEX steels

  • Frommeyer G
  • Brüx U
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High-strength TRIPLEX light-weight steels of the generic composition Fe-xMn-yAl-zC contain 18-28% manganese, 9-12% aluminium, and 0.7-1.2% C (in mass %). The microstructure is composed of an austenitic gamma-Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size kappa-carbides (Fe,Mn)(3) AlC1-x and alpha-Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation (gamma fcc) (epsilon hcp) amounts to Delta G(gamma ->epsilon) = 1757 J/mol and the stacking fault energy was determined to Gamma(SF) = 110 mJ/m(2). This indicates that the austenite is very stable and no strain induced epsilon-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(-1) 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 kappa-carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.

Author-supplied keywords

  • Shear band formation
  • Stacking fault energy
  • TRIPLEX steel
  • κ-carbide precipitates

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  • Georg Frommeyer

  • Udo Brüx

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