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

Abstract

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 γ-Fe(Mn, Al, C) solid solution matrix possessing a fine dispersion of nano size K-carbides (Fe,Mn)3 AlC1-x and α-Fe(Al, Mn) ferrite of varying volume fractions. The calculated Gibbs free energy of the phase transformation γfcc → εhcp amounts to ΔG γ-ε = 1757 J/mol and the stacking fault energy was determined to ΓSF = 110 mJ/m2. This indicates that the austenite is very stable and no strain induced ε-martensite will be formed. Mechanical twinning is almost inhibited during plastic deformation. The TRIPLEX steels exhibit low density of 6.5 to 7 g/cm3 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 103 s-1 is about 0.43 J/mm3. 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 κ-carbides coherent to the austenitic matrix. The high flow stresses and tensile strengths are caused by effective solid solution hardening and superimposed dispersion strengthening.