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-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.

Strain hardening in Fe–16Mn–10Al–0.86C–5Ni high specific strength steel

“…As a summary of the mechanical properties measured so far for high specific strength steels, Fig. 4c and d compare the tensile properties of our HSSS with that of Kim et al [27], as well as with the conventional FeeMneAleC based austenitic [5,11,24], duplex [20,25] and triplex [17,23,26] steels. As can be seen, the HSSS in the present study shows an outstanding combination of the specific yield strength (SYS) and uniform elongation, while exhibiting an acceptable combination of the yield strength-to-ultimate tensile strength (YS-to-UTS) ratio and the uniform elongation.…”

Overview of Lightweight Ferrous Materials: Strategies and Promises

“…2, where the reductions in density of Fe-Al and Fe-Mn-Al-C alloys are shown as a function of Al content. 18,19 Approximately 8 wt.% of Al causes density reduction of $10%. Manganese, which is often added in multiphase low-density steels, also reduces slightly the density of steels.…”

Shock and spall behaviors of a high specific strength steel: Effects of impact stress and microstructure

“…5 Moreover, lowdensity steels, mainly based on the Fe-Al-Mn-C alloy system, have been developed and widely studied due to their increased specific strength. [6][7][8][9][10][11][12][13] These low-density steels, the so-called "TRIPLEX" steels, are mainly composed of both the fcc austenite matrix and the bcc ferrite matrix and finely dispersed nanometer-sized j-carbides with the Fe; Mn ð Þ 3 AlC type. 11 More recently, a high specific strength steel (HSSS) with a new composition of Fe-16Mn-10Al-0.86C-5Ni has been developed, 13 in which the excellent combination of specific strength and elongation is achieved, when compared to other high-specific-strength alloys.…”