Influence of Al on internal friction spectrum of Fe–18Mn–0.6C twinning-induced plasticity steel Ilchan Jung, Seok-Jae Lee ⇑ and Bruno C. De Cooman Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Received 2 November 2011; revised 20 January 2012; accepted 20 January 2012 Available online 28 January 2012 The temperature dependence of the damping and the dynamic elastic modulus of Fe–18%Mn–0.6%C twinning-induced plasticity steel with different Al contents were investigated by the impulse excitation internal friction technique. The modulus effect was reduced and the Ne ´el temperature was lowered by the Al addition. The activation energy of the Finkelshtein–Rosin peak increased with Al addition. The Finkelshtein–Rosin peak became broader due to diffusional jumps of the interstitial C with different activation energies resulting from the interaction with substitutional Mn atoms. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Internal friction; Snoek relaxation; Finkelshtein–Rosin relaxation; Ne ´el temperature; Twinning-induced plasticity (TWIP) steel High Mn twinning-induced plasticity (TWIP) steel is attractive for automotive applications due to its superior combination of strength and ductility [1,2]. The stress–strain curve of TWIP steels containing solute C shows clear evidence of room temperature dynamic strain aging (DSA) [1,2]. DSA results in an increase in flow stress and stress hardening but a decrease in post- uniform elongation and a reduction of area at fracture [2]. Lee et al. [3] proposed that the DSA effect is due to the single diffusive jump of the C atom in a point defect complex in the stacking fault region. Al increases the stacking fault energy and the critical strain for the initiation of DSA and enhances the post-uniform elongation, resulting in good formability. However, the effect of Al on the diffusion of the C atom in Fe Mn Al C alloy has not yet been reported. The internal friction (IF) technique is a powerful tool to determine the activation energy of diffusion phenomena of intersti- tials (C, N) in alloys. It is also very sensitive to modulus changes resulting from magnetic ordering [4]. The Fin- kelshtein–Rosin (FR) peak, which has similarities to the well-known Snoek relaxation in body-centered cubic (bcc) metals, is observed in face-centered cubic (fcc) met- als and alloys contain interstitials. Whereas the Snoek peak is due to isolated interstital atoms and the elastic dipole associated with the local tetragonal lattice distor- tion caused by the presence of the interstitial atom, the FR peak is due to the stress-induced reorientation of the elastic dipole of substitutional–interstitial complexes [5]. An activation energy of 1.01 eV for the FR peak in Fe–18%Mn–0.6%C TWIP steel was reported using De- bye peak fitting of measurements carried out at a single frequency [3]. In the present study, a more accurate va- lue for the activation energy of the relaxation process giving rise to the FR peak was obtained by measuring the peak temperature at different frequencies. The addi- tion of Al was found to have a clear effect on the activa- tion energy. The broadening of the FR peak was also observed. In addition, the modulus anomaly due to the anti ferromagnetic to paramagnetic transition was observed and the addition of Al resulted in a reduction of the modulus anomaly. Three austenitic TWIP steels with 18%Mn, 0.6%C and x%Al (in mass%) with x = 0, 1.5 and 2.5, referred to as Fe18Mn0.6C–xAl in the following paragraphs, were used in the present study. The content of other alloying elements was less than 100 ppm. Hot-rolled materials were cold-rolled to 1.2 mm in thickness and recrystallization-annealed. The material preparation is discussed in detail by Chin et al. [6]. The three steels had a fully austenitic microstructure at room tempera- ture and when tested in uniaxial tensile deformation, TWIP behavior was observed for the three steels. The IF spectra were measured in vacuum in the free flexural vibration mode by the impulse excitation tech- nique. A detailed description of the equipment is given elsewhere [7]. Three types of rectangular specimens (65, 80 and 105 mm in length, and 20 mm in width) were used 1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2012.01.042 ⇑ Corresponding author. E-mail: seokjaelee@postech.ac.kr Available online at www.sciencedirect.com Scripta Materialia 66 (2012) 729–732 www.elsevier.com/locate/scriptamat