Temperature dependence of plastic deformation mechanisms in a modified transformation-twinning induced plasticity steel A. Asghari, A. Zarei-Hanzaki n , M. Eskandari School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran article info Article history: Received 19 January 2013 Received in revised form 17 March 2013 Accepted 26 April 2013 Available online 9 May 2013 Keywords: Steel Austenite Martensitic transformations Twinning Mechanical characterization abstract The plastic deformation behavior of a modified transformation-twinning induced plasticity steel is investigated in a wide range of temperature from 25 to 1000 1C through compression testing. The main emphasis is on different plastic deformation mechanisms involved. The results show that the flow stress is rigorously dependent on deformation temperature and this behavior is classified in three characteristic regions. The strain-induced martensite is detected as the chief plastic deformation mechanism from 25 1C to a comparably high temperature of 200 1C. The activation of deformation twinning as the second deformation mechanism starts with an ascending trend from 200 to 300 1C. This is followed by a descending trend in the range of 300–500 1C. In addition, the remarkable presence of deformation twins at temperatures above 200 1C has led to the formation of a new fine grain structure. The restoration processes as the third deformation mechanism are detected in the range of 700 to 1000 1C. The dynamic recrystallization is the most important softening mechanism for the experimental steel during hot compression from 900 to 1000 1C. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The environmental legislations to date push the industries to manufacture safe and green products. This has been a great driving force to develop and make use of various advanced steels such as dual phase, transformation induced plasticity (TRIP), complex phase and twinning induced plasticity (TWIP) steels [1,2]. An increasing attention toward advanced steels reveals that it has become the lightweight automotive material that best addresses society's need for reduced greenhouse gas emissions, without compromising safety, performance or affordability. Among the aforementioned steels, the TRIP/TWIP steels with high-Mn (15– 30 wt%) content provide a pronounced industrial potential due to their excellent strength–ductility combination and the high energy absorption capacity during impact loading such as crash in automobile industries. Outstanding mechanical properties have been derived from the strain-induced martensite and/or twinning materials which are called the transformation induced plasticity steel and/or the twinning induced plasticity steel. As is well established, the main governing factor responsible for the deformation mechanism in these steels is the stacking fault energy (SFE) of the austenitic matrix. The SFE in turn is a function of the alloy composition and deformation temperature. The low SFE (≤20 mJ/m 2 ) favors phase transformation whereas the twinning would suppress this transfor- mation at higher SFE values ( 420 mJ/m 2 ) [1]. Dislocation glide would be the main mechanism contributing to deformation where the SFE is high enough ( 460 mJ/m 2 ). Considering the effect of composition on the SFE, the alloy would mainly reveal TRIP effect where manganese content is lower than 20% while TWIP effect would be dominant where manganese content is higher than 25% [2]. Extensive researches have already been reported on high-Mn TRIP or TWIP steels, in particular their mechanical behavior [1–4]. It has been shown by Grassel et al. [1] that the TRIP steel (Fe–20Mn–3Si–3Al) exhibits a total elongation of about 82% as well as an ultimate tensile strength of 830 MPa during ambient deformation. In the other hand, the TWIP steel (Fe–25Mn–3Si–3Al) exhibits a total elongation of about 92% and ultimate tensile strength of 650 MPa during ambient defor- mation. In addition, Frommeyer [2] indicated that high-Mn TRIP steel exhibits a pronounced strain-hardening behavior along with a high tensile strength of 1100 MPa and elongation to failure of ε ¼ 0.45 while the TWIP steel shows a relatively low yield stress of 280 MPa and a moderate tensile strength of 650 MPa as well as extremely high elongation to failure of about 95%. It is recognized that TRIP steels shows better mechanical strength while TWIP steels yield better elongation to fracture [2]. Hence, the high ductility concurrent with the high strength level can be obtained in developed TRIP/TWIP steels. Such properties make these steels very attractive for automotive structures where high crashworthiness is required. Fig. 1 illustrates potential applications of TWIP/TRIP steels in a car body structure. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.106 n Corresponding author Tel.: +98 21 61114167; fax: +98 2188006076. E-mail address: zareih@ut.ac.ir (A. Zarei-Hanzaki). Materials Science & Engineering A 579 (2013) 150–156