Twinning and Tripping in 10% Mn steels Huseyin Aydin a,n , In-Ho Jung a , Elhachmi Essadiqi b , Stephen Yue a a McGill University, Department of Mining and Materials Engineering, Montreal, QC, Canada b Université Internationale de Rabat, UIR, Aerospace Engineering School, Rabat, Technopolis Shore Bypass Rabat-Salé, Morocco article info Article history: Received 14 August 2013 Received in revised form 22 October 2013 Accepted 29 October 2013 Available online 5 November 2013 Keywords: Twinning Strain induced transformation (SIT) Stacking fault energy (SFE) Retained austenite (RA) abstract In the present work, a medium Mn, Fe–Mn–C–Al–Si alloy was subjected to different heat treatment conditions and subsequent deformation to understand the effect of these processes on austenite. It was found that, after intercritical annealing, the microstructure was ferrite plus austenite duplex phase (FADP) regardless of cooling rate to room temperature. When cold rolled, the retained austenite of the FADP structures exhibited both twinning and strain induced transformation (SIT) to martensite. A detailed characterization of the co-existence of twinning and SITing after cold rolling is presented. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Over the last few decades, growing demands for weight saving and safety requirement have motivated new concepts of auto- motive steels to achieve improved mechanical properties in comparison with the existing Advanced High Strength Steels (AHSS). Among various developments, medium Mn content steels are considered to be potential candidates to achieve the perfor- mance targets of the so-called third generation AHSS [1]. These steels are currently one of the materials being increasingly devel- oped to harness the promise of second generation steels without high alloy costs and difficult processing issues associated with these latter steels. Indeed, several literature studies proved that the medium Mn steels containing 5–10 wt% Mn have enhanced mechanical proper- ties compared to the first generation AHSS due to the occurrence of deformation induced martensitic transformation [2–5]. It is also stated that the partitioning of Mn and C in the austenite during intercritical annealing are the two main contributions for the austenite stability, and mechanical stabilization of the austenite does not contribute to the austenite stability due to the very low dislocation density of the austenite grains [3–5]. Hence, these steels typically achieve very high strengths, but they have very limited strain hardening and, as a consequence, lower elongations compared to second generation AHSS. This paper describes a novel ferrite plus retained austenite microstructure based on the above alloying concepts. The behavior of retained austenite during room temperature deformation of three heat treated variants was investigated, with a focus on twinning and strain induced transformation characteristics. 2. Materials and experimental methods 2.1. Compositions The steel used throughout this work was supplied by CANMET- MTL (Hamilton, Ontario, Canada). Castings were done in an induction furnace. The composition (wt%) of the examined steel is shown in Table 1. This is one composition selected from a series of alloys which were designed on the basis of attaining a certain level of stacking fault energy (SFE) in the metastable austenite to promote twinning as opposed to transformation to martensite [2]. The thermodynamic modeling approach was adopted to calculate the magnitude of stacking fault energy and most of the data used in this study was taken from the literature and Fsteel database of FactSage (version 6.2) computational thermodynamic software [6]. 2.2. Processing The cast ingot was sectioned into plates having a thickness of 30 mm, and was homogenized for 3 h at 1200 1C, immediately hot rolled to 6 mm at temperatures between 1100 and 750 1C and then water quenched. The as-hot rolled plates were intercritically annealed at elevated temperatures and then either air (AC), furnace (FC) cooled or water quenched (WQ) prior to mechanical testing and microstructural characterization. Fig. 1 shows the detailed heat treatment process of the steel samples. The Contents lists available at 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.2013.10.088 n Corresponding author. Tel.: þ1 514 773 1559; fax: þ1 514 398 4492. E-mail address: huseyin.aydin@mail.mcgill.ca (H. Aydin). Materials Science & Engineering A 591 (2014) 90–96