Characterization and Prediction of Flow Behavior in High-Manganese Twinning Induced Plasticity Steels: Part I. Mechanism Maps and Work-Hardening Behavior A. SAEED-AKBARI, L. MOSECKER, A. SCHWEDT, and W. BLECK Thermodynamic stacking fault energy (SFE) maps were developed using the subregular solution model for the Fe-Mn-Al-C system. These maps were used to explain the variations in the work- hardening behavior of high-manganese steels, both through experiments and by comparison with the published data. The suppression of the transformation induced plasticity (TRIP) mechanism, the similarity between the shape of the work-hardening rate diagrams for the produced iso-SFE materials, and an earlier onset of stage C of work hardening by decreasing SFE were shown to be efficiently predictable by the given mechanism maps. To overcome the limitations arising from studying the deformation response of high-manganese steels by SFE values alone, for example, the different work-hardening rate of iso-SFE materials, an empirical criterion for the occurrence of short-range ordering (SRO) and the consequently enhanced work-hardening, was proposed. The calculated values based on this criterion were superimposed on the thermodynamics-based mechanism maps to establish a more accurate basis for material design in high-manganese iron-based systems. Finally, the given methodology is able to clarify the work-hardening behavior of high-manganese twinning induced plasticity (TWIP) steels across an extensive range of chemical compositions. DOI: 10.1007/s11661-011-0993-4 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION THE recently developed twinning induced plasticity (TWIP) steels are a class of high-manganese ( > 15 wt pct manganese) austenitic steels with superior mechanical properties such as ultrahigh strength and high formability. These steels have potential applica- tions in the auto industry, due to their increased strength-to-weight ratio and excellent energy absorption properties. [1–4] In these materials, a proper combination of alloying elements (such as manganese, aluminum, and carbon) increases the thermal or mechanical stability of the face-centered-cubic (fcc) austenite c phase, to the extent that no athermal or deformation-induced hexag- onal-close-packed (hcp) martensitic transformation can occur. The possible range of chemical compositions in which the hcp (e) martensite formation is suppressed can be efficiently predicted using the appropriate thermody- namic models and parameters and the resultant mech- anism maps. [5–7] The mechanism maps developed through the thermodynamics-based approach [5] not only show the existence range of transformation induced plasticity (TRIP) mechanisms, but also demonstrate the variations in the stacking fault energy (SFE) by chang- ing the chemical composition and temperature. There- fore, they were proposed as the practical tools for the tailored materials design in low-SFE austenitic systems. The main requirement for the activation of supportive deformation mechanisms for dislocation gliding, such as deformation-induced twinning and martensite forma- tion, is a low SFE value. Both TWIP and TRIP mechanisms (mechanical twinning and e martensite formation) are related to the dissociation of perfect dislocations into Shockley partials with a Burgers vector of type b ¼ 1 6 112 h i and, thus, are related to the energy of the created stacking faults (SFs). [8] The crystallographic changes associated with the formation of a twin or e martensite plate are explained in terms of the arrange- ment of SFs: identical Shockley partial dislocations are packed on every close-packed {111} plane for a twin and on every second close-packed {111} plane for e mar- tensite. [9,10] Therefore, the active plasticity mode is directly related to the SF probability and the SFE value. [3,11–13] In the proposed model by Mahajan and Chin, a three-layer fault formed by the dislocation reaction 1 2 01 1   þ 1 2 10 1   ! 3  1 6 11 2   can act as an embryonic twin. [14] Based on this model, the embryonic three-layer faults that are distributed throughout the slipped region grow into each other, leading to the formation of macroscopic twins. This approach was successfully employed by Kibey et al. [15] to establish a model for the prediction of twinning stress in fcc metals. Nevertheless, the specific dislocation interactions that A. SAEED-AKBARI, Postdoctoral Researcher and Research Team Leader, L. MOSECKER, Doctoral Candidate and Project Engineer, and W. BLECK, University Professor and Department Head, are with the Department of Ferrous Metallurgy, RWTH Aachen University, 52072 Aachen, Germany. Contact e-mail: alireza. saeed-akbari@iehk.rwth-aachen.de A. SCHWEDT, Head of the Scan- ning Electron Microscopy Group, is with the Central Facility for Electron Microscopy (GFE), RWTH Aachen University, 552074 Aachen, Germany. Manuscript submitted August 16, 2010. Article published online December 23, 2011 1688—VOLUME 43A, MAY 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A