Work hardening behavior of ultrafine-grained Mn transformation-induced plasticity steel Seawoong Lee, Seok-Jae Lee ⇑ , Bruno C. De Cooman Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea Received 18 June 2011; received in revised form 18 August 2011; accepted 19 August 2011 Available online 15 October 2011 Abstract Ultrafine grain refinement by intercritical annealing at 680 °C and 640 °C was investigated in a Fe–0.05%C–6.15%Mn–1.4%Si mul- tiphase TRIP steel. A large volume fraction of retained austenite was obtained at room temperature in both cases. Whereas a pronounced localization of the deformation during tensile testing appeared in the steel annealed at 640 °C, strain localization occurred only in the initial deformation stages in the steel annealed at 680 °C. The retained austenite was transformed to strain-induced martensite during tensile testing in the sample annealed at 680 °C. However, no martensitic transformation was observed in the sample annealed at 640 °C. The activation volume showed a sharp decrease during the tensile test and saturated to the same value in both cases. Two dif- ferent dislocation structures were observed in the ferrite grains of the samples annealed intercritically at 680 °C after tensile deformation, but only the dislocation-free structure of ferrite was observed in the sample annealed at 640 °C. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ultra-fine grain size; Localized deformation; Grain boundary thickening 1. Introduction The strong interest in ultrafine-grained (UFG) steels is due to their high strength, but their low ductility has remained a major obstacle to their use. It has been sug- gested that the presence of carbide precipitates, second- phase particles or a non-uniform grain size distribution could be used to overcome the lack of ductility [1–4]. Miller [5] first studied transformation-induced plasticity (TRIP) steels with 5–7 mass% Mn in detail. This is a promising UFG high-strength alloy characterized by a ductility due to a large volume fraction of retained austenite, which can transform to strain-induced martensite [6–8]. Lee et al. [6] reported that the presence of austenite islands with the proper stability at room temperature contributed to enhance the mechanical properties of UFG TRIP steel. Deformation localization was, however, a characteristic of these UFG TRIP steels. The latter characteristic is not compatible with applications such as automotive panels. The deformation behavior of UFG TRIP steel was there- fore investigated to find out why the occurrence of the localized deformation is controlled by the intercritical annealing temperature and why the localization does not lead to early fracture. From a microscopic point of view, plastic deformation results from the motion of dislocations. In order to glide and generate plastic deformation, dislocations have to over- come an energy barrier. Fig. 1a shows a schematic diagram illustrating the energy barrier required for the motion of a dislocation. In the presence of stress, part of this energy is provided by the mechanical work done by the applied stress s  V  , where s  and V  are known as the effective stress and the activation volume, respectively. The remaining part of the energy required for dislocation motion can be provided by thermal energy generated by lattice vibrations. The activation volume is typically used to identify the microscopic deformation mechanism responsible for dislo- cation glide, such as kink-pair formation and forest dislo- cation cutting. Whereas a small activation volume of the 1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.08.030 ⇑ Corresponding author. E-mail address: seokjaelee@postech.ac.kr (S.-J. Lee). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 59 (2011) 7546–7553