An assessment of the contributing factors to the superior properties of a nanostructured steel using in situ high-energy X-ray diffraction S. Cheng a,b, * , Y.D. Wang c , H. Choo a,b , X.-L. Wang b , J.D. Almer d , P.K. Liaw a , Y.K. Lee e a Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA b Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 39831, USA c School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China d Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA e Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, South Korea Received 14 September 2009; received in revised form 9 December 2009; accepted 14 December 2009 Available online 19 January 2010 Abstract In contrast to most nanostructured materials, outstanding mechanical property has been demonstrated in a nanostructured metasta- ble austenitic steel, owing to the new characteristics of deformation-induced martensitic transformation. In this paper, by employing an in situ high-energy X-ray diffraction technique, we explore these characteristics by examining factors from the load partitioning, Lu ¨ ders banding, to texture development. It was found that the martensitic transformation was mainly driven through Lu ¨ ders band propagation. Marked load transfer takes place from austenite to martensite as Lu ¨ ders band propagates, and continues into the homogeneous defor- mation regime. The texture development is mostly contributed by martensitic transformation, but dislocation-based plasticity also plays a role. The effective load partitioning along with the deformability of martensite promotes sample ductility. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Synchrotron high-energy X-ray diffraction; Nanostructured steel; Martensitic transformation; Luders banding; Texture 1. Introduction Grain-size refinement has historically been applied in metals and alloys to improve their strength, as known from the Hall–Petch relation. In traditional materials, tensile fracture toughness is sometimes enhanced through grain- size refinement as well [1]. However, most metals and alloys with nanostructure or sub-micron grain size developed in recent years showed very low tensile ductility. This has posed a serious problem to the utility of this group of mate- rials, since the attained high strength can hardly be exploited without reasonable tensile ductility. The poor ability of enduring plasticity was mainly attributed to weak work-hardening capability [2]. To date, after extensive effort, a few nanostructured alloy systems have been dem- onstrated with outstanding comprehensive properties [3– 8]. For instance, superior tensile behavior has recently been reported in a nanostructured austenitic steel [8]. The strat- egy used in this alloy increases the strength by grain-size refinement, while simultaneously maintaining a high work-hardening rate through deformation-induced mar- tensitic transformation [8]. However, due to the much refined grain size (also the much enhanced yield stress), many aspects of martensitic transformation are changed. For instance, in its coarse-grained (CG) counterpart, the metastable austenite (c phase, fcc) has been found to trans- form into two types of martensites, i.e. one with hexagonal close-packed (e phase, hcp) structure and the other with body-centered cubic (a 0 phase, bcc) structure, whereas the c ? e transformation is largely suppressed in nanostruc- tured samples, leaving the transformation product fully a 0 1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2009.12.028 * Corresponding author. Address: Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA. Tel.: +1 865 974 2683; fax: +1 865 974 4115. E-mail address: scheng1@utk.edu (S. Cheng). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 58 (2010) 2419–2429