Full length article Decoupling the contributions of constituent layers to the strength and ductility of a multi-layered steel Moo-Young Seok a , Jung-A Lee a , Dong-Hyun Lee a , Upadrasta Ramamurty b , Shoichi Nambu c, ** , Toshihiko Koseki c , Jae-il Jang a, * a Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea b Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India c Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan article info Article history: Received 3 June 2016 Received in revised form 2 September 2016 Accepted 5 September 2016 Keywords: Multi-layered steel Nanoindentation Tensile strength Ductility Martensitic phase transformation abstract Multi-layered steel (MLS) consisting of alternating soft/ductile austenitic and hard/brittle martensitic stainless steel layers is a new class of hybrid material for structural application as it offers excellent combinations of strength and ductility. In this study, the contributions of each of the constituent layers to the overall strength and ductility of an MLS (having tensile strength > 1.4 GPa and ductility > 20%) were examined by recourse to nanoindentation experiments on each of them. By adapting two different indenter tip radii for the spherical nanoindentation experiments, constituent layers' stress-strain re- sponses within the plastic regime were obtained and then compared with the macroscopic flow curve of the MLS that was obtained through tensile tests, to show that the strength contributions of the con- stituent steels to the global strength of MLS is as per the rule of mixtures. In order to examine the sources of tensile ductility of the MLS, sharp tip nanoindentation experiments were conducted on specimens extracted from tensile coupons that were subjected to predetermined plastic strains a priori. Results of these experiments show that the tensile failure occurs at a strain at which hardness of the austenitic layer, which is found to be dependent on the prior-plastic strain, is almost equal to the strain- independent hardness of the martensitic layer. The results are discussed in terms of martensitic trans- formation within austenitic layer and the role of the mechanical environment change imposed by the neighboring martensite layers on it. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Steels occupy the preeminent position in the structural mate- rials category, and considerable efforts are continually being made to enhance their mechanical performance without compromising on the inherent advantages they offer, such as the low cost, wide availability, and amenability to high volume manufacturing oper- ations such as stamping. These efforts led to the development of a variety of advanced high strength steels (so-called AHSS) such as dual phase (DP), complex phase (CP), transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP) steels. The microstructures of these steels consist two or more constituent phases, whose morphology and volume fractions are optimized such that enhanced combinations of high strength and ductility are available in the same material. Such steels are especially essential for environment-friendly automobiles whose structural integrity is maximum. This field of research, however, appears to have matured with further advances only leading to marginal benefits. One way of alleviating this is through the hybrid materials approach wherein two or more distinct steels are combined so as to obtain a material with far superior properties than the constituents. Such a “disruptive technology” concept has been employed to manufac- ture a multi-layered steel (MLS) that consists of alternating layers of hard, but relatively less-ductile martensitic stainless steel and soft, but ductile austenitic stainless steel. It was demonstrated that such a material can be extremely strong (tensile strength in excess of 1.2 GPa) and at the same time considerably ductile (at least 15% failure strain). Such strength-ductility combination is way beyond the reported trade-off between these properties in conventional * Corresponding author. ** Corresponding author. E-mail addresses: nambu@metall.t.u-tokyo.ac.jp (S. Nambu), jijang@hanyang.ac. kr (J.-i. Jang). Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat http://dx.doi.org/10.1016/j.actamat.2016.09.007 1359-6454/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Acta Materialia 121 (2016) 164e172