Interphase Strain Gradients in Multilayered Steel Composite from Microdiffraction
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INTRODUCTION
HIGH strength and high ductility structural materials are very attractive because of their potential industrial applications, for example, as lighter and tougher automobile materials for improved safety as well as environmental friendliness. Recently, it was found that multilayered steels consisting of alternating layers of ultra-high strength martensitic steel and ductile steel have the potential to achieve a combination of extremely high strength and elongation which is beyond the established trade-off relationship in conventional advanced high strength steels (AHSS),[1] as shown in Figure 1. In this concept,[2] we successfully fabricated multilayered steel sheets consisting of martensitic stainless steel SS420J2 and austenitic stainless steel SS304 and achieved a tensile strength exceeding 1200 MPa and 15 pct uniform elongation as shown in Figure 1. It has been reported that the tensile strength of multilayered composites generally follows the rule of mixtures, while the ductility sometimes falls far below the value based on the rule of mixtures.[3] The deteriorated tensile ductility of multilayered composites is presumably due to the following factors: the limited bond strength of
ROZALIYA I. BARABASH and ZHENZHEN YU, Research Staffs, and ZHILI FENG, Senior Researcher, are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831. Contact e-mail: [email protected] OLEG M. BARABASH, Research Professor, is with The University of Tennessee, Knoxville, TN 37996. MAYUMI OJIMA, Assistant Professor, JUNYA INOUE, SHOICHI NAMBU, and TOSHIHIKO KOSEKI, Professors, are with The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. RUQING XU, Beam Scientist and Research Staff, is with the Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted June 5, 2013. Article published online November 12, 2013 98—VOLUME 45A, JANUARY 2014
the interface between the dissimilar materials, the sensitivity of the brittle layer to cracking fracture, the ratio of tensile strength between dissimilar materials, the ease of interfacial delamination, and the thickness of the brittle layer. In a previous study[4], we investigated the effect of interfacial bonding strength on the tensile elongation of multilayered steel composites and confirmed that a sufficiently strong bonding interface between dissimilar steels exists. This was achieved by a combination of the rolling process and proper heat treatments, which suppress delamination at the interface between dissimilar steels as well as localized necking resulting in increased total ductility. In addition, another previous study[5] using a fracture mechanics-based approach revealed that tensile ductility could be enhanced considerably by suppressing brittle rupture of the brittle layer by adopting proper geometrical arrangements such as the brittle layer thickness and the volume fraction of each layer. All previous results suggest that the uniformity of the deformation in a multilayered composite is the key to achieving an extremely h
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