Crystallographic and Microstructural Studies of Lath Martensitic Steel During Tensile Deformation
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SINCE lath martensitic steels exhibit high strength and hardness owing to their fine microstructure, supersaturated solid solution carbon, and high dislocation density, it is expected to be increasingly employed for high-strength applications. Thus, numerous studies have been conducted to clarify its transformation and microstructural properties and resultant mechanical properties.[1–7] Swarr and Krauss[4] reviewed the strengthening mechanism of lath martensitic steel in relation to the complexity of its microstructure. Morito et al. and Kitahara et al. systematically clarified the constituent microstructures (such as prior austenite grains, packets, blocks, and subblocks) of lath martensitic steel by performing variant analysis using electron backscattering diffraction (EBSD).[5–7] From these investigations, the Kurdjumov–Sachs (K–S) relationship has generally been accepted as the relationship between the crystal orientations of prior austenite and martensite. Owing to symmetry, the martensite has 24 variants of the K–S orientation relationship, which evolve from a single crystal (grain) of austenite. The martensitic transformation induces four-level hierarchical microstructures, laths, subblocks, blocks, and packets, within a prior austenite grain. A martensite lath is a single crystal with high dislocation density and a habit plane close to both the {110} martensite and {111} austenite planes. A subblock is a bundle of laths with a single variant, and a HYUNTAEK NA, Postdoctoral Researcher, SHOICHI NAMBU, Lecturer, MAYUMI OJIMA, Assistant Professor, JUNYA INOUE, Associate Professor, and TOSHIHIKO KOSEKI, Professor, are with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Contact e-mail: [email protected] Manuscript submitted August 21, 2013. Article published online July 17, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
block is a bundle of two specific subblocks whose misorientation is approximately from 4 to 10 deg. A packet is a bundle of blocks that develop with a common invariant {111} austenite plane. In contrast, since lath martensitic steel exhibits significantly low ductility and toughness (premature brittle fracture under small tensile strain), it has been difficult to directly and comprehensively observe its plastic deformation behavior. Therefore, cold rolling has been the only method used to investigate the deformation behavior of martensitic steel during large deformation.[8–12] Vylezhnev et al.[8] investigated the plastic deformation of martensitic steel with respect to the defect density (such as dislocation density) of the crystal structure by X-ray diffraction (XRD). Khlebnikova et al.[9] confirmed that a h100i {110} texture similar to that of body-centeredcubic (bcc) metals is typically observed in lath martensitic steel after a large rolling reduction. Schastlivtsev et al.[10] reported that the primary slip planes in lath martensitic steel are close to the {110} and {112} planes, which is a
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