Composite Behavior of Lath Martensite Steels Induced by Plastic Strain, a New Paradigm for the Elastic-Plastic Response
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LATH martensitic steels are widely used iron-based alloys with outstanding mechanical properties. They contain carbon varying between a few hundredths to a few tenths of weight percentage and a variety of different alloying elements in small quantities. Their strength is induced by the transformation of the FCC c to the BCC a phase by fast cooling.[1] Coherency strains in the quenched alloy induce huge dislocation densities which are the source of the alloy’s strength.[1] Although this alloy has been used since the existence of steel, the way its microstructure functions is still unclarified. A typical lath martensite consists of blocks of lamellar plates, where the blocks form packets.[2–4] The blocks
TAMA´S UNGA´R, Professor Emeritus, and GA´BOR RIBA´RIK, Associate Professor, are with the Department of Materials Physics, Eo¨tvo¨s University, PO Box 32, Budapest 1518, Hungary, and also with the Laboratory of Excellence ’’DAMAS’’, Universite´ de Lorraine, Nancy-Metz, 57045 Metz, France. Contact e-mail: ungar@ludens. elte.hu STEFANUS HARJO, Senior Research Scientist, and TAKURO KAWASAKI, Research Scientist, are with the J-PARC Center, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. YO TOMOTA, Professor Emeritus, is with the Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan, and also with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. ZENGMIN SHI, Assistant Professor, is with the College of Materials and Chemical Engineering, China Three Gorges University, Yichang 443002, P.R. China. Tama´s Unga´r and Stefanus Harjo have contributed equally. Manuscript submitted June 12, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
are subdivided into sub-blocks, where the smallest constituents are lamellar plates called martensite laths. The hierarchy of packets, blocks, sub-blocks, and laths is shown schematically in Figure 1. Within the packets, the laths are parallel lamellae each having crystal orientations separated by different twin-related boundaries. The 110 oriented lath planes align coherently with one of the 111 type planes of the primary austenite. Within the primary austenite grain boundary, several packets of different crystallographic orientations can coexist,[2–4] see Figure 1(a). Despite the rather large yield stress of martensitic steels, they do show some ductility.[1,5–7] Recent microscale deformation experiments[8,9] have attempted to reveal the microscopic mechanisms controlling plastic deformation in lath martensite. Micropillars with a single martensite block have shown perfectly ideal stress-strain behavior with no strain hardening and a flow stress of ~1.2 GPa. Micropillars with two or more blocks, however, have shown significant strain hardening with a similarly large yield stress.[8] Microscale tensile experiments have shown relatively small flow stress values of the order of ~350 MPa, when the active slip systems were in-lath-plane, whereas the flow stress al
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