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UCTION

LATH martensite steel is widely used in high-strength structural materials. It is obtained by quenching to room temperature (RT) from a temperature at which the austenitic phase is stable. The martensitic phase transformation produces a fine-grained structure with an extremely high dislocation density (>1015 m2).[1] The microstructure of lath martensite typically comprises

STEFANUS HARJO, TAKURO KAWASAKI, WU GONG, and KAZUYA AIZAWA are with the J-PARC Center, Japan Atomic Energy Agency 2-4 Shirane Shirakata, Tokai-mura, Naka-gun, Ibaraki, 319-1195, Japan. Contact e-mail: [email protected] YO TOMOTA is with the Research Center for Structure Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. GEZA TICHY is with the Department of Materials Physics, Eo¨tvo¨s University, P.O. Box 32, Budapest, 1518, Hungary. ZENGMIN SHI is with the College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue Rd, Xiling, Yichang, Hubei, China. TAMAS UNGA´R is with the Department of Materials Physics, Eo¨tvo¨s University and also with the Materials Performance Centre, The University of Manchester, M13 9PL, Manchester, UK. Manuscript submitted December 15, 2016.

METALLURGICAL AND MATERIALS TRANSACTIONS A

several packets with different crystallographic orientations in a prior austenite grain, where the packets are formed by several blocks.[2,3] The blocks are subdivided into sub-blocks with the same variant, and the smallest constituents are plate-like crystals called laths with sizes of several tens to several hundreds of nm. The elastic limit of an as-quenched Fe-18Ni lath martensite steel is relatively low (300 MPa), and the tensile strength is 760 MPa at a nominal strain of approximately 1.5 pct.[4] This indicates a very high level of work hardening after yielding at the beginning of plastic deformation. Cold rolling was reported to increase the elastic limit substantially, resulting in higher 0.2 pct proof stress with increasing equivalent plastic strain.[4] To explain this deformation behavior, the changes in dislocation density (q) in the cold-rolled and tensile-deformed lath martensitic Fe-18Ni alloys were measured by X-ray diffraction (XRD)[4] and neutron diffraction (ND)[5] based on the classical Williamson–Hall (W–H) plot.[6] The q values were found to decrease with plastic deformation, as evidenced by the decrease in the slopes of the classical W–H plots with plastic deformation. In general, the change in flow stress (Dr) attributed to dislocations can be evaluated using Taylor’s equation[7]: p Dr ¼ r  r0 ¼ a l MT b q; ½1

where r is the flow stress attributed to dislocations, r0 is the sum of the friction stress of dislocations and the stress attributable to the effect of solute element strengthening, a is a geometric coefficient between zero and unity, l is the shear modulus, MT is the Taylor factor, which accounts for the effect of texture, and b is the Burgers vector. The value of a is usually assumed to be unchanged during deformation; hence