Ultrafine Structure and High Strength in Cold-Rolled Martensite
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STRUCTURAL refinement by plastic deformation to high strain has been explored extensively in metallic materials because of the close relationship between a fine structural scale and high strength. The structural refinement has its cause in the formation and multiplication of dislocations that interact and are stored in the form of low-angle dislocation boundaries and high-angle boundaries, which together subdivide the structure on a finer and finer scale as the strain is increased.[1] An enhancement of the structural refinement may therefore be achieved if the initial structure can be tailored in a way that it can enhance the dislocation density at a given strain. Three such ways are explored in the present study, as follows: To introduce a high dislocation density in the initial
structure through a martensitic transformation To increase the dislocation density by reducing the
grain size of the initial structure To increase the dislocation density by introducing
elements in solid solution X. HUANG and N. HANSEN, Senior Scientists, are with the Department of Wind Energy, Danish-Chinese Center for Nanometals, Technical University of Denmark, Risø Campus, DK-4000 Roskilde, Denmark. Contact e-mail: [email protected] S. MORITO, Associate Professor, is with the Department of Materials Science, Shimane University, Matsue 690-8504, Japan. T. MAKI, Executive Advisor, is with the Nippon Steel Corporation, Futtsu, Chiba 293-8511, Japan. Manuscript submitted February 3, 2012. Article published online July 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
The effect of a martensitic transformation has been demonstrated[2,3] by cold rolling of low-carbon steels to a reduction of 50 pct, which leads to an ultrafine deformation structure. In parallel, a fine deformation structure has been produced by cold rolling of unalloyed ultralow carbon (0.002 mass pct) lath martensite to a thickness reduction of 80 pct.[4,5] In the latter study, it was also observed that the martensitic structure by rolling is transformed into a typical cell-block structure that characterizes deformed face-centered cubic (fcc) and body-centered cubic (bcc) metals of medium to high stacking fault energy.[1] A reduction in the grain size of initial structure is expected to increase the dislocation density at a given strain.[6–8] The effect is present at low and medium strains due to an increase in the dislocation density in the form of statistically stored and geometrically necessary dislocations.[9] At large strain an effect of grain size can also be found. For example, it has been observed in pure nickel, cold-rolled to strain of evM = 4.5 that the dislocation density increases and the boundary spacing decreases when the grain size is reduced from 500 lm[10] to 70 lm.[11] As to the effect of elements in solid solution, it is well established that their presence leads to structural refinement related to a reduction in dynamic recovery thereby increasing the dislocation density at a given strain. The present study follows the previous findings in order to carry ou
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