Deformation Behavior of Cementite in Deformed High Carbon Steel Observed by X-ray Diffraction with Synchrotron Radiation
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INTRODUCTION
IT is well-known that severe plastic deformed pearlitic steel can exhibit very high tensile strength. To reveal the mechanism for such high strengthening, the microstructure of the deformed pearlitic steel has been studied by many researchers. It has been reported that microstructural evolution occurred in lamellar structure,[1–4] and that partial dissolution of cementite occurred in heavy cold drawn pearlitic steel. Complete dissolution of cementite plate occurred in pearlitic steel deformed by high torsion strain has been observed by transmission electron microscopy (TEM).[5] Moreover, it has been found that a high amount of carbon diffused in adjacent ferrite lamellar by electron energy loss spectroscopy.[6] The nanoscale analysis done by atom
AKIRA TANIYAMA is with the Advanced Technology Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba, 293-8511, Japan. Contact e-mail: [email protected] TORU TAKAYAMA is with the Nippon Steel & Sumikin Technology Co. Ltd, 1-8 Fuso-cho, Amagasaki, Hyogo, 660-0891, Japan. MASAHIRO ARAI is with the Nippon Steel & Sumikin Research Institute Corporation, 1-8 Fuso-cho, Amagasaki, Hyogo, 660-0891, Japan. TAKANARI HAMADA is with the Yawata Works [Kokura Area], Nippon Steel & Sumitomo Metal Corporation, 1, Konomi-machi, Kokurakita-ku, Kitakyushu, 803-0803, Japan. Manuscript submitted February 11, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
probe field ion microscopy (APFIM)[7] and three dimensional atom probe (3DAP)[8–11] proposed that the carbon content in cementite has decreased to almost 50 pct of stoichiometric concentration (25 mol pct) after severe deformation; the carbon concentration in ferrite has been estimated to be 0.5 mol pct at a true strain larger than 3.4.[11] Furthermore, complete dissolution of cementite has been observed at a true strain value of 5.1.[9] Mo¨ssbauer spectroscopy studies have proposed that 30 to 50 pct in volume of cementite dissolved during deformation.[10,12,13] The authors have reported X-ray diffraction analysis of drawn pearlitic steel with synchrotron radiation. They have suggested that a unit cell of ferrite in pearlite transforms from body centered cubic (bcc) structure to body centered tetragonal (bct) after severe deformation at a true strain larger than 1.5.[14] The carbon content in ferrite at a true strain value of 3.52 has been estimated to be 0.13 mass pct (0.6 mol pct). Recently, similar experimental results have been reported by other researchers.[15] To understand dissolution mechanisms of cementite during severe deformation, it is very important to reveal internal structure changes of cementite as well as ferrite. High-resolution TEM observation has been applied to observe deformed cementite structure in detail.[16–18] It has been reported that severe deformed cementite changed to be nanoscale crystalline grain and to be a partially amorphous microstructure. Thus, high-resolution TEM is a useful technique to observe nanoscale
structure of crystals directly;
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