Electron diffraction study of aging processes in Fe-1.83 wt pct C martensite at room temperature
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INTRODUCTION
O V E R the past 2 decades, especially in the last one, remarkable advances have been made in the study of tempering of ferrous martensites. Complicated but important reactions taking place upon the tempering have been used to strengthen and toughen quenched steels. It is Widely known that some reactions occur below conventional tempering temperature (373 to 873 K)Y -71 Tempering below these temperatures is usually called "aging."v.2] Unlike tempering at higher temperatures, where obvious microstructural changes occur, aging only produces very weak diffraction effects and fine scale microstructures ( - 1 nm), which are not so easily revealed. Therefore, the mechanism of the latter reaction and the details of the microstructures of the aged martensites are still not fully understood. Tweedlike modulated microstructures have commonly been observed in high resolution transmission electron microscope (TEM) images of Fe-C [lm,6,7]and Fe-X-C tS,s,91alloy martensites by many workers, but various origins have been proposed for the microstructures. Nagakura and co-workers [3,41 ascribed the microstructures to a periodic arrangement of randomly distributed carbon clusters on {102} planes along the (102) direction, noting that diffuse spikes and satellite reflections at the spike tips were observed around fundamental reflections along the (102) directions in the corresponding electron diffraction (ED) patterns. Taylor et al.[5], showing that the diffuse spikes appear along the (203) directions, considered the microstructures to be caused by a spinodal decomposition on {203} planes with a wavelength of about 1 nm, the plane being different from
S.B. REN, formerly Visiting Researcher, The Institute of Scientific and Industrial Research, Osaka University, is Postdoctoral Researcher, Department of Physics, Nanjing University, Nanjing 210008, People's Republic of China. T. TADAKI, Associate Professor, is with the Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567, Japan. K. SHtMIZU, Professor Emeritus, Osaka University, is Professor, Department of Materials Science and Engineering, Kanazawa Institute of Technology, Nonoichi, Ishikawa 921, Japan. X.T. WANG, Professor, is with the Department of Materials Science and Engineering, Xi'an Jiatong University, Xi'an 710049, People's Republic of China. Manuscript submitted June 14, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
the { 102} planes by Nagakura et al. However, they did not mention explicitly the satellite reflections, which are expected to appear in spinodal decomposition. According to Ohmori and Tamura, r7] the modulated microstructures are caused by the existence of dislocation lines with edge components along (112) directions. They emphasized that the projections of the dislocation lines onto the Ewald sphere are normal to the (102) or (203) directions. However, they recognized only diffuse spikes and not satellite reflections at the spike tips. On the other hand, theoretical studies by KhachaturyanVO] and Ren and Wang
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