Formation mechanism of bainitic ferrite in an Fe-2 Pct Si-0.6 Pct C alloy
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INTRODUCTION
THE mechanism of bainite transformation in steels has not yet been established despite much work both experimentally and theoretically; two completely different models, i.e., a diffusional and a displacive mechanism, have been proposed. II-sJ The diffusional school proposes that the ferritic component of bainite develops by a diffusional ledge mechanism. On the other hand, the displacive school claims that the bainite transformation occurs diffusionlessly, as far as the solvent and substitutional solute atoms are concerned. For clarifying the transformation mechanism, it is of great importance to examine the thickening process and the carbon concentration of bainitic ferrite. If bainitic ferrite forms by the diffusional mechanism with a local equilibrium condition, it has the carbon concentration in equilibrium with respect to austenite. Moreover, it can grow until the average carbon concentration of the untransformed austenite reaches the equilibrium value with respect to ferrite, if the so-called "solute drag-like effect "t1'2'5l is absent. On the other hand, if bainitic ferrite develops by a displacive mechanism, the carbon concentration of it can be higher than the equilibrium value with respect to austenite. Moreover, it may not thicken after growing to a limited size due to a loss of coherency of the interphase boundaries.I61 The measurements of the thickening process and carbon concentration of bainitic ferrite are usually difficult KANEAKI TSUZAKI, Associate Professor, and TADASHI MAKI, Professor, are with the Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan. AKI KODAI, formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan, is Researcher, with the Material Research Department, Akashi Technical Institute, Kawasaki Heavy Industries, Ltd., Kawasaki-cho, Akashi 673, Japan. This article is based on a presentation made at the Pacific Rim Conference on the "'Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
for the bainite transformation of conventional carbon steels because of the rapid precipitation of carbides and the absence of untransformed austenite within a bainitic packet or sheave. However, unlike the cases of conventional carbon steels, it is now well known that in the carbon steels containing about 2 mass pct silicon, carbide-free, lath-shaped, bainitic ferrite forms and is separated one from another by carbon-enriched retained austenite in early stages of transformation around 700 K. 17-111In this case, we can measure the carbon concentration of retained austenite by, e.g., X-ray diffraction analysis. Then, because it is carbide free, we can estimate the carbon concentration of bainitic ferrite from the mass balance of carbon, when the volume fractions of bainitic ferrite and austenite are obtained. Moreover, because bainitic fer
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