Variant Selection in Grain Boundary Nucleation of Upper Bainite |
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Authors: | T Furuhara H Kawata S Morito G Miyamoto T Maki |
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Affiliation: | (1) Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan;(2) Department of Materials Science and Technology, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan;(3) Present address: Steel Research Laboratories, Nippon Steel Corporation, Futtsu 293-8511, Japan;(4) Department of Materials Science, Shimane University, Matsue 690-8504, Japan |
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Abstract: | The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct)
alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron
microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship
with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite
are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation
from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area
by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation
of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse
blocks for small undercooling.
This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during
First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati,
Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.
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