Variant Selection in Grain Boundary Nucleation of Upper Bainite
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INTRODUCTION
BAINITE has become more important in recent steel technology, because it is dominant in the microstructure of high-strength structural steels for ship building or for line pipe with good welding performance, high-strength and high-ductility steels for automobile parts, and so on. An upper bainite structure, which forms with lathshaped bainitic ferrite[1] at temperatures typically above 623 K, consists of the substructures similar to lath martensite.[2] A prior austenite grain is divided into packets, each of which consists of a group of laths with the same parallel close-packed plane relationship in the Kurdjumov–Sachs (K-S) orientation relationship. In general, a bainite packet is partitioned into several blocks, each of which contains laths of a single variant of the K-S relationship. The packet or block size is important in lath martensite for both strengthening[3–5] and toughening.[3,6,7] Recently, the substructure of lath martensite was studied in detail by local orientation measurement with electron backscatter diffraction in scanning electron microscopy T. FURUHARA, Professor, and G. MIYAMOTO, Assistant Professor, are with the Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: furuhara@imr. tohoku.ac.jp H. KAWATA, formerly Graduate Student, Department of Materials Science and Technology, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan, is Researcher, Steel Research Laboratories, Nippon Steel Corporation, Futtsu 293-8511, Japan. S. MORITO, Associate Professor, is with the Department of Materials Science, Shimane University, Matsue, 690-8504, Japan. T. MAKI, Professor Emeritus, Department of Materials Science and Technology, Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan, is Executive Advisor, Steel Research Laboratories, Nippon Steel Corporation. 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. Article published online March 21, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
(SEM-EBSD).[8–10] It was confirmed that blocks and packets are refined with an increase of carbon content,[9] as previously reported by one of the present authors.[11] More interestingly, preferential formation of specific variant pairs that are misoriented by a relatively low angle (approximately, 10 deg) in a packet was revealed for low carbon alloys,[9] as also confirmed recently in an Fe-Ni alloy.[10] Application of SEM-EBSD was extended to study the crystallography of bainite structures,[8,12,13] although details of the bainite block/packet structures were not discussed. Variations of the block/packet sizes with transformation temperature, carbon content,[2] and ausforming[14] were first examined systematically by the present authors. At a higher transformation temperature or carbon content, where a driving for
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