Crystallography and Interphase Boundary of Martensite and Bainite in Steels
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IN recent years, the importance of martensite and bainite structure has increased in regard to the strengthening of practical steels. Lath martensite and ‘‘bainitic ferrite,’’ which is a ferrite component in the bainite structure, are both lath-shaped, fine crystals containing high densities of dislocations. There is a variety of crystalline size in lath martensite and bainite structures. A hierarchy of lath martensite structure is clearly identified, particularly in low-carbon steels.[1] A prior austenite (c) grain is divided into ‘‘packets,’’ each of which consists of a group of martensite laths with the same parallel close-packed plane relationship in the Kurdjumov–Sachs (K–S) orientation relationship, and was denoted as the ‘‘CP group’’ recently. In general, a packet is partitioned into several blocks, each containing laths of a single variant of the K–S relationship. Blocks and packets are mostly surrounded by high-angle boundaries, whereas lath boundaries inside a block are of low-angle type. Local orientation measurement using electron backscatter diffraction (EBSD) in scanning electron microscopy (SEM) has been successful for examining crystallography of phase transformations in steels.[2] It was recently reported that a block of
TADASHI FURUHARA and GORO MIYAMOTO are with the Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577 Japan. Contact e-mail: [email protected] TADACHIKA CHIBA is with the Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya, Japan. TAKESHI KANESHITA is with the Department of Metallurgy, Tohoku University, Sendai, Japan. HUIDONG WU is with the School of Materials Science and Engineering, Tsinghua University, Beijing, China. This article is based upon a presentation made at the International Conference on Solid-Solid Phase Transformations in Inorganic Material (PTM2015) held in Whistler, BC, Canada from June 28 to July 3, 2015. Manuscript submitted May 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
low-carbon lath martensite consists of two ‘‘sub-blocks’’ misoriented by about 10 deg.[3,4] A similar substructure was reported in the upper bainite structure.[5] More comprehensive study[6] revealed that variant selection in Bain correspondence (Figure 1(a)) is important, particularly in bainite structure formed at high temperature, resulting in coarse-grained structure consisting laths belonging to a single Bain variant, i.e., a ‘‘Bain’’ group, with lower strength/toughness balance.[1] Table I summarizes grouping for CP and Bain groups for 24 possible K-S variants with misorientations from V1 taken as a reference. Figure 1(b) shows the CP and Bain grouping in the cube axes for all the variants plotted on the 001c stereographic projection. It is clearly seen that the variants belonging to the same Bain group are misoriented to each other by rather small angles. Those crystalline sizes in martensite and bainite structure reflect strength directly. As in ferrite and pearlite, grain refinement of packet
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