Role of Different Kinds of Boundaries Against Cleavage Crack Propagation in Low-Temperature Embrittlement of Low-Carbon
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INTRODUCTION
HIGH-STRENGTH martensitic steels usually exhibit brittle fracture, in particular at low temperatures, that is the so-called low-temperature embrittlement. Because low-temperature embrittlement may result in sudden and serious accidents, it is necessary to improve the toughness of martensitic steels at low temperatures. It is well known that low-temperature embrittlement can be suppressed by grain refinement,[1–5] and grain boundaries can act as obstacles for cleavage crack propagation. Lath martensite structure, which is a typical microstructure in low and medium carbon martensitic steels, consists of several structural units with different length scales, i.e., lath, block, packet, and prior austenite grain.[6,7] The martensite lath is a single crystal of martensite with a thickness of about 0.2 lm. The block contains many laths having nearly identical orientation. Assuming Kurdjumov–Sachs (K–S) orientation MIZUKI TSUBOI is with the Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. Contact e-mail: [email protected] AKINOBU SHIBATA, and NOBUHIRO TSUJI are with the Department of Materials Science and Engineering, Kyoto University, and also with Elements Strategy Initiative for Structural Materials, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan. DAISUKE TERADA is with the Department of Mechanical Science and Engineering, Chiba Institute of Technology, Tsudanuma, Narashino, Chiba 275-0016, Japan, and also with the Elements Strategy Initiative for Structural Materials, Kyoto University. Manuscript submitted October 21, 2016. Article published online May 8, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
relationship between austenite and martensite ({111}A// {011}M, h110iA//h111iM, where subscripts A and M indicate austenite and martensite, respectively), for example, 24 different variants of martensite (i.e., 24 different orientations of blocks) can form within one austenite grain. The packet consists of several blocks having the same habit plane. Consequently, the boundaries inside lath martensite structure, such as lath boundary, block boundary, packet boundary, and prior austenite grain boundary, have different crystallographic features. Matsuda et al.[8] reported that the cleavage facets size correlated well with martensite block size. Zhijun et al.[9] also found that the cleavage crack propagations deflected significantly at block boundaries. On the other hand, Wang et al.[10] observed that the propagation direction of cleavage cracks was changed significantly at packet boundaries, but not at block boundaries. Hanamura et al.[11] proposed that prior austenite grain boundaries could prevent the propagation of cleavage cracks effectively because the facet size of fracture surface was in good agreement with prior austenite grain size. In addition, Morris et al.[12–14] have suggested a very important idea from a crystallographic point of view that the boundaries which separate different martensite vari
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