Strengthening effect of twin boundaries in bcc crystal evaluated through a micro-bending test
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Strengthening effect of twin boundaries in bcc crystal evaluated through a micro-bending test
Yuki Karasawa1, Tso-Fu Mark Chang1, Akinobu Shibata2 and Masato Sone3 1
Department of Materials Science & Engineering, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8503, Japan 2 Department of Materials Science and Engineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan 3 Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
ABSTRACT In the present study, the strengthening effect of nano-scale twins in body-centered cubic (bcc) crystal was evaluated using micro-sized cantilever type specimen which contained the nanotwinned region (midrib) in ferrous lenticular martensite. The SEM observations of the micro-sized specimen after bending deformation indicated that midrib can act as barriers against dislocations, resulting in slip localization and non-localization across midrib. The load-displacement curve obtained by bending test showed that twin boundaries significantly enhance the critical resolved shear stress of bcc.
INTRODUCTION Grain refinement is an important method in industries to improve mechanical properties of materials such as strength or ductile-brittle transition. Refinement by twin boundaries (TBs) as well as by general grain boundaries gives rise to the increase of strength [1,2]. Lu et al. [3-5] have succeeded in fabricating fully twinned copper by pulse electrodeposition, where twin thickness is in nanometer scale, and systematically studied the mechanical properties of the fabricated nanotwinned copper. According to their results, nanotwinned copper accomplishes ultrahigh strength without losing ductility significantly. In fact, ductility of nanotwinned copper improves with a decrease of twin thickness [5]. These results might be attributed to the
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coherency of TBs and partial dislocations gliding on TBs. For face-centered cubic (fcc) metals, similar results of strengthening by nanotwins have been reported [6,7]. In this context, introduction of nanotwins into materials is a more effective way to improve the strength, in terms of retaining ductility, rather than general grain refinement where the elongation dramatically decreases with an increase of strength. However, for body-centered cubic (bcc) metals, it is difficult to introduce a high density of twins into matrices during any processes because of those high stacking fault energies. One way to achieve the bcc materials with nanotwins is a martensite transformation. Lenticular martensite is a kind of Į’ martensite (bcc or body-centered tetragonal (bct)) transformed from austenite (fcc). It is composed of three substructures; midrib, twinned region and untwinned region [8]. Midrib, in the middle part of martensite plate, is an aggregation of very fine transformation twins of which thicknesses are about tens of nanometer, as shown in figure 1. In the present study, we explore the effect of TBs (midrib) on the strength of bcc crys
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