Topic: Bulk Nanostructured Metals processing, structure evolution, and mechanical properties

Speaker: Prof. Nobuhiro Tsuji
　　　Department of Materials Science and Engineering, Kyoto University
　　　Yoshida Honmachi, Sakyo-ku, Kyoto, 612-8492, Japan
　　　Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University

Abstract：

Bulk nanostructured metals (or ultrafine grained (UFG) metallic materials), of which mean grain size of the matrix is smaller than 1 m, perform superior and interesting mechanical properties, and they are expected advanced high strength materials in future. In this lecture, processing, structure evolution, and origin of mechanical properties obtained mainly in the speaker's group are introduced. Severe plastic deformation (SPD) that can give huge amounts of plastic strains to bulky metallic materials made a big breakthrough in studies of nanostructured metals [1,2]. Since SPD processes are not realistic for continuous mass-production in big materials industries, we have also developed several thermo-mechanical controlled processes (TMCP) that can fabricate UFG metals without huge plastic strains [3-5]. The formation mechanism of UFGs during SPD can be understood in terms of grain subdivision [6,7]. Some of the developed TMCPs use this principle, while some utilize combining phase transformation and plastic deformation [5,8]. Bulk nanostructured metals usually show very high strength, high fatigue strength and good low-temperature toughness, but their tensile ductility, especially uniform elongation, is limited in may cases [9]. This is attributed to the early plastic instability caused by high flow stress and limited strain-hardening capability of the UFG materials [9,10]. Therefore, an important strategy for managing both strength and ductility in nanostructured metals is to increase the strain-hardening capability by controlling microstructures, such as, dispersing nano-particles in the UFG matrix, making the nanostructure multi-phased, etc. [10]. Nanostructures fabricated by SPD naturally have characteristics of deformation structures, which is one of the main microstructural reasons of limited ductility, as strain-hardened metals usually do not show large tensile elongation. Recently, we have succeeded in obtaining fully-recrystallized nanostructures with mean grain sizes much smaller than 1 m in some alloys [11-15]. Such materials show both high strength and enough strain-hardening ability leading to large uniform elongation. Bulk nanostructured metals often show peculiar phenomena that are never found in conventionally coarse-grained metals, such as yield-drop phenomena universally appearing in almost all kinds of UFG metals [9,11,14,15,16], hardening-by-annealing & softening-by-deformation [17], and extra-hardening in Hall-Petch relationship [18]. Such characteristic properties are also discussed in the lecture.

References:
[1]  A.Azushima, et al.: CIRP Annals, 57 (2008), pp.716-735.
[2] N.Tsuji, Y.Saito, S.H.Lee, Y.Minamino: Adv. Eng. Mater., 5 (2003), pp.338-344.
[3] R.Ueji, N.Tsuji, Y.Minamino, N.Tsuji: Acta Mater., 50 (2002), 4177.
[4] Y.Okitsu, N.Takata, N.Tsuji: Scripta Mater., 60 (2009), pp.76-79.
[5] L.Zhao, N.Park, Y.Tian, A.Shibata, N.Tsuji: Scientific Reports, 6 (2016), 39127.
[6] X.Huang, N.Tsuji, N.Hansen, Y.Minamino: Mater. Sci. Eng., A340 (2003), pp.265-271.
[7] "Nanostructured Metals and Alloys", edited by Sung H.Whang, Woodhead Publishing (2011), Chapter 2 by N.Tsuji.
[8] N.Tsuji, T.Maki: Scripta Mater., 37 (2009), pp.1044-1049.
[9] N.Tsuji, Y.Ito, Y.Saito and Y.Minamino: Scripta Mater., Vol.45, No.12 (2002), pp.893-899.
[10] N.Tsuji, N.Kamikawa, R.Ueji, N.Takata, H.Koyama and D.Terada: ISIJ International, Vol.48, No.8 (2008), pp.1114-1121.
[11]  R.Saha, R.Ueji and N.Tsuji: Scripta Mater., Vol.68, Issue 10 (2013), pp.813-816.
[12]  Y.Z. Tian, L.J. Zhao, S. Chen, A. Shibata, Z.F. Zhang, N. Tsuji: Scientific Reports, Vol.5 (2015), 16707.
[13]  R.Zheng, T.Bhattacharjee, A.Shibata, T.Sasaki, K.Hono, M.Joshi and N.Tsuji: Scripta Mater., 131 (2017), pp.1-5.
[14]  S.Yoshida, T.Bhattacharjee, Y.Bai and N.Tsuji: Scripta Mater., 134 (2017), pp.33-36.
[15]  Y.Z.Tian, S.Gao, L.J.Zhao, S.Lu, Z.F.Zhang and N.Tsuji: Scripta Mater., 142 (2018), pp.88-91.
[16] D.Terada, M.Inoue, H.Kitahara, N.Tsuji: Mater. Trans., 49 (2008), pp.41-46.
[17] X.Huang, N.Hansen, N.Tsuji: Science, 312 (2006), pp.249-251.
[18] N.Kamikawa, X.Huang, N.Tsuji, N.Hansen: Acta Mater., 57 (2009), pp.4198-4208.

List of Tsuji's papers: http://www.researcherid.com/rid/F-7951-2010