Structural metallic materials are required to have ultrahigh strength nowadays by social demands of light-weighting for huge constructions and transportation devices like automobiles. Ultrafine grained (UFG) metallic materials (or bulk nanostructured metals) with average grain sizes much smaller than 1 µm are candidates of ultrahigh-strength metals. Nanostructured metals and alloys usually show strength 2-4 times higher than that of conventional metals with coarse grain sizes larger than several tens micro-meter. However, their tensile ductility, especially uniform elongation, is limited in many cases, which is attributed to the early plastic instability caused by high flow stress and limited strain-hardening capability of the UFG materials. Therefore, an important strategy for managing both strength and ductility in nanostructured metals is to increase strain-hardening capability by controlling microstructures, such as, dispersing nano-particles in the UFG matrix, making the nanostructure multi-phased, etc. UFG metals fabricated by severe plastic deformation (SPD) naturally have characteristics of deformation structures. This is one of the main microstructural reasons of their limited ductility, as strain-hardened metals usually do not show large tensile elongation. In order to remove the deformed characteristics, annealing processes are usually applied. But in many cases of the SPD processed materials, the mean grain size becomes over 1 µm when fully annealed microstructures are obtained, so that their strength decreases very much. Recently, on the other hand, we have succeeded in obtaining fully recrystallized nanostructures in some alloys. The recrystallized nanostructured metals, especially some kinds of steels, show both high strength and enough strain-hardening ability, resulting in large uniform elongation. It has been found that the excellent mechanical properties of recrystallized nanostructured metals could be explained by enhancement of strain-hardening due to the activation of different (and unexpected) deformation modes. Examples of nanostructured steels managing both high strength and large tensile ductility will be introduced in the lecture. Department of Materials Science and Engineering, Kyoto University, Japan. Abstract Managing Both UltraPossibilities of Advanced Steels Session 1 ｜Nobuhiro Tsuji received his PhD degree from Department of Materials Science and Technology, Kyoto University, Japan, in 1994. He worked as an assistant professor and then an associate professor in Osaka University, Japan, from 1994 to 2009. He has been a full professor for physical metallurgy of structural metallic materials at Department of Materials Science and Engineering in Kyoto University since March 2009. His research interests have been (i) correlation between nano/micro-structures and mechanical properties of structural materials; (ii) fundamental mechanisms of deformation, recrystallization, and phase transformation during thermomechanical processing of metals; and (iii) fabrication and properties of nanostructured metals. Particularly his works on bulk nanostructured metals (ultrafine grained metallic materials) are well recognized. Total citations for his 482 papers published in scientific journals are about 24,000 times and the h-index is 75 (according to Scopus). NIMS Award Symposium 2023-High Strength and Large Ductility Nobuhiro Tsuji Professor Deformation and FractureInvited Talk S1-1 23
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