講座題目：Fully Recrystallized Bulk Nanostructured Metals for Managing Both High Strength and Large Ductility
講座人：Prof. Nobuhiro Tsuji
Department of Materials Science and Engineering
Kyoto University, Japan
Bulk nanostructured metals (or ultrafine grained (UFG) metallic materials), of which mean grain size of the matrix is smaller than 1 μm, usually perform surprisingly high strength, high fatigue strength and good low-temperature toughness, but their tensile ductility, especially uniform elongation, is limited in many cases. This 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 the strain-hardening capability by controlling microstructures, such as, dispersing nano-particles in the UFG matrix, making the nanostructure multi-phased, etc. Nanostructures fabricated by severe plastic deformation (SPD) naturally have characteristics of deformation structures. This is one of the main microstructural reasons of 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 after fully annealed microstructures are obtained, so that the strength decreases very much. Recently, on the other hand, we have succeeded in obtaining fully recrystallized nanostructures in some alloys. The materials show both high strength and enough strain-hardening ability, resulting in large uniform elongation. All these ways to manage strength and ductility will be presented in the lecture.
Nobuhiro Tsuji received his Bachelor, Master and PhD degrees from Department of Materials Science and Technology, Kyoto University, Japan in 1989, 1991, and 1994, respectively. He worked in Osaka University from 1994 to 2009, during which he developed the accumulative roll bonding (ARB) process with his colleagues that is known as a severe plastic deformation process applicable to continuous production of large bulk nanostructured materials. He has been working as a full professor at Department of Materials Science and Engineering in Kyoto University since March 2009. His research interests include structural metallic materials, bulk nanostructured metals, microstructure and mechanical properties of materials, metal working process, physical and mechanical metallurgy. Total citations for his 377 papers have reached over 13,000 times and the h-index is 50 so far (according to publons on August 7, 2019).