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Materials Frontier No.181

Title:Suppression of weld degradations by grain boundary engineering
Speaker:Prof. Hiroyuki Kokawa, University of Bristol, UK. 
Venue:Room 500,Materials Building A 
Inviter:Prof.LI Zhuguo
   Dr. Kokawa is a Professor in Department of Materials Processing at Graduate School of Engineering of Tohoku University in Japan. He received his Ph. D. degree in materials science from Tohoku University in 1979. His doctoral work involved examination of grain boundary sliding and dislocation structures of metallic materials from crystallographic viewpoint by transmission electron microscopy. Following graduation, he joined Welding Engineering and Technology Group in Department of Materials Processing at Tohoku University, where he is currently the leader of the group. He has performed considerable research regarding welding metallurgy, especially on grain boundary engineering and friction stir welding. He has authored or co-authored over 200 publications. He spent a sabbatical year as a visiting scientist at University of Toronto, Canada in 1989. He has received Silver Medal from Honda Memorial Foundation in 1981, Professor Masubuchi Award from the American Welding Society in 1992, Best Paper of the Year Award of Japan Welding Society(JWS) in 1992, JIM Meritorious Award from Japan Institute of Metals(JIM) in 1995, Nishiyama Commemorative Prize from Iron and Steel Institute of Japan(ISIJ) in 1998, Best Author of the Year Award of JWS in 2004, Scientific Achievement Award of JWS in 2006, District Contribution of Welding Award in 2010 and Fellow of JWS in 2010. He was President of JWS (till April 13, 2016), Executive Board Member of Japan Welding Engineering Society(JWES) and Japan Institute of Light Metals(JILM), and Supervisor of JIM.
   Grain boundary phenomena strongly depend on grain boundary structure and character, i.e., coincidence site lattice (CSL) boundaries, as contrasted with random boundaries, are highly resistant to intergranular degradations. Grain boundary engineering (GBE) primarily intends to prevent the initiation and propagation of intergranular degradation along random boundaries by frequent introduction of CSL boundaries into the grain boundary networks in materials [1]. A high frequency of CSL boundaries by GBE processing leads to high resistance to grain boundary degradations. Annealing twins bring CSL boundaries into austenitic stainless steels. By twin-induced GBE utilizing optimized single-step thermomechanical processing consisting of a slight strain followed by annealing, a very high frequency of CSL boundaries was introduced into austenitic stainless steels [2,3]. The resulting steels indicated remarkably high resistance to intergranular corrosion even to weld-decay [4] and knife-line attack during welding. GBE could have a high potential to suppress variety of grain boundary-related degradations of welded austenitic stainless steels [1].
[1] H. Kokawa, Sci. Technol. Weld. Join., 16 (2011), 357.
[2] M. Shimada, H. Kokawa, Z.J. Wang, Y.S. Sato, I. Karibe, Acta Mater., 50 (2002), 2331.
[3] M. Michiuchi, H. Kokawa, Z.J. Wang, Y.S. Sato, K. Sakai, Acta Mater., 54 (2006), 5179.
[4] H. Kokawa, M. Shimada, M. Michiuchi, Z.J. Wang, Y. S. Sato, Acta Mater., 55 (2007), 5401.

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