Thermal Stability of Microstructure in Grain Boundary Character Distribution-Optimized and Cold-Worked Austenitic Stainl
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1215-V16-15
Thermal Stability of Microstructure in Grain Boundary Character Distribution-Optimized and Cold-Worked Austenitic Stainless Steel Developed for Nuclear Reactor Application Shinichiro Yamashita1, Yasuhide Yano1, Ryusuke Tanikawa2, Norihito Sakaguchi2, Seiichi Watanabe2, Masanori Miyagi3, Shinya Sato3, Hiroyuki Kokawa3 1 Oarai Research and Development Center, Japan Atomic Energy Agency (JAEA), 4002, Naritacho, Oarai-machi, Ibaraki, 311-1393, Japan 2 Center for Advanced Research of Energy Conversion Materials, Hokkaido University, N-13, W-8, Kita-ku, Sapporo 060-8628, Japan 3 Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-602 Aramaki-aza-Aoba, Aoba-ku, Sendai, 980-8579, Japan
ABSTRACT Grain boundary character distribution-optimized (GBCD) Type 316 corresponding austenitic stainless steel and its cold-worked form (GBCD+CW) are prospective materials to be considered for next generation nuclear energy systems. Specimens of these steels were thermally-aged at 973 K for 1 and 100 h and then examined by transmission electron microscopy (TEM) to evaluate microstructural stability during heat treatment high temperature. TEM results revealed that microstructures of both specimens types prior to ageing contained step-wise boundaries which were composed of coincidence site lattice (CSL) boundaries. The GBCD+CW specimens had dislocation cells and networks as well as deformation twins whereas the GBCD one possessed few dislocations. After thermal ageing, the precipitates were formed on not only random grain boundaries but also on dislocations, and they contribute to prevent significant microstructural change such as recrystallization and dislocation recovery. INTRODUCTION Austenitic stainless steels have been commonly used as structural materials in powergenerating industries due to their high ductility and fracture toughness, but meanwhile successive efforts to improve their properties have also been made to meet the severe requirement for structural materials used in future nuclear reactors, such as advanced light water reactors and fast breeder reactors [1,2]. Among several approaches to improve the material property, grain boundary structure studies using commercial austenitic stainless steels have suggested prospective and practical methods to modify the bulk property of the steels; for example, the optimal distribution of coincidence site lattice (CSL) boundaries and consequent discontinuity of random boundary network was demonstrated to enhance intergranular corrosion resistance [3-6]. Of particular interest is that this property is relatively easy to obtain through simple thermo-mechanical treatment process without any change of the original chemical composition. In this study, the thermal stability of microstructures of the grain boundary character distribution-optimized (GBCD) Type 316 austenitic stainless steel and its cold-worked corresponding form (GBCD+CW) was investigated, mainly focusing on the effects of the
additional cold-working and precipitate formation
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