题目: Synergistic effects of radiation damage of helium on ferritic/martensitic steels

报告人: Ph.D Yong Dai
        Laboratory for Nuclear Materials, Paul Scherrer Institut, 
        5232 – Villigen PSI, Switzerland

时间: 2015年11月2日（周一）上午9：00

地点: 李薰楼249房间

摘要: Various ferritic/martensitic (FM) steels have been irradiated to doses of 5-20 dpa with 300-1800 appm He in a temperature range of 80-460 °C in the irradiation experiments of SINQ (Swiss spallation neutron source) Target Irradiation Program (STIP). Dedicated post-irradiation examinations include mechanical tests such as tensile, 3-point bend, Charpy impact, small punch and hardness tests and microstructural analyses with transmission electron microscopy, positron annihilation spectroscopy and atom probe tomography have been performed on the irradiated specimens.

The results of mechanical testing indicate that irradiated FM steels may fail with different fracture modes, ductile, quasi-cleavage and intergranular. The fracture modes depend essentially on parameters such as irradiation dose (dpa), helium concentration, irradiation temperature, test temperature, strain rate, etc. The deformation mechanisms behind the fracture modes are the function of defect structure (defect clusters and loops) and bubble structure induced by irradiation and test temperature, which may involve strongly dislocation motion as in the ductile and quasi-cleavage fracture cases or very limited dislocation activity as for intergranular fracture. In quasi-cleavage fractured specimens defect-free dislocation channels were observed, while in ductile fractured specimens they were hardly detected. For the first time, deformation twins were observed widely in specimens failed in a very brittle manner, mostly in intergranular fracture mode and occasionally in cleavage fracture mode. The formation of twins is preceded by a phase transformation mechanism. The defect structure inside twins remains no change. The intergranular fracture could be attributed to a combined effect of strong hardening in matrix, contributed by both defect clusters and high-density small helium bubbles, and grain-boundary softening, by helium.

报告人简介：Dai Yong is currently a Senior Scientist, Team Leader of Spallation Materials Technology, in the Laboratory for Nuclear Materials in Paul Scherrer Institute (PSI), Switzerland. His main research interests include helium embrittlement, radiation damage, liquid metal (Hg, Pb-Bi eutectic) corrosion and embrittlement effects. He received his B.Sc. degree in physics from Anhui University in 1983, his M.S. degree in Solid State Physics from the Institute of Solid State Physics, Chinese Academy of Sciences in 1986, and his Ph.D. degree in Physics from Ecole Polytechnique Federal de Lausanne (EPFL) in 1995. He has three years research experience at Forschungszentrum Jülich, Germany as a Guest Scientist, studying helium embrittlement in austenitic stainless steels.

Since 1995, he has been working as a senior researcher at Paul Scherrer Institute in Switzerland and responsible for target and structural materials studies at PSI. Since 1996, he has been in charge of the SINQ Target Irradiation Program (STIP) which is joined by international laboratories and universities such as CEA (France), CIAE (China), CRPP/EPFL (Switzerland), FZJ (Germany), JAEA (Japan), LANL(USA), ORNL (USA), UCSB (USA). Due to his excellent work, Dr. Dai has been awarded the honorary professor of many institutes and universities such as Institute of Modern Physics, Institute of Solid State Physics and University of Science and Technology of China.