PHASE INSTABILITY AND TOUGHNESS CHANGE DURING HIGH-TEMPERATURE EXPOSURE OF VARIOUS STEELS FOR THE FIRST WALL STRUCTURAL-MATERIALS OF A FUSION-REACTOR

The objective of the present research is to clarify the phase instability, particularly, the precipitation behavior of carbide and nitride during the long term aging in the non-irradiation state of the materials proposed for the first wall structural component of fusion reactors, such as a type 316 austenitic steel, its modified steels, ferritic heat resisting steels and reduced radio-activation materials. The effect of the precipitation behavior on the toughness is also investigated. It is noticed that the toughness was much deteriorated by the formation of large amounts of coarse carbides within grains and on grain boundaries during 2.88 x 10(4) ks (8000 h) aging at 873 K and that intergranular fracture occurred by the impact test at room temperature even in the type 316 steel.