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Fig. 8-2 Microstructure of the DS-Cu/stainless steel joint observed with an optical microscope

Fig. 8-3 Comparison of tensile strength and three-point bend fracture energy of the DS-Cu base metal and joint materials Tensile strength of the joint is the same as DS-Cu, however, the fracture energy of the direct joint is 20% of the DS-Cu. It is improved up to 50% by inserting a gold foil.

Materials science has always played an indispensable role in the progress of science and technology. For nuclear fusion reactors to be built in the future, special materials will be needed to withstand both the severe irradiation environment created by high-energy neutrons reaching the first wall which faces the plasma, and the high heat load which accompanies them. It is also required that these materials have good thermal conductivity to efficiently transfer the heat to the coolant. Alumina-dispersion-strengthened copper (DS-Cu) is a promising candidate material for the first wall and the divertor of the International Thermonuclear Fusion Experiment Reactor (ITER). Although the DS-Cu is good from the view point of both thermal conductivity and strength, it is rather poor in its resistance to corrosion, irradiation and heat load. To overcome these weak points, technology has been developed to bond DS-Cu with austenitic 316 stainless steel to make use of the excellent properties of both of these materials. Conventional welding cannot be used to join these different materials because of the large difference in their melting temperatures which would cause degradation of the joint under neutron irradiation and because of the embrittlement caused by formation of intermetallic compounds at the interface. Another problem is that alumina segregates and precipitates at the grain boundary when the material is melted. Therefore, a solid-state diffusion bonding technique has been applied, in which DS-Cu and the stainless steel have been tightly contacted to make atoms of each material diffuse into the other and mix below the melting temperature. In the course of development, it was discovered that defects at the joint interface can be reduced by inserting a gold foil there. Figs. 8-2 and 8-3 show the microstructure and joint strength in the case of direct joint formation, and with joint formation using a gold foil insert, respectively. Defects are observed at the interface in the case of the direct joint while they are reduced when the gold foil is inserted. Consequently the strength of the joint is enhanced, with each material exhibiting its own positive properties.

Reference H. Nishi et al., Application of Solid State Diffusion Bonding to Fusion Reactor, Genshiryoku Kogyo, 42 (9), 18 (1996).

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