Research On Practicability Of Applying Vacuum Diffusion Bonding To China Low Activation Martensite Steel

As the primary candidate structural material for the first wall or test blanketmodule in fusion energy systems, the reliable bonding of reduced activationferritic/martensitic steel to the similar or dissimilar materials has been the spot innuclear fusion research. In this paper, by means of OM, SEM, EDS and mechanicaltesting, the practicability of applying vacuum diffusion bonding to China lowactivation martensite steel (CLAM steel) was investigated, exploring the effect ofsurface state and interlayer type on the microstructure and mechanical performance ofthe joint.The valid bonding between CLAM steels can be obtained by direct diffusionbonding, and no diffusion pores were observed at the interface. Different surfacepreparation had no significant influences on the tensile strengths of the joints at roomtemperature. In the case of direct diffusion bonding, the residual stress level at theinterface was high, leading to the low impact toughness.In contrast to direct diffusion bonding, when CLAM steels were diffusionbonded with nickel-based amorphous foil, Kirkendall voids occurred at theintermediate interlayer. And near the interface, the CLAM steel exhibited equiaxedgrains rather than martensite laths, which was suggested to be related to therecrystallization induced by diffusion of nickel along prior austenite boundaries. Thejoint tensile strength was relevant to the compositions of nickel-based amorphous foil,and its impact toughness was not improved by using intermediate interlayer.When using pure nickel as the interlayer, the Kirkendall voids near the interfacevanished, as well as equiaxed grains. However, at the center of nickel layer, coarse(Fe,Cr)3C was observed. The (Fe,Cr)3C distributed in a direction parallel to theinterface. In the case where interlayer was Cu/Ni composite layer, the precipitation of(Fe,Cr)3C was suppressed by the addition of Cu. There were no obviousmicrostructure defects at the joint interface, and the joint strength reached up to93%of that of the base material. These two types of interlayers did not improve the impacttoughness either.Finally, it can be concluded that the regular interlayers cannot meet therequirement of impact toughness for vacuum diffusion bonded joint, even despitetheir low activations. It is necessary to develop novel interlayers, whose compositions are similar to those of CLAM steel, in order to reduce the difference in microstructurebetween interlayer and CLAM steel, in addition to the consideration of low activation.