Investigation Of Irradiation Behavior Of Ni-Si C_np Composites

The structural materials for the primary loop of Molten Salt Reactor（MSR） will face the extreme environments, such as high temperature, high neutron irradiation and severe corrosion. The nickel-based Hastelloy N alloy was considered to be the primary choice of metallic structural materials in MSR due to its outstanding corrosion resistance to fluoride molten salts. However, due to the large neutron absorption cross section of nickel（Ni）, helium can be produced via reaction between Ni and neutron, resulting in the aggregation of helium bubbles in the grain boundary（GB） and the subsequent helium embrittlement and swelling problem, which can seriously deteriorate the mechanical property of the material. Recently, silicon carbide nanoparticles(SiC_NP) with excellent corrosion resistance to molten fluoride salt have been successfully reinforced into Ni metal by ball milling and spark plasma sintering（SPS）. The silicon carbide nanoparticle reinforced nickel-based alloys(Ni-SiC_NP composites) are expected to show good resistance to helium embrittlement and swelling. In order to evaluate the application prospect of this material in MSR, it is essential to carry out the helium ion irradiation experiment and study the irradiation damage of the material. Irradiation induced microstructural evolution is generally believed to be affected by many factors, i.e. temperature, fluence and flux. Compared to the effects of fluence and temperature, the flux effect has been rarely reported. Hence, the study of ion flux effect in our Ni-SiC_NP composite will also contribute to the further understanding of irradiation damage mechanism in general.In this study, the pure Ni and Ni-1wt.%SiC_NP composite samples were irradiated with helium ions at 600 oC. TEM and nanoindentation were used to study the irradiation induced microstructural changes and material hardening. The TEM results showed that in the same material, the size of helium bubbles increased with increasing ion dose. The size and number density of helium bubbles in the Ni-1wt.%SiC_NP composite were smaller and larger than those in the pure Ni, respectively. The adsorption of He atoms in the interface of dispersed SiC_NP and Ni matrix was believed to be the main reason for this, which indicated that the addition of SiC_NP in Ni can effectively restrain the growth of helium bubbles. Besides that, a large number of nano-scaled spherical and polygonal helium bubbles were observed where the peak damage of the Ni-1wt.%SiC_NP composite was when irradiated respectively at higher and lower ion flux. The shape change of the helium bubbles was attributed to the variation of internal pressure. In addition, the variation of ion flux affected the microstructure of the Ni-1wt.%SiC_NP composite after irradiation. The size and number density of helium bubbles decreased and increased respectively with increasing flux. However, at the same dose, the volume fractions of the helium bubbles were comparable at different fluxes. Therefore, it was concluded that the variation in flux had no obvious influence on the irradiation hardening of the material. Finally, the nanoindentation results showed that at both ion fluxes, the nanohardness increased with the increasing ion fluence. This was attributed to the irradiation induced helium bubbles, which can act as obstacles to the free motion of dislocations.