Defect structures in nickel and SUS304SS formed by the collapse of cavitation bubbles

A mercury target in high-power spallation neutron sources is subjected to pressure waves induced by a proton beam. The subsequent formation and collapse of cavitation bubbles lead to cavitation damage on the target vessel, especially the beam window. The cavitation damage in Ni and austenitic stainless steel SUS304SS were studied by using an electro-Magnetic IMpact Testing Machine (MIMTM) developed to simulate the damage. The existence of dislocations, stacking fault tetrahedra and vacancies was detected by positron annihilation lifetime measurements in Ni, and non-cellular dislocation structures were observed by transmission electron microscopy in Ni and SUS304SS. In addition, a high density of twins was observed in SUS304SS. These results were compared with those of high-speed compression tests using a high-speed projectile, proving that the cavitation damage caused by MIMTM corresponded to high-speed deformation.