Low-temperature mechanical properties of superconducting radio frequency cavity materials

Low-temperature mechanical behaviors have been investigated for the constituent materials of superconducting radio frequency cavities. Test materials consist of small grain Nb, single crystal Nb, large grain Nb (bicrystal), Ti45Nb–Nb weld joint (e-beam welded), and Ti–316L bimetal joint (explosion welded). The strength of all test metals displayed strong temperature dependence and the Ti–316L bimetal showed the highest strength and lowest ductility among the test materials. The fracture toughness of the small grain Nb metals decreased with decreasing test temperature and reached the lower shelf values (30–40 MPa √m) at or above 173 K. The Ti45Nb base and Ti45Nb–Nb weld metals showed much higher fracture toughness than the small grain Nb. An extrapolation and comparison with existing data showed that the fracture toughness of the small grain Nb metals at 4 K was expected to be similar to those at 173 and 77 K. The results from optical photography at a low magnification and fractography by a scanning electron microscope were consistent with corresponding mechanical properties.