Enhanced Radiation Tolerance in Sputtered Cu/V Multilayers

High energy particle (neutron, proton and He ions) irradiation to materials typically leads to deteriorating properties, including void swelling, blistering, embrittlement, fracture and exfoliation of surfaces. This dissertation examines size dependent radiation damage in nanostructured metallic multilayers synthesized by the magnetron sputtering technique at room temperature. It reveals the roles of interface in achieving enhanced radiation tolerance in metallic materials. The microstructure and mechanical properties of as-deposited Cu/V multilayer films are systemically investigated, providing the basis for studying radiation damage mechanisms. Sputter-deposited Cu/V multilayers are subjected to helium (He) ion irradiation at room temperature with a peak dose of 6 displacements per atom (dpa). The average helium bubble density and lattice expansion induced by radiation decrease significantly with decreasing h, where h is individual layer thickness. The magnitude of radiation hardening decreases with decreasing h, and becomes negligible when h is 2.5 nm or less. The interactions between interfaces and radiation induced point defects and the evolution of microstructurs and mechanical behavior are discussed. This study indicates that nearly immiscible Cu/V interfaces spaced a few nm apart can effectively reduce the concentration of radiation induced point defects. Dose dependent radiation damage at room temperature in these Cu/V multilayers is systematically investigated with a peak dose in the range of 1-12 dpa. Peak bubble density increases with increasing dose, but it is much lower in Cu/V 2.5 nm multilayers than that in Cu/V 50 nm specimens. A similar radiation hardening trend is observed in multilayers irradiated at different fluences. Radiation hardening increases with dose and seems to reach saturation at a peak dose of 6 dpa. Negligible hardening for fine ( h less than/equal to 2.5 nm) multilayers is observed at all dose levels. Thermal stability of Cu/V multilayers is revealed by in situ annealing inside a transmission electron microscope. During isothermal annealing at 600 degrees C grain boundary grooving occurs across layer interfaces in Cu/V 50 nm specimens, whereas Cu/V 5 nm multilayers appear rather stable. Annealing of Cu/V multilayers at 400 degrees C leads to hardening of multilayers, whereas softening occurs in Cu/V multilayers annealed at 600 degrees C. The evolution of mechanical properties during annealing is correlated to the degradation of the layer interface and the consequent reduction of interface resistance to the transmission of single dislocation.