UCLA

High heat flux facing components (HHFFCs) under extreme thermal loading often fail due to surface damage and crack propagation as a result of combinations of mechanical and thermal loading. Body centered cubic (BCC) refractory metals, such as tungsten (W) and 9Cr steels, are good candidates for HHFFCs due to their high melting temperature and excellent mechanical properties. However, applications of these materials in extreme thermal and radiation environments are limited by their brittle response at low temperatures, and by thermal creep and degradation of their mechanical properties as a result of re-crystallization at high-temperature. The main objective of this thesis is to investigate the influence of high temperature rate-dependent plasticity (creep), under both cyclic and steady loading, on the evolution of residual stresses, plastic strain accumulation, and thermal shock resistance of tungsten, tungsten foam, and 9Cr steels (F82H and Eurofer). The approach is mainly experimental, where constitutive modeling of material behavior and Finite Element (FE)analysis are used to analyze and understand experimental results. This work is focused on designing controlled thermomechanical experiments to investigate solid W and W foam fracture under a variety of constrains and loading conditions. We also construct multiphysics FE models of stress evolution in large-scale fusion energy structures (First Wall and Blanket (FW/B)), where thermal, mechanical, and radiation effects are taken into consideration. Special sample fixtures are designed and fabricated to apply certain constrains and boundary conditions under high heat flux thermal loading. Additionally, a heat flux sensor is designed to properly measure the incident heat flux on samples tested in the High Energy Flux Test facilitY (HEFTY). Extensive experimental testing revealed that W foam can be used as a sacrificial (armor) material for high heat flux applications because of its resilience. The majority of foam samples survived severe cyclic plasma thermal loading, showing no significant damage. Micro-cracks were observed generally at ligament triple junctions. Largescale cracks were not observed in any of the tested foam samples. The presence of microcracks in foam samples as compared to massive cracks in solid W samples is indicative of thermomechanical resilience. The accommodation of many micro-cracks in ligaments is possible without adverse effects on component integrity, as compared to massive through-the-thickness cracks that can lead to rupture and coolant ingress into the core plasma. Temperature measurements have shown that W foam can provide a degree of heat shielding for the substrate material due to a lower effective thermal conductivity. Thus, a plasma facing component may include a top non-structural layer (armor) of W foam, bonded to a structural solid W substrate. Multi-physics FE simulations of large-scale fusion energy structures (FW/B modules) reveal that gradients in the irradiation field variables (dpa and helium generation) result in a fanning deformation mode of the entire structure. Extreme deformation is observed for free side walls at 6 years of operation. The stress and plastic strain in the FW/B structure are found to increase due to radiation gradient effects. Constraining the side walls increases the bending type deformation and displacements, with the maximum displacement at the midplane of the FW. The build-up of residual stresses in the FW/B structure can cause crack propagation upon cool-down and during cyclic reactor operations. Potential failure concerns can be alleviated by design optimization. This may include controlling the cooling patterns of the structure to maintain a high yield strength in desired areas and to allow reduction in the yield strength in other areas that are under extreme tensile stresses. Three distinct dimensional stability modes have been revealed in the current study: (1) "self-similar growth" for unconstrained modules; (2) "bending" for constrained modules; and (3) "fanning" resulting from radiation damage gradients. We also show that the maximum temperature limit of 9Cr steels in a fusion energy environment is around 530 �C, and is dictated by the accumulation of large plastic creep strains, driven by swelling-induced stresses.