UC Berkeley

As fusion energy is being developed, a fundamental mechanistic understanding of how materials will behave when exposed to the intense reactor environment is imperative. F82H is one of the most mature candidate materials for the structural vessel of the pioneering ITER experiment. The alloy is a tempered martensitic steel developed to withstand the fusion reactor environment however much is still not known about it. Particularly, the mechanical effects of carbide nucleation in the steel has not been deeply studied across the microstructure. To investigate this, quenched and tempered F82H needs to be mechanically investigated using small scale mechanical testing to probe individual boundaries. Small scale mechanical testing probes micron-scale volumes of material in order to understand specific material defect interactions. Using such resolved techniques will allow for conclusions to be formed about carbide precipitation and the effects on mechanical properties within different microstructural features of F82H. In general, it was found that carbide nucleation on the different features of the martensitic microstructure have widely varying effects in terms of mechanical behavior which can be explained through preferred nucleation of carbides to high angle grain boundary interfaces. In static tension loading, the nucleation of carbides along high angle grain boundaries strengthened these boundaries, while low angle grain boundaries often initiated fracture. In dynamic loading, the nucleation of carbides along high angle grain boundaries prevented their interaction with damping mechanisms. On the other hand, carbides away from high angle grain boundaries were able to interact with dislocations causing a large increase to the damping capabilities of F82H.