Thermal Stability, Radiation Defect Production, and Gas Behavior in Fine-Grained Refractory Metals and Alloys

摘要

The unique behaviors of fine-grained metals have attracted significant interest in recent years due to their increased hardness and enhanced radiation tolerance manifested through annihilation of defects and He gas retention. However, their far from equilibrium state arising from their high interfacial densities renders fine-grained metals unstable at high temperatures, potentially vitiating the potential benefits of nanostructuring. Targeted doping has emerged as a strategy for alleviating thermal instabilities through reduction of the intrinsic driving force for grain growth by targeting the excess interfacial free energy or limiting the kinetic behavior. However, the addition of dopants has implications for the mechanical behavior and radiation tolerance of fine-grained metals. In this dissertation, numerous topics related to thermal stability, radiation defect production, and gas behavior are investigated in fine-grained BCC refractory metals and alloys. First, in situ TEM is utilized to study the transitions in the dominant thermodynamic stabilization mechanism in a fine-grained Mo alloy. In situ heating of a Mo-Au nanometallic multilayer microstructure demonstrated a transition from thermodynamically dominated stabilization at low temperatures to a kinetically stabilized microstructure at higher temperatures, coincident with predicted Au segregation and diffusion at those temperatures. Then, the influence of dopants and dispersoids on microstructural stability and defect production in fine-grained W alloys under ion irradiation are investigated through in situ TEM, with results indicating that the addition of dopants stabilizes the microstructure against irradiation induced grain growth and modifies the stress state around grain boundaries, augmenting their effective sink strength. Finally, the influence of helium gas implantation at grain boundaries on mechanical behavior in fine-grained tungsten is investigated through TEM and nanoindentation. The formation of helium filled cavities at the grain boundaries is shown to correlate to a significant reduction in material strength along with a reduction in dislocation activity and the initiation of abnormal dislocation behavior.