STEM-Based Techniques to Characterize Nanoscale Point Defects Formed Under Molten Salt Corrosion

Extreme nuclear reactor environments require materials to maintain their integrity all while a range of processes act in unison to degrade their performance. Integral features of these processes are that defects produced under irradiation directly limit or accelerate the corrosion rates. Moreover, in molten-salt reactor environments, previous experimental work has shown that Cr dealloying is strongly coupled to the microstructure evolution of these alloys. Connecting corrosion attack with alloy microstructure such as grain boundaries and accumulating point defects is imperative to understanding underlying mechanisms. However, the trends associated with varying salt chemistry, temperature, and irradiation dose are not intuitive, and a mechanistic understanding of the associated thermodynamics and kinetics remains unknown. A close inspection and tracking of point defects such as vacancies and interstitials would be essential for the understanding of material behavior under complex molten salt corrosion environments. The recent developments in 4D-STEM with high-speed direct electron detectors and atomic resolution STEM allow mapping vacancy distributions at the nanoscale and their associated strains, both of which have far-reaching implications for detailed analysis of complex irradiation / corrosion damage.<br/><br/>This work aims to understand, through experiments and modeling, the mechanisms that govern corrosion in LiF-NaF-KF eutectic salts (FLiNaK), and how they are correlated to microstructure evolution in the metal alloy. Here, implement techniques including 4D-STEM strain mapping and high-resolution STEM-EDX/EELS elemental mapping combined with computational modeling based on density functional theory to analyze the migration of these point defects with respect to local changes in composition around corrosion pores. Using this approach, we explore the defect map surrounding boundaries and interfaces where rapid transport occurs during a series of diffusional process. At elevated temperature, formation of a passive oxide layer is suited to protecting the metal-salt interface, however, oxide layers formed in oxygenated molten salt environments are neither stable nor protective. It is understood that Cr tends to leach into the salt, in combination with the breakdown of these protective layers, and the reaction is accelerated at discrete sites such as surface pitting and further corrosion in a creviced region obtained from microscopic SEM observations of the sample surface and the cross-section. Moreover, alloying elements (Cr oxidants) are selectively removed via a redox reaction to form metal halides which may act as a mechanical barrier to continued corrosion attack. Phase field modeling has shown that these microstructural coupling trends vary for selective dissolution from a binary (e.g. Cr dissolution from NiCr). Further, we conduct simultaneous EDX-STEM and 4D-STEM mapping in situ at elevated temperatures to track time / temperature sensitive corrosion behavior within an isolated salt-filled pore. Concentrations and distributions of point defects that form by surface diffusion or by bulk lattice diffusion in response to Cr migration and leaching at the metal-salt interface are measured. This extensive study fundamentally improves the understanding of the complex corrosion and irradiation processes and provides a new pathway for engineering materials designed in future nuclear energy systems. The project is part of FUTURE Energy Frontiers Research Center (EFRC), which aims to study how the coupled extremes of irradiation and corrosion work in concert to modify the evolution of materials by coupling experiments and modeling that target fundamental mechanisms.