Michael Moorehead

Refractory high-entropy alloys (RHEAs) are emerging materials with potential for use under extreme conditions. As a newly developed material system, a comprehensive understanding of their long-term stability under potential service temperatures remains to be established. This study examined a titanium-vanadium-niobium-tantalum alloy, a promising RHEA known for its superior high-temperature strength and room-temperature ductility. Using a combination of advanced analytical microscopies, Calculation of Phase Diagrams (CALPHAD) software, and nanoindentation, we investigated the evolution of its microstructure and mechanical properties upon aging at 700°C. Trace interstitials such as oxygen and nitrogen, initially contributing to solid solution strengthening, promote phase segregation during thermal aging. As a result of the depletion of solute interstitials within the metal matrix, a progressive softening is observed in the alloy as a function of aging time. This study, therefore, underscores the need for a better control of impurities in future development and application of RHEAs.

Insufficient availability of molten salt corrosion‐resistant alloys severely limits the fruition of a variety of promising molten salt technologies that could otherwise have significant societal impacts. To accelerate alloy development for molten salt applications and develop fundamental understanding of corrosion in these environments, here an integrated approach is presented using a set of high‐throughput (HTP) alloy synthesis, corrosion testing, and modeling coupled with automated characterization and machine learning. By using this approach, a broad range of CrFeMnNi alloys are evaluated for their corrosion resistances in molten salt simultaneously demonstrating that corrosion‐resistant alloy development can be accelerated by 2 to 3 orders of magnitude. Based on the obtained results, a sacrificial protection mechanism is unveiled in the corrosion of CrFeMnNi alloys in molten salts which can be applied to protect the less unstable elements in the alloy from being depleted, and provided new insights on the design of high‐temperature molten salt corrosion‐resistant alloys.

Although zirconium-niobium (Zr-Nb) alloys are commonly used over Zircaloys as fuel claddings in light-water reactors, the fundamental understanding of Nb effects on oxidation and hydrogen pickup mechanisms, especially under irradiation, is still lacking. This study aims at filling this knowledge gap by coupling state-of-the-art experimental and modeling approaches on a set of Zr-xNb (x = 0.5/1.0) model alloys, with different heat treatments and irradiation dose levels to understand the effect of Nb redistribution on corrosion kinetics. To investigate the effect of irradiation on Nb distribution in a Zr-Nb metal substrate, Zr-Nb alloys have been irradiated by 2-MeV protons up to 1 dpa (flat region, about 1.3 × 1019 ions/cm2) at 350°C at the University of Wisconsin Ion Beam Laboratory. Nb redistribution after irradiation has been characterized by scanning transmission electron microscopy and atom probe tomography with a focus on radiation-induced platelets and solute concentrations. Results from atom probe tomography show that indeed solute concentration in solid solution decreases with irradiation dose, which may explain the enhanced corrosion resistance of irradiated Zr-Nb alloys via space-charge effects in the oxide. To understand the correlation between oxide space charge and corrosion kinetics, X-ray absorption near-edge spectroscopy was performed to measure the Nb oxidation states as a function of exposure time and oxide depth in two different annealed Zr-1.0Nb samples, corroded in high-temperature water up to 240 days. X-ray absorption near-edge spectroscopy results show that Zr-1.0Nb with lower, sub-parabolic, corrosion kinetics contains less Nb concentration in solid solution. Lastly, the observed corrosion kinetics are rationalized based on the coupled current charge compensation model in terms of Nb redistribution and its compensation effect on oxide space charges. The effect of irradiation on corrosion kinetics of Zr-Nb alloys is discussed in the light of experimental and modeling results.