Congratulations to Dr. Annie Barnett on successfully defending her thesis, entitled “The Role of Local Chemical Order in Defect Formation and Irradiation Response in Multi-Principal Element Alloys”. Check out the abstract, below. Congratulations, Annie! We can’t wait to see what you do next!

Abstract: The compositional complexity of multi-principal element alloys (MPEAs) has motivated researchers to seek ways to understand and potentially control the arrangement of atoms sitting on a crystal lattice due to their promising mechanical and thermal properties. Thus, this thesisinvestigates how local chemical order governs defect formation and irradiation response in MPEAs, establishing mechanistic links between atomic-scale chemistry, defect evolution, and radiation tolerance. Firstly, hybrid Monte-Carlo-molecular dynamics (MCMD) simulations were developed to predict ordering phenomena in FCC MPEAs. This was coupled to diffraction-based analysis using precession-electron diffraction (PED) as well as multi-slice diffraction simulations to understand the relationship between ordering, diffuse electron scattering and nanoscale lattice instabilities. These lattice instabilities were exposed via in-situ ion irradiation by accelerating nucleation of the frustrated phase. Bulk characterization of this microstructure was completed via DMA and MPMS measurements, and the resulting structural signatures were interpreted within the context of prior diffuse scattering studies. We found that the accumulation of stacking faults serves as favorable nucleation sites for the new phase, producing satellite reflections and enhancing diffuse electron scattering. This is important because, while these processes mirror transformations in conventional TRIP materials, chemical fluctuations frustrate long-range martensitic transformation, confining it to nanoscale domains. Building on this understanding of chemical order, the influence of chemical order on grain boundary (GB) structure and stability was investigated in the same class of alloys. This effort was conducted entirely through molecular dynamics simulations and focused on the role of local chemical order in stabilizing grain boundary complexions under irradiation-relevant environments. Interstitial atoms were deposited into a grain boundary structure, which prompts GB structural evolution to relieve stresses caused by the evolving GB density. Interestingly, we observed that the structural transformations in chemically complex environments differed greatly from those in dilute and pure materials. Specifically, these models demonstrated that chemical order could stabilize GB motifs which act as more efficient sites for defect absorption under irradiation. In the final portion of this research, refractory MPEAs were studied due to their candidacy in modern nuclear energy systems that require high-temperature stability. The irradiation response of dilute and chemically complex refractory alloys was compared using in- and ex-situ 1 MeV Kr ion irradiation at 300 °C. In combination with post-irradiation atom probe tomography and MD simulations, we were able to confirm why the two alloy classes developed distinct defect morphologies which reflect their lattice characteristics. In dilute alloys, the relatively smooth energy landscape facilitates 1D interstitial diffusion, with defect evolution governed primarily by solute–vacancy binding. In contrast, the distorted lattice of MPEAs stabilizes low-energy interstitials, producing non-equilibrium chemical ordering and reduced interstitial–vacancy recombination. These findings indicate that chemical order in single-phase MPEAs can be tuned through the energetics of self-interstitial atoms, providing a future pathway for alloy design given desired order environments. Collectively, these findings elucidate the central role of local chemical order in controlling microstructural stability and provide a foundation for the rational design of compositionally complex alloys for radiation-relevant environments.