Dr. Ing. Ahmed Tamer AlAsqalani, Ph.D.

This study investigates the intricate mechanisms that govern irradiation damage in high-entropy ceramic materials. Specifically, we synthesized (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy carbide ceramics (HECC) with a single-phase rock-salt structure using spark plasma sintering. These ceramics were then subjected to irradiation with 1.08 MeV C ions, resulting in a dose of 7.2 dpa (dpa: displacements per atom) at both room temperature (RT) and 500 ◦C. To understand the resulting damage structure, we analyzed bulk irradiated HECC samples using Grazing Incidence X-ray Diffraction (GIXRD) and Transmission Electron Microscope (TEM) at both irradiation temperatures. GIXRD analysis revealed an average tensile strain out-of-plane of 0.16% for RT irradiation and 0.14% for irradiation at 500 ◦C. In addition, TEM analysis identified a buried damaged band, approximately 970 nm thick, under both irradiation temperatures. By employing the bright field TEM imaging technique under kinematic two-beam conditions, dislocation loops of both a/3 〈111〉{111} and a/2 〈110〉{110} types within the damaged band were observed. Furthermore, our analysis indicated an increase in the average size of the total dislocation loops within the band from 1.2 nm to 1.4 nm as the density decreased. Importantly, no amorphization, precipitates, or voids were detected in the damaged band under both irradiation temperatures. Density functional theory (DFT) simulations indicated that carbon predominantly resides in 〈110〉split interstitial sites causing lattice expansion, while vacancies, particularly Nb, induced compression along the c-axis. Carbon atoms tend to bond when collectively present in the <110> split interstitial sites, contributing to the formation of interstitial loops.

Layered carbides and borides, blending ceramic and metallic characteristics, present compelling prospects for future nuclear reactor applications due to their complex nanolaminate crystal structure. To explore the behavior of Fe2AlB2 under different conditions, helium irradiation is conducted across saturation fluences and under different annealing and irradiation temperatures. Room temperature (RT) irradiation causes some changes: a-lattice expands 0.3 %, c-lattice contracts 0.07 %, and an embedded amorphous layer into the matrix is formed. 400 °C annealing keeps an amorphous structure whereas 800 °C annealing fully recrystallizes with bubble growth. Increasing the irradiation temperature to 400 °C with the same fluence, smaller bubbles form without an amorphous layer, indicating defect recombination depends on helium attachment to vacancies. Under irradiation at 700 °C, faceted bubbles develop exclusively within Fe2AlB2, with no such bubbles in Al2O3. These faceted bubbles align themselves along (110) planes. Simultaneously, irradiation prompts tangled dislocations within the damaged layer, fostering the congregation of polygonal bubbles along grain boundaries (GBs) and creating denuded areas at GBs. Fe2AlB2 demonstrates remarkable oxidation resistance under high-temperature irradiation, maintaining surface stability without the cracks seen in ion-irradiated Ti3SiC2 and Ti2AlC at RT. Fe2AlB2 shows promise for use in reactors operating at temperatures above 500 °C due to its resilience to irradiation. Further investigation is warranted for its application in reactors operating in environments exceeding this threshold temperature.

Zr/Nb6 multilayers of 6 nm periodicity, with well-composition-modulated structures, were prepared by magnetron sputtering. Their microstructure and scratch properties were investigated using HAADF-STEM, XRD, AFM, and triboindenter. The Zr/Nb interfaces have various orientations along the growth direction. The hardness (H) and reduced elastic modulus (E) are measured as 6.6 GPa, and 176.3 GPa, respectively, resulting in a high ratio of H/E, compared to other multilayer systems such as Ta/Co and Ag/Cu, which indicates superior tribological performance. The coefficient of friction (COF) was 0.27, and the elastic recovery was observed along the scratch path. Extensive large molecular dynamics simulations (MD) were conducted to investigate the impact of different Zr/Nb interface orientations on the friction/wear behavior of Zr/Nb6 multilayers. The primary cause of plastic deformation of the Nb layer was dislocations and BCC twinning, while Zr layers deform via dislocations and intrinsic stacking faults. The Zr/Nb6 exhibited better tribological properties, such as lower COF, higher scratch hardness, and improved wear resistance compared to their single-crystal counterparts. The Pitsch-Schrader interface showed the lowest COF value, whereas Rong–Dunlop and Zhang-Killy orientations exhibited better wear resistance. The interface structure was analyzed, and its blocking strength was discussed. These findings contribute to understanding the relationship between Zr/Nb interface and wear performance and tailoring them to achieve desired properties for specific applications.

The study explored the microstructure evolution of 6H-SiC that underwent sequential iron and helium ion irradiation with energies of 2.5 MeV and 500 keV, respectively, at room temperature, followed by annealing at 1500°C for two hours. Following irradiation, the entire damaged layer underwent amorphization. However, during subsequent annealing, epitaxial recrystallization took place, resulting in the formation of defected polycrystalline 6H-SiC characterized by the presence of Fe-rich clusters, cavities, and stacking faults. Fe-rich cavities were found to predominantly form at the edges of the stacking faults, as revealed by XTEM. The interaction of microstructural defects is further investigated via first-principles calculations. The periphery of the stacking faults has been identified as the primary location for the emergence of vacancy clusters, serving as favorable sites for the accumulation of point defects, including Fe atoms. This behavior can be attributed to the combined effects of mechanical and electronic energy relaxation mechanisms. Mechanically, the presence of stacking faults allows for the release of elastic energy that had been stored at the boundary. Electronically, the energy relaxation arises from the saturation of C- and Si-dangling bonds. Both of these processes contribute to the observed behavior, highlighting the intricate interplay between mechanical and electronic factors in the system. The low point defect migration energy barriers in the vicinity of the stacking faults promise high recombination, which can limit cavity growth and enhance radiation resistance. The study not only offers valuable insights into the mechanism of cavity/stacking faults interaction, contributing to a better understanding of radiation damage in 6H-SiC but also demonstrates that 6H-SiC material containing stacking faults could serve as a viable alternative to 3C-SiC for nuclear application.

Microstructure of radiation-induced Iron phases were investigated in a 6H-SiC subjected to Iron and Helium bombardment with a damage level of 8 dpa. The microstructural evolution before and after annealing was investigated by combining transmission electron microscopy (TEM, STEM-EDS), automated crystal phase and orientation imaging (ACOM-TEM), secondary ion mass spectroscopy (SIMS), and atomic scale simulations. The irradiation amorphized the entire damaged layer which contains an embedded band of He bubbles located at peak damage concentration. After annealing, the amorphous layer recrystallized into a polycrystalline 6H-SiC where the Fe profile significantly changed to form Fe-rich clusters. ACOM-TEM reveals the formation of large cubic FeSi clusters and small bcc-Fe precipitates located at the 6H-SiC grain boundaries. The type and size distribution of the precipitates greatly depend on the Fe profile. Fe-Si compounds form around the Fe peak concentration, while, bcc Fe precipitates tend to be more homogeneously distributed. Density functional theory (DFT) calculations demonstrate that the formation of Fe dimers and trimers in the 1st nearest neighbor is energetically favorable. A combined Monte Carlo/Classical molecular dynamic (MMC/MD) technique reveals that the Fe atoms prefer to form large clusters in accordance with experimental results. MD annealing simulations reveal the formation of stable bcc Fe at high temperatures. The phase transition starts at the cluster-matrix interface around 620 K and the cluster is fully transformed at 700 K.

SiC is considered a perspective material in advanced nuclear systems as well as for electronic or spintronic applications, which require an ion implantation process. In this regard, two sets of 6H-SiC samples were implanted with i) 2.5 MeV Fe ions and ii) 2.5 MeV Fe ions and co-implanted 500 keV He ions at room temperature and then annealed at 1500 degrees C for 2 h. The microstructure evolution and Fe diffusion behavior before and after annealing were characterized and analyzed. After annealing, Fe concentration is enhanced close to the surface in the Fe-implanted sample, whereas in the co-implanted system, Fe atoms are redistributed into two distinct, spatially separated regions (close to the surface, and around the He-induced defects). The reason behind this finding is explained from an energetic point of view by using ab initio simulations. Technologically, the preexisting cavities can be used to control the Fe diffusion.