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High-entropy alloys (HEAs) are proposed as potential structural materials for advanced nuclear systems, but little is known about the response of matrix chemistry in HEAs upon irradiation. Here, we reveal a substantial change of matrix chemical concentration as a function of irradiation damage (depth) in equiatomic NiCoFeCr HEA irradiated by 3 MeV Ni ions. After ion irradiation, the matrix contains more Fe/Cr in depth shallower than ~900–1000 nm but more Ni/Co from ~900–1000 nm to the end of the ion-damaged region due to the preferential diffusion of vacancies through Fe/Cr. Preferential diffusion also facilitates migration of vacancies from high radiation damage region to low radiation damage region, leading to no void formation below ~900–1000 nm and void formation around the end of the ion-damaged region at a fluence of 5 × 1016 cm−2 (~123 dpa, displacements per atom, peak dose under full cascade mode). As voids grow significantly at an increased fluence (8 × 1016 cm−2, 196 dpa), the matrix concentration does not change dramatically due to new voids formed below ~900–1000 nm.
The effort to develop metallic alloys with increased structural strength and improved radiation performance has focused on the incorporation of either solute elements or microstructural inhomogeneities to mitigate damage. The recent discovery and development of single-phase concentrated solid-solution alloys (SP-CSAs) has prompted fundamental questions that challenge established theories and models currently applicable to conventional alloys. The current understanding of electronic and atomic effects, defect evolution, and microstructure progression suggests that radiation energy dissipates in SP-CSAs at different interaction strengths via energy carriers (electrons, phonons, and magnons). Modification of electronic- and atomic-level heterogeneities and tailoring of atomic transport processes can be realized through tuning of the chemical complexity of SP-CSAs by the selection of appropriate elements and their concentrations. Fundamental understanding of controlling energy dissipation via site-to-site chemical complexity reveals new design principles for predictive discovery and guided synthesis of new alloys with targeted functionalities, including radiation tolerance.
Single-phase concentrated solid solution alloys (SP-CSAs) are newly emerging advanced structural materials, which are defined as multiprincipal element solid solutions. SP-CSAs with more than four components in equimolar or near-equimolar ratios are also referred to as high-entropy alloys due to their high configurational entropy. SP-CSAs are potential structural materials in advanced nuclear energy systems due to their attractive mechanical properties. Therefore many investigations have been carried out to study the irradiation-induced structural damage and defect behavior in SP-CSAs. This paper reviews recent experimental results on the irradiation responses of various SP-CSAs, focusing on the accumulation of irradiation-induced structural damage, void swelling resistance, and solute segregation behavior. In addition, the characteristic defect behavior in SP-CSAs derived from ab initio and molecular dynamics simulations, as well as the challenges in the applications of SP-CSAs for the nuclear energy systems are briefly discussed.
Here, we demonstrate the enhanced imaging capabilities of an aberration corrected scanning transmission electron microscope to advance the understanding of ion track structure in pyrochlore structured materials (i.e., Gd2Ti2O7 and Gd2TiZrO7). Track formation occurs due to the inelastic transfer of energy from incident ions to electrons, and atomic-level details of track morphology as a function of energy-loss are revealed in the present work. A comparison of imaging details obtained by varying collection angles of detectors is discussed in the present work. A quantitative analysis of phase identification using high-angle annular dark field imaging is performed on the ion tracks. Finally, a novel 3-dimensional track reconstruction method is provided that is based on depth-dependent imaging of the ion tracks. The technique is used in extracting the atomic-level details of nanoscale features, such as the disordered ion tracks, which are embedded in relatively thicker matrix. Another relevance of the method is shown by measuring the tilt of the ion tracks relative to the electron beam incidence that helps in knowing the structure and geometry of ion tracks quantitatively.
Historically, alloy development with better radiation performance has been focused on traditional alloys with one or two principal element(s) and minor alloying elements, where enhanced radiation resistance depends on microstructural or nanoscale features to mitigate displacement damage. In sharp contrast to traditional alloys, recent advances of single-phase concentrated solid solution alloys (SP-CSAs) have opened up new frontiers in materials research. In these alloys, a random arrangement of multiple elemental species on a crystalline lattice results in disordered local chemical environments and unique site-to-site lattice distortions. Based on closely integrated computational and experimental studies using a novel set of SP-CSAs in a face-centered cubic structure, we have explicitly demonstrated that increasing chemical disorder can lead to a substantial reduction in electron mean free paths, as well as electrical and thermal conductivity, which results in slower heat dissipation in SP-CSAs. The chemical disorder also has a significant impact on defect evolution under ion irradiation. Considerable improvement in radiation resistance is observed with increasing chemical disorder at electronic and atomic levels. The insights into defect dynamics may provide a basis for understanding elemental effects on evolution of radiation damage in irradiated materials and may inspire new design principles of radiation-tolerant structural alloys for advanced energy systems.
Thin films of nanocrystalline ceria on a Si substrate have been irradiated with 3 MeV Au+ ions to fluences of up to 1x1016 ions cm-2, at temperatures ranging between 160 to 400 K. During the irradiation, a band of contrast is observed to form at the thin film/substrate interface. Analysis by scanning transmission electron microscopy in conjunction with energy dispersive and electron energy loss spectroscopy techniques revealed that this band of contrast was a cerium silicate amorphous phase, with an approximate Ce:Si:O ratio of 1:1:3.
Chemical disorder in ion-irradiated SiC and GaN has been examined by means of transmission electron microscopy. Radial distribution functions obtained by a quantitative analysis of electron diffraction intensities revealed that homonuclear bonds, which do not exist in the crystalline state, are formed in both ion-irradiated specimens. The origin of the homonuclear bonds is quite different between SiC and GaN. The constitute elements mix on the atomic-scale in amorphous SiC, while phase separation induced by irradiation is attributed to the formation of self-bonded Ga atomic pairs in amorphous/nanocrystalline GaN.
Thin films nano-crystalline zirconia of ~ 300 nm thick were deposited on Si substrate, and the samples were irradiated with 2 MeV Au+ ions at temperatures of 160 and 400 K, up to fluences of 35 displacements per atom. The films were then studied using glancing incidence x-ray diffraction, Rutherford backscattering, secondary ion mass spectroscopy and transmission electron microscopy. During the irradiation, cavities were observed to form at the zirconia/silicon interface. The morphology of the cavities was found to be related to the damage state of the underlying Si substrate. Elongated cavities were observed when the substrate is heavily damaged but not amorphized; however, when the substrate is rendered amorphous, the cavities become spherical. As the ion dose increases, the cavities then act as efficient gettering sites for the Au. The concentration of oxygen within the cavities determines the order in which the cavities getter. Following complete filling of the cavities, the interface acts as the secondary gettering site for the Au. The Au precipitates are determined to be elemental in nature due to the high binding free energy for the formation of Au-silicides.
Dense networks of amorphous GaSb nanofibers were fabricated by ion irradiation of bulk GaSb, and following formation, they were thermally annealed at a low temperature. Contrary to expectations, annealing of the GaSb fibers at just 50% of their melting temperature resulted in complete chemical decomposition of the nanofibers into core-shell structures consisting of crystalline Sb cores surrounded by amorphous shells. In this study, we investigate the transition of the single-phase nanofibers to their core-shell configuration, and we analyze the unique, temperature-dependent phase decomposition process. Thermodynamic considerations are discussed, and a model is presented to explain the thermally induced decomposition of the GaSb semiconductor fibers into core-shell structures, based upon the singular interaction of several size-dependent material properties.
Electron-beam-induced effects in preamorphized Sr2Nd8(SiO4)6O2 were investigated in situ using transmission electron microscopy with 200-keV electrons at temperatures ranging from 380 to 780 K. Within the electron-irradiated area, epitaxial recrystallization was observed from the amorphous/crystalline interface toward the surface, with the rate of recrystallization increasing as temperature increased from 380 to 580 K. Structural contrast features (i.e., O deficient amorphous material), as well as recrystallization, were observed outside of the irradiation area at temperatures from 680 to 780 K. Ionization-induced processes and local nonstoichiometry induced by oxygen migration and desorption are possible mechanisms for the electron-beam- induced recrystallization and for the formation of the structural contrast features, respectively.
Experimental charged-particle irradiations and multi-scale computer simulations have been used to investigate the primary damage state and evolution of damage in silicon carbide as functions of temperature and charged-particle mass and energy. Atomistic simulations of energetic C and Si collision cascades, similar to those created by reactor neutrons, indicate that single interstitials, vacancies, antisite defects, and small defect clusters are produced. The point defects are dominated by close Frenkel pairs, and atomistic simulations indicate that the activation energies for recombination of most close pairs range from 0.24 to 0.38 eV, which suggest significant reduction in defect survivability at room temperature. Atomistic simulations have also determined that the activation energies for long-range diffusion of C and Si interstitials are 0.7 and 1.5 eV, respectively. Using these activation energies and ab initio results as input parameters, a kinetic Monte Carlo (MC) simulation model has been developed to study isochronal annealing of defects in SiC between cascade events. The defects are produced by a 10 keV Si cascades in a molecular dynamics (MD) simulation cell, and these defects are then accurately transferred to defect lattice sites in the Monte Carlo model to investigate defect recovery. By transferring defects states back and forth between the MD and MC environments, damage accumulation can be investigated as a function of temperature. Charged particle irradiations are often used to simulate radiation damage from neutrons and radioactive decay; however, at extreme charged-particle fluxes used in irradiation studies to simulate radiation damage in nuclear materials, the ratio of ionization rate to displacement rate can have a significant impact on observed temperature-dependent processes, which can affect both interpretation and model development. At high charged-particle fluxes, the defect recovery rates in SiC increase nearly linearly with the ratio of ionization rate to displacement rate. A fundamental understanding of these ionization effects is needed if charged particle irradiation results are to be used to develop predictive models of damage evolution in nuclear materials, such as SiC, as functions of time, temperature and dose rates.
Experimental and computational results on ion-beam-induced defect production, damage accumulation, and thermal recovery in SiC are reviewed. The accumulation and recovery of disorder on the Si and C sublattices have been determined experimentally by ion-channeling methods. Atomic-level simulations are used to determine defect production, cascade-overlap effects, and defect migration energies. Energetic Si and C collision cascades, with energies up to 50 keV, primarily produce single interstitials, mono-vacancies, antisite defects, and small defect clusters. Overlapping of Si and C cascades results in the interaction of defects and clusters that stimulates cluster growth and produces long-range structural disorder. For energetic Au cascades, nanoscale amorphous clusters are produced directly within about 25% of the Au cascades, along with point defects and smaller clusters. The disordering behavior and the changes in volume and elastic modulus obtained experimentally and from molecular dynamics simulations are in good agreement, thus providing atomic-level interpretation of experimentally observed features. Simulations of close-pair production and recombination in SiC indicate that the activation energies for recombination of most close pairs range from 0.24 to 0.38 eV. Multiaxial channeling measurements indicate annealing below 300 K results in relaxation of some interstitials to lower-energy configurations. Long-range migration energies for interstitials and vacancies have likewise been determined by computational methods.
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