A series of experiments has been conducted in which thin foils containing large polycrystals of Ni (single crystals from the perspective of transmission electron microscopy) have been irradiated with 300 key Ni ions at temperatures from 25 degrees to 475 degrees C. The aim was to examine the fundamental aspects of the build-up of extended defects in a "simple" system with no implantation of foreign species and without the likelihood of segregation, precipitation or formation of new phases. Experiments were carried out using the MIAMI-2 facility in which the development of radiation damage is observed (and recorded) whilst ion-irradiating in-situ in a transmission electron microscope. Surprisingly, all irradiations of the electrochemically-thinned foils of Ni resulted in the accumulation of dislocations to form low-angle grain boundaries such that single crystal material was converted into a series of grains, each typically less than 200 nm in width but generally more than 1 mu m in length with the long axis approximately parallel to the edge of the foil. The early stages of this process have been modelled using Molecular Dynamics simulations and an interpretation of this process of radiation-induced grain-boundary formation is discussed in terms of the coupled effects of irradiation, temperature and stress induced by the radiation damage. The stress arises due to swelling in the thin irradiated region of the jet-polished specimens (with a wedge-shaped radial cross section) which is constrained by deeper-lying unirradiated material. The position in which a grain boundary forms is determined by the interaction of glisssile dislocations with the stress induced by the radiation-damaged layer and that from a neighbouring boundary.

Ti and Cr beryllides are materials of potential importance as neutron multipliers for tritium breeding in nuclear fusion reactors. The n-to-2n transmutation reaction also produces tritium and helium, which form bubbles. Neutron irradiation from fusion plasma also introduces point defects. The effect on beryllides' microstructure is important for understanding their mechanical properties and evolution in the fusion reactor environment. This study determines and describes the microstructural features that occur in TiBe12 and CrBe12 when He and fast-particle-induced point defects are introduced at fusion reactor neutron breeder relevant temperatures. Beryllide samples were implanted with 300kV He at temperatures between $387-900^{\circ}C$, sectioned down through the implantation surface with a focused ion beam post-irradiation, and the resulting microstructures examined using transmission electron microscopy, electron-dispersive spectroscopy (EDS) and precession diffraction mapping. Nanometre bubbles grew in both TiBe12 and CrBe12 at $600^{\circ}C$; larger (100 nm and over) bubbles, some faceted, grew at $900^{\circ}C$. Some bubbles in CrBe12 were lined with Cr, some oxidised. TiBe12 developed planar faults, on $\{110\}$ planes at $600^{\circ}C$ and below but on $\{111\}$ at $900^{\circ}C$. Faults were preferentially associated with large bubbles. The displacement vectors of faults on the $\{110\}$ planes had some commonality with previous studies that found displacement vectors of families $R=\langle 011\rangle$ and $R= \langle 110\rangle$; the present study also found faults that matched neither previously found type. CrBe12 also developed planar faults with an appearance quite different from the typical striped appearance of planar stacking faults; their nature remains unknown. Oxide particles were found in both beryllides, most prominently in CrBe12.

Polycrystalline 3C SiC was irradiated and observed in-situ via Transmission Electron Microscopy with a 20 keV He ion beam at 40 0, 80 0, 10 0 0 and 120 0 degrees C at the Microscopes and Ion Accelerators for Materials Investigations facility. During the 40 0, 80 0, 10 0 0 and 120 0 degrees C irradiations, black-spot damage was observed at 3.1, 1.1, 2.1 and 2.1 dpa respectively. Helium bubbles were observed after 6.3 dpa at 400 degrees C and 2.1 dpa at 800 degrees C, and He platelets were seen after 1.1 dpa at 80 0, 10 0 0 and 120 0 degrees C but not observed during the 400 degrees C irradiation. This work shows for the first time, the preferential nucleation of platelets within stacking faults in 3C SiC. The dependence of He platelet diameter with temperature and dose has also been observed.(c) 2021 Elsevier B.V. All rights reserved.

High-entropy alloys (HEAs) and some complex alloys exhibit desirable properties and significant structural stability in harsh environments, including possible applications in advanced reactors. Energetic ion irradiation is often used as a surrogate for neutron irradiation; however, the impact of ion electronic energy deposition and dissipation is often neglected. Moreover, differences in recoil energy spectrum and density of cascade events on damage evolution must also be considered. In many chemically complex alloys, the mean free path of electrons is reduced significantly, thus their decreased thermal conductivity and slow dissipation of localized radiation energy can have noticeable effects on displacement cascade evolution that is greatly different from metals with high thermal conductivity. In this work, nanocrystalline HEAs of Ni20Fe20Co20Cr20Cu20 and nonequiatomic (NiFeCoCr)97Cu3, both having much lower room-temperature thermal conductivity than pure Ni or Fe, are chosen as model HEAs to reveal the role that electronic energy loss during ion irradiation has in complex alloys. The response of nanocrystalline HEAs is investigated under irradiation at room temperature using MeV Ni and Au ions that have different ratios of electronic energy to damage energy, which is the energy dissipated in displacing atoms. Different from previously reported amorphization of nanocrystalline SiC, experimental results on these HEAs show that, similar to the process in nanocrystalline oxide materials, both inelastic thermal spikes via electron-phonon coupling and elastic thermal spikes via collisions among atomic nuclei contribute to the overall grain growth. The growth follows a power law dependence with the total deposited ion energy, and the derived value of the power-exponent suggests that the irradiation-induced instability at and near grain boundaries leads to local rapid atomic rearrangements and consequently grain growth. The high power-exponent value can be attributed to the sluggish diffusion and delayed defect evolution arising from the chemical complexity intrinsic to HEAs. This work calls attention to quantified fundamental understanding of radiation damage processes beyond that of simplified displacement events, especially in simulating neutron environments.

MAX phases have recently attracted significant attention for potential nuclear applications due to their novel properties such as unique hexagonal-compact nanolayered crystal structure, high-machinability due to lower hardness levels than conventional ceramics, and high-chemical inertness. In order for MAX phases to be used in nuclear reactors, two aspects deserve detailed investigations: (i) their phase stability at high-temperatures and (ii) microstructural defect formation and recovery induced by energetic particle irradiation. To date, degradation mechanisms of MAX phases at high-temperatures and following irradiation are largely unexplored fields of research. This work focuses on the evaluation of two Ti-based MAX phases—Ti2AlC and Ti3SiC2—within the context of extreme environments. To accomplish this, a one-of-a-kind comparison between neutron irradiations, performed over a decade of research at the high flux isotope reactor, and heavy-ion irradiations, carried out in situ in a transmission electron microscope, has been conducted. The results show Ti-based MAX phases are prone to accelerated decomposition under the conditions investigated. This questions the hypothesis that MAX phases exhibit high phase stability, especially when used in future nuclear energy systems where energetic particle irradiation is a dominating degradation mechanism.

Using in-situ transmission electron microscopy (TEM) with ion irradiation, we investigated the microstructural changes in silicon carbide nanowhiskers (SiC NWs) which were used as a model system for nanoporous SiC. Irradiations were carried out using 6 keV He ions at temperatures between 500 and 1000°C and doses up to 20 dpa. These results are compared with the irradiation effects in SiC thin foils under the same conditions to establish differences in their response to radiation damage. The irradiation temperature played a significant role in the evolution of different microstructures; at 500°C, small defect clusters were observed in the NWs together with a segregation of carbon at the surface of the NWs mapped using energy-filtered TEM (EFTEM). At 800°C, small He bubbles (2–4 nm in diameter) were observed in the NW matrix while He platelets and bubble discs formed in the foils. At 1000°C, several changes were observed in the NWs including bubbles at twin boundaries, voids and oxygen-rich precipitates. The large surface area to volume ratio enhances defect recombination supressing the defect density in the SiC NWs compared to the foils indicating high radiation tolerance; however, elemental segregation and precipitation may limit its application in advanced nuclear reactors.

We have used both in situ radiation damage techniques and direct observations of ex-reactor materials to study radiation damage mechanisms in a range of zirconium-niobium (Zr-Nb) alloys with different initial microstructures. The aim has been to determine the relative stability of the different phases present under in-service conditions, including oxides and second phase particles (SPPs), and how damage to these phases alters the chemistry of the surrounding alloy matrix. A monoclinic-to-cubic transformation of the bulk oxide is observed by in situ ion irradiation experiments, followed by irradiation-induced grain growth. The possibility of radiation-induced stabilization of this cubic phase thus needs to be considered as an additional process that can occur in the regions of oxides exposed to high fluxes in service and may further affect the corrosion rates. In situ studies of β-Nb and Laves phase SPPs under ion irradiation showed that they behaved differently as a function of ion fluence and irradiation temperatures. The β-Nb SPPs show good stability under both ion and neutron irradiation to high damage levels and over a wide temperature range. The formation in flux, by a combination of irradiation-enhanced oxygen diffusion and the direct effects of radiation, of oxides that are both less well textured and with a more disrupted grain structure will also contribute to different corrosion rates in reactor. Finally, high-resolution energy-dispersive X-ray and atom probe tomography analysis were used to study changes to both SPP and matrix chemistry as result of radiation damage.

Once nanomaterials have been synthesized, inducing further structural modifications is challenging. However, being able to do so in a controlled manner is crucial. In this context, germanium nanowires are irradiated in situ within a transmission electron microscope (TEM) by a 300 keV xenon ion beam at temperatures ranging from room temperature (RT) to 500 °C. The ion irradiation is performed in situ and the evolution of nanowires during irradiation is monitored. At 300 °C and below, where the temperature is low enough to allow amorphization, the ion beam causes the formation of nanostructures within the nanowires. Formation of nanopores and swelling of nanowires is observed for a very low fluence of 2.2 × 10 14 and up to 4.2 × 10 15 ions cm −2 . At higher fluences, the thickness of the nanowires decreases, the nanowires lose their wire‐like cylindrical shape and the nanostructuring caused by the ion beam becomes more complex. The nanostructures are observed to be stable upon crystallization when the nanowires are annealed at 530 °C. Furthermore, in situ imaging allows the growth of nanopores during irradiation to be followed at RT and at 300 °C providing valuable insights into the mechanism responsible for the nanostructuring.

Two metallic alloys in the quaternary system Fe?Cr?Mn?Ni were irradiated in situ within a transmission electron microscope (TEM) using Xe+ heavy ions in the temperature range of 293?873 K and in the regime of low- (30 keV) and medium-energies (300 keV) with respective maximum doses of around 40 and 140 dpa. The first alloy is the FeCrMnNi high-entropy alloy (HEA) synthesised with the alloying elements close to equimolar composition. The second alloy is a commercial austenitic stainless steel AISI-348 (70.5Fe-17.5Cr-1.8Mn-9.5Ni wt.%), selected as the ?low-entropy? counterpart of the FeCrMnNi HEA. Microstructural characterisation was carried out in the TEM with in situ heavy ion irradiation to investigate the role of entropy on radiation induced segregation and precipitation (RIS and RIP). The results demonstrated that among all the irradiation cases investigated, the FeCrMnNi HEA had its random solid solution matrix phase preserved in 80% of the experiments whilst the austenite matrix of the AISI-348 steel underwent RIP in 80% of the cases. It is therefore demonstrated that small differences between two alloys can lead to different radiation responses, confirming the trend that, by tuning the elemental composition superior radiation resistance can be achieved in metallic alloy systems, but emphasising that some of the constitutive core-effects of HEAs are still in need of further confirmation especially when the application of HEAs in energetic particle irradiation environments is considered.

In this work, germanium nanowires rendered fully amorphous via xenon ion irradiation have been annealed within a transmission electron microscope to induce crystallization. During annealing crystallites appeared in some nanowires whilst others remained fully amorphous. Remarkably, even when nucleation occurred, large sections of the nanowires remained amorphous even though the few crystallites embedded in the amorphous phase were formed at a minimum of 200 °C above the temperature for epitaxial growth and 100 °C above the temperature for random nucleation and growth in bulk germanium. Furthermore, the presence of crystallites was observed to depend on the diameter of the nanowire. Indeed, the formation of crystallites occurred at a higher annealing temperature in thin nanowires compared with thicker ones. Additionally, nanowires with a diameter above 55 nm were made entirely crystalline when the annealing was performed at the temperature normally required for crystallization in germanium (i.e. 500 °C). It is proposed that oxygen atoms hinder both the formation and the growth of crystallites. Furthermore, as crystallites must reach a minimum size to survive and grow within the amorphous nanowires, the instability of crystallites may also play a limited role for the thinnest nanowires.

Different types of glass-ceramic composites are being researched around the world as potential matrices for nuclear waste conditioning. To demonstrate their long-term durability and build a safety case for geological disposal, accelerated irradiation tests simulating He accumulation and the effects of recoil nucleus damage are needed. To study the behaviour of He in these materials, particularly, in zirconolite-based glass-ceramics, transmission electron microscopy (TEM) with in-situ ion irradiation was employed. This research work provides a detailed overview of the various parameters such as the implantation temperature, He concentration, structural disorder, grain boundaries, glass-ceramic interfaces, ballistic collisions etc on He accumulation and bubble formation in these materials. These results are then discussed in the context of nuclear waste disposal in ceramics and glass-ceramic-matrix based materials.

We have investigated the microstructural and crystallographic evolution of nanocrystalline zirconia under heavy ion irradiation using in-situ transmission electron microscopy (TEM) and have studied the atomic configurations of defect clusters using aberration-corrected scanning transmission electron microscopy (STEM). Under heavy ion irradiation the monoclinic-ZrO2 is observed to transform into cubic phase, stabilised by the strain induced by irradiation-induced defect clusters. We suggest that the monoclinic-to-cubic transformation is martensitic in nature with an orientation relationship identified to be (100)m∥(100)c and [001]m∥[001]c. By increasing the damage dose, both the formation of voids and irradiation-induced grain growth were observed. A model for the formation of voids is proposed, taking defect interactions into consideration. The study has also demonstrated that high resolution orientation mapping by transmission Kikuchi diffraction (TKD) combined with in-situ irradiation in a TEM is a powerful method to probe the mechanisms controlling irradiation-induced processes, including grain boundary migration, phase transformations and texture evolution.

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The irradiation stability of a material in the temperature range from 300°C to 500°C represents an important step for a variety of nuclear applications. Here we used dual-beam irradiation to investigate the response of an amorphous silicon oxycarbide (SiOC) to extreme environments (temperature and irradiation). The amorphous SiOC was prepared in thin film form by sputtering and then fabricated into cross-sectional transmission electron microscopy (TEM) specimens, and finally irradiated with helium (He) and krypton (Kr) ions inside a TEM. In-situ TEM observations revealed that He bubble and void formation are highly suppressed after irradiation up to 95 dpa with simultaneous He implantation up to 231 at.%. Atomic pair-distribution functions suggested that the amorphous structures are almost the same before and after irradiation, and no crystallization, nor phase separation was detected. This study demonstrates the stability of amorphous SiOC under dual-beam irradiation at nuclear reactor operation temperatures, suggesting that this material is applicable as a structural material for advanced nuclear reactors.

The irradiation stability of a material in the temperature range from 300 degrees C to 500 degrees C represents an important step for a variety of nuclear applications. Here we used dual-beam irradiation to investigate the response of an amorphous silicon oxycarbide (SiOC) to extreme environments (temperature and irradiation). The amorphous SiOC was prepared in thin film form by sputtering and then fabricated into cross-sectional transmission electron microscopy (TEM) specimens, and finally irradiated with helium (He) and krypton (Kr) ions inside a TEM. In-situ TEM observations revealed that He bubble and void formation are highly suppressed after irradiation up to 95 dpa with simultaneous He implantation up to 231 at.%. Atomic pair-distribution functions suggested that the amorphous structures are almost the same before and after irradiation, and no crystallization, nor phase separation was detected. This study demonstrates the stability of amorphous SiOC under dual-beam irradiation at nuclear reactor operation temperatures, suggesting that this material is applicable as a structural material for advanced nuclear reactors.

Thin foils of AISI 316L stainless steel were irradiated in-situ in a transmission electron microscope (TEM) with 325 keV Xe ions at 550°C at three different fluxes to study flux effects. The kinetics of the radiation-induced precipitation and the evolution of the precipitates were found to be correlated with the irradiation flux. At lower fluxes (1 and 2 × 1012 ions·cm-2·s-1), cascade mixing played an important role in the accumulation of point defects within the austenite matrix, facilitating the formation of clusters which act as sinks for heterogeneous nucleation of precipitates with high areal density. At the highest flux (4 × 1012 ions·cm-2·s-1) the cascade mixing favours the recombination of vacancies and interstitials which supresses the growth of existing precipitates beyond a certain total damage level. The results agree with a previous radiation-induced precipitation (RIP) model proposed by Wiedersich, Okamoto and Lam and further studied by Bruemmer, but a small modification is proposed when the flux is closer to the vacancy-interstitial recombination limit.

The self-healing capability of point and extended defects that are introduced by energetic particle irradiation is a desired behavior to be attained in the design and selection in potential materials for application in extreme environments. Nanoporous materials have a potential for achieving higher radiation tolerance due to the presence of many active unsaturable surfaces to which defects may diffuse and thus be effectively annihilated. The effects of heavy ion collisions in the lattice of functional AISI-316 steel nanoparticles (NPs).which serve as a model for the ligaments in a nanoporous.are herein investigated in situ within a transmission electron microscope. Comparisons are made directly with AISI-316 steel in the form of foils, and the results show that the fewer radiation-induced defect clusters form in the NPs and that small NPs (r < 50 nm) were observed to accumulate fewer defects when compared to larger NPs. Post-irradiation analytical characterization within a scanning transmission electron microscope revealed that the AISI-316 steel NPs may develop a radiation-induced self-passivation driven by a solute-drag mechanism: an effect that can potentially enhance their radiation corrosion resistance in the expected extreme conditions of a reactor. The capability of an NP to self-heal irradiation-induced point defects is investigated using the cellular model for active internal and surface sinks. The design of functional nanoscale materials for extreme environments is discussed.

This work addresses the impact of radiation damage on the leaching of International Simple Glass (ISG). Pristine glass and specimens irradiated with multienergy Au ions were leached for 82 days at ...