High energy neutron and proton radiation can induce serious damage in structural metals, including void swelling and embrittlement. Hence the design of advanced metallic materials with significantly enhanced radiation tolerance is critical for the application of advanced nuclear energy systems. The goals of this dissertation are to examine the fundamental physical mechanisms that determine the responses of certain metallic multilayers, with ultra-high density interface structures, to plastic deformation and high fluence He ion irradiation conditions. This dissertation focuses on the investigation of mechanical and radiation responses of Al/Nb and Fe/W multilayers. Radiation induced microstructural evolution in Cu and Cu/Mo multilayer films are briefly investigated for comparisons. Al/Nb multilayer films were synthesized by magnetron sputtering at room temperature. The interface is of Kurdjumov-Sachs orientation relationship. In situ nanoindentation inside a transmission electron microscope (TEM) reveal that interfaces act as strong barriers for dislocation transmission and dislocations climb along the Al/Nb interfaces at a much higher velocity than in bulk. The evolution of microstructure and mechanical properties of Al/Nb multilayers has been investigated after helium ion irradiations: 100 keV He+ ions with a dose of 6x10^16/cm2. When layer thickness, h, is greater than 25 nm, hardness barely changes, whereas radiation hardening is more significant at smaller h. This study shows that miscible fcc/bcc interface with large positive heat of mixing is not stable during ion irradiation. In parallel we investigate sputtered Fe/W multilayers. Film hardness increases with decreasing h, and approaches a maximum of 12.5 GPa when h = 1 nm. After radiation, radiation hardening is observed in specimens when h >/= 5 nm, however, hardness barely changes in irradiated Fe/W 1 nm specimens due to intermixing. In comparison, Cu/Mo 5 nm multilayers with immiscible interface has also been investigated after helium ion irradiations. Interfaces exhibit significantly higher helium solubility than bulk. He/vacancy ratio affects the formation and distribution of He bubbles. The greater diameter of He bubbles in Cu than Mo originates from the ease of bubble growth in Cu via punching of interstitial loops. Finally, helium bubble migration and growth mechanisms were investigated in irradiated Cu (100) single crystal films via in situ heating inside a TEM. The activation energy for bubble growth is ~ 0.02 eV at low temperature. At higher temperatures, the activation energy for bubble coalescence is ~ 0.22 eV inside crystal, and 0.34 eV close to surface. The migration mechanisms of helium bubbles involve continuous as well as Brownian movement.

No description available.

The irradiation of metallic glasses to induce nanocrystallization was studied in two metallic glass compositions, Cu50Zr45Ti5 and Zr55Cu30Al10Ni5. Atomic mobility was described using a model based on localized excess free volume due to displace- ment cascades created by energetic particle irradiation. Due to the di erence in cascade size among di erent masses of projectiles, a mass-dependent study was per- formed. Metallic glass ribbon samples produced by melt-spinning were bombarded with electron, He, Ar, and Cu particles. Electron irradiation and characterization was performed "in-situ" by means of transmission electron microscopy. The di erent metallic glasses showed dissimilar levels of radiation stability under electron irradi- ation by Cu50Zr45Ti5 forming crystals 1-10 nm in diameter embedded in the amor- phous matrix after about 30 minutes of irradiation, while Zr55Cu30Al10Ni5 showed no such crystallization. Increasing projectile mass caused an increase in the maximum nanocrystal diameter up to approximately 100 nm in Cu irradiated Zr55Cu30Al10Ni5. Studies of di raction patterns of irradiated specimens showed nucleation of Cu10Zr7 phases in both specimens, as well as evidence of CuZr2 in Cu50Zr45Ti5 and both CuZr2 and NiZr2 in Zr55Cu30Al10Ni5. Crystal sizes in irradiated Zr55Cu30Al10Ni5 specimens showed bimodal distribution with many large (50-100 nm) crystals and many small (1-5 nm) crystals. The small crystals in irradiated Zr55Cu30Al10Ni5 were determined to be NiZr2 phase because of the low abundance of Ni. After exposure to 2 keV Ar ions, areas of composition roughly Cu10Zr7 were found by energy-dispersive X-ray spectroscopy but no crystallization was found. Further crystallization was achieved in decomposed specimens after electron irradiation. This shows that atomic segregation is a necessary step before nucleation in metallic glasses.

No description available.

The effect of ion irradiation and annealing on the microstructure and migration behaviour of implanted Sr and Ag in SiC have been investigated. SiC is used as the main barrier for fission products in modern high temperature gas cooled reactors. An understanding of the transport behaviour of the implanted ions under irradiation by swift heavy ions (SHI) will shed some light into SiC’s effectiveness in the retention of fission products. The diffusion behaviour of silver (Ag) and strontium (Sr) implanted separately into SiC was investigated after irradiation by xenon ions and isochronal annealing methods from 1100 ˚C up to temperatures of 1500 ˚C in step of 100 ˚C for 5 hours. Ion implantation and ion irradiation were performed at room temperature. The implantation fluences in all cases were in the order of 2×10 16 ions per cm 2 . Some of the implanted samples were then irradiated by SHI at different fluences (i.e. 3.4×10 14 and 8.4×10 14 ions per cm 2 ). The implantation depth profiles before and after irradiation and annealing were determined by Rutherford backscattering spectroscopy (RBS). The microstructure of SiC individually implanted with Ag and Sr were investigated using Raman spectroscopy and scanning electron microscopy (SEM). Implantation of Ag and Sr amorphized the SiC, while SHIs irradiation of the as-implanted SiC resulted in limited recrystallization of the initially amorphized SiC. Annealing at 1100 °C caused more recrystallization on the un-irradiated but implanted samples compared to SHI irradiated samples. This poor recrystallization of the irradiated SiC samples was due to the amount of impurities (i.e. concentration of Ag or Sr atoms) retained after annealing at 1100 o C. Raman and SEM results showed that annealing of the un-irradiated but implanted samples at 1100 °C resulted in large average crystal size compared to the irradiated samples annealed in the same conditions. RBS results showed that SHI irradiation alone induced no change in the implanted Ag and Sr. However, annealing the SHI irradiated samples iscohonally up to 1500 ˚C showed a strong diffusion and release of Ag and Sr as compared to the un-irradiated but implanted samples. The differences in the migration behavior of Ag and Sr is due to the difference in SiC structure and recrystallization in the irradiated and un-irradiated but implanted samples. / Thesis (PhD (Physics))--University of Pretoria, 2019. / National Research Foundation (NRF) and The World Academy of Sciences (TWAS). / Physics / PhD (Physics) / Unrestricted

The multi-scale modeling of ion-solid interactions presented can lead to a fundamentally new approach for understanding temperature evolution and damage formation. A coupling of the Monte Carlo code, SRIM, to a C FEM heat transfer code was performed, enabling a link between the damage cascade event to the subsequent heat transfer. Modeling results indicate that for 1 MeV Ni ion irradiation in Ni52.5Nb10Zr15Ti15Pt7.5, the heat transfer rate is too large for direct crystallization. Although the damage cascade induces a peak temperature of 5300 K, within 6 picoseconds the temperature is below the glass transition temperature. This result implies that there is a cooling rate of 10^14 K/s, which is much greater than the critical cooling rate for this material. Ion irradiation was performed to compare modeling with experiment. No evidence of direct crystallization is observed under TEM. Nanocrystals are formed as a consequence of series of multistage phase transitions. This provides evidence that the energy dissipation occurs too quickly for direct crystallization, as crystals are found in regions having undergone substantial compositional changes. A host of conventional electron microscopy methods were employed to characterize the structural changes induced by 1 MeV Ni ion irradiation in Ni52.5Nb10Zr15Ti15Pt7.5 and identify the phases that form, Ni3Nb, Ni3Ti and Ni3Zr. Scanning TEM analysis revealed Pt segregation near crystal regions due to irradiation. Due to a lack of Pt crystal phases observed and high concentrations of Pt in crystal regions it is postulated that Pt is substituting for Ni to form (Ni,Pt)3Nb and (Ni,Pt)3Ti.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第20484号 / エネ博第353号 / 新制||エネ||70(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

The overall purpose of this dissertation is to develop a multi-scale framework that can simulate radiation defect accumulation across a broad range of time and length scales in metals. In order to accurately describe defect accumulation in heterogeneous microstructures and under complex irradiation conditions, simulation methods are needed that can explicitly account for the effect of non-homogeneous microstructures on damage accumulation. In this dissertation, an advanced simulation tool called spatially resolved stochastic cluster dynamics (SRSCD) is developed for this purpose. The proposed approach relies on solving spatially resolved coupled rate equations of standard cluster dynamics methods in a kinetic Monte Carlo scheme. Large-scale simulations of radiation damage in polycrystalline materials are enabled through several improvements made to this method, including a pseudo-adaptive meshing scheme for cascade implantation and implementation of this method in a synchronous parallel kinetic Monte Carlo framework. The performance of the SRSCD framework developed in this dissertation is assessed by comparison to other simulation methods such as cluster dynamics and object kinetic Monte Carlo and experimental results including helium desorption from thin films and defect accumulation in neutron-irradiated bulk iron. The computational scaling of the parallel framework is also investigated for several test cases of irradiation conditions. SRSCD is next used to investigate radiation damage in three main types of microstructures, using α-iron as a test material: iron thin films, coarse-grained bulk iron, and nanocrystalline iron. SRSCD is used to investigate the mechanisms involved with defect accumulation in irradiated materials, such as effective diffusivity of helium in thin films and the effect of grain boundary sink strength on defect accumulation in nano-grained metals, and to predict defect populations in irradiated materials for comparison with experiments. Particular emphasis is placed on the role of microstructural features such as free surfaces and grain boundaries in influencing damage accumulation. Finally, the methodology developed in this dissertation is applied in the context of multiscale modeling and experimental design. To complete the multi-scale transition between defect-level behavior and macroscopic material property changes caused by irradiation, the relationship between mechanical loading and radiation damage is investigated. The impact of radiation damage on hardening of irradiated materials is investigated by using the results of SRSCD as inputs into polycrystalline crystal plasticity simulations. This is carried out in bulk iron by fitting hardening models to experimental data from neutron irradiation of iron and then used to predict hardening under irradiation conditions beyond what has already been accomplished in experimental studies. In addition, SRSCD is used to demonstrate the temperature shift required to achieve equivalent damage accumulation in irradiation conditions with significantly differing dose rates, such as in the case of using ion irradiation to simulate damage from neutron irradiation. In this dissertation, the development of SRSCD and its application in a multi-scale framework to predict macroscopic material property changes in metals represents a significant improvement over the state of the art due to improved simulations of defect accumulation and direct upscaling of results into polycrystalline plasticity models. The tools and understanding of defect behavior developed here will allow predictive modeling of metal degradation in reactor-relevant damage environments, including the defected microstructure and macroscopic material property changes due to irradiation.

It is a well-known fact in the materials science community that the use of low-energy atom impacts during thin film deposition is an effective tool for altering the growth behavior and for increasing the crystallinity of the films. However, the manner in which the incident atoms affect the growth kinetics and surface morphology is quite complicated and still not fully understood. This provides a strong incentive for further investigations of the interaction among incident atoms and surface atoms on the atomic scale. These impact-induced energetic events are non-equilibrium, transient processes which complete in picoseconds. The only accessible technique today which permits direct observation of these events is molecular dynamics (MD) simulations. This thesis deals with MD simulations of low-energy atom impact phenomena on metal surfaces during crystal growth. Platinum is chosen as a model system given that it has seen extended use as a model surface over the past few decades, both in experiments and simulations. In MD, the classical equations of motion are solved numerically for a set of interacting atoms. The atomic interactions are calculated using the embedded atom method (EAM). The EAM is a semi-empirical, pair-functional interatomic potential based on density functional theory. This potential provides a physical picture that includes many-atom effects while retaining computational efficiency needed for larger systems. Single adatoms residing on a surface constitute the smallest possible clusters and are the fundamental components controlling nucleation kinetics. Small two-dimensional clusters on a surface are the result of nucleation and are present during the early stages of growth. These surface structures are chosen as targets in the simulations (papers I and II) to provide further knowledge of the atomistic processes which occur during deposition, to investigate at which impact energies the different kinetic pathways open up, and how they may affect growth behavior. Some of the events observed are adatom scattering, dimer formation, cluster disruption, formation of three-dimensional clusters, and residual vacancy formation. Given the knowledge obtained, papers III and IV deal with growth of several layers with the aim to study the underlying mechanisms responsible for altering growth behavior and how the overall intra- and interlayer atomic migration can be controlled by low-energy atom impacts. / <p>On the day of the defence date the status of article II was Accepted.</p>

Ion irradiation of carbon nanotubes is a tool that can be used to achieve modification of the structure. Irradiation stability of carbon nanotubes was studied by ion and electron bombardment of the samples. Different ion species at various energies were used in experiments, and several defect characterization techniques were applied to characterize the damage. Development of dimensional changes of carbon nanotubes in microscopes operated at accelerating voltages of 30 keV revealed that binding energy of carbon atoms in CNs is much lower than in bulk materials. Resistivity measurements during irradiation demonstrated existence of a quasi state of defect creation. Linear relationship between ID/IG ratio and increasing irradiation fluence was revealed by Raman spectroscopy study of irradiated carbon buckypapers. The deviations from linear relationship were observed for the samples irradiated to very high fluence values. Annealing of irradiated samples was able to reduce the value of ID/IG ratio and remove defects. However, annealing could not affect ID/IG ratio and remove defects in amorphized samples. The extracted value of activation energy for irradiated sample was 0.36 ±0.05 eV. The value of activation energy was in good agreement with theoretical studies.