<p> This thesis reports on the investigation of damage production in GaAs at low temperature using the channeling-backscattering technique.</p> <p> The study has been divided into two parts; first, the investigation of damage produced by 2 MeV helium ions in unimplanted and previously implanted samples with varied doses of 40 keV nitrogen and bismuth. The helium beam damage has been found to depend on the initial state of damage of the samples. In the second part the damage production due to 40 keV N+, As+, Sb+ and Bi+ ion implantation has been investigated. A comparison with damage production due to the corresponding 80 keV diatomic implants has also been carried out. No enhancement in the damage production was noticed due to the molecular implants.</p> / Thesis / Master of Engineering (MEngr)

Metallic alloys are important structural and cladding materials for current and future reactors. Understanding radiation-induced damage on metallic alloys is important for maintaining the safety of nuclear reactors. This dissertation mainly focuses on radiation-induced primary damage in iron-based metallic. Systematic molecular dynamics simulations were conducted to study the alloying element effects on the primary damage in Fe-based alloys, including defect production and dislocation loop transformations, and their connections with defect thermodynamics. First, effects of alloying elements on the primary damage in three Fe-based ferritic alloy systems were studied, with a particular focus on the production behaviors of solute interstitials. The production behaviors of solute interstitials include over-production or under-production, compared with their solute concentration in the Fe matrix. The three alloy systems are: (1) a Fe-Cr alloy system; (2) a Fe-Cu alloy system; and (3) an ideal but artificial Fe-Cr alloy system, which is used as a reference system. It is found that the number ratio of solute interstitials to the total interstitials is distinct in these alloys. The solute interstitials are over-produced in the Fe-Cr systems but under-produced in the Fe-Cu system, compared with solute composition in the alloys. The defect formation energies in both dilute and concentrated alloys, interstitial-solute binding energies, liquid diffusivities of Fe and solute atoms, and heat of mixing have been calculated for both Fe-Cr and Fe-Cu alloys. Among these factors, our analysis shows that the relative thermodynamic stability between Fe self-interstitials and solute interstitials plays the most important role on the production behaviors of solute interstitials. Next, to obtain a correlation that can quantitatively estimate the solute interstitial fraction in the Fe-based alloys, molecular dynamics simulations were conducted to simulate the cascade damage in a series of "artificial" Fe-Cr alloys with tunable binding energies between a substitutional solute (Cr) atom and a Fe self-interstitial atom (SIA). To achieve this, the Fe-Cr cross pair interaction in the interatomic potential was modified by multiplying a scaling factor so that the solute-SIA binding energy varies linearly from positive to negative values. It is found that the solute interstitial fraction has a strong correlation with the solute-SIA binding energy, and the correlation can be approximately described by a Fermi-Dirac-Distribution-like equation. The independent defect production results reported in literature are found to align well with this correlation. The correlation may be used to estimate the solute interstitial fraction in a wide range of Fe-based alloys simply based on the solute-SIA binding energy, without conducting laborious cascade simulations. Furthermore, primary damage was further investigated in Fe-tungsten (W) alloys to investigate the atomic size effect. The large difference in atomic size between Fe and W can introduce both global volume expansion and local lattice distortion in the Fe matrix. In order to understand how oversized W influences the defect production behaviors in Fe-based alloys, molecular dynamics simulations were conducted to study the primary damage in three systems at 300 K: (a) unstrained pure Fe, (b) Fe-5at.%W alloy, and (c) strained pure Fe with the same volume expansion as the Fe-5%W. The investigation of defect production behaviors include the production of Frenkel pairs, and cluster formation preference. Based on the total number of Frenkel pairs, it indicates that the global volume expansion introduced by oversized W and external strain can lead to enhanced defect production. Meanwhile, the defect cluster analysis in all three systems indicates that the local lattice distortion induced by oversized W can significantly influence the morphologies and size distributions of defect structures. Defect formation energies were calculated to interpret the different defect production behaviors in these systems. Finally, radiation can produce not only point defects but also both <100> and ½<111> type dislocation loops in pure Fe and Fe-Cr alloys. However, contradictory experimental results have been reported on how the Cr concentration affects the ratio of <100> to ½<111> dislocation loops. In this section, molecular dynamics simulations were conducted to study how Cr concentration affects the formation probability of <100> dislocation loops from overlapping cascades on a pre-existing ½<111> dislocation loop in a series of Fe-Cr alloys with 0 – 15%Cr at 300 K. Our atomistic modeling directly demonstrates that the ratio of <100> to ½<111> dislocation loops decreases with the increasing Cr concentration, which is consistent with many experimental observations. Next, independent molecular statics calculations show that the formation energies of both <100> and ½<111> dislocation loops increase with the increasing of Cr content. However, the former has a much faster increase rate than the latter, indicating that the formation of <100> loops becomes energetically more and more unfavorable than ½<111> loops as the Cr content increases. The results provide a thermodynamics-based explanation for why Cr suppresses the formation of <100> dislocation loops in Fe-Cr alloys, which can be applied to all <100> loop formation mechanisms proposed in literature. The possible effects of other alloying elements on the formation probability of <100> loops in Fe-based alloys are also discussed. / Doctor of Philosophy / Metallic alloys are important structural and cladding materials for current and future nuclear reactors. The understanding of radiation-induced damage in metallic alloys is important for the safe operation of nuclear reactors. This dissertation mainly focuses on radiation-induced primary damage in iron-based ferritic alloys. Systematic molecular dynamics simulations were conducted to study how different alloying elements influence the primary damage behaviors in iron-based alloys, including defect production behaviors and dislocation loop transformations. The relations between defect production and defect thermodynamics are also studied. First, molecular dynamics simulations were conducted to study the effects of alloying elements on the primary damage behavior in three Fe-based ferritic alloy systems (Fe-Cr, Fe-Cu, and ideal Fe-Cr), with a particular focus on the production behaviors of solute interstitials. It is found that the number ratio of solute interstitials to the total interstitials has distinct behavior in these alloys. In the Fe-Cr alloys, the ratio of Cr interstitials is much higher than the Cr concentration in the Fe-Cr alloys. By contrast, in the Fe-Cu alloys Cu interstitials are barely produced. In the ideal alloy system, the fraction of solute interstitials is close to the solute concentration in the alloys. Among all the factors we have investigated, it is found the relative thermodynamic stability between Fe self-interstitials and solute interstitials plays the most important role on affecting the production behaviors of solute interstitials. Next, to obtain a quantitative correlation that can predict the solute interstitial fraction in the Fe-based alloys, molecular dynamics simulations were conducted to simulate the cascade damage in a series of "artificial" Fe-Cr alloys with tunable binding energies between a substitutional solute (Cr) atom and a Fe self-interstitial atom (SIA). It is found that the solute interstitial fraction has a strong correlation with the solute-SIA binding energy, and the correlation can be approximately described by an analytical equation. The correlation may be used to estimate the solute interstitial fraction in a wide range of Fe-based alloys simply based on the solute-SIA binding energy, without conducting laborious cascade simulations. Furthermore, primary damage was further investigated in iron-tungsten (Fe-W) alloys. W is about 10.5% larger in atomic radius or 34.8% larger in atomic volume than Fe. The oversize W can introduce both global volume expansion and local lattice distortion in the Fe matrix. Through molecular dynamics simulations in a series of model systems for comparison, it is found that oversized W can lead to enhanced defect production. In addition, it is found that oversized W can significantly influence the morphologies and size distributions of defect clusters. Finally, molecular dynamics simulations were conducted to study how Cr concentration affects the formation probability of <100> and ½<111> dislocation loops in a series of Fe-Cr alloys. Our results demonstrate that the ratio of <100> to ½<111> dislocation loops decreases with the increasing Cr concentration, which is consistent with many experimental observations. The formation energies of both <100> and ½<111> dislocation loops indicate that the formation of <100> loops becomes energetically more and more unfavorable than ½<111> loops as the Cr content increases. The results provide a thermodynamics-based explanation for why Cr suppresses the formation of <100> dislocation loops in Fe-Cr alloys.

AA7075 is a precipitation hardening structural aluminum alloy, which has garnered considerable interest in automotive industry, primarily due its lightweighting capacity compared to many other aluminum alloys from 2xxx and 6xxx series. However, the damage evolution in AA7075 is quite complex due to the presence of different second phase particles in the microstructure and their contribution on damage evolution is largely unknown at large plastic strains. The common second phase particles are η precipitates, θ precipitates and Fe-rich intermetallic particles. The current work presents an extensive multiscale numerical framework, which in conjunction with complementary experiments, is applied to study strain localization, void nucleation, growth, and coalescence in a particle rich matrix. Experimentally, void nucleation is observed to be driven by particle decohesion and particle fracture. Nanoscale molecular dynamics (MD) simulation is carried out to estimate interface properties of the three distinct particle types. The extracted properties are used as input for real particle field 2D and 3D microstructure based finite element (FE) models. The stochastic nature of particle fracture is described using a Weibull distribution, while the effect of grains is incorporated in terms of their Taylor factors. Ductile matrix is described using the well known Gurson Tvergaard Needleman (GTN) void damage model. Complementary experiments included uniaxial tensile tests carried out in-situ in Scanning Electron Microscope (SEM) and X-ray Computed Tomography (XCT), ex-situ high resolution XCT and Electron Back Scattered Diffraction (EBSD) tests. The FE models with three distinct particle stoichiometries and three competing damage mechanisms, show good agreement with experimental observations. Particle fracture marginally dominates particle decohesion. At low plastic strains, void nucleation is initiated by decohesion and fracture of larger Fe-rich particles, which facilitate formation of localized deformation bands. At large plastic strain, elevated stresses within the localized bands facilitate decohesion and fracture of more resistant η and θ precipitates. Due to their inherent larger size and more irregular morphology, θ precipitates contribute to voiding more than η precipitates. Under uniaxial tensile loads, void growth takes place in the middle of the specimen, driven by higher triaxiality stress state in the middle, relative to the surface. Void coalescence occurs along deformation bands driven by higher stresses due accumulated plastic strain within the bands, in a process known as void sheeting. / Thesis / Doctor of Philosophy (PhD)

4H-SiC n-channel power MESFETs with nitrogen-doped epitaxially grown channel and nitrogen n+-implanted source/drain ohmic contact regions, with and without p-buffer layer fabricated on semi-insulating substrates exhibited hysteresis in the drain I-V characteristics of both types of devices at 300 K and 480 K due to traps. However, thermal spectroscopic measurements could detect the traps only in the devices without p-buffer. In this study the two-dimensional device simulator, MediciTM, and optical admittance spectroscopy (OAS) measurements are used to help resolve the discrepancy in the initial experimental characterization results and interpret the results. Device simulations also showed hysteresis in the drain I-V curves of both types of devices at 300 K and 480 K. Simulations suggest that, in addition to the SI substrate traps, which are known to be major cause of hysteresis in MESFET drain I-V characteristics, acceptor traps due to source/drain residual implant lattice damage could also contribute to the hysteresis observed in the drain I-V characteristics of the experimental MESFETs. Although surface traps are known to cause hysteresis in the I-V curves of MESFETs, their presence was not observed in the experimental devices. The results of the OAS measurements showed several peaks in the spectra of the devices without p-buffer, while in the spectra of the devices with p-buffer the peaks were generally non-existent or reduced. This demonstrates that the peaks observed in the OAS spectra are largely due to substrate traps and that the p-buffer layer is effective in isolating the channel from the substrate. A peak centered around 1.51 eV below the conduction band, which has also been observed in the literature after He+-implantation, is consistently observed in the spectra of both types of devices although it appears reduced in the spectra of the devices with buffer. In this dissertation it is shown that it is likely the traps responsible for this peak could contribute to the hysteresis observed at 300 K and could be solely responsible for the hysteresis observed at high temperatures such as 480 K, since simulations suggest that hysteresis due to semi-insulating substrate traps disappear at high temperatures such as 480 K.

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