Effects of dislocation emission from a mode I crack and of pinning distances on the behavior of the crack and on fracture toughness in aluminum were studied by using the Molecular Statics Technique with atomic interactions described in terms of the Embedded Atom Method. It was found that aluminum is a ductile material in which the cracks generate dislocations, blunting the cracks. The blunting and the dislocation shielding reduce the local stress intensity factor. Also, twinning, which has not been observed experimentally in Aluminum due to the high stacking fault, was obtained in the simulation. Probably, the low temperature facilitates twin formation. The applied stress intensity factor required to propagate the crack tip increases at first, and then becomes constant as the maximum distance that the first dislocation can travel away from the crack tip increases. These effects can be attributed to dislocation shielding and crack blunting. The maximum distance of the emitted dislocations from the crack tip is the equilibrium distance for the largest simulation performed (400,000 atoms) while for the smaller simulations the dislocations are hindered by the fixed boundary condition of the model. On the other hand, the total local stress intensity factor at the crack tip and the local stress intensity factor along the slip plane remain basically constant as the maximum distance of the emitted dislocations from the crack tip increases. For distances larger than , these local stress intensity factors start to increase slightly. / Master of Science

Molecular dynamics simulations of the diffusion process in ordered B2 compounds at high temperature were performed using an embedded atom interatomic potential developed to fit NiAl properties. Diffusion in the bulk occurs through a variety of cyclic mechanisms that accomplish the motion of the vacancy through nearest neighbor jumps restoring order to the alloy at the end of the cycle. The traditionally postulated six-jump cycle is only one of the various cycles observed and some of these are quite complex. Diffusion at the grain boundary mainly takes place through sequences of coordinated nearest neighbor jumps yielding to a rearrangement of the grain boundary structure. Two distinct mechanisms resulting in a structural unit migration of the vacancy are also identified. The results are analyzed in terms of the activation and configuration energies calculated using molecular statics simulations. / Master of Science

Silver (Ag) is a precious metal with a low stacking fault energy that is known to form copious nanoscale coherent twin boundaries during magnetron sputtering synthesis. Nanotwinned Ag metals are potentially attractive for creating new interface-dominated nanomaterials with unprecedented mechanical and physical properties. Grain-boundary segregation of solute elements has been found to increase the stability of interfaces and hardness of nanocrystalline metals. However, heavily alloying inevitably complicates the underlying deformation mechanisms due to the hardening effects of solutes, or a change of stacking fault energies in Ag caused by alloying. For the above reasons, we developed a microalloying (or doping) strategy by carefully selecting Cu as the primary impurity – a solute that is predicted to have no solid-solution strengthening effect in Ag when its content is below 3.0 wt.%. Neither will Cu affect the stacking fault energy of Ag at a concentration <1.0 wt.%. Moreover, Cu atoms are ~12% smaller than Ag ones, and Ag-Cu is an immiscible system, which facilitates the segregation of Cu into high-energy interface sites such as grain-boundaries and twin-boundary defects. In this thesis, large-scale hybrid Monte-Carlo and molecular dynamics simulations are used to study the unexplored mechanical behavior of Cu-segregated nanocrystalline nanotwinned Ag. First, the small-scale mechanics of solute Cu segregation and its effects on incipient plasticity mechanisms in nanotwinned Ag were studied. It was found that solute Cu atoms are segregated concurrently to grain boundaries and intrinsic twin-boundary kink-step defects. Low segregated Cu contents (< 1 at.%) are found to substantially increase twin-defect stability, leading to a pronounced rise in yield strength at 300 K. Second, atomistic simulations with a constant grain size of 45 nm and a wide range of twin boundary spacings were performed to investigate the Hall-Petch strength limit in nanocrystalline nanotwinned Ag containing either perfect or kinked twin boundaries. Three distinct strength regions were discovered as twin boundary decreases, delineated by normal Hall-Petch strengthening with a positive slope, the grain-boundary-dictated mechanism with near-zero Hall-Petch slope, and twin-boundary defect induced softening mechanism with a negative Hall-Petch slope. Third, by systematically studying smaller grain sizes, we find that the “strongest” size for pure nanotwinned Ag is achieved for a grain size of ~16 nm, below which softening occurs. The controlling plastic deformation mechanism changes from dislocation nucleation to grain boundary motion. This transition decreases to smaller grain sizes when Cu contents are segregated to the interfaces. Our simulations show that continuous Hall-Petch strengthening without softening, down to grain sizes as small as 6 nm, is reached when adding Cu atoms up to 12 at. %. For Cu contents ≥ 15 at. %, however, the predominant plastic deformation mechanism changes to shear-band induced softening. The present thesis provides new fundamental insights into solute segregation, and strengthening mechanisms mediated by grain boundaries and twin boundaries in face-centered cubic Ag metals, which is expected to motivate experimental studies on new nanotwinned metals with superior mechanical properties controlled by microalloying.

Generation and migration of helium and other point defects under irradiation causes ferritic steels based on the Fe-Cr system to age and fail. This is motivation to study point defect migration and the He equation of state using atomistic simulations due to the steels' use in future reactors. A new potential for the Fe-Cr-He system developed by collaborators at the Lawrence Livermore National Laboratory was validated using published experimental data. The results for the He equation of state agree well with experimental data. The activation energies for the migration of He- and Fe-interstitials in varying compositions of Fe-Cr lattices agree well with prior work. This research did not find a strong correlation between lattice ordering and interstitial migration energy

Dans les alliages 3d, les propriétés magnétiques des solutés peuvent être extrêmement sensibles aux environnements chimiques locaux, et avoir un impact crucial sur diverses propriétés thermodynamiques et cinétiques. Afin de comprendre les propriétés fondamentales de ces alliages, la première partie de ce travail est dédiée à l’étude ab-initio des effets de l’environnement chimique local sur l’état magnétique des solutés de Mn dans le Fe-Mn. Diverses configurations contenant du Mn, isolé ou sous forme d’amas, en présence de lacunes ou d’impuretés interstitielles sont étudiées et leur configuration magnétique de plus basse énergie est déterminée. Un modèle effectif d’interactions est paramétré à partir des données ab-initio afin d’étudier les propriétés des alliages Fe-Mn à température finie. Les propriétés clés sont identifiées et le modèle est validé à basse température en reproduisant les résultats ab-initio. L’utilisation de ce modèle couplé à des simulations Monte Carlo permet de simuler l’évolution chimique des alliages Fe-Mn en fonction de la température et de la concentration en Mn, tout en relaxant la structure magnétique en temps réel. Afin d’illustrer les possibles applications de ce modèle, diverses propriétés sont étudiées telles que la dépendance en concentration de la température de Curie ou encore l’évolution en température de l’énergie de mélange et de l’ordre atomique à courte distance. Puisque dans ces alliages la diffusion est en général régie par mécanisme lacunaire, nous proposons aussi un formalisme prenant en compte explicitement les effets de l’ordre magnétique local sur les propriétés des lacunes. Par simulations Monte Carlo de traceurs, cette approche prédit la dépendance en température de l’auto-diffusion dans le Fe en excellent accord avec les études expérimentales. La déviation de la loi d’Arrhénius proche de la température de Curie est directement prédite, ainsi que le changement de pente entre les régimes ferromagnétique et paramagnétique. La précision du modèle de Ruch, couramment utilisé dans la littérature, est discutée au vu des résultats obtenus. Enfin, cette approche est appliquée à la diffusion d’un soluté de Mn dans le Fe pur et comparée aux résultats expérimentaux. / In 3d alloys, magnetic properties of solutes can be extremely sensitive to local chemical environments and can have a crucial impact on various thermodynamic and kinetic properties. In order to properly understand the fundamental properties of these alloys, the first part of this work is dedicated to the study of the effects of local chemical environment on the magnetic state of Mn solutes in bcc Fe-Mn by means of Density Functional Theory. Namely, configurations containing Mn, being isolated or forming a cluster, and in the presence of vacancies or interstitial impurities are investigated and their lowest-energy magnetic configuration is determined. The ab-initio data produced are then used to parameterize an effective interaction model in order to study the properties of Fe-Mn alloys at finite temperature. The key features of Fe-Mn alloys are identified, and the model is validated at low temperature by reproducing ab-initio predictions. Using this model coupled to Monte Carlo simulations, we simulate the chemical evolution of Fe-Mn properties depending on temperature and Mn concentration while relaxing the magnetic structure on-the-fly. In order to illustrate the validity and the applicability of the model, we examine certain finite temperature properties of bcc Fe-Mn alloys such as the concentration dependence of the Curie temperature or the temperature evolution of the mixing energy and the atomic short-range order. Since diffusion in Fe and Fe-Mn alloys is generally ruled by vacancy-mechanism, we also propose a formalism to take explicitly into account the properties of vacancies in the interaction model and the effect of local magnetic state on these properties. Using tracer diffusion Monte Carlo simulations, this approach predicts the temperature dependence of self-diffusion in bcc Fe in excellent agreement with experimental results, including the deviation from Arrhenius law around the Curie temperature and the change of slope between the ferromagnetic and paramagnetic regimes. The accuracy of the widely used Ruch model is discussed in the light of the present results. Finally, we apply this approach to the diffusion of a Mn solute in bcc Fe and compare with experimental results.

ymmetric tilt boundary [2 1 10] (01 12) in HCP titanium has been investigated by computer simulation methods using the embedded atom potential. The structure and energies of the considered boundary and the energies of formation of vacancies in it have been calculated by the method of molecular-static simulation. The stability of the boundary at elevated temperatures has been investigated by the molecular dynamics method, and the coefficients of grain-boundary diffusion have been calculated.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-02T21:28:34Z No. of bitstreams: 1 EduardoDiniz.pdf: 974608 bytes, checksum: 134920c92050dd5dee6af94ec24287a3 (MD5) / Made available in DSpace on 2017-06-02T21:28:34Z (GMT). No. of bitstreams: 1 EduardoDiniz.pdf: 974608 bytes, checksum: 134920c92050dd5dee6af94ec24287a3 (MD5) Previous issue date: 2006-02-22 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this work static atomistic simulations were performed to investigate two systems: rare-earth trifluorides, REF3, with tysonite structure (RE = La, Ce, Pr and Nd) and rare-earth mixed metallic oxides, RE(TiTa)O6, with RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er and Yb. In the rare-earth trifluorides was investigated the pressure induced structural phase transition identified by X-ray powder difraction in RE = La and Ce. The results shown that the crystalline structure is possibly tetragonal, in concordance with quantum mechanical calculations. For this, an interionic potential set that describes better the structural data of each compound and the physical observable was obtained. Besides, it was observed that critical pressure values were closed. The study involving the rare-earth mixed metallic oxides was focused in determining their bulk and structural properties. Thus, firstly a set of reliable interionic potentials was looked for to reproduce the crystalline structure and the physical observable, trough the investigation of three methodologies for obtaining interionic potentials, assuming the methodology that reproduces more reliable the dielectric constants. Furthermore, others bulk properties as lattice energy formation, elastic constants and bulk modulii were calculated. All then shown a rare-earth ionic radii dependence that revealed the morphotropic phase boundary between aeschynite and euxenite crystalline structures presented by these crystals. / Neste trabalho foram empregadas simulações atomistícas estáticas para se investigar dois sistemas: trifluoretos de terra-rara, REF3, com estrutura tisonita (RE = La, Ce, Pr e Nd) e óxidos metálicos mistos de terras-raras, RE(TiTa)O6, com RE = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er e Yb. Nos trifuoretos de terra-rara foi investigada a transição de fase estrutural que ocorre para altas pressões identificada por difração de raios-X em pó para RE = La e Ce. Os resultados mostraram que a estruturas cristalinas à altas pressões são possivelmente tetragonais, concordando com os resultados obtidos por cálculos quânticos. Para tanto, obtivemos um conjunto de potenciais interiônicos que proporcionassem uma descrição confiável da estrutura cristalina de cada composto, bem como o cálculo de observáveis físicos que produzissem valores próximos aos observados experimentalmente. O estudo envolvendo os óxidos metálicos mistos de terras-raras, dedicou-se em determinar suas propriedades de bulk e estruturais. Para isso, primeiramente obtivemos um conjunto de potenciais interiônicos confiáveis para se reproduzir a estrutura cristalina e os observáveis físicos, através da investigação de três metodologias para a obtenção de potenciais interiônicos, optando-se por aquela que melhor reproduziu as constantes dielétricas. Além da constante dielétrica, outros observáveis como a energia de formação da rede, constantes elásticas e módulo de bulk, foram calculados. Todos apresentaram uma dependência com o raio iônico do íon terra-rara que evidenciou o contorno de fase morfotrópico entre as estruturas aeschinita e euxenita apresentada por estes compostos.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-06-06T17:35:07Z No. of bitstreams: 1 EdsonFirmino.pdf: 1970355 bytes, checksum: d3777c3685602b1f299f41ead9f30f1f (MD5) / Made available in DSpace on 2017-06-06T17:35:07Z (GMT). No. of bitstreams: 1 EdsonFirmino.pdf: 1970355 bytes, checksum: d3777c3685602b1f299f41ead9f30f1f (MD5) Previous issue date: 2007-02-16 / Fundação de Amparo à Pesquisa e ao Desenvolvimento Científico e Tecnológico do Maranhão (FAPEMA) / In this work we have performed static atomistic simulations to investigate the structural and bulk properties of the complex double perovskites with rock-salt ordering with the Ba2BiM+5O6 stoichiometry, where M+5 = Ta5+, Sb5+ and Nb5+, under pressure changes. For this we have adopted a Buckingham potential model for the ionic interactions and the shell model in order to model the atomic polarizations. In the case of Ba2BiTaO6 and Ba2BiNbO6 compounds, we assumed potential parameters listed in the literature and in the case of Ba2BiSbO6 compound, new potential parameters for the Sb5+¡O interaction were founded. The results observed with theses potentials for the cases of Ba2BiTaO6 and Ba2BiSbO6 materials, shown that the potential sets are sensitive to the structural phase transition observed experimentally between the phases R¹3 and I2=m at high pressures. Thus it was possible to de ne the critical pressure for Ba2BiSbO6. Some elastic and mechanical properties of these materials had been calculated and had shown these transitions clearly, where it was observed the Young's modulus collapse at the critical pressure and discontinuities in the shear modulus and in the longitudinal and transversal sound propagation speeds. The elastic constants had not presented collapse and the volume practically did not change at the transition, what as expected from the experimental results. For the case of Ba2BiNbO6 material, once that there are not experimental results, the aim was to predict their structural characteristics. There were tested four potential sets presented in the literature using as input initial cell parameters data generated by the SPUDS software. Two of these potential sets exhibited the same phase transition that Ba2BiTaO6 and Ba2BiSbO6 present. / Neste trabalho foram empregadas simulações atomísticas estáticas para investigar as propriedades estruturais e de bulk das perovskitas complexas duplas com ordenamento do tipo NaCl com a estequiometria Ba2BiM5+O6, onde M5+ = Ta5+, Sb5+ e Nb5+ sob varia ção de pressão. Para tanto assumimos um modelo de potencial de Buckingham para as interações entre os íons e o modelo de casca esférica para tratar as polarizações dos átomos. No caso dos compostos Ba2BiTaO6 e Ba2BiNbO6 potenciais adotados da literatura foram empregados e para o Ba2BiSbO6, o potencial para a interação Sb5+¡O foi encontrado. Os resultados obtidos com os potenciais para os casos do Ba2BiTaO6 e Ba2BiSbO6, mostraram que ambos são sensíveis à transição de fase estrutural observada experimentalmente entre as fases R¹3 e I2=m que ocorre para altas pressões, de nindo o ponto de transição para o Ba2BiSbO6. Algumas propriedades elásticas e mecânicas destes compostos foram calculadas e mostraram claramente esta transição, onde foi observado o colapso do módulo de Young na pressão crítica e descontinuidades no módulo de cisalhamento e nas velocidades, longitudinal e transversal, de propagação do som dos compostos. As componentes das constantes elásticas não apresentaram colapsos, e o volume praticamente não variou na transição, o que era esperado dos resultados experimentais, implicando em um módulo de bulk praticamente contínuo na transição. Já no caso do Ba2BiNbO6, como não se dispunha de resultados experimentais, o principal objetivo foi predizer suas características estruturais. Testou-se quatro conjuntos de potenciais encontrados na literatura utilizando-se como dados iniciais os parâmetros de rede e posições atômicas gerados pelo software SPUDS. Dois destes conjuntos de potenciais mostraram a mesma transição de fase que o Ba2BiTaO6 e Ba2BiSbO6.

In 2005, Park et al. demonstrated that the 2-fold surface of a d-AlNiCo quasicrystal exhibits an 8-fold frictional anisotropy, as measured by atomic-force microscopy, between the periodic and aperiodic directions [40, 41]. It has been well known that quasicrystals exhibit lower friction than their crystalline counterparts [38, 18, 51, 30, 12, 54]; however, the discovery of the frictional anisotropy allows for a unique opportunity to study the effect of periodicity on friction when chemical composition, oxidation, and wear are no longer variables. The work presented herein is focused on obtaining an understanding of the mechanisms of friction and the dependence of friction on the periodicity of a structure at the atomic level, focusing on the d-AlNiCo quasicrystal studied by Park et al. Using the LAMMPS [44] package to simulate the compression and sliding of an 'adamant' tip, see section 3.3, on a d-AlNiCo quasicrystalline approximant substrate, we have demonstrated, in preliminary results, an 8-fold frictional anisotropy, but in more careful studies the anisotropy is found to be much smaller. The simulations were accomplished using Widom-Moriarty pair potentials to define the interactions between the atoms [36, 56, 55, 9]. The studies presented in this work have shown a clear velocity dependence on the measured frictional response of the quasicrystalline approximant's surface. The final results show between a 1.026-fold and 1.127-fold anisotropy between sliding in the periodic and 'aperiodic' directions, depending on the sliding velocity.

Thesis (P.hD (Physics)) --University of Limpopo, 2013 / Titanium dioxide (TiO2) nanoparticles, nanowires, nanosheets and nanoporous are of great interest in many applications. This is due to inexpensive, safety and rate capability of the material. It has being considered as a replacement of graphite anode material in rechargeable lithium batteries. Much experimental work on pure and lithiated nanostructures of TiO2 has been reported, mostly with regards to their complex microstructures. In this work we employ molecular dynamics (MD) simulation to generate models of TiO2 nano-architectures including: nanosheet, nanoporous, nanosphere and bulk. We have successfully recrystallised all four nanostructures from amorphous precursors; calculated radial distribution functions (RDFs), were used to confirm crystallinity. Configuration energies, calculated as a function of time, were used to monitor the recrystallisation. Calculated X-Ray Diffraction (XRD) spectra, using the model nanostructures, reveal that the nanostructures are polymorphic with TiO2 domains of both rutile and brookite in accord with experiment. Amorphisation and recrystallisation was successful in generating complex microstructures. In particular, bulk and nanoporous structures show zigzag tunnels (indicative of micro-twinning) while nanosphere and nanosheet shows zigzag and straight tunnels in accord with experiment. All model nanostructures of TiO2 were lithiated with different lithium content. RDFs, microstructures, configuration energies, calculated as a function of time and XRDs of all lithiated structures are presented. / University of Limpopo Research Office,The Royal Institution(Ri),Granfield University,Materials Modelling Centre,UCL,and the CHPC