The intermetallic compound TiAl possesses a unique complex of properties that include sufficiently low material density, high values of the strength-to-ductility ratio, high elastic moduli, high oxidation resistance, low creep rate, and improved fatigue characteristics. These properties make TiAl alloys very attractive, particularly for structural applications for aerospace and aeronautic industries, where, at certain temperatures, they might be capable of replacing heavy nickel-based superalloys. However, so far applications of TiAl alloys have been limited by their poor ductility. Many of the recent studies have focused on the source of this limited ductility and on methods to improve this property. It has been found out experimentally that the strength and ductility of $gamma$-TiAl alloys can be affected by many different parameters, including alloy stoichiometry, heat treatment, deformation temperature, impurity content, grain size, and ternary element additions. In this thesis we present the results of our computer simulations of deformation and fracture in TiAl. In contrast to many previous studies our simulations include the interaction of the crack with point defects in the lattice. We use the molecular statics technique with atomic interactions described in terms of the embedded atom method. We simulate the crack propagation along (100), (001), (110) and (111) planes in TiAl. The cleavage along (100) and (001) planes shows purely brittle behavior, whereas the cleavage along (110) and (111) planes is accompanied by extensive dislocation emission. Our studies of the crack interaction with point defects reveal that vacancies and antisites near the crack tip can influence the amount of plastic deformation. Another important observation is that the antisite formation energy near the crack tip is generally lower than in the perfect lattice. This observation suggests the formation of relatively disordered zones near the crack tip at high temperatures, and leads us to a formulation of a new mechanism of a brittle-to-ductile transition in TiAl. / Ph. D.

"Cellulose nanocrystals, one of the most abundant materials in nature, have attracted great attention in the biomedical community due to qualities such as supreme mechanical properties, biodegradability, biocompatibility and low density. In this research, we are interested in developing a bio-inspired material-by-design approach for cellulose-based composites with tailored interfaces and programmed microstructures that could provide an outstanding strength-to-weight ratio. After a preliminary study on some of the existing biomaterials, we have focused our research on studying the nanostructure and nanomechanics of the bamboo fiber, a cellulose-based biocomposite, designed by nature with remarkable strength-to-weight ratio (higher than steel and concrete). We have utilized atomistic simulations to investigate the mechanical properties and mechanisms of interactions between cellulose nanofibrils and the bamboo fiber matrix which is an intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). Our results suggest that the molecular origin of the rigidity of bamboo fibers comes from the carbon-carbon or carbon-oxygen covalent bonds in the main chain of cellulose. In the matrix of bamboo fiber, hemicellulose exhibits larger elastic modulus and glass transition temperature than lignin whereas lignin shows greater tendency to adhere to cellulose nanofibrils. Consequently, the role of hemicellulose is found to enhance the thermodynamic properties and transverse rigidity of the matrix by forming dense hydrogen bond networks, and lignin is found to provide the strength of bamboo fibers by creating strong van der Waals forces between nanofibrils and the matrix. Our results show that the amorphous region of cellulose nanofibrils is the weakest interface in bamboo microfibrils. We also found out that water molecules enhance the mechanical properties of lignin (up to 10%) by filling voids in the system and creating hydrogen bond bridges between polymer chains. For hemicellulose, however, the effect is always regressive due to the destructive effect of water molecules on the hydrogen bond in hemicellulose dense structure. Therefore, the porous structure of lignin supports the matrix to have higher rigidity in the presence of water molecules. "

In this thesis, current driven domain-wall motion is studied using atomistic simulations with the exchange coupling modeled by the Heisenberg Hamiltonian under the nearest-neighbor approximation. The investigations may be divided into two parts, each concerned with how different aspects of the systems affect the domain-wall motion. The first part deals with domain-wall width dependence of the velocity in a three dimensional geometry with simple cubic crystal structure and uniaxial anisotropy. Results from this part showed that the velocity has a minor domain-wall width dependence. For a fixed current density, the velocity increased with domain-wall width, though only from 61.5 a/ns to 64.5 a/ns as the domain-wall width was increased from 3 to 25 atoms. The second part of the investigations deals with phenomena involving mixed cubic and uniaxial anisotropy, the non-adiabaticity parameter as well as the geometry of the system. The discussion includes an account of how the spin-transfer and cubic anisotropy torques contribute to the motion for different values of the non-adiabaticity parameter. In comparing a one dimensional atomic chain and a three dimensional system with simple cubic crystal structure, but otherwise with the same material properties, results showed a difference in how the two systems responded to currents. This difference is not accounted for by the micromagnetic theory, and its origin was unable to be determined.

Atomistic computer simulations were performed using embedded atom method interatomic potentials in α-Fe with impurities and defects. The effects of intergranular carbon on fracture toughness and the mechanisms of fracture were investigated. It was found that as the average grain size changes the dominant energy release mechanism also changes. Because of this the role of the intergranular carbon changes and these mechanisms compete affecting the fracture toughness differently with changing grain size. Grain boundary accommodation mechanisms are seen to be dominant in the fracture of nanocrystalline α-Fe. To supplement this work we investigate grain boundary sliding using the Σ = 5,(310)[001] symmetrical tilt grain boundary. We observe that in this special boundary sliding is governed by grain boundary dislocation activity with Burgers vectors belonging to the DSC lattice. The sliding process was found to occur through the nucleation and glide of partial grain boundary dislocations, with a secondary grain boundary structure playing an important role in the sliding process. Interstitial impurities and vacancies were introduced in the grain boundary to study their role as nucleation sites for the grain boundary dislocations. While vacancies and H interstitials act as preferred nucleation sites, C interstitials do not. / Ph. D.

No description available.

Nanowires are an exciting class of novel materials that have potential applications in areas including biological sensing, photonics, and electronics. The promise of these future applications relies on the production of nanowires of controlled size, shape, and crystal structure, in reasonable quantities, and further, ultimately requires that the nanowires be mechanically stable in the application environment. This research is aimed at understanding the mechanical behavior of metallic nanowires, through the use of atomistic simulations. At the nanometer scale, where the surface-area-to-volume ratio is substantial, the effects of free surfaces have a primary influence on the physical properties of a material. Surface energy arises from unsatisfied bond coordination at the surface of a solid and results in a surface stress as the surface atoms contract into the bulk of the material to increase their local electron density. The magnitude of surface energy and surface stress is dependent on the orientation of the surface and the compliance of the structure. In bulk materials, the effects of surfaces are negligible; however, at the nanometer scale, surface effects become quite significant. In metallic nanowires, these surface effects strongly influence mechanical properties, and the characteristics of plastic deformation. The mechanical testing of nanowires is precluded by the difficulties of accurately applying and measuring forces on the nanometer scale. For this reason, computational simulations are a primary tool for investigating the mechanical behavior of nanowires. In this work, we have performed atomistic simulations to examine the mechanical response of silver nanowires. We have conducted studies to determine the deformation characteristics of experimentally observed nanowire geometries subjected to tensile and bending loads. We have also developed a technique to probe the energetics of mechanical deformation, in order to elucidate the energetically favored deformation pathways in nanowires. Our results show that nanowires may be tailored for specific mechanical requirements based on geometry and free surface orientation and provide insight to the effect of free surfaces in the mechanical deformation of nanometer scale structures.

With the emergence of nanotechnology, mankind has obtained the capability to manipulate materials at nanoscale and this led to the invention of a new group of novel materials like carbon nanotubes, graphene and quantum nanodots. Silicene nanoribbons (SiNRs) are one of the newest members of this nanomaterial family which has been synthesized very recently by deposition on silver substrates. A SiNR sheet is made up of a layer of two dimensional honeycomb structure solely composed of silicon atoms. In this thesis, structural and mechanical properties of SiNR are being investigated with the help of classical empirical molecular dynamics simulation technique. In the first part of this thesis, structural properties of SiNR sheets have been investigated. This investigation has been carried out by performing classical molecular dynamics simulations using atomistic many-body potential energy functions at many different SiNR sheet lengths and widths, at low and room temperatures with and without periodic boundaries. It has been found that SiNR sheets do not have perfectly flat honeycomb geometry. It has also been found that finite length models are more likely to form tubular structures resembling distorted silicon nanotubes at room temperature. In the second part of this thesis, mechanical properties of SiNRs have been investigated. Mechanical properties of silicene nanoribbons of varying width have been investigated under 5% and 10% uniaxial strain via classical Molecular-Dynamics simulations at 1 K&deg / and 300 K&deg / temperatures by the aid of atomistic many-body potential energy functions. It has been found that under strain, SiNRs show such material properties: they are very ductile, they have considerable toughness and despite their low elasticity, they have a very long plastic range before fragmentation.

Adverse changes in the Earth's climate are thought to be due to the output of carbon dioxide from power stations. This has led to the development of many new materials to remove CO2 from these gas streams. Polymers of intrinsic microporosity (PIMs) are a novel class of polymers that are rigid with sites of contortion. These properties result in inefficient packing and so lead to large pore volumes and high surface areas. The inclusion of Tröger’s base, a contortion site made up of two nitrogen atoms, is thought to lead to increased uptake of CO2. The combination of electrostatic interactions with strong van der Waals forces should interact favourable with the quadrupole moment of CO2.Here a molecular simulation study of a selection of these polymers is presented. The study begins by developing a quick screening method on single polymer chains. This shows that the high surface area and adsorption affinity are a result of the contorted nature of PIMs along with the inclusion of groups such as Tröger’s base.The creation of atomistic models that reproduce the space packing ability of these polymers is also explored. Methods developed for PIMs in literature are investigated along with a new method developed during this study. GCMC simulations are then used to investigate the adsorption of CO2. In this study it is seen that that these polymers possess a well percolated network of both ultramicropores and supermicropores with a significant fraction of these pores being close to the kinetic diameter of CO 2. It is posited that these pores may be the result of the inclusion of Tröger’s base. It is also shown that this produces a particularly favourable site for adsorption. The phenomenon of swelling as a result of CO2 adsorption is also investigated using a variety of methods that make use of the output from the GCMC simulations. It was found that swelling is negligible for pressures of up to 1 bar. This result is important as swelling in the polymer can lead to a reduction in selectivity and an increase in permeability, which can affect the overall material’s performance.

Ordered intermetallics based on transition metal aluminides have been proposed as structural materials for advanced aerospace applications. The development of these materials, which have the advantages of low density and high operating temperatures, have been focused on the aluminides of titanium, nickel and iron. Though these materials exhibit attractive properties at elevated temperatures, their utilization is limited due to their propensity for low temperature fracture and susceptibility to decreased ductility due to environmental effects. A major embrittlement mechanism at ambient temperatures in these aluminides has been by the loss of cohesive strength at the interfaces (intergranular failure). This study focuses on this mechanism of failure, by undertaking a systematic study of the energies and structures of specific grain boundaries in some of these compounds. The relaxed atomistic grain boundary structures in B2 aluminides, FeAl, NiAl and CoAl and <I>L</I>1₀ γ-TiAl were investigated using molecular statics and embedded atom potentials in order to explore general trends for a series of B2 compounds as well as TiAl. The potentials used correctly predict the proper mechanism of compositional disorder of these compounds. Using these potentials, point defects, free surface energies and various grain boundary structures of similar energies in three B2 compounds, FeAl, NiAl and CoAl were studied. These B2 alloys exhibited increasing anti-phase boundary energies respectively. The misorientations chosen for detailed study correspond to the Σ5(310) and Σ5(210) boundaries. These boundaries were investigated with consideration given to possible variations in the local chemical composition. The effects of both boundary stoichiometry and bulk stoichiometry on grain boundary energetics were also considered. Defect energies were calculated for boundaries contained in both stoichiometric and off-stoichiometric bulk. The surface energies for these aluminides were also calculated so that trends concerning the cohesive energy of the boundaries could be studied. The implications of stoichiometry, the multiplicity of the boundary structures and possible transformations between them for grain boundary brittleness are also discussed. / Ph. D.

Although aluminides with the B2 crystal structures have good properties for high temperature applications, the strong ordered bonds that make them durable at high temperature also make them too brittle at room temperature for industrial fabrication. In order to better understand this lack of ductility, molecular statics simulations of planar fault defects and dislocation core structures were conducted in a series of B2 aluminides with increasing ordering energy (FeAl, NiAl, CoAl). The simulation results in NiAl were compared with in-situ straining observations of dislocation motion. The dislocations simulated were of (100) and (111) types. The simulations results obtained indicate a strong influence of the planar fault energies on the mobility of the dislocations. As the cohesive energy increases from FeAl to CoAl, antiphase boundary and unstable stacking fault energies increase resulting in more constricted dislocation core spreadings. This constriction of the cores decreases the mobility of dislocation with planar core structures and increases the mobility of dislocations with non-planar cores. The (100) screw dislocations were found with planar cores in {110} planes for FeAl, NiAl and CoAl. For very high APB values, the cores were very compact, as predicted by the Peierls- Nabarro model. As the APB energies decrease, increasingly two dimensional spreading of the cores was observed and ultimately dislocation dissociation into partials. As a result of the deviation of the stable planar fault energy from the APB fault, the partials were not exact 1/2(111) but deviate to the point corresponding to the actual minima of the γ-surfaces for these compounds. Alloying NiAl with Fe was found to promote the dissociation of the (100) dislocation. The in-situ straining of a single crystal of NiAl only revealed the motion of (100) dislocations. Both in-situ observations and atomistic simulations agreed on the zig-zag shape of the (100) dislocation with an average screw orientation. In this configuration, the mobility of the dislocation is severely reduced. / Ph. D.