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Twins are a pair of adjoining crystal grains related to each other by a special symmetry. They are frequently observed in bulk materials and nanomaterials. The formation of twins is an important topic in materials science and engineering because it affects material behaviors such as plastic deformation of metals, yield strength, and band gap energy in nanoscale semiconductors. Because of these unique phenomena and properties that the twinning can bring to the materials, it is of interest to investigate the formation of twins. Our primary objective in this dissertation is to study twinning in nanowires. Both gold and platinum <111> oriented nanowires were fabricated by similar solution-phase chemical synthesis methods. High-resolution transmission electron microscopy and electron diffraction patterns were carried out to analyze the structures of the nanowires. Nanodiffraction was used to demonstrate twinning is a general structural feature of the growth of gold nanowires growing in a <111> direction.  A model was proposed to explain the conditions under which twinning is energetically favored during nanowire growth. The model, which is based on a maximum rate hypothesis, considers the nanowire geometry and the relative surface and stacking fault energies and predicts twins should appear in gold nanowires but not in platinum nanowires, in agreement with experimental observations. During the structural characterization of gold nanowires, our interest is to resolve 3D structure of twinning. However, the structure of twinning in gold nanowires is very fine and the average spacing between twin boundaries is only 0.57nm (+/- 0.38 nm); therefore, regular 3D electron microscopy technique is unable to reconstruct these defected structures. Here we present a stereo vision technique to reconstruct 3D atomic non-periodic structures containing defects. The technique employs intrinsic atomic planes as epipolar planes to achieve the alignment accuracy needed to reconstruct a crystal with atomic resolution. We apply it to determine the 3D geometry and atomic arrangements of twinning in gold nanowire. In addition, an iterated cross-correlation algorithm was developed to analyze electron diffraction fully automatically to facilitate structural analysis of nanowires. A time-temperature-transformation diagram of platinum nanowires in chemical synthesis was determined to help optimize the fabrication process of the nanowires. / Ph. D.

Natural and hot isostatically pressed dolomite aggregates were experimentally deformed at effective pressures of Pe = 50 ?? 400 MPa, temperatures of 400 ?? 850??C, and strain rates of ε& = 1.2x10-4 s-1 to 1.2x10-7 s-1. Coarse- and fine-grained dolomite deformed at low temperature (T ≤ 700??C for coarse-grained natural dolomite, T < 700??C for fine-grained natural and synthetic dolomite) exhibit mechanical behavior that is nearly plastic; differential stresses are insensitive to strain rate, fitted either by a power law no⎟⎟⎠⎞⎜⎜⎝⎛−=??σσεε31&& with n values that range from 12 to 49 or an exponential law ([31exp )] σσαεε−=o&& with exponential law term α values from 0.023 to 0.079 MPa-1. Microstructures of samples deformed at low temperatures include mechanical twins, and undulatory extinction suggesting that twin glide and dislocation slip are the predominant deformation mechanisms. At high temperatures (T ≥ 800??C) flow strengths of coarse- and fine-grained dolomite depend more strongly on strain-rate and exhibit pronounced temperature dependencies. Microstructures of coarse-grained dolomite samples deformed at T ≥ 800??C include undulatory extinction and fine recrystallized grains suggesting that recovery and dynamic recrystallization contribute to dislocation creep at these conditions. By comparison with lower temperature deformation, mechanical twinning is unimportant. Fine-grained synthetic dolomite deformed at high temperature (T ≥ 700??C) exhibits nearly linear (Newtonian) viscous behavior, with n = 1.28 (??0.15) consistent with grain boundary (Coble) diffusion creep. At low temperatures (T ≤ 700??C) coarse-grained dolomite exhibits higher strengths at higher temperatures which cannot be described by an Arrhenius relation, while fine-grained dolomite strengths show little or no temperature dependence. At high temperatures (T ≥ 800??C), dislocation creep of coarse-grained dolomite can be described by a thermally activated power law ⎟⎟⎠⎞⎜⎜⎝⎛−⎟⎟⎠⎞⎜⎜⎝⎛−=RTHno*31exp??σσεε&& with H*/n = 60 kJ/mol, or by an exponential law ()[]⎟⎟⎠⎞⎜⎜⎝⎛−−=RTHo*31expexpσσαεε&& with H*/α = 25447 kJ/mol. At high temperatures, diffusion creep of fine-grained synthetic dolomite can be described by ⎟⎟⎠⎞⎜⎜⎝⎛−⎟⎟⎠⎞⎜⎜⎝⎛−⎟⎠⎞⎜⎝⎛Ω=RTHdno*313exp??σσεε&& with H* = 280 ??45 kJ/mol. Taken together, the flow laws for coarse- and fine-grained dolomites constrain the high temperature conditions over which crystal plasticity, dislocation creep, and diffusion creep dominate.

Natural and hot isostatically pressed dolomite aggregates were experimentally deformed at effective pressures of Pe = 50 ?? 400 MPa, temperatures of 400 ?? 850??C, and strain rates of &#949;& = 1.2x10-4 s-1 to 1.2x10-7 s-1. Coarse- and fine-grained dolomite deformed at low temperature (T &#8804; 700??C for coarse-grained natural dolomite, T < 700??C for fine-grained natural and synthetic dolomite) exhibit mechanical behavior that is nearly plastic; differential stresses are insensitive to strain rate, fitted either by a power law no&#9119;&#9119;&#9120;&#9118;&#9116;&#9116;&#9117;&#9115;&#8722;=??&#963;&#963;&#949;&#949;31&& with n values that range from 12 to 49 or an exponential law ([31exp )] &#963;&#963;&#945;&#949;&#949;&#8722;=o&& with exponential law term &#945; values from 0.023 to 0.079 MPa-1. Microstructures of samples deformed at low temperatures include mechanical twins, and undulatory extinction suggesting that twin glide and dislocation slip are the predominant deformation mechanisms. At high temperatures (T &#8805; 800??C) flow strengths of coarse- and fine-grained dolomite depend more strongly on strain-rate and exhibit pronounced temperature dependencies. Microstructures of coarse-grained dolomite samples deformed at T &#8805; 800??C include undulatory extinction and fine recrystallized grains suggesting that recovery and dynamic recrystallization contribute to dislocation creep at these conditions. By comparison with lower temperature deformation, mechanical twinning is unimportant. Fine-grained synthetic dolomite deformed at high temperature (T &#8805; 700??C) exhibits nearly linear (Newtonian) viscous behavior, with n = 1.28 (??0.15) consistent with grain boundary (Coble) diffusion creep. At low temperatures (T &#8804; 700??C) coarse-grained dolomite exhibits higher strengths at higher temperatures which cannot be described by an Arrhenius relation, while fine-grained dolomite strengths show little or no temperature dependence. At high temperatures (T &#8805; 800??C), dislocation creep of coarse-grained dolomite can be described by a thermally activated power law &#9119;&#9119;&#9120;&#9118;&#9116;&#9116;&#9117;&#9115;&#8722;&#9119;&#9119;&#9120;&#9118;&#9116;&#9116;&#9117;&#9115;&#8722;=RTHno*31exp??&#963;&#963;&#949;&#949;&& with H*/n = 60 kJ/mol, or by an exponential law ()[]&#9119;&#9119;&#9120;&#9118;&#9116;&#9116;&#9117;&#9115;&#8722;&#8722;=RTHo*31expexp&#963;&#963;&#945;&#949;&#949;&& with H*/&#945; = 25447 kJ/mol. At high temperatures, diffusion creep of fine-grained synthetic dolomite can be described by &#9119;&#9119;&#9120;&#9118;&#9116;&#9116;&#9117;&#9115;&#8722;&#9119;&#9119;&#9120;&#9118;&#9116;&#9116;&#9117;&#9115;&#8722;&#9119;&#9120;&#9118;&#9116;&#9117;&#9115;&#8486;=RTHdno*313exp??&#963;&#963;&#949;&#949;&& with H* = 280 ??45 kJ/mol. Taken together, the flow laws for coarse- and fine-grained dolomites constrain the high temperature conditions over which crystal plasticity, dislocation creep, and diffusion creep dominate.

The thesis describes the EU/EC Twinning and TAIEX Instruments in the context of the management of these instruments by the Czech authorities and of the capacity for the Czech Republic to participate in these instruments as a 'service provider'. The main target is to provide the basic information about IPA/ENPI, Twinning and TAIEX, which should be further updated based on practical experience of the Czech experts involved in TW/TAIEX activities.

Crystals with a hexagonal close-packed crystal structure are inherently anisotropic, and have a limited number of independent slip systems, which lead to strong deformation textures and reduced formability in polycrystalline products. In magnesium (Mg), all of the easy slip systems have a Burgers vector in the <a> direction making twinning necessary for arbitrary shape changes. The most common twinning system which allows extension along the c-axis is {10-12}<10-11>. A good predictor of slip is the global Schmid factor, which resolves the externally applied force onto the slip plane and direction of a crystal. The critically resolved shear stress (CRSS) at which a grain twins is not readily measured by experiment and the CRSS for twin initiation often appears larger than for twin propagation. In polycrystals, twin variants with both low and high Schmid factors have been observed indicating that this Schmid factor is inappropriate to predict twinning and more local effects play an important, though still uncertain role. In this work, experiments were devised to dynamically study extension twinning in a polycrystalline Mg alloy AZ31B with a strong basal rolling texture by tensile deformation parallel to the plate normal. Three-dimensional X-ray diffraction using a synchrotron source was used to map the centre-of-mass positions, orientations, and grain-resolved elastic strain tensors of over 1000 grains in-situ up to a true strain of 1.4%. The majority of twins formed in grains with a high local Schmid factor; however, low-ranked twin variants were common. The average grain-resolved stress did not always select the highest twin variant and resulted in some negative Schmid factors. The internal stress state of parent grains and twinned grains did not differ significantly within the large measurement uncertainties. The misorientations between grains ideally oriented for twinning and their nearest neighbours could not explain cases of no twin activity. Results suggest that the controlling factors for twin formation are much more local and not captured within the spatial resolution of the technique. Complimentary measurement of the strain rate sensitivity during twinning, by instantaneous strain rate change tests, suggest that basal slip is a part of this local process. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-03-12 13:40:26.039

D.Sc. / Please refer to full text to view abstract

The research has been analyzed by using scanning electron and transmission electron microscopy (SEM and TEM).To research microstructure in single crystal alumina induced by Vickers indentation and the effect of alumina microstructure induced by deformation twinning. Sapphire is a kind of alumina single crystal called Corundum. It has fine machinery, optical, chemistry and anti-radiationary nature and been used widespread in industry world in recent years. Mechanical twinning is the main mode of plastic deformation. It has two types (1) basal twinning (2) rhombohedral twinning. When author pressed 20 N on (0001) plane, it produces B (0001), R1 ( 102) and R1 ¡¨ ( 104) in cross- section. But when pressing 20 N in (1 10) plane, it produces B (0001), B 1 ( 101) and R2 ( 012) in cross-section. In order to understand the role of pressure direction in twinning at room temperature, this research will discuss what effects will be produced twinning microstructure of alumina sample and derivation of dislocation microstructure by using scanning electron and transmission electron microscopy.

Molecular Dynamics (MD) and Molecular Statics (MS) simulations have always proven to be powerful tools to study material behavior at the lowest length scale. However, one of the greatest challenges in material modelling is the effective upscaling of relevant material properties from atomic scale to continuum scale. One such challenge is how best to capture the role of twinning in higher scale plasticity. Capturing twin nucleation in fullield crystal plasticity is a long-standing problem in materials science. The challenge resides mainly in the biased regional lattice transformation associated with twin formation in defiance of its obedience to a threshold stress law which could be fulfilled in regions where twinning is deferred. Hence, determining a favorable site for nucleation of a twin variant remains a daunting task. This dissertation is an attempt to understand twin nucleation in hexagonal close packed (HCP) metals using a technique called Nudged Elastic Band (NEB) method. NEB calculations are performed between two stable configurations and use a number of intermediate images to describe the transition. Results of calculations demonstrate that the role of stress and atomic structure in twin nucleation could be understood in terms of the minimum energy path, energy barrier, and relaxed energy. This method allows to gauge the transformations that the lattice could experience when evolving from a given thermodynamic state to a final state of a predefined twin embryo under various boundary conditions. This allows direct comparisons between various cases with respect to twin nucleation and hence provides a measure of the material plasticity. Such results are crucial in higher scale modelling of the material.

This thesis is focused on determining the structures of two anti-carbohydrate antibodies to understand how they achieve their specificity toward antigen. First, the structure of the antigen-binding fragment from the monoclonal antibody S64-4 in complex with a pentasaccharide bisphosphate fragment from chlamydial lipopolysaccharide (LPS) has been determined by x-ray diffraction to 2.6 Å resolution. Like the well-characterized antibody S25-2, S64-4 displays a pocket formed by the residues of germline sequence corresponding to the heavy and light chain V gene segments that binds the terminal Kdo (3-deoxy-α-D-manno-oct-2-ulopyranosonic acid) residue of the antigen; however, although S64-4 shares the same heavy chain V gene segment as S25-2, it has a different light chain V gene segment. The new light chain V gene segment codes for a combining site that displays greater avidity, different specificity, and allows a novel antigen conformation that brings a greater number of antigen residues into the combining site than possible in S25-2. Further, while antibodies in the S25-2 family use complementarity determining region (CDR) H3 to discriminate among antigens, S64-4 achieves its specificity via the new light chain V gene segment and resulting change in antigen conformation. These structures reveal an intriguing parallel strategy where two different combinations of germline-coded V gene segments can act as starting points for the generation of germline antibodies against chlamydial antigens and show how anti-carbohydrate antibodies can exploit the conformational flexibility of this class of antigens to achieve high avidity and specificity independently of CDR H3. Second, the structure of a rabbit, single chain variable fragment against terminal mannose-6-phosphate (Man6P) residues, termed scFv M6P-1, has been determined by x-ray diffraction to 2.7 Å resolution with Man6P in the binding site. The Man6P pathway is the predominant pathway that transports acid hydrolases from the trans-Golgi to endosomes. Newly synthesized hydrolases first require the generation of Man6P markers before they can be transported. Maintaining a full complement of hydrolases within lysosomes is essential as failure to do so results in a number of different lysosomal storage diseases. Due to its specificity, scFv M6P-1 is able to diagnose lysosomal storage diseases mucolipidosis II and mucolipidosis III. scFv M6P-1 is also able to purify Man6P containing proteins which may be useful for enzyme replacement therapies. Additionally, scFv M6P-1 is one of the first structures of an antibody fragment that exhibits high specificity for a single carbohydrate residue and is one of the first structures of a rabbit antibody fragment. The specificity of scFv M6P-1, which gives it these unique attributes, is revealed in the structure where multiple hydrogen bonds are seen between the antibody’s heavy chain and the mannose ring while two salt bridges are observed between the antibody’s light chain and the phosphate moiety. Finally, scFv M6P-1 binds in such a way as to allow binding to proteins possessing terminal Man6P residues. Crystallographic challenges that arose during this research included poor crystal growth as well as twinning and these are explored while the structure of scFv M6P-1 complex with Man6P is analysed. / Graduate / 0487 / 0982 / 0307 / dyl.w.evans@gmail.com