Thesis (Ph. D.)--Washington State University, August 2009. / Title from PDF title page (viewed on Sept. 9, 2009). "School of Mechanical and Materials Engineering." Includes bibliographical references (p. 180-188).

Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2005. / Includes bibliographical references (p. 27-28).

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (p. 138-144).

This thesis describes applications of the ac Josephson effect. Firstly, results are presented from bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Au multilayer external capacitor, to make a junction with a hysteretic current voltage characteristic operating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at 72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junction shunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measured capacitance was less than that expected from a calculation of the parallel plate shunt capacitance. The explanation was thermal noise suppression of the hysteresis and the junction saw the shunt capacitor as a distributed impedance rather than a lumped circuit element. It was found during these investigations that the influence of the SrTiO3 substrate on the intrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at 4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm was patterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. The dispersion relation consisted of two branches, one at low frequencies with a high resonator capacitance per unit length and a high frequency branch with a low resonator capacitance per unit length. This was due to the frequency dependence of the permittivity of bulk SrTiO3, which drops above the soft optic phonon frequency. From the dispersion relation, the permittivity of bulk SrTiO3 was 750 and the soft optic phonon frequency was 145GHz. The ac Josephson effect was exploited to measure the permittivity of thin films of SrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators. The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independent between 100GHz and 900GHz and to decrease with film thickness. The permittivity of the 50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nm film was tunable with a dc voltage bias between 245 and 112 at 30K and 116GHz. The grain boundary capacitance was used to probe grain boundary current transport. The capacitance per unit area scaled inversely with resistance area product and increased linearly with critical current density, for undoped and Ca doped YBa2Cu3O7-d grain boundaries on 24° bicrystals. This behaviour could not be explained by tunneling models of grain boundary current transport, and requires current flow over a fraction of the area of the grain boundary.

The thesis addresses two topics in the study of material properties as determined by the microstructure. The first topic involves percolation as a tool in relating the grain boundary structure to global properties such as fracture and corrosion resistance. The second investigates optimization techniques in order to find the space of values that properties of a material can take, from consideration of the microstructure. In part I, the applicability of standard lattice percolation models to a random 2-D grain structure is explored. A random network based on the triangle lattice is proposed as a more appropriate model, and results in a higher percolation threshold (0.711 compared with 0.653 for the standard hexagonal lattice). The triple junction constraint inherent in grain boundary structures is subsequently applied to the new network. This results in a lowering of the percolation threshold to 0.686, which turns out to be very close to the value obtained from the hexagonal lattice under the same constraint. In Part II, an efficient method for finding the closure of a bi-objective optimization problem involving two material properties is formulated. The method is based upon two algorithms developed to find the Pareto front in multi-objective problems — the weighted sum, and the normal boundary intersection methods. The resultant procedure uses quadratic programming (QP) to find as many points on the closure as possible, changing to the less efficient sequential quadratic programming (SQP) only where necessary to find points on concave, or almost concave, regions. Improvements on the method are demonstrated using extra linear constraints in the generalized weighted sum (GWS) algorithm, and multiple GWS trials at each stage. Optimization using only the linear part of the problem is shown to give excellent results for the particular example used, and may help to achieve an approximate closure more quickly for larger problems. An adaptation of a common Pareto front method from genetic algorithms — the maxmin algorithm — is also demonstrated. The efficiency of the method is found to be reasonable for finding closures in the test case. Other general optimization techniques for the form of the problem in-hand are explored for completeness of the study. These include a survey of unconstrained techniques that might be useful in large-scale problems, and an in-depth application of QP for the constrained case.

In an effort to understand intrinsic grain boundary brittleness, computer simulation of grain boundary structures in ordered NiAl was carried out. The considered boundaries were boundaries of low reciprocal coincident site density Σ. The structural unit model was used to obtain a general picture of the grain boundary features in NiAl. Relation between the grain boundary stoichiometry and the energy was studied and it was found that the grain boundary energy increases with a higher aluminum content in the grain boundary. Thus, excess of aluminum in the grain boundary seems to make the material brittle. The grain boundaries obtained are relatively dense and grain boundary brittleness would not be expected to originate from large interstitial holes. However, the structures computed show a large spread in energy values and it is proposed that the observed brittleness is due to the appearance of high energy structures when dislocations arrive at the boundary. / Master of Science

Theoretical and experimental studies of grain boundary structures were carried out to understand the grain boundary properties of Ni₃Al. Using various techniques of transmission electron microscopy, the geometry of grain boundaries was determined from a number of grain boundaries to obtain a distribution of grain boundary types in Ni₃Al. Five macroscopic parameters, including a misorientation between two grains and a grain boundary plane orientation, were considered in the grain boundary type. A distribution of grain boundary types in ductile Ni₃Al contained more low Σ boundaries than brittle Ni₃Al. In a distribution of grain boundary plane orientations, the trend of having high density plane of coincident sites was maintained on low Σ boundaries up to Σ = 9 and random orientations of grain boundary planes became dominant after Σ = 11. In ductile Ni₃Al, grain boundary accommodations of dislocations in the vicinity of grain boundaries were found, which were not shown in brittle Ni₃Al. Using CBED techniques, symmetry information of a single crystal was obtained. However, an extension to bicrystals was not successful. In order to study the possible disordering and segregation near grain boundaries, two different atomistic computer simulations, namely, molecular statics simulation and two dimensional lattice gas model, were carried out. Both results showed a similar trend of disordering near the grain boundary regardless of grain boundary composition and atomic interaction energy assumptions. However, segregation behavior near grain boundaries showed strong influences from grain boundary compositions and atomic interaction energy assumptions. / Ph. D.

not OCRd / Master of Science

A theoretical and experimental study of Grain boundary structure in B doped Ni₃Al was carried out. Geometrical modelling was done to obtain the grain boundary structure in Ll₂ compounds for different misorientations between two grains. The size of interstitial sites in these boundaries was calculated to understand the segregation of B to the boundaries. Ratios of the B-B equilibrium interatomic distance to the nearest neighbour distances in various (A₃B) Ll₂ compounds were calculated, in an attempt to predict the grain boundary fracture behaviour in these compounds. Selected area channeling patterns were obtained from a number of grains to solve for the orientation relationship between grains, in order to predict the structure of the boundaries between them. A distribution of grain boundary structure that is different from the distribution for an undoped Ni₃Al is obtained in this work and is compared with results from other investigators. / M.S.

Depuis des décennies, le rôle joué par les joints de grains dans le comportement plastique des matériaux cristallins interroge mécaniciens, métallurgistes, spécialistes des matériaux. Après les avancées décisives de la plasticité individuelle des cristaux et de leur comportement collectif au sein d’agrégats, les effets complexes des interfaces sur le comportement effectif constituent l’enjeu restant à maîtriser. Car si les joints peuvent aussi bien être sites d’endommagement que réseau de consolidation dans une structure polycristalline, ils contribuent toujours fortement aux propriétés macroscopiques de cette dernière. Le travail réalisé est une contribution à l’étude du comportement des joints lors de traitements thermomécaniques induisant leur mobilité. On parle de migration.Nous avons étudié les caractéristiques essentielles de la migration de joints de grains à partir du mécanisme de SIBM (croissance de grains sans germination après faible déformation plastique). La migration des joints de grains sous recuit après déformation a été suivie in situ en MEB. Les évolutions microstructurales sont analysées en MEB et en AFM. Les principales forces et caractéristiques contribuant à la migration des joints sont étudiées. Pour les forces, un diagramme de migration reliant le déplacement des joints à l’énergie de courbure et à l’énergie stockée par déformation plastique a permis d’évaluer la pertinence de l’expression usuelle de la force motrice de migration et d’en discuter de possibles ajustements. Pour les caractéristiques, nous avons confirmé l’effet de surface de la gravure thermique et celui plus intrinsèque des jonctions triples sur la mobilité des joints de grains. / For many decades, the role of grain boundaries in the plastic behavior of crystalline materials questions engineers, metallurgists, materials scientists. After the decisive progress in plasticity of individual crystals and for their collective behavior in aggregates, the complex effects of boundaries on the actual behavior are the remaining challenge to master. If boundaries can as well be considered as damage sites as strengthening network in a poly-crystalline structure, they always contribute greatly to the macroscopic properties of the latter. This work is a contribution to the study of the behavior of boundaries during thermomechanical treatments inducing mobility. We talk about migration.We studied the essential characteristics of the migration of grain boundaries from SIBM mechanism (Strain Induced Boundary Migration). The migration of grain boundaries under annealing after deformation was monitored in situ SEM. Microstructural changes are analyzed by SEM and AFM. The main strengths and characteristics contributing to the boundary migration are studied. For strengths, a migration diagram linking the displacements of grain boundaries to the curvature energy and the stored energy from plastic deformation allowed to assessing of the relevance of the usual expression for the migration driving force and to discussing possible adjustments. For characteristics, we confirmed the surface effect of the thermal grooving and the more intrinsic one of triple junctions on the mobility of grain boundaries.