Topological analysis of the grain boundary space

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from PDF version of thesis.Includes bibliographical references (p. 117-125).Grain boundaries and their networks have a profound influence on the functional and structural properties of every class of polycrystalline materials and play a critical role in structural evolution and phase transformations. Recent experimental advances enable a full crystallographic characterization, including the boundary misorientation and inclination parameters, of grain boundaries. Despite these advances, a lack of appropriate analytical tools severely undermines our ability to analyze and exploit the full potential of the vast amounts of experimental data available to materials scientists. This is because the topology of the grain boundary space is unknown and even a well-studied part of the complete grain boundary space, the misorientation space, is relatively poorly understood. This thesis summarizes efforts to improve the representation of misorientation information and to understand the topology of the complete grain boundary space. First, the topology of the space of misorientations is discussed with a focus on the effect of symmetries on the minimum embedding dimensions in Euclidean space. This opens the door to a new method of representation of misorientation information in which grain boundaries can be uniquely colored by their misorientations. Second, conditions under which the topology of the grain boundary space has been resolved are presented. Resolving the topology of the complete grain boundary space not only facilitates statistical analysis of grain boundaries, but can also help describe the structure-property relationships of these interfaces.by Srikanth Patala.Ph.D

Basis functions on the grain boundary space: Theory

With the increasing availability of experimental and computational data concerning the properties and distribution of grain boundaries in polycrystalline materials, there is a corresponding need to efficiently and systematically express functions on the grain boundary space. A grain boundary can be described by the rotations applied to two grains on either side of a fixed boundary plane, suggesting that the grain boundary space is related to the space of rotations. This observation is used to construct an orthornormal function basis, allowing effectively arbitrary functions on the grain boundary space to be written as linear combinations of the basis functions. Moreover, a procedure is developed to construct a smaller set of basis functions consistent with the crystallographic point group symmetries, grain exchange symmetry, and the null boundary singularity. Functions with the corresponding symmetries can be efficiently expressed as linear combinations of the symmetrized basis functions. An example is provided that shows the efficacy of the symmetrization procedure

Molecular insights on the solvent effect of methanol additive in glycine polymorph selection

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (p. 79-85).In an effort to improve control and design in organic crystallization, the effect of solvent on polymorph selection has gained tremendous interest in recent years. In this thesis, molecular simulation techniques are used to gain insight into the solvent effect on glycine crystallization in water-methanol mixtures. We report the validation of the Optimized Potential for Liquid Simulations (OPLS) force field and parameters with modified Lennard-Jones parameters for hydrogens attached to a-carbon in glycine zwitterion. Solution and interface simulations in water and 50% v/v water-methanol solutions reveal the mechanism through which methanol additive results in the crystallization of the least stable [Beta]-glycine polymorph. Free energy calculations through the Umbrella Sampling method show an increased stability of the centrosymmetric dimer structure ([alpha]-glycine growth unit) in the presence of the methanol additive. Even though the dimer structure is more stable in water-methanol mixtures, a higher fraction of glycine monomers were observed in water-methanol mixtures. It is revealed through thermodynamic arguments that a drastic decrease in solubility results in a higher fraction of glycine monomers in water-methanol mixtures. It was hypothesized in previous studies that the presence of monomer units docking onto the (010) interface of [alpha]-glycine inhibits further growth due to exposed ammonium groups at the interface. The effect of solvent on crystal growth inhibition is explored by the interface simulations of a-glycine in water-methanol mixtures. When the monomer units are docked onto the interface, water is shown to be more effective than methanol in inhibiting crystal growth of (010) interface of [alpha]-glycine. This study sheds light on the role played by the solvent on glycine polymorph selection in water-methanol solutions.by Srihanth Patala.S.M

A high-throughput technique for determining grain boundary character non-destructively in microstructures with through-thickness grains

Grain boundaries (GBs) govern many properties of polycrystalline materials. However, because of their structural variability, our knowledge of GB constitutive relations is still very limited. We present a novel method to characterise the complete crystallography of individual GBs non-destructively, with high-throughput, and using commercially available tools. This method combines electron diffraction, optical reflectance and numerical image analysis to determine all five crystallographic parameters of numerous GBs in samples with through-thickness grains. We demonstrate the technique by measuring the crystallographic character of about 1,000 individual GBs in aluminum in a single run. Our method enables cost- and time-effective assembly of crystallography–property databases for thousands of individual GBs. Such databases are essential for identifying GB constitutive relations and for predicting GB-related behaviours of polycrystalline solids.United States. Department of Energy. Office of Basic Energy Sciences (award no DE-SC0008926)MIT International Science and Technology InitiativesNational Science Foundation (U.S.) (grant DMR-1003901