Static abnormal grain growth (SAGG) was studied in Al-Mg alloy AA5182 sheet by varying four processing parameters: deformation temperature, strain rate, annealing temperature, and annealing time. SAGG is a secondary recrystallization process related to geometric dynamic recrystallization (GDRX) and requires both deformation at elevated temperature and subsequent static annealing. A minimum temperature is required for both SAGG and GDRX. Recrystallized grains only develop at strains larger than the critical strain for SAGG, [epsilon]SAGG. The size of the recrystallized grains is inversely related to and controlled by the density of SAGG nuclei, which increases as local strain increases. The results of this study suggest that SAGG is controlled by two thermally-activated mechanisms, dynamic recovery and recrystallization. During deformation, dynamic recovery increases as deformation temperature increases or strain rate decreases, increasing the critical strain for SAGG. SAGG is subject to an incubation time that decreases as annealing temperature increases. SAGG can produce grains large enough to reduce yield strength by 20 to 50 percent. The results of this study suggest strategies for avoiding SAGG during hot-metal forming operations by varying processing conditions to increase [epsilon]SAGG. / text

The structure and properties of chemical vapor deposited zinc sulfide (CVD ZnS) were assessed before and after heat treatments, involving different annealing and hot isostatic pressing (HIPing) profiles. Samples were characterized using optical microscopy, SEM, TEM, electron diffraction, polycrystalline and powder x-ray diffraction, x-ray chemical microanalysis, photoluminescence, ultraviolet through longwave infrared transmission, and mechanical testing. Before heat treatment, CVD ZnS consists of lamellar twinned structures in 10 to 100 nm layers aggregated into domains which compose grains typically 5 to 10 μm in diameter with an overall crystallographic texture on the {100} planes. The scattering behavior of CVD ZnS was investigated and described by a surface scattering model based on internal surface roughness and refractive index variations due to onedimensional stacking disorder. The two to five percent hexagonality measured by x-ray diffraction is believed to form due to oxygen impurities at the twin boundaries which cause nanostructural polytypism and result in differential refractive index and scattering. CVD ZnS variants in low temperature deposited red ZnS and sulfur precursor elemental ZnS are examined as well. Color in CVD ZnS is believed to be due to band edge position, probably due to oxygen content, and not directly related to the hydride absorption at 6 μm. After annealing or hot isostatic pressing above 850 °C for sufficient time, CVD ZnS recrystallizes and becomes strongly textured on the {111} planes. This recrystallization is required to remove stacking disorder, resulting in a structure with less than half a percent hexagonality and low visible scattering. The recrystallization is believed to proceed by diffusing the oxygen at the nano-twin boundaries back into the lattice, thus unpinning the boundaries and allowing them to move and grow into the tabular recrystallized morphology by polytype induced exaggerated grain growth. The presence of active metals like platinum, silver, copper, or nickel during hot isostatic pressing causes a reaction with sulfur and lowers the temperature required for recrystallization. The optical scattering model is consistent in describing standard CVD ZnS, elemental ZnS, and multispectral recrystallized ZnS as having successively lower birefringence at internal surfaces.

No description available.

Microstructural evolution in a naturally-aged and cold-rolled AA6451 aluminum alloy during a non-isothermal annealing process, which leads to significant grain refinement, is investigated through: (a) conducting a comprehensive experimental analysis and (b) developing a computational modeling technique. The underlying mechanisms of annealing have been investigated through analysing interactive phenomena between precipitation and concurrent recovery and recrystallization. It is shown that the interactions between solute elements, clusters, and fine precipitates with dislocations restrict dynamic and static recovery during deformation and subsequent annealing. Inhibition of recovery favours recrystallization that initiates at 300oC and progresses through a nucleation and growth mechanism. Despite localized inhomogeneities, nucleation mainly occurs in non-recovered high energy sites which are uniformly distributed within the entire structure. Growth of the recrystallized nuclei is restricted by pinning precipitates that undergo a concurrent coarsening process. The fine, uniform distribution of recrystallized nuclei and their limited growth result in the formation of a fine-grained microstructure, after completion of recrystallization. The acquired knowledge has been used to develop a computational modeling technique for simulating microstructural evolution of the alloy. Microstructural states are simulated by integrating analytical approaches in a Monte Carlo algorithm. The effects of deformation-induced and pre-existing inhomogeneities, as well as precipitate coarsening and grain boundary pinning on the competitive recovery-recrystallization process are included in the simulation algorithm. The developed technique is implemented to predict the microstructural evolution during isothermal and non-isothermal annealing of AA6xxx sheets. A good quantitative agreement is found between the model predictions and the results from the experimental investigations.

The present research program is a renewed attempt at explaining the transformation mechanisms. The experimental investigations has focused on a model analysis of transformations which occur in single crystals, with stable orientations, i.e. Goss{110}<001> and brass{110}<112>, the deformation is carried out by channel-die compression to simulate the rolling process of thin sheets. Next, the samples were annealed at temperatures of primary recrystallization. The analysis of crystallographic transformations was conducted on metals from a wide spectrum of stacking fault energy: low - Cu-2%Al, average- Cu and Ni to high Al and Al-1%Mn. At work were analyzed the mechanisms controlling the initial stages of recrystallization. Detailed analysis of disorientation across the recrystallization front clearly showed that the initial grain orientations were not accidental. The axes of disorientation in the relationship across the front of recrystallization were near normal in {111} planes, but only sporadically covered with the <111> direction. The distribution of the recrystallization angle rotation in relation to the preferences presented through the formation of two maxima values near 30 ° and 45-55 °.

Antifreeze proteins (AFPs) are produced by a variety of organisms to either protect them from freezing or help them tolerate being frozen. Recent structural work has shown that AFPs bind to ice using ordered surface waters on a particular surface of the protein called the ice-binding site (IBS). These 'anchored clathrate' waters fuse to particular planes of an ice crystal and hence irreversibly bind the AFP to its ligand. An AFP isolated from the perennial ryegrass, Lolium perenne (LpAFP) was previously modelled as a right-handed beta helix with two proposed IBSs. Steric mutagenesis, where small side chains were replaced with larger ones, determined that only one of the putative IBSs was responsible for binding ice. The mutagenesis work also partly validated the fold of the computer-generated model of this AFP. In order to determine the structure of the protein, LpAFP was crystallized and solved to 1.4 Å resolution. The protein folds as an untwisted left-handed beta-helix, of opposite handedness to the model. The IBS identified by mutagenesis is remarkably flat, but less regular than the IBS of most other AFPs. Furthermore, several of the residues constituting the IBS are in multiple conformations. This irregularity may explain why LpAFP causes less thermal hysteresis than many other AFPs. Its imperfect IBS is also argued to be responsible for LpAFP's heightened ice-recrystallization inhibition activity. The structure of LpAFP is the first for a plant AFP and for a protein responsible for providing freeze tolerance rather than freeze resistance. To help understand what constitutes an IBS, a non-ice-binding homologue of type III AFP, sialic acid synthase (SAS), was engineered for ice binding. Point mutations were made to the germinal IBS of SAS to mimic key features seen in type III AFP. The crystal structures of some of the mutant proteins showed that the potential IBS became less charged and flatter as the mutations progressed, and ice affinity was gained. This proof-of-principle study highlights some of the difficulties in AFP engineering. / Thesis (Ph.D, Biochemistry) -- Queen's University, 2012-07-18 15:24:42.082

No description available.

Neutral diorganotin compounds of the 1,3-dithiole-2-thione-4,5-dithiolate ligand have been prepared. The syntheses of Me₂Sndmit, Et₂Sndmit, Bu₂Sndmit, (C₈H₁₇)₂Sndmit, (C₁₀H₂₁)₂Sndmit and (C₁₄H₂₉)₂Sndmit are described. For the purposes of indicating the formation of different structural phases, D.S.C. powder patterns are reported for initial and recrystallised samples of Me₂Sndmit, Et₂Sndmit and Bu₂Sndmit. Since Et₂Sndmitshowed different tin environments by solid phase state N.M.R. and clearly different powder patterns after recrystallisation, the compound was recrystallised from various solvents in an attempt to determine how many crystalline forms exist. To date, two forms have been identified: orthorhombic and monoclinic. The single crystal X-ray structure analyses of these are described. It has also been shown that upon heating a transition from the orthorhombic to the monoclinic form occurs at 140°C. The crystal strict of Me₂Sndmit has also been determined and is reported, along with Mossbauer parameters for Me₂Sndmit and Me₂Sndmio (dmio = 1,3-dithiole-2-one-4,5-dithiolate). Anionic organotin bis-dmit complexes of the form [RSn(dmit)₂][Q]⁺ have also been prepared, with a view to investigating their electrochemical properties. The syntheses of complexes with R chain length ranging from 4 to 18 carbons are described. The problem of formation of [Sn(dmit)₃][Q]₂ is also discussed. D.S.C. curves and cyclic voltammograms for [C₁₄H₂₉Sn(dmit)₂][C₁₄H₂₉NMe₃] and [NEt₄][(dmit)₂SnC₁₀H₂₀Sn(dmit)₂][NEt₄] are also described. Chiral macrocycles have potential uses as catalysts for asymmetric bond-forming reactions and as selective to metal ions. A chiral macrocyclic derivative of dmit was synthesised from methyl-4,6-<I>O</I>-benzylidene-2,3-bis-<I>O</I>-[(2-iodoethoxyethyl)-ethyl]-α-D-glucopyranoside. Since reaction of this with dmit proved to be unexpectedly problematic, attempted reactions of the sugar derivative with Na₂dmit<I> in situ</I>, isolated Na₂dmit, [Zn(dmit)₂][NEt₄]₂, [Ph₂Sn(dmit)I][NEt₄] and Cs₂dmit are described.

The goal of our research is to generate cryopreservation agents derived from antifreeze glycoproteins. One postulated mechanism of cell cryo-injury is ice recrystallization. It is known that simple saccharides and cryopreservation agents (DMSO) display ice recrystallization inhibition (IRI). This study assessed the cytotoxicity and cryopreservation ability of these sugars in relation to their IRI. It was determined that compounds with greater IRI have increased cytotoxicity yet confer cryoprotection. To further investigate how structure is affecting IRI activity, several galactopyranoside derivatives were synthesized. A series of deoxy and α-Callyl- deoxy galactopyranoses were prepared. Testing determined that removal of any hydroxyl group removes IRI. 3-deoxy-β-thiophenyl galactose was also synthesized and had surprisingly better IRI than β-thiophenylgalactose. Also, 6-azido galactose had similar IRI to 6-deoxy galactose. Lastly, a series of β- thioalkylgalactosides was synthesized and testing gave contradicting results which suggest that predicting IRI based on hydrophilicity is more complicated than initially hypothesized.

The cryopreservation of cells and tissues requires the addition of a cryoprotectant in order to prevent cellular damage caused by ice. Unfortunately, common cryoprotectants such as DMSO and glycerol exhibit significant toxicity which makes their use unfeasible for many clinical procedures. Our laboratory is interested in the development of alternative, non-toxic cryoprotectants which possess ice recrystallization inhibition (IRI) activity. Potent IRI activity has recently been discovered in certain small molecules, but the structural features required for this process are unclear. Herein we report the development of a library of O-aryl glycosides in order to probe the importance of electron density and hydrophobic moieties for IRI activity. It was found that the degree of electron density at the anomeric oxygen does not correlate with IRI ability in para-substituted aryl glycosides, nor does changing the position of the aryl substituent impart a predictable effect on activity. However, the addition of hydrophobic alkyl or acyl chains was beneficial for IRI activity; generally, increasing chain length was found to correlate with increasing activity. In some instances, an optimal alkyl chain length was identified, after which continued lengthening results in a loss of potency. We conclude from this study that a certain extent of hydrophobic character is beneficial for the IRI activity of aryl glycosides, and that a balance between hydrophobicity and hydrophilicity is required for optimum IRI ability. It is hoped that these findings will aid future efforts towards the rational design of novel cryoprotectants.