Understanding the microstructure and fatigue behavior of magnesium alloys

This thesis is mainly focused on the study of magnesium alloy’s fatigue behavior. The experimental material is obtained from racing wheels. Racing wheels manufactured from metal are cast, forged, machined, or manufactured from a combination of these methods. For many years, aluminum was chosen as the material of racing wheels because it is relatively convenient to manufacture and more economical. However aluminum wheels are relatively heavy because of the density to strength ratio of the element. Therefore it is necessary to choose a lighter and stronger material for solving this problem. Introducing magnesium alloy would be a recommended way as magnesium alloy wheels can be used on racing vehicles. However, magnesium alloy wheels have some drawbacks. First of all, magnesium is relatively expensive. In addition, magnesium is a quite brittle metal due to the fact that it does not have five different slip systems available at room temperature and it does not satisfy Von Mises criterion for full plasticity. There would be considerable risks involved with failure if it is not being designed and manufactured to provide adequate strength. This study will cover the microstructure of magnesium alloys and the fatigue properties of magnesium. The specimens are mainly provided by Dymag who manufacture magnesium racing wheels. The microstructure has been examined by optical microscopy. Electron imaging methods such as Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD) have been used to characterize the microstructure. Fatigue testing has been performed by a rotating-bending test machine (or Gill machine). The fatigue fracture was also been examined by electron microscopy to determine the fatigue mechanism. The results show that fatigue behavior differs through the whole wheel cross section. After microstructures of different locations of the specimen have been examined by microscopy, it is found that these differences were caused by manufacture method. Therefore, future works will mainly focus on techniques to improve the fatigue behavior. In order to further understand the mechanical performance of the magnesium alloys studied here, the properties of the different phases in the microstructure have been determined by nanoindentation testing. This has allowed the Young’s modulus and hardness of the α-Mg and β-Mg17Al12 phases to be determined. The results are discussed in terms of the implications for the fatigue performance of the material

Luminescent Main-Chain Organoborane Polymers: Highly Robust, Electron-Deficient Poly(oligothiophene borane)s via Stille Coupling Polymerization

A series of polymers (<b>PB<i>n</i>T</b>, <i>n</i> = 2–5) with boron atoms incorporated into the conjugated polythiophene main chain have been prepared via Pd-catalyzed coupling of stannylated thienylborane monomers. The polymers exhibit excellent long-term chemical stability to air and moisture and remarkable thermal stability with decomposition temperatures reaching over 300 °C. The high stability is achieved by placing very bulky pendant groups, 2,4,6-tri-<i>tert</i>-butylphenyl (Mes*) and 2,4,6-tris­(trifluoro­methyl)­phenyl (^FMes), on boron that prevent attack by nucleophiles. All these polymers display strong absorptions in the visible region and intense fluorescence in both solution and the solid state with quantum yields of up to 38% and fast radiative decay constants (<i>k</i>_r) of up to 3.3 × 10⁸ s^–1. Density functional theory (DFT) studies on diborylated oligothiophene model compounds suggest that the strong absorption of the polymers results from π–π* transitions on the oligothiophene borane main chain with significant charge transfer to boron. The unusually intense luminescence in the solid state is favored by the rigid planar skeleton and steric shielding of the bulky pendent groups. The emission color can be tuned from blue to deep orange by varying the length of the π-conjugated oligothiophene spacer between the boron atoms. Spectroelectrochemical studies on a dimeric model compound in THF solution reveal reversible two-step reductions to give highly colored species, while the corresponding polymeric material precipitates at higher potentials after undergoing an initial reversible reduction. The LUMO energy levels of the polymers can be effectively lowered by introduction of electron-withdrawing pendent groups on boron, affording a versatile approach for development of electron-deficient boron-containing polymers with controllable electronic structures and photophysical properties. The facile modular synthetic approach combined with the exceptional stability opens the door to broad adoption of electron-deficient organoboranes in conjugated materials design and development

Direct Yeast Surface Codisplay of Sequential Enzymes with Complementary Anchor Motifs: Enabling Enhanced Glycosylation of Natural Products

Yeast surface display is an appealing technique for constructing multienzyme cascades. This technique is commonly achieved using a scaffold for the ordered arrangement of various enzymes. However, this method is typically complicated because scaffold use may engender extra metabolic burden on the cell host. Here, we established a direct yeast surface codisplay strategy by employing two complementary anchor motifs, Agα1 and Pir1. These motifs allow for the codisplay of sequential uridine diphosphate-glycosyltransferase (UGT) and sucrose synthase (SUS) on the surface of Pichia pastoris (syn. Komagataella phaffii) for the glycosylation of natural products. We manipulated the displayed stoichiometry, amount, and assembly order of UGT and SUS by coupling them with anchor motifs. Furthermore, their effect on enzyme activity was thoroughly investigated. The surface-codisplayed strain UGT-Pir-SUS-Agα exhibited greater thermostability than the single-displayed strains and their free counterparts. Moreover, the strain UGT-Pir-SUS-Agα was successfully applied to glycyrrhetinic acid (GA) glycosylation to produce GA-3-O-Glc, with sucrose being the sugar donor in this process. This generated 7.5- to 20- and 5.3-fold higher GA-3-O-Glc concentration compared with the free counterparts (enzyme mass loading of 20-fold in excess) and mixed single-displayed strains of UGT-Agα and SUS-Pir, respectively. This increase was due to the improved biochemical properties and substrate channeling effect of strain UGT-Pir-SUS-Agα. This controllable direct surface codisplay strategy, based on complementary anchor motifs, is readily extendable to other enzyme cascades

Polymorphisms in <em>CTLA4</em> Influence Incidence of Drug-Induced Liver Injury after Renal Transplantation in Chinese Recipients

<div><p>Genetic polymorphisms in cytotoxic T lymphocyte-associated antigen 4 (CTLA4) play an influential role in graft rejection and the long-term clinical outcome of organ transplantation. We investigated the association of 5 CTLA4 single-nucleotide polymorphisms (SNPs) (rs733618 C/T, rs4553808 A/G, rs5742909 C/T, rs231775 A/G, and rs3087243 G/A) with drug-induced liver injury (DILI) in Chinese renal transplantation (RT) recipients. Each recipient underwent a 24-month follow-up observation for drug-induced liver damage. The CTLA4 SNPs were genotyped in 864 renal transplantation recipients. A significant association was found between the rs231775 genotype and an early onset of DILI in the recipients. Multivariate analyses revealed that a risk factor, recipient rs231775 genotype (p = 0.040), was associated with DILI. Five haplotypes were estimated for 4 SNPs (excluding rs733618); the frequency of haplotype ACGG was significantly higher in the DILI group (68.9%) than in the non-DILI group (61.1%) (p = 0.041). In conclusion, CTLA4 haplotype ACGG was partially associated with the development of DILI in Chinese kidney transplant recipients. The rs231775 GG genotype may be a risk factor for immunosuppressive drug-induced liver damage.</p> </div

Synthesis, Structures, and Hydroboration of Oligo- and Poly(3-alkynylthiophene)s

3-Alkynyl-substituted terthiophenes and polythiophenes were synthesized, and their properties and behavior toward hydroboration with Mes₂BH were investigated. The alkynyl-substituted terthiophene was found to crystallize in an intriguing layered structure that mimics an interdigitated polymer wherein the terthiophenes form tightly π-stacked linear pseudopolymer strands. The heptynyl side chains of neighboring stacks interlock ideally, almost completely filling the void in-between substituents and thus allowing for minimal spacing between neighboring pseudopolymer strands. Poly­(3-alkynylthiophene)­s were accessed through Grignard metathesis polymerization (GRIM) of 2,5-dibromo-3-heptynylthiophene, which yielded polymers of moderate molecular weights (<i>M</i>_n = 6.3; PDI = 1.73) in yields of up to 82%. Further modification by hydroboration with Mes₂BH resulted in a material that is partially functionalized with vinylborane groups and as a result exhibits charge transfer bands in the UV–vis spectra. A single-crystal X-ray structure of the corresponding hydroborated terthiophene species shows extensive intermolecular interactions, resulting in π-stacks of π-dimers, despite the steric bulk of the dimesitylborane moieties

The distribution of haplotypes in 5 locus of CTLA-4 between bacterial infection and non- bacterial infection.

<p>The distribution of haplotypes in 5 locus of CTLA-4 between bacterial infection and non- bacterial infection.</p

The allele distribution of <i>CTLA4</i> polymorphisms in patients with viral infection and non-viral infection.

<p>OR: odds ratio, CI: confidence intervals.</p

The genotype distribution of the <i>CTLA4</i> polymorphisms in patients with viral infection and non-viral infection.

*<p>Fisher's Exact Test;</p><p>OR: odds ratio, CI: confidence intervals.</p

The distribution of haplotypes in 5 locus of CTLA-4 between viral infection and non-viral infection.

<p>The distribution of haplotypes in 5 locus of CTLA-4 between viral infection and non-viral infection.</p

The genotype distribution of the <i>CTLA4</i> polymorphisms in patients with bacterial infection and non-bacterial infection.

*<p>Fisher's Exact Test;</p><p>OR: odds ratio, CI: confidence intervals.</p