Development of low energy positron beams and their application to the study of the surface region of metals

Two low energy positron beams have been designed, built and their performance characteristics evaluated. Both are magnetically guided systems using a combination of a solenoid and Helmholtz coils. Slow positrons are produced by the moderation of fast positrons, from a 22Na source, in annealed polycrystalline tungsten mesh with efficiencies greater than 2 x 10-4. The original beam, built to high vacuum specifications, has been incorporated into a fully automated microcomputer controlled Doppler-broadening spectrometer system. The newer beam line is built, to UHV specifications, into a liquid helium cryostat. It also has a vertical geometry making the whole system far more versatile and allowing the study of liquid surfaces. Doppler-broadening analysis has been applied to measurements taken using both beams applied to pure metals (Mo and Ga). A two-state model incorporating diffusion of thermal positrons back to the surface has been found to be inadequate at low incident positron energies. In this regime epithermal positron and positronium emission is significant. A model of simple back-scattering of epithermal positrons was found to be successful in fitting the experimental lineshape parameters. Both fast and slow positron techniques have been applied to the study of inert gas precipitates in metals. Using conventional methods a detailed Doppler-broadening study has been carried out on the anealing of bulk Cu samples containing 3 atomic % Kr in the form of a high concentration of solid precipitates at 300K. Melting of the Kr and bubble growth are clearly seen. Deconvolution of the annihilation lineshape indicates that positrons are trapped at the Cu-Kr interface.Slow positrons have been used to profile the defect distribution of Mo implanted with a high dose of Kr ions. Using the simplest model of a step-function distribution there is reasonable agreement with the expected Kr profile.<p

Nano-scale behavior of irradiated nano-structured alloys

Future fast neutron fusion and fission nuclear systems will be subjected to levels of radiation damage from fast neutrons which is significantly higher than the current generation of nuclear power stations. This will require innovative materials solutions to allow long term mechanical stability of reactors. One proposed class of materials are nanostructured alloys where the large number of interfaces allow for recombination defects and reduce the degree of radiation hardening seen. However their response under irradiation has not thoroughly been studied. In this work, two irradiated nanostructured alloys have been studied W-5%Re in both a nanostructured and annealed variant and a novel Hf-Ti-Ta-V-Zr high entropy alloy. I will outline the benefits nanostructured materials offer under irradiation and some of the problems and challenges in measuring their mechanical properties after irradiation and relating this to the nano-structure using XRD, TEM, HR-EBSD and atom probe tomography. Rolled tungsten 5 wt% rhenium sheet was studied in two microstructural variants: (a) as received with a high dislocation density (mean value of 1.4×1014lines/m2), measured using HR-EBSD, and pancake shaped grains with a thickness of≈200nm and (b) annealed at 1400oC for 24 hours to produce equiaxed grains with average grain size of ≈90 µm and low dislocation density (with a mean value of 4.8×1013 lines/m2). Both materials were ion implanted with 2MeV W+ ions at 300oC to damage levels from 0.07, to 33 displacements per atom (dpa). Nanoindentation was used to measure the change in hardness after implantations. Irradiation induced hardening saturated in the as-received material at an increase of 0.4dpa from the unimplanted hardness of 8GPa at 0.4dpa. In the annealed material saturation does not occur by 13dpa and the hardness change of 1.3GPa from the unimplanted hardness of 6.2GPa was over four times higher. At 33dpa both material types showed a further increase in hardening. In these samples Atom probe tomography showed clustering of Re in ≈4nm precipitates with a rhenium concentration of ≈11%. In both cases the number density and volume fraction are similar at ≈3100 x1000/µm3 and volume fraction of ≈13%. These differences in radiation response are likely to be due to the high damage sink density in the as-received microstructure in the form of dislocation networks, as even in the as-received material the average grain size is too large to provide sufficient sinks. Initially this provides a large sink network for radiation damage resulting in less hardening in the rolled material. However at 33dpa the formation of rhenium clusters occurs at similar levels in both material conditions. These dominate the hardening mechanisms and result in secondary hardening at high damage levels. The difficulties in extracting hardness values from 200nm deep ion implanted layers will be discussed, with reference to minimizing the influence of the substrate material and how changes in pile up effects in irradiated materials can change mechanical responses, and proposed methods to minimize these. High entropy alloys have been proposed as potential nuclear materials as high configurational entropy may provide resistance to radiation damage. We have produced a novel high entropy alloy (Hf-Ti-Ta-V-Zr) in which is single phase on casting but two high entropy phases (one bcc and one hcp) are produced during heat treatment. This material then has a nano-lamella structure with an average lamella thickness of 200nm. Samples of the as cast single phase material, the dual phase high entropy alloy and single crystal vanadium were ion irradiated with V+ ions at 300oC to a dose of 5e14 ions/cm2. In the vanadium control samples the hardness as measured using CSM-nanoindentation was seen to increase from 2GPa in the unimplanted condition to 3.5GPa in the ion irradiated condition. The high entropy alloy in both the as cast and heat treated condition showed no increase in hardness after irradiation, demonstrating the intrinsic resistance to radiation damage of HEA’s. These studies show the ability of nanostructured alloys to have improved irradiation hardening resistance over conventional alloys. However challenges still remain in the production of large scale engineering components in such materials

Using coupled micropillar compression and micro-Laue diffraction to investigate deformation mechanisms in a complex metallic alloy Al13Co4

In this investigation, we have used in-situ micro-Laue diffraction combined with micropillar compression of focused ion beam milled Al13Co4 complex metallic alloy to study the evolution of deformation in Al13Co4. Streaking of the Laue spots showed that the onset of plastic flow occured at stresses as low as 0.8 GPa, although macroscopic yield only becomes apparent at 2 GPa. The measured misorientations, obtained from peak splitting, enabled the geometrically necessary dislocation density to be estimated as 1.1 x 1013 m-2

Data on a new beta titanium alloy system reinforced with superlattice intermetallic precipitates.

The data presented in this article are related to the research article entitled "a new beta titanium alloy system reinforced with superlattice intermetallic precipitates" (Knowles et al., 2018) [1]. This includes data from the as-cast alloy obtained using scanning electron microscopy (SEM) and x-ray diffraction (XRD) as well as SEM data in the solution heat treated condition. Transmission electron microscopy (TEM) selected area diffraction patterns (SADPs) are included from the alloy in the solution heat treated condition, as well as the aged condition that contained < 100 nm B2 TiFe precipitates [1], the latter of which was found to exhibit double diffraction owing to the precipitate and matrix channels being of a similar width to the foil thickness (Williams and Carter, 2009) [2]. Further details are provided on the macroscopic compression testing of small scale cylinders. Of the micropillar deformation experiment performed in [1], SEM micrographs of focused ion beam (FIB) prepared 2 µm micropillars are presented alongside those obtained at the end of the in-situ SEM deformation as well as videos of the in-situ deformation. Further, a table is included that lists the Schmidt factors of all the possible slip systems given the crystal orientations and loading axis of the deformed micropillars in the solution heat treated and aged conditions

Structural and functional insight into human O-GlcNAcase

O-GlcNAc hydrolase (OGA) removes O-linked N-acetylglucosamine (O-GlcNAc) from a myriad of nucleocytoplasmic proteins. Through co-expression and assembly of OGA fragments, we determined the three-dimensional structure of human OGA, revealing an unusual helix-exchanged dimer that lays a structural foundation for an improved understanding of substrate recognition and regulation of OGA. Structures of OGA in complex with a series of inhibitors define a precise blueprint for the design of inhibitors that have clinical value

Финансовые риски в деятельности организации (на примере ООО «Зубная аптека»)

Объектом исследования является деятельность ООО "Зубная аптека" (г. Томск) Предметом исследования являются финансовые риски в деятельности организации. Целью выпускной квалификационной работы является разработка рекомендаций по снижению финансовых рисков в деятельности организации на примере ООО "Зубная аптека". В результате исследования: рассмотрена сущность финансового риска, методики оценки финансовых рисков, рассмотрены механизмы нейтрализации финансовых рисков; изучена характеристика деятельности ООО "Зубная аптека", проведен анализ и оценка финансовых рисков, разработаны общие рекомендации по снижению финансовых рисков в деятельности организации.The object of the study is the activity of LLC "Dental Pharmacy" (Zubnaya Apteka LLC (Tomsk). The subject of the study is the financial risks in the organization's activities. The purpose of the final qualification work is to develop recommendations for reducing financial risks in the organization's activities on the example of LLC "Dental Pharmacy". As a result of the study: the essence of financial risk, methods of assessing financial risks, mechanisms for neutralizing financial risks are considered; the characteristics of the activities of LLC "Dental Pharmacy" are studied, the analysis and assessment of financial risks are carried out, general recommendations for reducing financial risks in the organization's activities are developed