Microstructural Alterations of Commercial Metallic Alloys by Friction Surfacing

The process of friction surfacing has been known in its basic arrangement for around 70 years, but only a limited amount of scientific studies has been carried out, and industrial use is marginal. Lately, the emerging shortness of energy and resources draws attention to alternative production routes, and interest in the friction surfacing process has revived. The process’ advantages base on the fact that the deposited material does not undergo the liquid state, but instead is “forged” onto the substrate. Little is understood of the metallurgical mechanisms involved. This work aims at revealing some of these mechanisms by electron-microscopy investigations, and at evaluating the microstructure resulting from friction surfacing as to the coatings’ behaviour under tribological loading. The materials tested are 6082 aluminium alloy, NiAl-bronze (CuAl10Fe5Ni4), tool steel (≈X50CrMoV5-1) and the alloy Cr60Ni40. The aluminium alloy, which is most easily processed by friction surfacing, undergoes dynamic and static recovery. It is a precipitation hardening alloy, and since the temperatures occurring during friction surfacing do not match the thermal sequence for artificial ageing, the material’s hardness is adversely affected. The other three alloys undergo major phase transformations during processing. The bronze assumes a high temperature bcc structure while being deposited. During cooling, small globular or acicular fcc α grains crystallise, followed by precipitation of second (κ-) phases in spherical shape, due to decreasing solubility. The tool steel completely converts into its fcc austenite state, followed by martensitic transformation during cooling. When several layers are deposited overlapping each other, both tempering and secondary hardening effects arise. The non weldable and non deformable Cr60Ni40 alloy consists of a Cr- and a Ni-rich phase during processing. In the coatings, an fcc supersaturated solid solution and a bcc phase rich in Cr, incorporating fine Ni-precipitates, prevail. Supersaturation of solid solution, spheroidisation of second phases and low grain size due to recrystallisation were found to be very beneficial for the wear resistance of all alloys against surface fatigue under cavitation. Sliding wear tests revealed comparable behaviour of the conventional and the friction surfaced material state. For the feasibility of friction surfacing, a material’s properties at the temperatures reached during the process are essential. In particular, it must be possible to transfer a sufficient volume of stud tip material into an adequate plasticised state. This opens up possibilities to join novel material combinations and achieve coatings with advantageous microstructures compared to conventional techniques.Das Reibauftragschweißen ist in seinen Grundzügen bereits seit etwa 70 Jahren bekannt. Der Prozess ist dennoch nicht erschöpfend erforscht und wird selten industriell eingesetzt. Die zunehmende Knappheit von Energie und Ressourcen treibt aktuell die Auslotung alternativer Fertigungsmethoden voran, wodurch das Interesse am Reibauftragschweißen wieder belebt wurde. Die Vorteile des Prozesses sind dadurch bedingt, dass in der festen Phase gefügt wird. Die metallurgischen Vorgänge hierbei sind für die meisten Werkstoffe nicht untersucht. Ziel der vorliegenden Arbeit ist das Verständnis der Mikrostrukturentwicklung vier verschiedener Werkstoffe während des Reibauftragschweißens, sowie der daraus resultierenden Eigenschaften der Schichten unter tribologischer Beanspruchung. Hierfür wurden in erster Linie Elektronen-Mikroskopie und Verschleißprüfung herangezogen. Die untersuchten Werkstoffe sind die Aluminiumlegierung 6082, eine NiAl-Bronze (CuAl10Fe5Ni4), ein Werkzeugstahl (≈X50CrMoV5-1) sowie die Legierung Cr60Ni40. Die Aluminiumlegierung durchläuft dynamische und statische Rekristallisation. Da es sich um eine ausscheidungshärtende Legierung handelt, und die Temperaturen beim Reibauftragschweißen nicht mit denjenigen einer gezielten Wärmebehandlung übereinstimmen, wirkt sich der Prozess negativ auf die Schichthärte aus. Die übrigen drei Werkstoffe durchlaufen wesentliche Phasenumwandlungen. Die Bronze nimmt bei der erreichten Temperatur eine krz Struktur an, aus welcher beim Abkühlen kleine globulare und nadelförmige kfz α Körner kristallisieren, gefolgt von der löslichkeitsbedingten Ausscheidung globularer κ-Phasen. Der Werkzeugstahl erreicht den kfz Austenit-Zustand, mit anschließender martensitischer Umwandlung während des Abkühlens. Beim Auftragen mehrerer überlappender Lagen wurden sowohl Anlass-, als auch Sekundärhärtungsvorgänge beobachtet. Die als nicht schweiß- und umformbar geltende Legierung Cr60Ni40 besteht während des Prozesses aus einer Cr- und einer Ni-reichen Phase. In den Schichten existieren anschließend ein übersättigter kfz Mischkristall und eine Cr-reiche krz Phase mit feinen Ni-Ausscheidungen. Die Übersättigung von Mischkristallen, eine Einformung von Phasen und Kornfeinung durch Rekristallisation wirken sich bei allen untersuchten Werkstoffen positiv auf die Verschleißbeständigkeit unter Kavitation aus. In Gleitverschleißuntersuchungen zeigen die Schichten ein vergleichbares Verhalten wie der jeweilige konventionelle Werkstoffzustand. Für die Anwendbarkeit des Reibauftragschweißens sind die Materialeigenschaften bei den erhöhten Prozesstemperaturen relevant. Insbesondere muss es möglich sein, ein ausreichendes Materialvolumen in der Pinspitze zu plastifizieren. Ist dies der Fall, eröffnet das Reibauftragschweißen Möglichkeiten neuartiger Materialkombinationen, sowie Beschichtungen mit einer im Vergleich zum konventionellen Auftragschweißen vorteilhaften Mikrostruktur

Reducing Fragmentation in University Pre-Service Teacher Education: Conditions and Strategies

We report insights from a design research study in higher education that aims at overcoming pre-service teachers’ experiences of fragmentation in their educational programmes. The design approach started from the assumption that fragmentation could be reduced by initiating and conducting boundary crossing between practices in subject matter and subject matter didactics courses at university. Following this assumption, the design principle of boundary crossing by design(ing) for interlinking subject matter and subject matter didactics (domain-specific pedagogy) was implemented in two subjects in pre-service teacher education at university, one in mathematics and one in English language teaching. The linking between subject matter and subject matter didactics we strive for is two-fold: On the one hand, it requires curricular and organisational dovetailing of the courses involved (boundary crossing by design); on the other hand, it requires a study space where students are urged to try to interlink the courses’ contents in their thinking and acting (boundary crossing by designing). Following this design principle, a nested design approach is developed in which students’ designs of teaching practice is interlocked with the design of courses at university level. In this paper, we illustrate two kinds of findings by empirical examples: a conditional model for the intertwined realisation of the two types of linking, and interlinking strategies as heuristics for the pre-service teachers’ thinking and acting

Development of a ballistic transport model for ion-induced kinetic electron emission

Die folgende Arbeit behandelt die Beschusswinkelabhängigkeit der inneren und externen Elektronenemissionsausbeute sowie das allgemeine Transportverhalten von kinetisch induzierten heißen Elektronen im Festkörper. Zur Untersuchung der Beschusswinkelabhängigkeit wird der von Duvenbeck et al. angenommene, rein diffusive Transport elektronischer Anregung im Festkörper verwendet. In Kombination mit einem thermionischen Richardson-Dushman-Formalismus kann so die externe Elektronenemission in guter Übereinstimmung mit experimentellen Daten bestimmt werden. Zur Bestimmung der inneren Elektronenemission wird ein Hybrid-Modell entwickelt, welches alle Schichten des Modellkristalls als Quellen heißer Elektronen annimmt. Diese Elektronen werden ballistisch an die Ober- und Unterseite des Kristalls transportiert und tragen dort zur Emission bei. Um nicht nur die Größenordnung der Ausbeute, sondern auch die Beschusswinkelabhängigkeit der inneren Emission reproduzieren zu können, wird berücksichtigt, dass zusätzlich zur Energie auch Impuls an die Elektronen übertragen wird. Unter der Annahme, dass sich alle heißen Elektronen nach der Anregung ballistisch in Richtung des Projektils bewegen, kann eine qualitative Übereinstimmung mit experimentellen Daten erreicht werden. Im zweiten Teil der Arbeit wird das ballistische Transportverhalten angeregter Elektronen mit Hilfe der Boltzmann-Transport-Gleichung näher untersucht. Hierzu werden verschiedene Arten der Anregung (thermische, isotrope und an-isotrope Verteilung angeregter Elektronen) initialisiert und die räumliche und zeitliche Entwicklung der damit verbundenen Anregungsenergie im Modellsystem untersucht. Zur Charakterisierung des Transportes der Anregungsenergie wird ein "effektiver" Diffusionskoeffizient eingeführt. Hierbei zeigt sich für eine thermische Anregung eine Abhängigkeit des effektiven Diffusionskoeffizienten von der Elektronentemperatur (und somit auch der Anregungsenergie), welche ein Sättigungsverhalten erkennen lässt. Für eine Peak-artige Form der Anregung durch Elektronen mit genau vorgegebener Energie zeigt sich ebenfalls eine Abhängigkeit von der Anregungsenergie. Es zeigt sich jedoch auch, dass neben dem Energiewert auch die Ursprungsrichtung der angeregten Elektronen einen Einfluss auf den Wert des effektiven Diffusionskoeffizienten hat.The following work discusses the impact angle dependence of both the internal and external electron emission yield as well as the general transport behavior of kinetically induced hot electrons in solids. The impact angle dependence is investigated by application of a purely diffusive transport model for the electronic excitations in solids, proposed by Duvenbeck et al.. The use of a thermionic Richardson-Dushman model combined with the diffusive transport enables the determination of the external electron emission yield in good agreement with experimental data. For the prediction of the internal electron emission a Hybrid model is developed, which treats every layer of the model crystal as a source of hot electrons. These electrons are then transported in a ballistic way to the top and bottom side of the crystal where they contribute to the emission yields. This way the correct order of magnitude for the internal electron emission yield can be predicted. To reproduce the impact angle dependence as well, the transfer of both the energy as well as the momentum is taken into account. Therefore the direction of motion of the excited electrons is assumed to be the same as the impact direction of the projectile. With this assumption the experimental data can reproduced qualitatively. The second part of this thesis investigates the ballistic transport phenomena of excited electrons by implementation of the Boltzmann transport equation. After initiation of different kinds of excitation (thermal, isotropic and anisotropic distribution of excited electrons) the temporal and spatial development of the initialized excitation energy distribution is investigated and characterized by means of an "effective" diffusion coefficient. Considering the thermal excitation, the diffusion coefficients show a clear temperature dependence resulting in saturation. The isotropic and anisotropic excitation profiles show a dependence of the diffusion coefficient from the energy of the excited particles as well. It becomes apparent that both the excitation energy as well as the direction of propagation of the excited electrons has strong influence on the values of the resulting effective diffusion coefficient

Evaluation of the uniformity and stability of T-DNA integration and gene expression in transgenic apple plants

The generation of transgenic apple plants relies on the molecular analysis of transgene integration and expression based on polymerase chain reaction (PCR) analysis, blotting techniques and enzymatic assays on vitro leaves of putative transgenic regenerates. In order to assess the uniformity and the stability of transfer DNA (T-DNA) integration and gene expression, we studied 26 transgenic apple lines carrying the attacin E gene from Hyalophora cecropia , the \u3b2-glucuronidase gene, and the nptII gene. Plants were evaluated using standard molecular techniques, such as PCR, Southern blot, reverse transcription PCR (RT-PCR) and Enzyme Linked Immunosorbent Assay (ELISA), and propagated in vitro on non-selective antibiotic-free media for four years to mimic natural conditions in the field. In some T-lines transgene integration and expression did not remain stable; differences were also found between distinct plants of a single T-line. Individual plants with partially or completely silenced transgenes were identified as well as plants with non-detectable T-DNA. Several lines appeared chimeric or partially silenced. Although most molecular techniques can reliably detect the presence of transgenic cells, they often fail to detect mixtures of transformed and non-transformed cells, or cells with silenced transgenes. This should be taken into consideration, especially in the case of vegetatively propagated trees, where non-transformed or silenced plant parts could mistakenly be used as propagation material

Selective targeting of regulatory T cells with CD28 superagonists allows effective therapy of experimental autoimmune encephalomyelitis

CD4+CD25+ regulatory T cells (T reg cells) play a key role in controlling autoimmunity and inflammation. Therefore, therapeutic agents that are capable of elevating numbers or increasing effector functions of this T cell subset are highly desirable. In a previous report we showed that a superagonistic monoclonal antibody specific for rat CD28 (JJ316) expands and activates T reg cells in vivo and upon short-term in vitro culture. Here we demonstrate that application of very low dosages of the CD28 superagonist into normal Lewis rats is sufficient to induce T reg cell expansion in vivo without the generalized lymphocytosis observed with high dosages of JJ316. Single i.v. administration of a low dose of the CD28 superagonist into Dark Agouti (DA) rats or Lewis rats that suffered from experimental autoimmune encephalomyelitis (EAE) proved to be highly and equally efficacious as high-dose treatment. Finally, we show that T reg cells that were isolated from CD28-treated animals displayed enhanced suppressive activity toward myelin basic protein–specific T cells in vitro, and, upon adoptive transfer, protected recipients from EAE. Our data indicate that this class of CD28-specific monoclonal antibodies targets CD4+CD25+ T reg cells and provides a novel means for the effective treatment of multiple sclerosis and other autoimmune diseases

Threshold Values for Marine Litter: General discussion paper on defining threshold values for marine litter

The revised Commission Decision (EU) 2017/848 requires EU Member States to establish threshold values (TVs) for criteria of Descriptor 10 on marine litter. TVs which are now mandatory through the new provisions, are intended to contribute to MS determination of a set of characteristics for GES and enable their assessment of the extent to which GES is being achieved. The MSFD Technical Group on Marine Litter (TG-ML) has been mandated, through the MSFD 2016-2019 work program of the Common Implementation Strategy, to develop approaches for setting of TVs and to work towards agreed TVs. This documents sets out the scope of setting TVs for marine litter criteria, explores on general concepts of threshold setting and how those concepts can be translated to address the various hazardous effects of marine litter and evaluates potential options for setting TVs and which of them might be suitable to be applied to marine litter. While thresholds for ML criteria may follow the same basic concepts, they may each require specific approaches and need to be discussed separately. Therefore basic thoughts and questions are raised in order to be further discussed when working on the actual definition of TVs for litter in the different marine compartments and for marine litter impacts. Finally concluding recommendations for marine litter threshold setting are proposed.JRC.D.2-Water and Marine Resource