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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
91

Influência da velocidade de deslizamento, da carga normal e da transformação martensítica induzida por deformação plástica na resistência ao desgaste por deslizamento dos aços inoxidáveis austeníticos. / Influence of sliding velocity, applied load and strain-induced martensitic transformation on the sliding wear resistance of the austenitic stainless steels.

Moré Farías, María Cristina 02 December 2004 (has links)
Neste trabalho, estudaram-se os mecanismos de desgaste por deslizamento dos aços AISI 304 e AISI 316 em função da carga normal (de 6 N a 20 N) e da velocidade tangencial (de 0,07 m/s a 0,81 m/s). De acordo com o planejamento fatorial 23, realizaram-se ensaios de desgaste em equipamento convencional, do tipo pino-contra-disco. Foram usadas técnicas de análise das superfícies e das partículas após o desgaste: microscopia eletrônica de varredura (MEV), difração de raios-X, espectroscopia Mössbauer, rugosidade, temperatura e dureza instrumentada. Concluiu-se que os mecanismos de desgaste por deslizamento para os aços AISI 304 e AISI 316 são dominados pela deformação plástica (desgaste por oxidação de partículas metálicas, desgaste adesivo e desgaste misto). A variação da taxa de desgaste dos dois materiais dependeu do efeito da interação entre os níveis de carga normal e velocidade tangencial. Por meio do planejamento composto central, foi possível obter um modelo empírico da variação da taxa de desgaste em função da carga normal e da velocidade. / In this work, sliding wear mechanism of AISI 304 and AISI 316 austenitic stainless steels were studied as a function of the applied load (from 6 N to 20 N) and the tangential velocity (from 0.07 m/s to 0.81 m/s). Following the 23 factorial design, wear experiments were performed using a conventional pin-on-disc machine. Worn surfaces and wear debris analysis techniques were used: scanning electron microscopy (SEM), X-ray diffraction, Mössbauer spectroscopy, stylus profiling, surface temperature measurement and instrumented indentation. Plasticity-dominated wear (metallic debris oxidation, adhesive wear and mixed wear) are the sliding wear mechanisms for AISI 304 and AISI 316 steels. The wear rate behavior depended on interaction effect between applied load and tangential velocity. An empirical model of wear rate as a function of applied load and tangential velocity was obtained by means of the central composite design.
92

Analyse Multi-échelle de la Fatigue des Alliages à Mémoire de Forme / Multi-scale Analysis of the Fatigue of Shape Memory Alloys

Zheng, Lin 28 September 2016 (has links)
L’Alliage à Mémoire de Forme (AMF) est un matériau intelligent ayant de nombreuses applications dans l'industrie aérospatiale, le génie civil, ainsi que dans le domaine biomédical. Dans toutes ces applications, le matériau est soumis à un chargement cyclique ce qui le rend vulnérable vis-à-vis du phénomène de la fatigue. Une des questions importantes dans l'étude de la fatigue de l’AMF polycristallin est l'interaction entre l’endommagement local et la transformation de phase martensitique; cette transformation se déroule dans un mode homogène macroscopique ou un mode hétérogène se traduisant par la formation de bandes de Lüders en raison de la localisation de la déformation et du changement de phase. La formation et l'évolution de ces bandes influence fortement les mécanismes physiques de déformation ainsi que l’endommagement par fatigue du matériau. Dans la littérature, on ne trouve pas d’études permettant de faire le lien entre la formation et l’évolution des bandes de localisation et la fatigue du matériau. Dans cette thèse, des expériences systématiques de fatigue en traction sont réalisées sur les éprouvettes pseudo-élastiques du Nickel-Titane avec des observations optiques in-situ de l’évolution des macro-bandes. Ces observations ont permis de retracer l'histoire de la déformation locale dans les zones où la rupture se produit. Ces résultats expérimentaux permettent de mieux comprendre le comportement de fatigue ainsi que sa dépendance par rapport à la contrainte appliquée ainsi que la fréquence du chargement. En particulier, il a été prouvé que la déformation locale résiduelle représente un meilleur indicateur de l’endommagement du matériau que la déformation résiduelle nominale/globale de la structure. / Shape Memory Alloy (SMA) is a typical smart material having many applications from aerospace industry, mechanical and civil engineering, to biomedical devices, where the material’s fatigue is a big concern. One of the challenging issues in studying the fatigue behaviors of SMA polycrystals is the interaction between the material damage and the martensitic phase transformation which takes place in a macroscopic homogeneous mode or a heterogeneous mode (forming macroscopic patterns (Lüders-like bands) due to the localized deformations and localized heating/cooling). Such pattern formation and evolution imply the governing physical mechanisms in the material system such as the fatigue process, but there is still no fatigue study of SMAs by tracing the macro-band patterns and the local material responses. To bridge this gap, systematic tensile fatigue experiments are conducted on pseudoelastic NiTi polycrystalline strips by in-situ optical observation on the band-pattern evolutions and by tracing the deformation history of the cyclic phase transformation zones where fatigue failure occurs. These experimental results help to better understand the stress- and frequency-dependent fatigue behaviors. Particularly, it is found that the local residual strain rather than the structural nominal/global residual strain is a good indicator on the material’s damage leading to the fatigue failure, which is important for understanding and modeling the fatigue process in SMAs.
93

Selection mechanisms for microstructures and reversible martensitic transformations

Della Porta, Francesco M. G. January 2018 (has links)
The work in this thesis is inspired by the fabrication of Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub>. This is the first alloy undergoing ultra-reversible martensitic transformations and closely satisfying the cofactor conditions, particular conditions of geometric compatibility between phases, which were conjectured to influence reversibility. With the aim of better understanding reversibility, in this thesis we study the martensitic microstructures arising during thermal cycling in Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub>, which are complex and different in every phase transformation cycle. Our study is developed in the context of continuum mechanics and nonlinear elasticity, and we use tools from nonlinear analysis. The first aim of this thesis is to advance our understanding of conditions of geometric compatibility between phases. To this end, first, we further investigate cofactor conditions and introduce a physically-based metric to measure how closely these are satisfied in real materials. Secondly, we introduce further conditions of compatibility and show that these are nearly satisfied by some twins in Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub>. These might influence reversibility as they improve compatibility between high and low temperature phases. Martensitic phase transitions in Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub> are a complex phenomenon, especially because the crystalline structure of the material changes from a cubic to a monoclinic symmetry, and hence the energy of the system has twelve wells. There exist infinitely many energy-minimising microstructures, limiting our understanding of the phenomenon as well as our ability to predict it. Therefore, the second aim of this thesis is to find criteria to select physically-relevant energy minimisers. We introduce two criteria or selection mechanisms. The first involves a moving mask approximation, which allows one to describe some experimental observations on the dynamics, while the second is based on using vanishing interface energy. The moving mask approximation reflects the idea of a moving curtain covering and uncovering microstructures during the phase transition, as appears to be the case for Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub>, and many other materials during thermally induced transformations. We show that the moving mask approximation can be framed in the context of a model for the dynamics of nonlinear elastic bodies. We prove that every macroscopic deformation gradient satisfying the moving mask approximation must be of the form 1 + a(x) ⊗ n(x), for a.e. x. With regards to vanishing interface energy, we consider a one-dimensional energy functional with three wells, which simplifies the physically relevant model for martensitic transformations, but at the same time highlights some key issues. Our energy functional admits infinitely many minimising gradient Young measures, representing energy-minimising microstructures. In order to select the physically relevant ones, we show that minimisers of a regularised energy, where the second derivatives are penalised, generate a unique minimising gradient Young measure as the perturbation vanishes. The results developed in this thesis are motivated by the study of Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub>, but their relevance is not limited to this material. The results on the cofactor conditions developed here can help for the understanding of new alloys undergoing ultra-reversible transformations, and as a guideline for the fabrication of future materials. Furthermore, the selection mechanisms studied in this work can be useful in selecting physically relevant microstructures not only in Zn<sub>45</sub>Au<sub>30</sub>Cu<sub>25</sub>, but also in other materials undergoing martensitic transformations, and other phenomena where pattern formation is observed.
94

Influência da velocidade de deslizamento, da carga normal e da transformação martensítica induzida por deformação plástica na resistência ao desgaste por deslizamento dos aços inoxidáveis austeníticos. / Influence of sliding velocity, applied load and strain-induced martensitic transformation on the sliding wear resistance of the austenitic stainless steels.

María Cristina Moré Farías 02 December 2004 (has links)
Neste trabalho, estudaram-se os mecanismos de desgaste por deslizamento dos aços AISI 304 e AISI 316 em função da carga normal (de 6 N a 20 N) e da velocidade tangencial (de 0,07 m/s a 0,81 m/s). De acordo com o planejamento fatorial 23, realizaram-se ensaios de desgaste em equipamento convencional, do tipo pino-contra-disco. Foram usadas técnicas de análise das superfícies e das partículas após o desgaste: microscopia eletrônica de varredura (MEV), difração de raios-X, espectroscopia Mössbauer, rugosidade, temperatura e dureza instrumentada. Concluiu-se que os mecanismos de desgaste por deslizamento para os aços AISI 304 e AISI 316 são dominados pela deformação plástica (desgaste por oxidação de partículas metálicas, desgaste adesivo e desgaste misto). A variação da taxa de desgaste dos dois materiais dependeu do efeito da interação entre os níveis de carga normal e velocidade tangencial. Por meio do planejamento composto central, foi possível obter um modelo empírico da variação da taxa de desgaste em função da carga normal e da velocidade. / In this work, sliding wear mechanism of AISI 304 and AISI 316 austenitic stainless steels were studied as a function of the applied load (from 6 N to 20 N) and the tangential velocity (from 0.07 m/s to 0.81 m/s). Following the 23 factorial design, wear experiments were performed using a conventional pin-on-disc machine. Worn surfaces and wear debris analysis techniques were used: scanning electron microscopy (SEM), X-ray diffraction, Mössbauer spectroscopy, stylus profiling, surface temperature measurement and instrumented indentation. Plasticity-dominated wear (metallic debris oxidation, adhesive wear and mixed wear) are the sliding wear mechanisms for AISI 304 and AISI 316 steels. The wear rate behavior depended on interaction effect between applied load and tangential velocity. An empirical model of wear rate as a function of applied load and tangential velocity was obtained by means of the central composite design.
95

Thermodynamic Evaluation and Modeling of Grade 91 Alloy and its Secondary Phases through CALPHAD Approach

Smith, Andrew Logan, Mr. 07 May 2018 (has links)
Grade 91 (Gr.91) is a common structural material used in boiler applications and is favored due to its high temperature creep strength and oxidation resistance. Under cyclic stresses, the material will experience creep deformation eventually causing the propagation of type IV cracks within its heat-affected-zone (HAZ) which can be a major problem under short-term and long-term applications. In this study, we aim to improve this premature failure by performing a computational thermodynamic study through the Calculation of Phase Diagram (CALPHAD) approach. Under this approach, we have provided a baseline study as well as simulations based on additional alloying elements such as manganese (Mn), nickel (Ni), and titanium (Ti). Our simulation results have concluded that high concentrations of Mn and Ni had destabilized M23C6 for short-term creep failure, while Ti had increased the beneficial MX phase, and low concentrations of nitrogen (N) had successfully destabilized Z-phase formation for long-term creep failure.
96

Magnetic field-induced phase transformation and variant reorientation in Ni2MnGa and NiMnCoIn magnetic shape memory alloys

Karaca, Haluk Ersin 15 May 2009 (has links)
The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.
97

Thermomechanical response of laser processed nickel-titanium shape memory alloy

Daly, Matthew January 2012 (has links)
The exciting thermomechanical properties of nickel-titanium shape memory alloys have sparked significant research efforts seeking to exploit their exotic capabilities. Until recently, the performance capabilities of nickel-titanium devices have been inhibited by the retention of only one thermomechanical characteristic. However, laser processing technology promises to deliver enhanced material offerings which are capable of multiple functional responses. Presented in this thesis, is an investigation of the effects of laser processing on the thermomechanical behaviour of nickel-titanium shape memory alloys. In the context of this work, laser processing refers to removal of alloy constituents, as in the case of laser ablation, or alternatively, addition of elements through laser alloying. The effects of laser ablation on the composition, crystallography and phase transformation temperatures of a nickel-titanium strip have been studied. Application of laser energy was shown to ablate nickel constituents, induce an austenite-martensite phase change and cause an increase in phase transformation onset temperatures, which correlated well with reported findings. Laser processing of a nickel-titanium wire was shown to locally embed an additional thermomechanical response which manifested as unique shape memory and pseudoelastic properties. Localized alloying of ternary species via laser processing of nickel-titanium strip was investigated. Synthesis of a ternary shape memory intermetallic within the laser processing region was achieved through melting of copper foils. Results from thermoanalytical testing indicated that the ternary compound possessed a higher phase transformation temperature and reduced transformation hysteresis in comparison to the reference alloy. Indentation testing was used to demonstrate the augmented thermomechanical characteristics of the laser processed shape memory alloy. In order to demonstrate the enhanced functionality of laser processed nickel-titanium shape memory alloys, a self-positioning nickel-titanium microgripper was fabricated. The microgripper was designed to actuate through four different positions, corresponding to activation of three embedded shape memory characteristics. Thermoanalytical and tensile testing instrumentations were used to characterize the thermomechanical performance of the laser processed nickel-titanium microgripper. Results indicated that each of the laser processed microgripper components possessed unique mechanical and shape memory recovery properties.
98

Magnetic field-induced phase transformation and variant reorientation in Ni2MnGa and NiMnCoIn magnetic shape memory alloys

Karaca, Haluk Ersin 15 May 2009 (has links)
The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.
99

Synthesis, microstructure, and deformation mechanisms of CuZr-based bulk metallic glass composites

Song, Kaikai 27 November 2013 (has links) (PDF)
In the past, it has been found that CuZr-based BMG composites containing B2 CuZr crystals in the glassy matrix display significant plasticity with obvious work hardening. In this work, it was tried to provide a strategy for pinpointing the formation of CuZr-based BMG composites, to modify the microstructures of these composites, and to clarify their yielding and deformation mechanisms. In order to pinpoint the formation of CuZr-based BMG composites, the phase formation and structural evolution of 11 kinds of CuZr-based alloy systems, altogether 36 different compositions, during heating and quenching processes were investigated. An endothermic event between the crystallization and melting peaks was found to be associated with a eutectoid transformation of the B2 CuZr phase. With the addition of elements to the CuZr-based alloys, this endothermic peak(s) shifts to lower or higher temperatures, implying that minor element additions can change the thermal stability of the B2 CuZr phase. By considering the thermal stability of the supercooled liquid, i.e. its resistance against crystallization, and the thermal stability of the B2 CuZr phase, a new strategy to select compositions, which form metastable CuZr-based composites consisting of an amorphous phase and B2 CuZr crystals, is proposed. It is characterized by a parameter, K = Tf /TL, where Tf and TL are the final temperature of the eutectoid transformation during heating and the liquidus temperature of the alloy, respectively. Based on this criterion, the present CuZr-based alloys are classified into three types. For Type I alloys with lower K values, it is difficult to obtain bulk metallic glass (BMG) composites. For Type III alloys with higher K values, BMG composites with larger dimensions are prone to be fabricated, whereas only moderate-sized BMG composites can be obtained for Type II possessing intermediate K values. Accordingly, CuZr-based BMG composites containing B2 CuZr phase in the glassy matrix for different alloy systems were successfully fabricated into different dimensions. For the sake of controlling the formation of the B2 CuZr phase in the glassy matrix and then changing the deformability of CuZr-based BMG composites, different methods were also used to fabricate these composites by: (1) introducing insoluable/high-melting particles; (2) appropriate re-melting treatments of master alloys; and (3) a new flash heating and quenching method. It was demonstrated that the volume fraction, size and distribution of the B2 phase in the glassy matrix can be controlled as well using the methods above. In order to clarify the excellent mechanical properties of CuZr-based BMG composites, the yielding and plastic deformation mechanisms of CuZr-based BMG composites were investigated based on SEM, XRD, and TEM observations. With the volume fraction of amorphous phase (famor) decreasing from 100 vol.% to 0 vol.%, a single-to-“double”-to-“triple”-double yielding transition was found. For the monolithic CuZr-based BMGs and their composites with the famor ³ 97.5 ± 0.5 vol.%, only one yielding at a strain of ~2% occurs, which is due to the formation of multiple shear bands in the glassy matrix, and the associative actions of the shear banding and the martensitic transformation (MT), respectively. When the famor is less than 97.5 ± 0.5 vol.%, a “yielding” occurs at a low strain of ~1%, which results from the yielding of B2 CuZr phase and the onset of the MT within B2 CuZr phase. When the famor is larger than 55 ± 3 vol.%, a “yielding” observed at strains >8% is ascribed from the operation of dislocations with a high density as well as partial de-twinning. It was also found that with the famor decreasing, the deformation mechanism gradually changes from a shear-banding dominated process, to a process being governed by the MT in the crystalline phase, resulting in different plastic strains. Owing to the importance of the MT and the shear banding to the deformation of CuZr-based BMG composites, the details of the MT and the shear banding process were investigated. On one hand, it was found that the MT temperatures of CuZr-based martensitic alloys have a clear relationship with the respective electronic structure and the lattice parameter of the equiatomic CuZr intermetallics. The MT temperatures of the studied alloys can be evaluated by the average concentration of valence electrons. Additional elements with larger atomic radius can affect the stacking fault energy and the electronic charge density redistribution, resulting in the difference of the electronic structures. On the other hand, the formation and multiplication of shear bands for CuZr-based BMG composites is associated with the storage and dissipation of the partial elastic energy during the plastic deformation. When microstructural inhomogeneities at different length scales are introduced into the glassy matrix, the elastic energy stored in the sample-machine system during the plastic deformation is redistributed, resulting in a transition of shear banding process from a chaotic behavior to a self-organized critical state. All in all, our studies and observations provide an understanding of the formation, deformation, and microstrcutural optimization of CuZr-based BMG composites and give guidance on how to improve the ductility/toughness of BMGs. / In letzter Zeit zeigte sich, dass massive Cu-Zr-basierte metallische Glaskomposite, welche B2 CuZr-Kristallite in der amorphen Matrix enthalten, eine ausgeprägte Plastizität mit klarer Kaltverfestigung aufweisen. Im Rahmen dieser Arbeit wurde versucht, eine Strategie zur zielgenauen Einstellung der Phasenbildung und des dazugehörigen Gefüges von massiven CuZr-basierten Glas-Matrix-Kompositen bereitzustellen, sowie deren Fließ- und Verformungsmechanismen aufzuklären. Es wurden elf verschiedene CuZr-basierte Legierungssysteme, insgesamt 36 verschiedene Zusammensetzungen, während Heiz- und Abschreckprozessen untersucht, um die Phasenbildung samt Gefüge von massiven CuZr-basierten Glas-Matrix-Kompositen zielgenau einzustellen. Bei CuZr-basierten metallischen Gläsern kann eine endotherme Reaktion zwischen Kristallisation und Schmelzvorgang der eutektoiden Umwandlung von B2 CuZr zugeordnet werden. Mit Zugabe verschiedener Elemente zur CuZr-Basislegierung kann diese Umwandlung zu höheren bzw. niedrigeren Temperaturen verschoben werden. Bereits geringe Beimischungen beeinflussen die thermische Stabilität der B2 CuZr-Phase. Unter Berücksichtigung der thermischen Stabilität, sowie des Widerstands gegen Kristallisation der unterkühlten Schmelze und der B2 CuZr-Phase wurde eine neue Strategie zur Auswahl des Zusammensetzungsgebiets metastabiler CuZr-Legierungen verschiedener Durchmesser vorgeschlagen. Dieser Widerstand kann durch den Parameter K=Tf/TL beschrieben werden, wobei Tf die Endtemperatur der eutektoiden Umwandlung und TL die Liquidustemperatur sind. Basierend auf diesem Parameter können die untersuchten CuZr-basierten Legierungen in drei Klassen unterteilt werden. Für Legierungen vom Typ I mit niedrigeren K-Werten, ist es schwer massive metallische Glas-Komposite (BMG-Komposite) zu erhalten. Im Gegensatz dazu lassen sich für Legierungen vom Typ III, mit höheren K-Werten, BMG-Komposite mit größeren Probendurchmessern herstellen und Legierungen vom Typ II mit einem mittleren K-Wert mit moderaten Probendurchmessern erzeugt werden. Folglich wurden CuZr-basierte Glas-Matrix-Komposite verschiedener Legierungssysteme mit B2-Phase in der amorphen Matrix erfolgreich in unterschiedlichen Geometrien hergestellt. Zur Kontrolle der Ausbildung der B2-Phase in der amorphen Matrix wurden unterschiedliche Methoden verwendet, um duktile CuZr-basierte BMG-Komposite herzustellen: (1) Einbringen von unlöslichen, hochschmelzenden Partikeln; (2) geeignete Wiederaufschmelzbehandlungen der Vorlegierungen; (3) eine neue Schnellerhitzungs- und -Abschreckmethode. Es konnte gezeigt werden, dass der Volumenanteil, sowie die Größe und Verteilung der B2-Phase in der amorphen Matrix durch die oben genannten Methoden kontrolliert werden können. Um die mechanischen Eigenschaften hinsichtlich des Fließens und der plastischen Deformationsmechanismen von CuZr-basierten BMG-Kompositen aufzuklären, wurden diese näher mittels Rasterelektronenmikroskopie, Röntgenbeugung und Durchstrahlungs-elektronenmikroskopie untersucht. Mit sinkendem Volumenanteil der amorphen Phase (famor) von 100 vol.% auf 0 vol.% kann ein Übergang von einer über zwei zu drei Fließgrenzen beobachtet werden. Für monolithische CuZr-basierte BMGs und ihre Komposite mit einem Anteil famor ≥ 97.5 ± 0.5vol.% erfolgt das Fließen ab einer Stauchung von ~2% durch Ausbildung von mehreren Scherbänden in der amorphen Matrix bzw. dem Zusammenwirken des dazugehörigen Scherens und der Martensitumwandlung. Bei einem Anteil famor unter 97.5 ± 0.5 vol.% findet ein Fließen bei niedrigerer Stauchung von ~1% statt. Dies geschieht aufgrund des Fließens und der beginnenden martensitischen Umwandlungen der B2 CuZr-Phase. Bei einem Anteil famor größer als 55 ± 3 vol.% kann ein Fließen oberhalb einer Stauchung von 8% durch die Interaktion von Versetzungen bei hoher Versetzungsdichte sowie partiellem „Entzwillingen“, beobachtet werden. Es wurde herausgefunden, dass mit sinkendem famor der Verformungsmechanismus schrittweise von einem Scherband dominierten zu einem von der martensitischen Umwandlung dominierten Mechanismus übergeht. Dieser Übergang führt zu Unterschieden in der plastischen Verformung. Da für das Verformungsverhalten von CuZr-basierten BMG-Kompositen die deformationsinduzierte martensitische Umwandlung und die Entstehung sowie Ausbreitung von Scherbändern von herausragender Bedeutung sind, wurden sie näher untersucht. Einerseits wurde herausgefunden, dass die Umwandlungstemperatur der martensitischen Umwandlung von CuZr-basierten martensitischen Legierungen in klarer Beziehung zur entsprechenden Elektronenstruktur und der Gitterkonstanten der äquiatomaren intermetallischen CuZr-Phasen stehen. Die martensitischen Umwandlungstemperaturen der untersuchten Legierungen können über die mittlere Valenzelektronenkonzentration ausgewertet werden. Zusätzliche Elemente mit größerem Atomradius können die Stapelfehlerenergie und die Ladungsdichteverteilung ändern, was in unterschiedliche Elektronenstrukturen mündet. Andererseits ist die Entstehung und Vervielfachung von Scherbändern in CuZr-basierten BMG-Kompositen verbunden mit der Speicherung und Dissipation der partiellen elastischen Energie während der plastischen Verformung. Durch das Einbringen von Gefügeinhomogenitäten unterschiedlicher Größe in die Glasmatrix, wird die elastische Energie, die im System Probe-Maschine gespeichert ist, während der plastischen Deformation umverteilt. Dies führt zu einem Übergang des Schervorgangs von chaotischem Verhalten zu einem selbstorganisierten kritischen Zustand. Insgesamt stellen unsere Untersuchungen und Beobachtungen ein Verständnis der Ausbildung, Verfomung und Gefügeoptimierung von CuZr-basierten BMG-Kompositen bereit und sollen als Leitfaden zur Verbesserung der Duktilität bzw. Zähigkeit von BMGs dienen.
100

Development of the smart aircraft bolt.

Msibi, L. L. January 2002 (has links)
The work contained herein is in pursuance of the Development of the SMART aircraft bolt. Failure of the bolt in the aircraft wing is taken for granted in the project, and the consequent repairs are presently very costly. The SMART material investigated in this work is the TRIP steel, and any reference to SMART material, in this work, shall at all time mean TRIP steel. Investigation of the stresses pertaining to bolts in general is carried out, based on the bolt theories and using the finite element analysis. An optimal bolt based on impact resistance only has also been suggested. Metallurgical behaviour of materials similar to TRIP materials is also investigated, including a section dedicated only to TRIP steels. Therefore, the work contained herein acts as a good base for further research. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2002.

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