Selective laser melting (SLM) is a powder-based additive manufacturing technique consisting of the exact reproduction of a three dimensional computer model (generally a computer-aided design CAD file or a computer tomography CT scan) through an additive layer-by-layer strategy. Because of the high degree of freedom offered by the additive manufacturing, parts having almost any possible geometry can be produced by SLM. More specifically, with this process it is possible to build parts with extremely complex shapes and geometries that would otherwise be difficult or impossible to produce using conventional subtractive manufacturing processes. Another major advantage of SLM compared to conventional techniques is the fast cooling rate during the process. This permits the production of bulk materials with very fine microstructures and improved mechanical properties or even bulk metallic glasses. In addition, this technology gives the opportunity to produce ready-to-use parts with minimized need for post-processing (only surface polishing might be required). Recently, significant research activity has been focused on SLM processing of different metallic materials, including steels, Ti-, Ni- and Al-based alloys. However, most of the research is devoted to the parameters optimization or to feasibility studies on the production of complex structures with no detailed investigations of the structure-property correlation. Accordingly, this thesis focuses on the production and structure-property correlation of Al-12Si samples produced by SLM from gas atomized powders. The microstructure of the as-prepared SLM samples consists of supersaturated primary Al with an extremely fine cellular structure along with the residual free Si situated at the cellular boundaries. This microstructure leads to a remarkable mechanical behavior: the yield and tensile strengths of the SLM samples are respectively four and two times higher than their cast counterparts. However, the ductility is significantly reduced compared with the cast samples. The effect of annealing at different temperatures on the microstructure and resulting mechanical properties of the SLM parts has been systematically studied by analyzing the size, morphology and distribution of the phases. In addition, the mechanical properties of the SLM samples have been modeled using micro- structural features, such as the crystallite and matrix ligament sizes. The results demonstrate that the mechanical behavior of the Al-12Si SLM samples can be tuned within a wide range of strength and ductility through the use of the proper annealing treatment. The Al-Si alloys are generally used as pistons or cylinder liners in automotive applications. This requires good wear resistance and sufficient strength at the operating temperature, which ranges between 373 – 473 K. Accordingly, the tensile properties of the SLM samples were also tested at these temperatures. Changing the hatch style during SLM processing vary the texture in the material. Hence, samples with different hatch styles were produced and the effect of texture on their mechanical behavior was evaluated. The results show that the hatch style strongly influences both the mechanical properties and the texture of the samples; however no direct correlation was observed between texture and mechanical properties. The wear properties of the Al-12Si material was evaluated using pin-on-disc and fretting wear experiments. These experiments show that the as-prepared SLM samples exhibit better wear resistance than their cast counterparts and the SLM heat-treated samples. Finally, the corrosion investigations reveal that the SLM samples have similar corrosion behavior as the cast specimens under acidic conditions. A major drawback for the wide application of SLM as an industrial processing route is the limited size of the products. This is a direct consequence of the limited dimensions of the available building chambers, which allow for the production of samples with volumes of about 0.02 m3. A possible way to overcome this problem would be the use of the welding processes to join the small SLM objects to form parts with no dimensional limitations. In order to verify this possibility, friction welding was employed to join Al-12Si SLM parts. The results indicate that friction welding not only successfully permits the join materials manufactured by SLM, but also helps to significantly improve their ductility. This work clearly demonstrates that SLM can be successfully used for the production of Al-12Si parts with an overall superior performance of the mechanical and physical properties with respect to the conventional cast samples. Moreover, the mechanical properties of the SLM samples can be widely tuned in-situ by employing suitable hatch styles or ex-situ by the proper heat treatment. This might help the development of SLM for the production of innovative high-performance Al-based materials and structures with controlled properties for automotive and aerospace applications.

In this study a series of novel Fe-based materials derived from a bulk metallic glass-forming composition was investigated to improve the ductility of this high-strength glassy alloy. The interplay between the factors chemistry, structure and resulting mechanical properties was analyzed in detail. It has been recognized that subtle modifications of the chemical composition (carbon addition) lead to appreciable changes in the phase formation, which occurs upon solidification (from a single-phase structure to composite materials). As a consequence, significant differences in the mechanical response of the particular samples have been observed. The materials developed here were fabricated by centrifugal casting. To explore the structure features of the as-cast cylinders, manifold experimental techniques (X-ray diffraction, optical, as well as electron microscopy) were employed. The occurrence of the numerous reflections on the X-ray diffraction patterns has confirmed the crystalline nature of the studied Fe-based alloy systems. The subsequent extensive research on their deformation behavior (Vickers hardness and room temperature compression tests) has revealed that, although the glass-forming ability of the investigated compositions is not high enough to obtain a glassy phase as a product of casting, excellent mechanical characteristics (high strength - comparable to that of the reference bulk metallic glass (BMG) - associated with good ductility) were achieved for the “composite-like” alloys. In contrast, the single phase cylinders, subjected to compressive loading, manifested an amazing capacity for plastic deformation – no failure occurred. The fracture motives developed during deformation of the “composite-structured” samples were studied by scanning electron microscopy. The main emphasis has been put on understanding the mechanisms of crack propagation. Owing to the structural complexity of the deformed samples, it was crucial to elucidate the properties of the individual compounds. Based on the obtained results it was concluded that the coexistence of a soft f.c.c. γ-Fe phase in combination with a hard complex matrix is responsible for the outstanding mechanical response of the tested composites. While the soft particles of an austenite contribute to the ductility (they hinder the crack propagation and hence, cause unequivocal strain-hardening), the hard constituents of the matrix phase yield the strength.

Etwa 27000 Flugzeuge durchqueren täglich den Luftraum über Europa. Dieser weiter steigende Flugverkehr erfordert neue Richtlinien für die Luftfahrzeuge. Im Besonderen stehen CO2- und NOX-Emission, Kerosinverbrauch und Lärmbelastung unter Optimierungsbedarf. Diese Anforderungen wurden bis 2050 vom Advisory Council for Aerospace Research in Europe (kurz: ACARE) festgelegt und werden wissenschaftlich unterstützt [3, 4]. Um diese Ziele zu erreichen, gibt es verschiedene Forschungsprogramme, Clean Sky ist ein EU-Technologieprogramm davon. In diesem Projekt werden sechs Demonstrator-Programme entwickelt, von denen MTU Aero Engines eines gestaltet. Im Rahmen dieses Projektes wurde eine Weiterentwicklung des Getriebefan (Geared Turbofan-GTF) erreicht, bei dem Fan und Niederdruckturbine durch ein Getriebe voneinander entkoppelt sind. Durch die optimierte Drehzahl beider Komponenten (vergrößerter Fan - langsamer, Niederdruckturbine (LPT) - schneller) wird die Turbinenleistung gesteigert und gleichzeitig die Geräuschemission minimiert. Entwickelt wurde der GTF von Pratt & Whitney in Kooperation mit MTU Aero Engines. Herkömmliche Varianten sehen vor, dass die Niederdruckturbine u.a. den Fan antreibt und zwar nur so schnell, dass der äußere Radius des Fans die zulässige Geschwindigkeit nicht überschreitet. Die herkömmlich verwendeten Nickelbasislegierungen in der Niederdruckturbine haben mit 8 g/cm3 eine zu hohe Dichte um einige Anforderungen im ACARE wirtschaftlich erfüllen zu können. Bereits 1967 hat die US Airforce das große Potential zur Gewichtsreduzierung durch Titanaluminid-Legierungen (TiAl-Legierungen) mit einer Dichte von rund 4 g/cm3 im Hochtemperaturbereich der Flugzeugtriebwerke erkannt. Zwischen 1980 und 1990 entwickelte das General Electric-Forschungscenter die gamma-TiAl-Legierung Ti-48Al-2Cr-2Nb, welche als erste kommerzielle Titanaluminidlegierung in der Niederdruckturbine von Flugzeugtriebwerken eingesetzt wurde. Eine weitere Legierung dieser Werkstoffgruppe kam erst ca. 15 Jahre später zum Einsatz, die TNM-Legierung. Wie man an diesem Beispiel sehen kann, dauert die Integration neuer Werkstoffe in der Luftfahrt aufgrund der notwendigen Vorversuche und Sicherheitsaspekte teilweise 20 Jahre. Seit September 2014 kommt im Triebwerk PW1100G GTF von Pratt & Whitney die geschmiedete Version der TNM-Legierung zum Einsatz. MTU Aero Engines AG München baut hierfür die Niederdruckturbine. Durch die hervorragenden Hochtemperatureigenschaften der gamma-TiAl-Legierungen wie z.B. thermische Stabilität der Mikrostruktur, Resistenz gegen Titanfeuer und hohe spezifische Fes-tigkeit, konnten sich die Titanaluminide in Konkurrenz zu den Nickelbasislegierungen sehr gut platzieren. Deswegen werden die beiden gamma-TiAl-Legierungen (Ti-48Al-2Cr-2Nb, TNMTM) bereits in den letzten Stufen der Niederdruckturbine eingesetzt. Ein Nachteil der gamma-Titanaluminide ist die begrenzte Oxidationsbeständigkeit über 750 °C, wodurch das Einsatzfeld als Hochtemperaturwerkstoff stark begrenzt wird. Um das Anwen-dungspotential der gamma-Titanaluminide weiter zu steigern und auch bei Temperaturen über 750 °C einzusetzen, ist eine Steigerung der Oxidationsbeständigkeit notwendig. Die Oxidationsbeständigkeit kann durch das Aufbringen von Oxidationsschutzschichten wie z.B. Al2O3 erreicht werden. Welche neben der Korrosionsbeständigkeit auch die thermisch-mechanischen Anforderungen des Substrat-Schicht-Verbundes sicherstellen müssen. Zur Erhöhung der Temperaturbelastbarkeit von gamma-TiAl-Schaufeln können zur thermischen Isolation keramische Wärmedämmschichten (WDS) aufgebracht werden. Aufgrund der WDS können höhere Prozesstemperaturen realisiert und die Lebensdauer des Grundwerkstoffs verlängert werden. Die Lebensdauer der Wärmedämmschichten und das Betriebsverhalten werden unter anderem durch eine gute Haftung auf dem Untergrund, eine niedrige Wärmeleitfähigkeit und einen thermisch stabilen Phasenaufbau bestimmt. Die Kombination aus Oxidationsschutz und Wärmedämmung wird bereits für Nickelbasislegierungen in der Brennkammer und Hochdruckturbine der Flugzeugtriebwerke eingesetzt. Um gamma-Titanaluminide in weitere Stufen der Niederdruckturbine oder Hochdruckturbine einzubringen, müssen diese Temperaturen von mindestens 900 °C aushalten und erfordern ebenso Beschichtungen zum Oxidations- und Wärmeschutz. Diese Schutzschichten finden für gamma-Titanaluminide bisher jedoch noch keine Anwendung.

Introduction: Acellular scaffolds are increasingly used for the surgical repair of tendon injury and ligament tears. Despite this increased use, very little data exist directly comparing acellular scaffolds and their native counterparts. Such a comparison would help establish the effectiveness of the acellularization procedure of human tissues. Furthermore, such a comparison would help estimate the influence of cells in ligament and tendon stability and give insight into the effects of acellularization on collagen. Material and Methods: Eighteen human iliotibial tract samples were obtained from nine body donors. Nine samples were acellularized with sodium dodecyl sulphate (SDS), while nine counterparts from the same donors remained in the native condition. The ends of all samples were plastinated to minimize material slippage. Their water content was adjusted to 69%, using the osmotic stress technique to exclude water content-related alterations of the mechanical properties. Uniaxial tensile testing was performed to obtain the elastic modulus, ultimate stress and maximum strain. The effectiveness of the acellularization procedure was histologically verified by means of a DNA assay. Results: The histology samples showed a complete removal of the cells, an extensive, yet incomplete removal of the DNA content and alterations to the extracellular collagen. Tensile properties of the tract samples such as elastic modulus and ultimate stress were unaffected by acellularization with the exception of maximum strain. Discussion: The data indicate that cells influence the mechanical properties of ligaments and tendons in vitro to a negligible extent. Moreover, acellularization with SDS alters material properties to a minor extent, indicating that this method provides a biomechanical match in ligament and tendon reconstruction. However, the given protocol insufficiently removes DNA. This may increase the potential for transplant rejection when acellular tract scaffolds are used in soft tissue repair. Further research will help optimize the SDS-protocol for clinical application.

In dieser Arbeit wurden die mechanischen Eigenschaften der Magnesiumsandgusslegierung AZ91 in Abhängigkeit von den Prozessparametern untersucht. Es wurde gezeigt, dass durch die Anwendung von Filtration, Kornfeinung und Wärmebehandlung das Niveau der mechanischen Eigenschaften des Sandgusses dem des Druckgusses angepasst werden kann. In dieser Arbeit wurde außerdem der Einfluss der Prozessparameter auf die Spannungsrelaxations- und Dämpfungseigenschaften untersucht.

Den Kern der Untersuchungen bildete die Messung der mechanischen Spannungen in dünnen Kupferschichten während thermischer Zyklen und nach Belastung durch Vierpunktbiegung. Parallel dazu wurden Charakterisierungen der Korngröße, der Textur und korninnerer Defekte, inbesondere von Versetzungen durchgeführt. Bei den Spannungsmessungen fielen folgende Besonderheiten auf: (i) höhere Festigkeiten mit abnehmender Schichtdicke, (ii) hohe Verfestigungsraten bei der Abkühlung, (iii) Asymmetrie der Fließspannungen in Zug- und Druckrichtung und (iv) hohe Spannungen bei hohen Temperaturen. Da die klassischen Plastizitätsmodelle diese Besonderheiten zu erklären vermögen, wurden zwei neue Modelle entwickelt. Das erste beschreibt eine effektive Verfestigung auf der Basis elastischer Wechselwirkungen zwischen mobilen Versetzungen und sich an der Schicht/Substrat-Grenzfläche ansammelnder Versetzungen. Das zweite Modell bezieht sich auf das Korngrenzendiffusionskriechen und erklärt dessen Behinderung durch die unvollständige laterale elastische Relaxation auf einem Substrat haftender Körner. Das gesamte thermomechanische Verhalten kann nur aus dem Zusammenwirken verschiedener strukturbildender und Plastizitäts-Mechanismen beschrieben werden.

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.:Contents Abstract V Kurzfassung IX 1 Theoretical background 1 1.1 Development of metallic glasses 1 1.2 Formation of metallic glasses 3 1.2.1 Thermodynamic considerations 5 1.2.2 Kinetic considerations 7 1.2.3 Structural considerations 10 1.3 Mechanical properties of metallic glasses 14 1.4 Deformation mechanisms of metallic glasses 18 1.4.1 Shear transformation zone theory 18 1.4.2 Free volume model 20 1.4.3 Potential energy landscape theory 21 1.4.4 Cooperative Shearing Model 22 1.5 Strategies to improve the ductility of metallic glasses 24 1.5.1 Nano-scaled microstructural inhomogeneities 25 1.5.2 Micro-scaled microstructural inhomogeneities 28 1.5.3 CuZr-based BMG composites 31 2 Experimental techniques 37 2.1 Sample preparation 37 2.1.1 Arc melting/suction casting 37 2.1.2 Centrifugal casting 38 2.1.3 High-frequency melting/injection casting 39 2.1.4 Melt spinning 39 2.1.5 Ball milling and powder consolidation 40 2.2 Structure characterizations 41 2.2.1 X-ray diffraction 41 2.2.2 Optical microscopy and scanning electron microscopy 41 2.2.3 Transmission electron microscopy 42 2.3 Thermal analysis 43 2.3.1 Differential scanning calorimetry 43 2.3.2 Dilatometry 44 2.4 Measurement of the elastic constants 44 2.5 Compression and tensile tests 44 3 Strategy for pinpointing the formation of CuZr-based BMG composites 46 3.1 Theoretical analysis for the formation of CuZr-based BMG composites 46 3.2 Nature of the eutectoid B2 CuZr transformation 49 3.2.1 Shift of endothermic peak(s) related to the eutectoid B2 transformation 49 3.2.2 Thermal stability of the B2 CuZr phase 52 3.3 Formation of the amorphous phase and the B2 CuZr phase 54 3.4 A new parameter for pinpointing the formation of CuZr-based BMG composites 57 3.5 Conclusions 59 4 Synthesis of CuZr-based BMG composites 60 4.1 Formation of Type I alloys 60 4.2 Formation of Type II alloys 62 4.2.1 Formation and microstructures of the Cu50Zr50 BMG composites 62 4.2.2 Formation and microstructures of the Cu-Zr-Ti BMG composites 67 4.2.3 Formation and microstructures of the Cu-Zr-Al and Cu-Zr-Ag BMG composites 70 4.3 Formation of Type III alloys 74 4.4 Conclusions 76 5 Processing routes for CuZr-based BMG composites 78 5.1 Influence of the melting current/time 78 5.2 Adjusting the cooling rate 81 5.3 Re-melting of the pre-alloy 82 5.4 Introduction of boron nitride particles 84 5.5 Effect of TaW inoculation 87 5.6 “Flash annealing” 93 5.7 Conclusions 100 6 Yielding and deformation mechanisms of CuZr-based BMG composites 101 6.1 Formation and microstructures of Cu47.5Zr47.5Al5 BMG composites 101 6.2 Deformation behavior of Cu47.5Zr47.5Al5 BMG composites 105 6.3 Yielding and plastic deformation mechanisms 110 6.3.1 Yielding and plastic deformation during stage I 110 6.3.2 Yielding and plastic deformation during stage II 113 6.3.3 Yielding and plastic deformation during stage III 114 6.3.4 Plastic deformation during stage IV 118 6.3.5 Fracture behavior 120 6.4 Modeling of the “yielding” behavior 121 6.5 Conclusions 124 7 Martensitic transformation behavior in CuZr-based alloys 126 7.1 Electronic structures and martensitic transformation 126 7.1.1 Electronic structures of the B2 CuZr phase 127 7.1.2 Electronic structures of CuZr martensites 129 7.2 Effect of minor additions on the martensitic transformation 130 7.2.1 Formation of Cu-Zr-Ti crystalline samples 130 7.2.2 Effect of Ti element on the martensitic transformation 133 7.2.3 Effect of minor elements on the martensitic transformation temperature 135 7.3 Martensitic transformation in rapidly solidified alloys 139 7.3.1 Martensitic transformation in the as-cast Cu50Zr50 alloys 140 7.3.2 Martensitic transformation in the as-cast Cu-Zr-Al alloys 142 7.4 Conclusions 145 8 Shear banding process of CuZr-based BMG composites 146 8.1 Serrated flow in CuZr-based BMG composites 146 8.2 Statistical analysis of the serrations for brittle and ductile BMGs 148 8.3 Different statistical results of the serration events for CuZr-based BMG composites during deformation 152 8.4 Energy criteria for serrations in CuZr-based BMG and their composites 155 8.5 Conclusions 158 9 Summary and Outlook 160 Publications 162 Acknowledgements 163 References 164 Schriftliche Erklärung 191 / 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.:Contents Abstract V Kurzfassung IX 1 Theoretical background 1 1.1 Development of metallic glasses 1 1.2 Formation of metallic glasses 3 1.2.1 Thermodynamic considerations 5 1.2.2 Kinetic considerations 7 1.2.3 Structural considerations 10 1.3 Mechanical properties of metallic glasses 14 1.4 Deformation mechanisms of metallic glasses 18 1.4.1 Shear transformation zone theory 18 1.4.2 Free volume model 20 1.4.3 Potential energy landscape theory 21 1.4.4 Cooperative Shearing Model 22 1.5 Strategies to improve the ductility of metallic glasses 24 1.5.1 Nano-scaled microstructural inhomogeneities 25 1.5.2 Micro-scaled microstructural inhomogeneities 28 1.5.3 CuZr-based BMG composites 31 2 Experimental techniques 37 2.1 Sample preparation 37 2.1.1 Arc melting/suction casting 37 2.1.2 Centrifugal casting 38 2.1.3 High-frequency melting/injection casting 39 2.1.4 Melt spinning 39 2.1.5 Ball milling and powder consolidation 40 2.2 Structure characterizations 41 2.2.1 X-ray diffraction 41 2.2.2 Optical microscopy and scanning electron microscopy 41 2.2.3 Transmission electron microscopy 42 2.3 Thermal analysis 43 2.3.1 Differential scanning calorimetry 43 2.3.2 Dilatometry 44 2.4 Measurement of the elastic constants 44 2.5 Compression and tensile tests 44 3 Strategy for pinpointing the formation of CuZr-based BMG composites 46 3.1 Theoretical analysis for the formation of CuZr-based BMG composites 46 3.2 Nature of the eutectoid B2 CuZr transformation 49 3.2.1 Shift of endothermic peak(s) related to the eutectoid B2 transformation 49 3.2.2 Thermal stability of the B2 CuZr phase 52 3.3 Formation of the amorphous phase and the B2 CuZr phase 54 3.4 A new parameter for pinpointing the formation of CuZr-based BMG composites 57 3.5 Conclusions 59 4 Synthesis of CuZr-based BMG composites 60 4.1 Formation of Type I alloys 60 4.2 Formation of Type II alloys 62 4.2.1 Formation and microstructures of the Cu50Zr50 BMG composites 62 4.2.2 Formation and microstructures of the Cu-Zr-Ti BMG composites 67 4.2.3 Formation and microstructures of the Cu-Zr-Al and Cu-Zr-Ag BMG composites 70 4.3 Formation of Type III alloys 74 4.4 Conclusions 76 5 Processing routes for CuZr-based BMG composites 78 5.1 Influence of the melting current/time 78 5.2 Adjusting the cooling rate 81 5.3 Re-melting of the pre-alloy 82 5.4 Introduction of boron nitride particles 84 5.5 Effect of TaW inoculation 87 5.6 “Flash annealing” 93 5.7 Conclusions 100 6 Yielding and deformation mechanisms of CuZr-based BMG composites 101 6.1 Formation and microstructures of Cu47.5Zr47.5Al5 BMG composites 101 6.2 Deformation behavior of Cu47.5Zr47.5Al5 BMG composites 105 6.3 Yielding and plastic deformation mechanisms 110 6.3.1 Yielding and plastic deformation during stage I 110 6.3.2 Yielding and plastic deformation during stage II 113 6.3.3 Yielding and plastic deformation during stage III 114 6.3.4 Plastic deformation during stage IV 118 6.3.5 Fracture behavior 120 6.4 Modeling of the “yielding” behavior 121 6.5 Conclusions 124 7 Martensitic transformation behavior in CuZr-based alloys 126 7.1 Electronic structures and martensitic transformation 126 7.1.1 Electronic structures of the B2 CuZr phase 127 7.1.2 Electronic structures of CuZr martensites 129 7.2 Effect of minor additions on the martensitic transformation 130 7.2.1 Formation of Cu-Zr-Ti crystalline samples 130 7.2.2 Effect of Ti element on the martensitic transformation 133 7.2.3 Effect of minor elements on the martensitic transformation temperature 135 7.3 Martensitic transformation in rapidly solidified alloys 139 7.3.1 Martensitic transformation in the as-cast Cu50Zr50 alloys 140 7.3.2 Martensitic transformation in the as-cast Cu-Zr-Al alloys 142 7.4 Conclusions 145 8 Shear banding process of CuZr-based BMG composites 146 8.1 Serrated flow in CuZr-based BMG composites 146 8.2 Statistical analysis of the serrations for brittle and ductile BMGs 148 8.3 Different statistical results of the serration events for CuZr-based BMG composites during deformation 152 8.4 Energy criteria for serrations in CuZr-based BMG and their composites 155 8.5 Conclusions 158 9 Summary and Outlook 160 Publications 162 Acknowledgements 163 References 164 Schriftliche Erklärung 191

Background: To ensure adequate function after orthopedic tumor reconstruction, it is important to reattach the remaining soft tissue to the implant. This study aimed at obtaining mechanical properties of textile muscle-implant and muscle-bone connections in a preliminary test. Methods: Two groups of soft-tissue attachment were mechanically tested and compared: Native bone-muscle samples obtained from human femora and muscles attached to a prosthetic implant by means of Trevira® attachment tubes. Additionally, muscle samples were tested with muscle fibers aligned parallel and perpendicular to the tension load. A uniaxial load was exerted upon all samples. Results: Failure loads of 26.7 ± 8.8 N were observed for the native bone-muscle group and of 18.1 ± 9.9 N for the Trevira® group. Elongations of 94.8 ± 36.2 % were observed for the native bone-muscle group and 79.3 ± 51.8 % for the Trevira® group. The location of failure was mainly observed in the central area of the muscle fibers. Muscle fibers with parallel fiber orientation (47.6 ± 11.5 N) yielded higher tensile strength than those with perpendicular fiber orientation (14.8 ± 4.1 N). Conclusions: Our experiments showed that higher forces were transmitted in the origin and insertion areas than in areas of flat soft tissue reconstruction using attachment tubes. The data indicate that the tested material allows reattaching muscles, but without reinforcing the insertion site. Therefore, attachment tubes with region-dependent and potentially anisotropic material behavior might be advantageous to optimize muscle-bone load transmission after surgery, which may allow lower complication rates and shorter physical recovery.

The usage of more inexpensive silicon feedstock for the crystallization of multicrystalline silicon blocks promises cost reduction for the photovoltaic industry. Less expensive substrates made out of metallurgical silicon (MG-Si) are used as a mechanical support for the epitaxial solar cell. Moreover, conventional inert solar cells can be produced from up-graded metallurgical silicon (UMG-Si). This feedstock has higher content of impurities which influences cell performance and mechanical strength of the wafers. Thus, it is of importance to know these effects in order to know which impurities should be preferentially removed or prevented during the crystallization process. Solar cell processing steps can also exert a change in the values of mechanical strength of processed multicrystalline silicon wafers until the fabrication of a solar cell. Bending tests, fracture toughness and dynamic elastic modulus measurements are performed in this work in order to research the mechanical behavior of multicrystalline silicon crystallized with different qualities of silicon feedstock. Bending tests and residual stress measurements allows the quantification of the mechanical strength of the wafers after every solar cell processing step. The experimental results are compared with theoretical models found in the classical literature about the mechanical properties of ceramics. The influence of second phase particles and thermal processes on the mechanical strength of silicon wafers can be predicted and analyzed with the theoretical models. Metals like Al and Cu can decrease the mechanical strength due to micro-cracking of the silicon matrix and introduction of high values of thermal residual stress. Additionally, amorphous silicon oxide particles (SiOx) lower the mechanical strength of multicrystalline silicon due to thermal residual stresses and elastic mismatch with silicon. Silicon nitride particles (Si3N4) reduce fracture toughness and cause failure by radial cracking in its surroundings due to its thermal mismatch with silicon. Finally, silicon carbide (SiC) and crystalline silicon oxide (SiOx) introduce thermal residual stresses but can have a toughening effect on the silicon matrix and hence, increase the mechanical strength of silicon wafers if the particles are smaller than a certain size. The surface of as-cut wafers after multi-wire sawing presents sharp micro-cracks that control their mechanical behavior. Subsequent removal of these micro-cracks by texture or damage etching approximately doubles the mechanical strength of silicon wafers. The mechanical behavior of the wafers is then governed by defects like cracks and particles formed during the crystallization of multicrystalline silicon blocks. Further thermal processing steps have a minor impact on the mechanical strength of the wafers compared to as-cut wafers. Finally, the mechanical strength of final solar cells is comparable to the mechanical strength of as-cut wafers due to the high residual thermal stress introduced after the formation of the metallic contacts which makes silicon prone to crack.

The ferromagnetic bulk metallic glasses (BMGs) started to be investigated only in the last 10 years.They are difficult to cast, but their properties are uniques. The work deals with casting, mechanical and soft magnetic properties of new Fe-based BMGs. Such alloys can be cast directly in samples with various geometries and they can be use as magnetic parts in different devices.