High cycle fatigue tests were carried out on a medium strength continuous casting AA 5754 Al alloy, and new generation high strength AA 2026 and AA 2099 Al alloys. The effect of texture on fatigue properties and short crack behavior were studied. The strengthening mechanisms were also thoroughly investigated for the two high strength alloys.Texture played an important role in the anisotropy of fatigue strength for the AA 5754 Al alloy. Being a solution strengthened alloy, it had a fatigue strength of 120% σy. High strength Al alloys had a strong tendency for planar slip due to the high density of coherent and shearable precipitates in the alloys. Texture was a key factor controlling the crack initiation and propagation. The crack path and the possible minimum twist angles were measured using EBSD and calculated theoretically by a crystallographic model. Based on the micro-texture measured by EBSD, the crack paths were predicted for the AA 2099 alloy and confirmed by the observed values.The excellent balance of superior fatigue properties and high tensile strength of AA 2026 and AA 2099 was attributed to the reduced population of Fe-containing particles, homogeneously distributed precipitates and dislocations. The addition of Zr coupled with the optimized thermo-mechanical treatment strongly restrained the recrystallization, refined the grain structure and promoted the homogenization of the precipitates. Moreover, the retainment of the deformation texture developed during the hot extrusion provided significant orientation strengthening in the high strength Al alloys.Fatigue cracks tended to initiate at coarse second phase particles on sample surfaces and the crack population varied markedly with the applied stresses in the high strength Al alloys. The relationship between of the crack population and the applied stress level was studied and quantified by a Weibull distribution function. Since the measured cracknumbers were associated with the crack initiate sites (i.e., the weakest links) in an alloy, the fatigue weakest-link density, which is defined as the crack population per unit area when stress close to the ultimate tensile stress, and the weakest-link strength distribution can all be calculated and regarded as a property of the studied materials.

The behaviors of short fatigue crack (SFC) propagation through grain boundaries (GBs) were monitored during high cycle fatigue in an Al-Li alloy AA8090. The growth behaviors of SFCs were found to be mainly controlled by the twist components (α) of crack plane deflection across each of up to first 20 GBs along the crack path. The crack plane twist at the GB can result in a resistance against SFC growth; therefore SFC propagation preferred to follow a path with minimum α at each GB. In addition to the grain orientation, the tilting of GB could also affect α. An experiment focusing on quantifying GB-resistance was conducted on an Al-Cu alloy AA2024-T351. With a focused ion beam (FIB) and electron backscatter diffraction (EBSD), the micro-notches were made in front of the selected GBs which had a wide range of α, followed by monitoring the interaction of crack propagation from the notches with the GBs during fatigue. The crack growth rate was observed to decrease at each GB it had passed; and such growth-rate decrease was proportional to α. The resistance of the GB was determined to vary as a Weibull-type function of α. Based on these discoveries, a microstructure-based 3-D model was developed to quantify the SFC growth in high-strength Al alloys, allowing the prediction of crack front advancement in 3-D and the quantification of growth rate along the crack front. The simulation results yielded a good agreement with the experimental results about the SFC growth rate on the surface of the AA8090 Al alloy. The model was also used to predict the life of SFC growth statistically in different textures, showing potential application to texture design of alloys. Fatigue crack initiation at constituent particles (β-phase) was preliminarily studied in the AA2024-T351 Al alloy. Cross-sectioning with the FIB revealed that the 3-D geometry, especially the thickness, of fractured constituent particles (β-phase) was the key factor controlling the driving force for micro-crack growth. The resistance to micro-crack growth, mainly associated with crack plane twist at the particle/matrix interface, also influenced the growth behaviors of the micro-cracks at the particles on the surface.

Recycled concrete is among the rising eco-friendly construction materials which helps to reduce waste and the need for new natural resources. However, such concrete may present previous deterioration due to, for instance, alkali-silica reaction (ASR), which is an ongoing distress mechanism that may keep being developed in the recycled material. This work aims to evaluate the potential of further distress and crack development (i.e. initiation and propagation) of AAR-affected RCA concrete in recycled mixtures displaying distinct past damage degrees and reactive aggregate types. Therefore, concrete specimens incorporating two highly reactive aggregates (Springhill coarse aggregate and Texas sand) were manufactured in the laboratory and stored in conditions enabling ASR development. The specimens were continuously monitored over time and once they reached marginal (0.05%) and very high (0.30%) expansion levels, they were crushed into RCA particles and re-used to fabricate RCA concrete. The RCA specimens were then placed in the same previous conditions and the “secondary” ASR-induced development monitored over time. Results show that the overall damage in ASR-affected RCA concrete is quite different from affected conventional concrete, especially with regards to the severely damaged RCA particles, where ASR is induced by a reactive coarse aggregate, as the RCA particle itself may present several levels of damage simultaneously caused by past/ongoing ASR and newly formed ASR. Moreover, the influence of the original damage extent in such RCA concrete was captured by the slightly damaged RCA mixture eventually reaching the same damage level as the severely damaged mixture. Furthermore, the original extent of deterioration influence the “secondary” induced expansion and damage of RCA concrete since the higher the original damage level, the higher the cracks numbers and lengths observed in the RCA concrete for the same expansion level whereas wider cracks are generated by RCA having previously been subjected to slight damage thus indicating the difference in the distress mechanism as a function of original extent of damage. In addition, it has been found that distress on RCA containing a reactive sand generates and propagates from the residual mortar (RM) into the new mortar (NM) as opposed to RCA containing a reactive coarse aggregate, being generated and propagated from the original coarse aggregate (i.e. original virgin aggregate – OVA) into the NM. Likewise, RCA containing a reactive sand caused longer and higher number of cracks for the same “secondary” induced expansion than the RCA made of reactive coarse aggregate. Finally, novel qualitative and descriptive models are proposed in this research to explain ASR-induced distress generation and propagation on RCA mixtures made of reactive fine and coarse aggregates.

Die vorliegende Arbeit beschäftigt sich mit der Herstellung von CrNi-Stahl/Aluminium-Mischverbunden mit dem Ziel der Verbesserung der mechanischen Eigenschaften und der Erhöhung der Lebensdauer der Lötverbindungen. Da sich die Eigenschaften der Fügepartner stark unterscheiden, ist ein geeignetes Fügeverfahren erforderlich. Die wesentliche Herausforderung besteht in der Vermeidung der Bildung von dicken intermetallischen Schichten in der Reaktionszone, welche die mechanischen Eigenschaften der resultierenden Lötverbindungen verschlechtern. Dementsprechend wird ausgehend vom Stand der Technik ein Konzept zur vollständigen Untersuchung der Mikrostruktur-Eigenschafts-Beziehungen der Mischverbunde erarbeitet und umgesetzt. Die Mischverbunde werden durch Induktionslöten hergestellt, was einen lokalen Wärmeeintrag in die Fügestelle ermöglicht. Dadurch können Lötverbindungen mit dünnen Reaktionszonen erzeugt werden. Das Potenzial der Lötverbindungen wird anhand von Zugscher- und Ermüdungsversuchen aufgezeigt, die mit den Ergebnissen der Mikrostrukturanalyse und der fraktografischen Bewertung korreliert werden.:Abkürzungen und Formelzeichen iii Abkürzungen iii Formelzeichen iv Abbildungsverzeichnis v Tabellenverzeichnis xi 1 Einleitung und Motivation 1 2 Stand der Technik 2 2.1 Anwendungen von Stahl/Aluminium-Mischverbunden 2 2.2 Stoffschlüssiges Fügen von Stahl/Aluminium-Mischverbunden 3 2.2.1 Schweißen von Stahl/Aluminium-Mischverbunden 3 2.2.2 Löten von Stahl/Aluminium-Mischverbunden 8 2.2.2.1 Grundlagen 8 2.2.2.2 Verfahren und Lotwerkstoffe 10 2.3 Besonderheiten bei Stahl/Aluminium-Mischverbunden 15 2.3.1 Intermetallische Fe-Al-Verbindungen in der Reaktionszone 15 2.3.2 Kontrolle der Bildung und des Wachstums der Reaktionszone 20 2.3.3 Problematik bei mechanischer Charakterisierung der Mischverbunde 23 3 Folgerungen und Zielstellung 27 4 Experimentelle Durchführung 29 4.1 Grund- und Lotwerkstoffe 29 4.2 Benetzungs- und Lötversuche 31 4.3 Mikrostrukturelle Charakterisierung 34 4.3.1 Mikrostrukturanalyse 34 4.3.2 Mikrohärtemessung und Nanoindentation 35 4.3.3 Thermische Auslagerung 37 4.4 Mechanische Charakterisierung 39 4.4.1 Zugscher- und Warmzugscherversuche 39 4.4.2 Ermüdungsversuche 40 5 Ergebnisse 43 5.1 Benetzungsverhalten 43 5.2 Mikrostrukturelle Untersuchungen 45 5.2.1 Mikrostruktur der Lötverbindungen und Bildung der Reaktionszone 45 5.2.2 Mechanische Charakteristika der Gefügebestandteile 55 5.3 Festigkeitsuntersuchungen 59 5.3.1 Quasistatische Untersuchungen gelöteter Mischverbunde 59 5.3.2 Ermüdungsverhalten gelöteter Mischverbunde 69 5.4 Korrelation zwischen Reaktionszonendicke und Festigkeit 81 6 Diskussion der Ergebnisse 88 7 Zusammenfassung und Ausblick 92 8 Anlagen 93 8.1 Übersicht der Benetzungsproben auf Stahl X5CrNi18-10 93 8.2 Übersicht der Benetzungsproben auf AlMn1Cu 94 8.3 Beispiel der Ergebnisse der EDX-Analyse 95 8.4 Härteverlauf über die Reaktionszone 96 8.5 EBSD-Analyse der Lötverbindung 97 8.6 Mikrozugversuche 98 8.7 TEM-Untersuchungen der hergestellten Lötverbindungen 99 9 Literaturverzeichnis 102 10 Normenverzeichnis 112 11 Publikationen 113 / This thesis deals with the production of aluminum/stainless steel mixed joints in order to improve the mechanical properties and to extend the lifetime of the joints. Because of the different physical properties of the joining partners, a suitable joining technique is necessary. In comparison to welding, brazing offers significant advantages due to the lower liquidus temperature of the used brazing fillers. The main challenge is to prevent the formation of thick intermetallic layers in the reaction zone. These layers deteriorate the mechanical properties of the resulting joints predominantly. Correspondingly, a concept for the complete investigation of the microstructure-property relationships of the brazed joints is investigated. The joints are produced by induction brazing, which takes place in a short process time and allows a local heat input into the joint. Therefore, joints with a thin intermetallic layer in the reaction zone can be manufactured. The potential of the joints is demonstrated using monotonic tensile shear tests as well as fatigue tests. The achieved results are correlated with the results of the microstructural and fractographic analysis.:Abkürzungen und Formelzeichen iii Abkürzungen iii Formelzeichen iv Abbildungsverzeichnis v Tabellenverzeichnis xi 1 Einleitung und Motivation 1 2 Stand der Technik 2 2.1 Anwendungen von Stahl/Aluminium-Mischverbunden 2 2.2 Stoffschlüssiges Fügen von Stahl/Aluminium-Mischverbunden 3 2.2.1 Schweißen von Stahl/Aluminium-Mischverbunden 3 2.2.2 Löten von Stahl/Aluminium-Mischverbunden 8 2.2.2.1 Grundlagen 8 2.2.2.2 Verfahren und Lotwerkstoffe 10 2.3 Besonderheiten bei Stahl/Aluminium-Mischverbunden 15 2.3.1 Intermetallische Fe-Al-Verbindungen in der Reaktionszone 15 2.3.2 Kontrolle der Bildung und des Wachstums der Reaktionszone 20 2.3.3 Problematik bei mechanischer Charakterisierung der Mischverbunde 23 3 Folgerungen und Zielstellung 27 4 Experimentelle Durchführung 29 4.1 Grund- und Lotwerkstoffe 29 4.2 Benetzungs- und Lötversuche 31 4.3 Mikrostrukturelle Charakterisierung 34 4.3.1 Mikrostrukturanalyse 34 4.3.2 Mikrohärtemessung und Nanoindentation 35 4.3.3 Thermische Auslagerung 37 4.4 Mechanische Charakterisierung 39 4.4.1 Zugscher- und Warmzugscherversuche 39 4.4.2 Ermüdungsversuche 40 5 Ergebnisse 43 5.1 Benetzungsverhalten 43 5.2 Mikrostrukturelle Untersuchungen 45 5.2.1 Mikrostruktur der Lötverbindungen und Bildung der Reaktionszone 45 5.2.2 Mechanische Charakteristika der Gefügebestandteile 55 5.3 Festigkeitsuntersuchungen 59 5.3.1 Quasistatische Untersuchungen gelöteter Mischverbunde 59 5.3.2 Ermüdungsverhalten gelöteter Mischverbunde 69 5.4 Korrelation zwischen Reaktionszonendicke und Festigkeit 81 6 Diskussion der Ergebnisse 88 7 Zusammenfassung und Ausblick 92 8 Anlagen 93 8.1 Übersicht der Benetzungsproben auf Stahl X5CrNi18-10 93 8.2 Übersicht der Benetzungsproben auf AlMn1Cu 94 8.3 Beispiel der Ergebnisse der EDX-Analyse 95 8.4 Härteverlauf über die Reaktionszone 96 8.5 EBSD-Analyse der Lötverbindung 97 8.6 Mikrozugversuche 98 8.7 TEM-Untersuchungen der hergestellten Lötverbindungen 99 9 Literaturverzeichnis 102 10 Normenverzeichnis 112 11 Publikationen 113

<p dir="ltr">In structural metallic components, a material’s lifecycle begins with the processing route, to produce a desired structure, which dictates the in-service performance. The variability of microstructural features as a consequence of the processing route has a direct influence on the properties and performance of a material. In order to correlate the influence processing conditions have on material performance, large test matrices are required which tend to be time consuming and expensive. An alternative route to avoid such large test matrices is to incorporate physics-based process modeling and lifing paradigms to better understand the performance of structural materials. By linking microstructural information to the material’s lifecycle, the processing path can be modified without the need to repeat large-scale testing requirements. Additionally, when a materials system is accurately modeled throughout its lifecycle, the performance predictions can be leveraged to improve the design of materials and components.</p><p dir="ltr">Ni-based superalloys are a material class widely used in many critical aerospace components exposed to coupling thermal and mechanical loads due to their increased resistance to creep, corrosion, oxidation, and strength characteristics at elevated temperatures. Many Ni-based superalloys undergo high-temperature forging to produce a desired microstructure, targeting specific strength and fatigue properties in order to perform under thermo-mechanical loads. When in-service, these alloys tend to fail as a consequence of thermo-mechanical fatigue (TMF) from either inclusion- or matrix- driven failure. In order to produce safer, cheaper and more efficient critical aerospace components, the micromechanical deformation and damage mechanisms throughout a Ni-based superalloy’s lifecycle must be understood. This research utilizes process modeling as a tool to understand the damage and deformation of inclusions in a Ni-200 matrix throughout radial forging as a means to optimize the processing conditions for improved fatigue performance. In addition, microstructural sensitive performance modeling for a Ni-based superalloy is leveraged to understand the influence TMF has on damage mechanisms.</p><p dir="ltr">The radial forging processing route requires both high temperatures and large plastic deformation. During this process, non-metallic inclusions (NMIs) can debond from the metallic matrix and break apart, resulting in a linear array of smaller inclusions, known as stringers. The evolution of NMIs into stringers can result in matrix load shedding, localized plasticity, and stress concentrations near the matrix-NMI interface. Due to these factors, stringers can be detrimental to the fatigue life of the final forged component. By performing a finite element model of the forging process with cohesive zones to simulate material debonding, this research contributes to the understanding of processing induced deformation and damage sequences on the onset of stringer formation for Alumina NMIs in a Ni-200 matrix. Through a parametric study, the interactions of forging temperature, strain rate, strain per pass, and interfacial decohesion on the NMI damage evolution metrics are studied, specifically NMI particle separation, rotation, and cavity formation. The parametric study provides a linkage between the various processing conditions parameters influence on detrimental NMI morphology related to material performance.</p><p dir="ltr">The microstructural characteristics of Ni-based superalloys, as a consequence of a particular processing route, creates a variability in TMF performance. The micromechanical failure mechanisms associated with TMF are dependent on various loading parameters, such as temperature, strain range, and strain-temperature phasing. Insights on the complexities of micromechanical TMF damage are studied via a temperature-dependent, dislocation density-based crystal plasticity finite element (CPFE) model with uncertainty quantification. The capabilities of the model’s temperature dependency are examined via direct instantiation and comparison to a high-energy X-ray diffraction microscopy (HEDM) experiment under coupled thermal and mechanical loads. Unique loading states throughout the experiment are investigated with both CPFE predictions and HEDM results to study early indicators of TMF damage mechanisms at the grain scale. The mesoscale validation of the CPFE model to HEDM experimental data provides capabilities for a well-informed TMF performance paradigm under various strain-temperature phase profiles. </p><p dir="ltr">A material’s TMF performance is highly dependent on the temperature-load phase profile as a consequence of path-dependent thermo-mechanical plasticity. To investigate the relationship between microstructural damage and TMF phasing effects, the aforementioned CPFE model investigates in-phase (IP) TMF, out-of-phase (OP) TMF, and iso-thermal (ISO) loading profiles. A microstructural sensitive performance modeling framework with capabilities to isolate phasing (IP, OP, and ISO) effects is presented to locate fatigue damage in a set of statistically equivalent microstructures (SEMs). Location specific plasticity, and grain interactions are studied under the various phasing profiles providing a connection between microstructural material damage and TMF performance.</p>