The continued interest in the use of magnesium alloys for new applications demand the successful production of high quality wrought alloys. Magnesium Elektron seek to reliably produce high quality alloy billets by the DC casting method combined with ultrasonic inspection. The main objectives of this study are to characterize the defects which are currently found in the material and to understand the ability of the ultrasonic inspection technique currently employed to detect the defects. This study began by locating defects using the ultrasonic inspection method which were then characterised using X-ray Computed Tomography (XCT) 3D imaging technique. Attempts were then made to understand and simulate the mechanisms by which the defects form during the casting process. The simulations were used to investigate the flow patterns during casting and the growth kinetics of the intermetallic phase. The initial phase of this research established that the defects found comprised of an entrained oxide film entangled with an abundance of intermetallic phase particles. These defects were found to be present in the size range of 0.5 – 5 mm, and were deleterious to the materials mechanical properties. Greater understanding of the ultrasonic inspection process was achieved and informed improvements to assisting the production of high quality feedstock. Simulation of the formation of the defects indicated that there was a region in which the oxide films could form and be free to enter into the final cast product. Simulation of the growth of the intermetallic particles demonstrated that precipitation from the liquid occurs in the mould during which particles are carried by the melt flow and experiences a complex thermal history. The combination of the two phases was established to be due to entanglement of the oxide and particles which when combined will settle out of the melt as a single defect. Improved filtering and melt handling methods were recommended to eliminate the defects and reliably produce high quality alloys.

The present study concerns the application of LSM using an excimer laser to enhance the corrosion resistance of rare-earth Elektron 21 magnesium alloy. The alloy has been treated by an excimer laser to produce a highly homogeneous and refined microstructure for improvement of corrosion resistance. The laser surface treatment was applied on two different prepared surfaces of the alloy; i) a ground surface up to 1200 SiC grit; ii) a chemically cleaned surface using CrO3 +AgNO3 boiling solution. The intermetallic phases within the α-matrix that are believed to initiate corrosion have been dissolved by two methods. The first is by the excimer laser, where they were dissolved in the melted layers. The second is by a chemical dissolution prior LSM. Variation of the laser parameters such as changed laser influence (low, medium and high) and increased number of pulses, resulted in formation of thicker melted layers, but promoted the formation of porosity and micro-cracks particularly at overlap regions. The initial stage of this study was aimed at optimising the laser conditions for production of a uniform microstructure, with an increase in the corrosion resistance of the alloy being determined by potentiodynamic polarization measurements in sodium chloride solution. A laser fluence of 6 and 7 J/cm2 with 10, 20, 25, 40 and 50 pulses with a different overlap ratio of 7%, 20% and 50% were subsequently selected as the optimum condition to treat the surface of the alloy. After laser treatment, the top surfaces and the cross-sections of the alloy showed a relatively homogenous melted layer and a significant reduction in the number of large intergranular Mg-Zn-RE phase was achieved resulting in a significant improvement of the corrosion resistance of the alloy. This work also investigated the mechanism of corrosion and the interaction between the intergranular Mg-Zn-RE phase, the Zr-rich regions within the grains and the bulk Mg-rich matrix. The results obtained by scanning electron microscopy (SEM) / energy-dispersive X-ray (EDX) and scanning Kelvin prop forced microscopy (SKPFM) potential map measurements as well as transmission electron microscopy (TEM) / energy-dispersive X-ray (EDX) have shown the importance of the microstructure in the initiation of corrosion in 3.5 wt% NaCl solution, where the Zr-rich regions played a distinct role in the early stages of corrosion in this alloy. However, the obtained results have demonstrated that such laser melted layers improved the corrosion resistance of the alloy, but further work is still needed to obtain the fully understanding of such behaviour which can better the research results, particularly the selectively chemical dissolution of the second phases prior LSM.

Magnesium is the lightest structural metal and magnesium alloys are therefore obvious candidates in weight critical applications. The environmental imperative to reduce vehicle emissions has recently led to intensified research interest in magnesium, since weight reduction is one of the most effective ways of improving fuel efficiency. The hexagonal close-packed structure of magnesium results in poor room temperature formability. However, on heating, several magnesium alloys show superplastic properties, with the ability to deform to very high strains (up to 3000%). This opens up the possibility of forming complex components directly by superplastic forming (SPF). As a result, SPF of magnesium is a highly active research topic. The most widely used class of magnesium alloys contain aluminium as the major alloying addition, which has a relatively high solubility in magnesium, and manganese, which has a less solubility. The effect of these elements on the deformation behaviour and failure mechanisms operating in the superplastic regime is not yet well understood. The objective of this work was to gain fundamental insights into the role of these elements. To do this, alloys with different aluminium content (AZ31 and AZ61) and manganese levels have been studied in-depth.After casting, all alloys were subject to a hot rolling procedure that produced a similar fine grain size and texture in each material. Hot uniaxial testing was performed at temperatures between 300 to 450 degC and at two strain rates to investigate the material flow behaviour, elongation to failure and failure mechanism. All of the alloys exhibited flow curves characterised by an initial hardening and extensive flow softening region. Dynamic recrystallization did not occur, and the flow softening was attributed to grain growth and cavity formation. Increasing the level of aluminium in solution was observed to increase the grain growth rate, and also reduce the strain rate sensitivity. The elongation to failure, however, depended strongly on the manganese level but not on the aluminium content. This attributed to the role of manganese in forming coarse particles that act as sites for cavitation.To study cavity formation and growth, and its effect on failure, a series of tests were conducted to different strain levels followed by investigation of cavitation in 3-dimensions using X-ray tomography. New methods were developed to quantify the correlation between cavities and coarse particles using X-ray tomography data and it was shown that over 90% of cavities are associated with particles. Cavity nucleation occurred continuously during straining, with progressively smaller particles forming cavities as strain increased. The mechanism of cavity formation and growth was identified, and it has been demonstrated that particle agglomerates are effective sites for cavity formation even when the individual particles in the agglomerates are below the critical size predicted by theory for cavity nucleation sites. These results suggest that to improve the ductility of magnesium alloys in the superplasticity regime, it is most critical to minimise the occurrence of particle agglomerates in the microstructure.

Magnesium is the lightest metal with a very high specific strength. However, its practical applicability is limited by its toughness and reliability. Mg, being HCP has low ductility. This makes the improvement of toughness a grand challenge in Mg alloys. Friction stir processing (FSP) is a thermomechanical technique used to effect microstructural modification. Here, FSP was utilized to affect the toughness of WE43 sheets through microstructural modification. Room temperature Kahn-type tests were conducted to measure the toughness of WE43 sheets. Microscopic techniques (SEM, TEM) was utilized to study the effect of various microstructural factors like grain size, texture, constituent particles, precipitates on crack initiation and propagation. Tensile properties were evaluated by mini-tensile tests. Crack growth in WE43 sheets was also affected by mechanics and digital image correlation (DIC) was utilized to study the plastic zone size. The underlying mechanisms affecting toughness of these sheets were understood which will help in formulating ways in improving it. WE43 nanocomposites were fabricated via FSP. Uniform distribution of reinforcements was obtained in the composites. Improved mechanical properties like that of enhanced strength, increased hardness and stiffness were obtained. But contrary to other metal matrix composites which show reduction in ductility with incorporation of ceramic reinforcements, the nanocomposites showed good strength-ductility combination. The composites were precisely characterized and mechanisms governing this property were studied. The nano-length of the reinforcements was observed to be the main criteria and the dislocation-particle interaction, the main reason behind the strength-ductility property.

This diploma thesis deals with research and preparation of bulk materials based on the Mg–Al–Ti system. The theoretical part summarizes the basic knowledge about magnesium alloys, focusing mainly on Mg–Al and Mg–Ti systems. Furthermore, basic information on powder metallurgy methods was included here, from the production of powder materials, through their compaction, to heat treatment and spark plasma sintering (SPS). The theoretical part ends with literature review on the current research of the Mg–Al–Ti system. In the experimental part, bulk materials based on the Mg–Al–Ti system was prepared using traditional methods of powder metallurgy, as well as using the SPS method. The microstructure of the material, elemental and phase composition was examined in this thesis. Subsequently, Vickers hardness and flexural strength were measured, and fractographic observation of the fracture surface was performed. It was found that the aluminum was completely dissolved during the heat treatment, but the titanium particles remained almost intact in the material and worked as a particulate reinforcement. Materials prepared by methods of conventional powder metallurgy showed increased porosity compared to materials prepared by the SPS, resulting in lower hardness and flexural strength. The hardness increased with increasing the amount of aluminum and titanium and with the amount of magnesium phase . Fractographic observation of the fracture surface suggests that a diffuse connection between the reinforcement and the matrix may have occurred after the sintering process.

Title: Study of novel magnesium alloys with controlled microstructure and texture Author: Daria Drozdenko Department / Institute: Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University in Prague Supervisor of the doctoral thesis: Ing. Patrik Dobroň, Ph.D., Department of Physics of Materials. Abstract: The work elucidates the role of dislocation slip and twinning during plastic deformation in selected magnesium (Mg) alloys with controlled microstructure and texture. The acoustic emission (AE) technique was concurrently applied during deformation to determine the activity of particular deformation mechanisms. A detailed insight into microstructure was provided by electron microscopy. In order to obtain a comprehensive set of AE data for particular deformation mechanisms, Mg single crystals with various crystallographic orientations were channel-die and uniaxially compressed. The obtained results were applied on deformation mechanisms in polycrystalline textured Mg alloys. Particularly, the twinning - detwinning processes, in the sense of twin boundary mobility, during one cycle loading (pre-compression followed by tension) were described. Clear correlations between changes in the AE response and the inflection points on the deformation curve were found. An analysis of twin...

This dissertation proposes a multiscale Internal State Variable (ISV) inelasticity-corrosion damage model that is motivated by experimental microstructure-property relations of magnesium alloys. The corrosion damage framework was laid out based on observation of different corrosion mechanisms occurred on an extruded AM30 magnesium alloys. The extruded AM30 magnesium alloy was studied under two corrosion environments (cyclical salt spray and immersion) in order to observe the corrosion rates under different exposure environments. The coupons were examined at various times to determine the history effects of three corrosion mechanisms: (1) general corrosion; (2) pitting corrosion in terms of the nucleation rate, growth rate, and coalescence rate; and (3) intergranular corrosion. The multiscale ISV corrosion model was developed by bridging the macroscale corrosion damage to the mesoscale electrochemical kinetics, microscale material features, and nanoscale material activation energies. The corrosion testing results of Mg alloys (pure Mg, Mg-2% Al, and Mg-6% Al) were used to develop, calibrate, and validate the model, and good agreement was found between the model results and the corrosion testing data. Finally, the simultaneous effects of corrosion and cyclic loading were tested but not modelled for the extruded AM30 magnesium alloy by conducting fatigue experiments in a 3.5 wt.% NaCl solution environment. The corrosion fatigue life of the AM30 alloy was significantly reduced due to corrosion pit formation on specimen surface, hydrogen diffused into the material , and the fracture surface dissolved into the solution. The corrosion damage that arose on the fatigue specimens reduced the crack nucleation process and enhanced the crack propagation rate.

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Das Thema Klimaschutz und daraus folgend auch beispielsweise eine Einsparung fossiler Brennstoffe ist ein großer Forschungsschwerpunkt, wobei vor allem im Automobil und Transportbereich die Substitution der klassischen Materialien wie Stahl durch Leichtbauwerkstoffe, wie Aluminium- und Magnesiumlegierungen, von Interesse ist. Aufgrund der hexagonalen Kristallstruktur des Magnesiums, bringt die Verarbeitung nahezu einzigartigen Herausforderungen mit sich. Dabei wird durch das Hinzulegieren von Calcium, beispielsweise in ZAX-Legierungen – mit Zink, Aluminium und Calcium – die Umformbarkeit bei Raumtemperatur durch eine schwächer ausprägte Textur mit einem Basalpolsplit verbessert. [2] Um die ersetzten Bauteile in den Fahrzeugen zu verbinden, wird beispielsweise das Schweißen verwendet. Dieses Verfahren profitiert allerdings von einer artgleichen Schweißung, wo Grundwerkstoff und Schweißdraht ähnliche Zusammensetzungen besitzen. Zum Schweißen, und auch für eine drahtbasierte additive Fertigung, sind Drähte mit ca. 1 - 1,6 mm Durchmesser nötig, die ausschließlich über einen Ziehprozess erreicht werden können. Für die Herstellung der Vorprodukte von Drähten für das Ziehen rücken die Verfahren Strangpressen und Kaliberwalzen in den Fokus. Für letzteres fehlen allerdings grundsätzliche Erfahrungswerte und Forschungsergebnisse, die aber für eine Anwendung und Einführung in die Wirtschaft unabdingbar sind.