Magnesiumwerkstoffe sind aufgrund ihrer geringen Dichte und hohen spezifischen Festigkeit für den Leichtbau prädestiniert. Ziel der vorgelegten Arbeit ist es, durch die gleichzeitige Verbesserung des Korrosions- und Verschleißverhaltens neue Anwendungsfelder für Magnesiumlegierungen zu erschließen. Anhand der Magnesiumlegierung AZ91D wurden die Möglichkeiten einer lokalen beanspruchungsgerechten Modifikation von Struktur und Gefüge im oberflächennahen Bereich durch eine Elektronenstrahl(EB)-Flüssigphasen-Randschichtbehandlung ohne Zusatzstoff (EB-Umschmelzen) und unter Verwendung von Al-Si-Zusatzstoffen (EB-Umschmelzlegieren) aufgezeigt. Die mittels verschiedener hochfrequenter Strahlablenktechniken erzeugten Schichten weisen eine deutliche Gefügefeinung, neue Gefügemorphologien sowie eine veränderte Phasenverteilung und/oder -neubildung auf, sind riss- und porenfrei und haben eine ausgezeichnete schmelzmetallurgische Anbindung an den Grundwerkstoff. Anhand von Tauchversuchen und potentiodynamischen Polarisationsmessungen in verschieden konzentrierten NaCl-Lösungen konnte eine signifikante Verbesserung des Korrosionsverhaltens der generierten Schichten im Vergleich zum Ausgangszustand nachgewiesen werden. EB-umschmelzlegierte Schichten weisen im Ergebnis von Trockenverschleißtests (Stift-Scheibe) eine deutliche Reduzierung des spezifischen Verschleißkoeffizienten auf.

Magnesium and magnesium alloys are used in a wide variety of structural applications including automotive, aerospace, hand tools and electronic industries thanks to their light weight, high specific strength, adequate corrosion resistance and good castability. Al and Zn are the primary alloying elements in commercial Mg alloys and commonly used in automotive industries. AZ91 is one of the most popular Mg alloys containing 9% Al and 1% Zn. Hence, lots of research have been done during last decades on AZ91D. However, the existing data concerning mechanical properties and microstructural features showed large scatter and is even contradictory. This work focused on the correlation between the microstructure and the mechanical properties of as-cast AZ91 alloy. An exhaustive characterization of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 using optical and electron backscattered diffraction (EBSD) was performed. These microstructural parameters were correlated to offset yield point (Rp0.2), fracture strength and elongation to fracture. It was understood that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy at temperature range from room temperature up to 190oC. It was realized that by increasing the Mg17Al12 content above 11% a network of intermetallic may form. During deformation this rigid network should break before any plastic deformation happen. Hence, increase in Mg17Al12 content resulted in an increase in offset yield point. The presence of this network was supported by study of thermal expansion behaviour of the alloy containing different amount of Mg17Al12. A physically-based model was adapted and validated in order to predict the flow stress behaviour of as-cast AZ91D at room temperature up to 190ºC for various microstructures. The model was based on dislocation glide and climb in a single-phase (matrix) material containing reinforcing particles. The temperature dependant variables of the model were quite well correlated to the underlying physics of the material.

Today, there is an essential need for lightweight, energy-efficient, environmentally benign engineering systems, and this is the driving force behind the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges have motivated the use of magnesium alloys for lightweight structural systems. Magnesium has a density of 1.74 g/cm3, which is almost 30% less than that of aluminium, one quarter of steel, and almost identicalto polymers. The ease of recycling magnesium alloys as compared to polymers makes them environmentally attractive, but their poor mechanical performance is the primary reason for the limited adoption of these alloys for structural applications. The Mg-Al-Zn alloy AZ91D exhibits an excellent combination of strength, die-castability, and corrosion resistance. However, its mechanical performance with regard to creep strength, for example, at evaluated temperatures is poor. Moreover, very little is known about the correlation between its mechanical properties and microstructural features. This thesis aims to provide new knowledge regarding the role played by microstructure in the mechanical performance of the magnesium alloy. The properties/performance of the material in relation to process parameters became of great interest during the investigation. An exhaustive characterisation of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 was performed using optical and electron backscatter diffraction (EBSD). These microstructural parameters were correlated to the offset yield point (Rp0.2), fracture strength, and elongation to failure of the material. It was proposed that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy in a temperature range of room temperature to 190°C by forming a rigid network of intermetallic. The presence of this network was confirmed by studying the thermal expansion behaviour of samples of the alloy containing different amounts of Mg17Al12. A physically based constitutive model with a wide validity range was successfully adapted to describe the flow stress behaviour of AZ91D with various microstructures. The temperature-dependent variables of the model correlated quite well with the underlying physics of the material. The model was validated through comparison with dislocation densities obtained using EBSD. The influence of high-pressure die-cast parameters on the distortion and residual stress of the cast components was studied, as were distortion and residual stress in components after shot peening and painting. Interestingly, it was found that intensification pressure has a major effect on distortion and residual stresses, and that the temperature of the fixed half of the die had a slight influence on the component's distortion and residual stress. / Numera finns det ett väsentligt behov av lätta, energieffektiva och miljövänliga tekniksystem. Detta behov är drivkraften för utveckling av ett brett utbud av material för energigenerering, energilagring, framdrivning och transport. Dessa utmaningar motiverade användningen av magnesiumlegeringar för lättviktskonstruktioner. Magnesium har en densitet på 1,74 g/cm3, vilket är ca 30% lägre än för aluminium, en fjärdedel av densiteten för stål och nästan i nivå med många polymerer. Då magnesiumlegeringar dessutom är lätta att återvinna, jämfört med polymerer, gör det dem miljömässigt attraktiva. Låga mekaniska egenskaper är den främsta orsaken till begränsad användning av dessa legeringar för lastbärande tillämpningar. Mg-Al-Zn-legeringen AZ91D uppvisar en utmärkt kombination av styrka, gjutbarhet och korrosionsbeständighet. Dess mekaniska egenskaper vid förhöjd temperatur, som tex kryphållfasthet, är låga. Dessutom är korrelationen mellan mikrostruktur och mekaniska egenskaper oklar. Denna avhandling syftade till att ge ny kunskap om mikrostrukturens roll för magnesiumlegeringars mekaniska egenskaper. Slutligen var materialets egenskaper i förhållande till processparametrar vid tillverkningen av stort intresse. En omfattande karaktärisering av kornstorleks-, sekundära dendritarmavstånds (SDAS)-fördelning och fraktion av Mg17Al12 utfördes med hjälp av optisk mikroskopering och diffraktion av bakåtspridda elektroner (EBSD). Mikrostrukturen korrelerades till sträckgränsen (Rp0.2), brottstyrkan och brottförlängningen. Det föreslogs att den intermetalliska fasen, Mg17Al12, spelar en viktig roll vid bestämning av legeringens mekaniska och fysikaliska egenskaper vid temperaturintervall från rumstemperatur upp till 190°C genom att bilda ett styvt nätverk av intermetaller. Uppkomsten av ett sådant nätverk stöddes genom en studie av den termiska expansionen av legeringen för olika fraktioner av Mg17Al12. En fysikalisk konstitutiv modell med ett brett giltighetsområde användes framgångsrikt för att beskriva det plastiska flytbeteendet hos AZ91D för olika mikrostrukturer. De temperaturberoende variablerna i modellen korrelerade ganska väl med materialets underliggande fysik. Modellen validerades genom att jämföra dislokationstätheten som predikterades av modellen och den med EBSD uppmätta dislokationstätheten. Påverkan av pressgjutningsparametrar på geometrisk tolerans och restspänning hos de gjutna komponenterna studerades. Vidare studerades geometrisk tolerans och restspänning av komponenter efter pening och målning. Intressant nog hade eftermatningsfasen en stor effekt på geometrisk tolerans och restspänningar. Dessutom hade temperaturen på den fasta formhalvan av verktyget även ett visst inflytande på komponentens geometriska tolerans och restspänning.

Magnesiumwerkstoffe sind aufgrund ihrer geringen Dichte und hohen spezifischen Festigkeit für den Leichtbau prädestiniert. Ziel der vorgelegten Arbeit ist es, durch die gleichzeitige Verbesserung des Korrosions- und Verschleißverhaltens neue Anwendungsfelder für Magnesiumlegierungen zu erschließen. Anhand der Magnesiumlegierung AZ91D wurden die Möglichkeiten einer lokalen beanspruchungsgerechten Modifikation von Struktur und Gefüge im oberflächennahen Bereich durch eine Elektronenstrahl(EB)-Flüssigphasen-Randschichtbehandlung ohne Zusatzstoff (EB-Umschmelzen) und unter Verwendung von Al-Si-Zusatzstoffen (EB-Umschmelzlegieren) aufgezeigt. Die mittels verschiedener hochfrequenter Strahlablenktechniken erzeugten Schichten weisen eine deutliche Gefügefeinung, neue Gefügemorphologien sowie eine veränderte Phasenverteilung und/oder -neubildung auf, sind riss- und porenfrei und haben eine ausgezeichnete schmelzmetallurgische Anbindung an den Grundwerkstoff. Anhand von Tauchversuchen und potentiodynamischen Polarisationsmessungen in verschieden konzentrierten NaCl-Lösungen konnte eine signifikante Verbesserung des Korrosionsverhaltens der generierten Schichten im Vergleich zum Ausgangszustand nachgewiesen werden. EB-umschmelzlegierte Schichten weisen im Ergebnis von Trockenverschleißtests (Stift-Scheibe) eine deutliche Reduzierung des spezifischen Verschleißkoeffizienten auf.

Magnesium alloys have emerged as a promising material for light-weighting due to their potential for higher weight saving compared with advanced high-strength steel, aluminium alloys and glass fibre reinforced polymer composites based on equal stiffness or strength. Even though magnesium alloys offer low density, high strength to weight ratio and excellent machinability their poor creep resistance and low yield strength at elevated temperatures (~150oC) restricts their use in automotive powertrain applications. Possible ways of improving the creep resistance include development of creep resistant alloys and/or reinforcing the alloy with ceramic particulates, fibres/whiskers. Several magnesium alloys such as – Mg-Zr based alloys, Mg-Al-RE alloys (RE: rare earth), Mg-Al-Ca alloys, Mg-Al-Sr alloys, Mg-Al-Sr-Ca alloys, Mg-Al-Si alloys offer improvement in creep resistance to different extent. However, these alloys are relatively expensive than the widely used Mg-Al-Zn alloys. Some of these alloys are unsuitable for die casting application due to cracking or die filling problems for example, in Ca and Sr containing alloys their content must be controlled to avoid castability problems. Discontinuous reinforcement of magnesium alloys opens up wide variety of casting techniques for production. Most of the components do not require high performance capability all throughout the component. Squeeze casting technique can be used to economically produce selectively reinforced composites having complex near-net-shape components. By selectively reinforcing only the regions of stress concentration, material property can be optimised at the same time lowering both the cost of manufacturing and machining. In the present study, wear behaviour of Saffil short fibre reinforced AZ91D Mg alloy composite prepared by squeeze casting is explored. The Mg-Saffil composite had a two dimensional planar random fibre orientation which was inherent to the fabrication process of porous fibre preform. In the as-polished specimen fibres protrude out of the Mg matrix due to differential polishing. The Mg-Saffil composites were subject to unidirectional sliding wear against steel counterface under nominally dry condition in ambient atmosphere at low velocities, to simulate the piston reversal at the top dead centre of an engine wherein lubrication starvation results in wear of engine cylinder. The fibres protruding out of the magnesium matrix prevent the softer Mg matrix coming in contact with the counterface material. The worn surface was examined by means of scanning electron microscopy and the physical and chemical changes caused by the wear processes were characterised using different spectroscopic techniques. The effect of fibre distribution and fibre orientation on wear of the composite was studied. Depending on the radius of curvature of the counterface, inhomogeneities in the fibre distribution up to a certain length scale were permissible without deteriorating the wear resistance of the composite. The normal fibre orientation proved deleterious to the wear of the counterface. But wear of the composite was independent of the fibre orientation of the wear surface. In contrast to unreinforced AZ91 Mg alloy which undergoes extensive wear, the steel counterface was machined by the hard alumina fibres protruding out of the matrix. With progressive sliding a discontinuous patch of transferred material formed on the worn surface. Eventually, an oxidised iron-rich transfer layer formed on the worn surface due to compaction of the transferred material and wear debris under the combined action of applied normal load and frictional force. The coefficient of friction plateaus following an initial rapid increase with the increase in the areal coverage of the transfer layer. The abrupt increase in friction coincides with the change in contact at the sliding interface from protruding alumina fibres/steel counterface to mostly between the transferred layer and steel counterface. The increase in friction due to the iron-rich transfer layer formed on the worn surface induced bending stresses in the fibre. A correlation between the incipient fibre fracture and build-up of the transfer layer was observed. Wear of the composite was governed by the dawn of the fibre fracture event, which in turn leads to three-body wear. Diamond-like carbon coatings are well-known for their low friction, high hardness and elastic modulus, chemical inertness and optical transparency. They have found widespread use due to their superior tribological characteristics as protective coatings for magnetic storage media (hard-disk drives), in micro-electromechanical devices (MEMS), biomedical applications (joint implants, artificial heart valves), optical windows (anti-reflection coating) and razor blades. The physical and mechanical properties of these coatings can be tailored by controlling the sp3 /sp2 ratio and modification via alloying with metals such as W, Ti, Cr, Al; or non-metallic elements such as B, N, F, Si. A tungsten doped hydrogenated diamond-like carbon coated steel counterface was used to rule out or minimise any chemical and physical interaction between the composite and the counterface. The composite exhibited a higher wear resistance when slid against a DLC coated steel counterface due to ease of interfacial sliding between the carbon-rich transfer layer and the DLC coated counterface. The transition from ultra-mild to mild wear was not altogether suppressed but delayed to higher loads, prolonging the ultra-mild wear regime. As a result of reduced friction the point of maximum shear stress recedes from the surface, which manifests as subsurface cracks. The dominant wear mechanism of Mg-Saffil composite sliding against the DLC coated counterface was delamination wear.