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.

Ni-Mn-Ga alloys close to the stoichiometric composition Ni2MnGa belong to the quite new family of ferromagnetic shape memory alloys. These alloys are characterized by the magnetic field induced strain (MFIS) based on the comparably easy motion of twin boundaries under a magnetic field. They are mostly chosen as a potential candidate for practical application especially promising for actuators and sensors because they are showing the largest MFIS so far. Depending on the chemical composition and heat treatment, at least three martensitic structures can be distinguished in the Ni-Mn-Ga system. However, the effect mentioned above only exists in two modulated structures. Since for the intended application of MFIS in technology polycrystalline materials seem to be more appropriate in contrast to single crystals, the specific polycrystalline aspects are considered. Factors important for decreasing the twinning stress and increasing the twinning strain of polycrystalline Ni-Mn-Ga alloys are texturing, adjusting the structure by annealing and training by thermomechanical treatments. To achieve pronounced MFIS in polycrystals, fabrication processes are needed to produce specific strong textures. The material texturing has been obtained by directional solidification and plastic deformation by hot rolling and hot extrusion as well as high pressure torsion (HPT). To examine the texture of coarse-grained Ni-Mn-Ga alloys (due to a solidification process or dynamic recrystallization), diffraction of synchrotron radiation and neutrons was applied. The texture results show that the texture of Ni-Mn-Ga subjected to directional solidification, hot rolling and hot extrusion is a fibre or weak biaxial texture. However, local synchrotron measurements reveal that the global fibre texture of the hot extruded sample is a ”cyclic” fibre texture, i.e. it is composed of components related to the radial direction rotating around the extrusion axis. This allows finding regions with a strong texture component. The texture after HPT is characterized by a strong cube with the cube favourably oriented. The initial microstructure of the Ni-Mn-Ga alloys is a typical self-accommodated microstructure of martensite. High resolution EBSD mappings show macro, micro twins and two types of microstructure. The twin plane is determined to be {110). In a typical martensitic transformation the high-temperature phase has a higher crystallographic symmetry than the low-temperature phase. Consequently, austenite may transform to several martensitic variants, the number of which depends on the change of symmetry during transformation. Generally, in a cubic-to-tetragonal transformation (5M case) three variants can form with the c-axis oriented close to the three main cubic axes of austenite. However, close examination of the high resolution EBSD mapping reveals that more than just three orientations, as expected from the Bain model, exist in Ni50Mn29Ga21. Each of three Bain variants may be split in some twin relations in different regions of the sample which differ from each other by about few degrees creating a much higher number of variants. The training process, as the last step in the preparation procedure of Ni-Mn-Ga alloys, consists of multi-axis compression finally leading to a single-variant state. Compression of polycrystalline samples leads to motion of those twin boundaries changing the volume fraction of particular martensitic variants in such a way that the shortest axis (c-axis) becomes preferentially aligned parallel to the compression axis. It allows reducing the twinning stress and maximizing the twinning strain. To understand the training process in more detail, the interaction of the twin variants with the neighbourhood of parent austenite grains was investigated.

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.

Magnetische Formgedächtnislegierungen sind Festkörper, die eine Phasenumwandlung erster Ordnung von einer hochsymmetrischen Phase (Austenit) zu einer niedersymmetrischen Phase (Martensit) durchlaufen. Dies kann in der Nähe von Raumtemperatur stattfinden und sowohl durch Temperaturänderung, als auch durch äußere Magnetfelder, mechanische Spannungen oder hydrostatischen Druck induziert werden. Daraus ergeben sich funktionale Eigenschaften, wie der magnetokalorische und der elastokalorische Effekt, eine magnetfeldinduzierte Dehnung und ein großer Magnetowiderstand. Zwillingsgrenzen im Martensit können durch äußere Magnetfelder bewegt werden, was zu großen reversiblen Längenänderungen führt. Der Ablauf der Phasenumwandlung und das Gefüge des Martensits werden dabei durch die elastischen Randbedingungen an der Phasengrenze bestimmt. In dieser Arbeit werden deshalb die Nukleation und das Wachstum des Martensits untersucht. Als Modellsystem werden epitaktische Schichten der Heuslerlegierung Ni-Mn-Ga verwendet. In der martensitischen Phase weist diese Legierung eine modulierte Kristallstruktur auf, die im Konzept des adaptiven Martensits durch eine Verzwillingung auf der atomaren Skala interpretiert werden kann. Im ersten Teil wird mit Röntgenbeugung die modulierte Struktur untersucht. Die Intensität der Überstrukturreflexe wird mit einer kinematischen Beugungssimulation verglichen. Dabei wird nachgewiesen, dass es sich um ein nanoverzwillingtes Gefüge mit einer hohen Dichte an Stapelfehlern handelt. Im zweiten Teil wird das martensitische Gefüge mit Elektronenbeugung im Rasterelektronenmikroskop und Texturmessungen durch Röntgenbeugung untersucht. Das martensitische Gefüge kann im Rahmen der phänomenologischen Martensittheorie quantitativ erklärt werden. Daraus ergibt sich ein geometrisches Modell des martensitischen Nukleus und seiner Wachstumsstadien. Die Phasenumwandlung wird temperaturabhängig im Elektronen- und im Atomkraftmikroskop untersucht und mit dem geometrischen Modell verglichen. Die begrenzte Gültigkeit des geometrischen Modells an makroskopischen Zwillingsgrenzen und an der Grenzfläche zum Schichtsubstrat werden diskutiert. Schließlich kann die Bildung des gesamten hierarchischen Zwillingsgefüges erklärt werden. Im dritten Teil wird die Energiebarriere der Nukleation untersucht. Da die Umwandlung bei konstanter Temperatur abläuft, wird geschlussfolgert, dass Autonukleationsprozesse zu einer starken Verringerung der Nukleationsbarrieren führen. Schließlich wird gezeigt, dass durch Nanoindentation die Nukleation gezielt beeinflusst werden kann. / Magnetic shape memory alloys are solids that undergo a first order phase transition from a high symmetry phase (austenite) into a low symmetry phase (martensite). This can happen close to room temperature and can be induced by changes of temperature, external magnetic fields, mechanical stresses or hydrostatic pressure. This leads to functional properties like the magnetocaloric and elastocaloric effect, a magnetic-field-induced strain and giant magnetoresistance. Twin boundaries in the martensite can be moved by external magnetic fields, which leads to giant reversible length changes. The process of the phase transition and the microstructure of martensite are determined by the elastic boundary conditions at the phase interface. In this work, nucleation and growth of the martensite are studied. Epitaxial films of the Heusler alloy Ni-Mn-Ga are used as a model system. This alloy exhibits a modulated crystal structure which is interpreted as twinning on the atomic scale in the framework of adaptive martensite. In the first part, the modulated structure is studied by X-ray diffraction. The intensity of the superstructure is compared to a kinematic diffraction simulation and it is shown that it is a nanotwinned microstructure with a high density of stacking faults. In the seond part, the martensitic microstructure is studied by electron diffraction in the scanning electron microscope and by texture measurements using X-ray diffraction. The martensitic microstructure can be explained quantitatively in the framework of the phenomenological theory of martensite. This leads to a geometrical model of the martensitic nucleus and its growth stages. The phase transformation is studied as a function of temperature in the scanning electron microscope and atomic force microscope and is compared to the geometric model. The limits of the geometrical model at macroscopic twin boundaries and at interfaces to the substrate are discussed. Finally, the formation of the entire twin microstructure can be explained. In the third part, the energy barrier of nucleation is studied. The transformation is isothermal which leads to the conclusion that autonucleation processes decrease the nucleation barrier significantly. Finally, the influence of nanoindentation on the nucleation is shown.

This work has been devoted to the study of phase transformations involving chemical ordering and magnetic properties evolution in bulk Fe-Pt alloys composed of nanometer-sized grains. A comprehensive study of phase transformations and ordering in Fe-Pt alloys is performed by a combination of in-situ neutron powder diffraction and thermal analysis. The dependence of ordering processes on the alloy composition and initial microstructure (homogeneous A1 phase or multilayer-type) is established. Through the use of mechanical alloying and subsequent heat treatment it has been possible to achieve the formation of chemically highly ordered L10 FePt and, in the case of the Fe-rich and Pt-rich compositions, L12 Fe3Pt and FePt3 phases, respectively. Whereas in Pt-rich alloys the decoupling effect of the FePt3 phase leads to coercivity improvement, in Fe-rich nanocomposites a peculiar nanometer scale multilayer structure gives rise to remanence enhancement due to large effects of exchange interactions between the crystallites of the phases. The structure, magnetic properties and magnetisation reversal processes of these alloys are investigated. Experimentally observed phenomena are understood on the basis of a simple two-particle interaction model. Neutron diffraction has also been used for the investigation of the magnetic structure of ordered and partially ordered nanocrystalline Fe-Pt alloys. It has been shown that the magnetic moment of Fe atoms in L10-type Fe Pt alloys is sensitive to the compositional order. The results are compared to density functional calculations.

In this study polycrystalline NiAl has been subjected to torsion deformation. Torsion has been used because of its characteristics. By this deformation mode high shear strains (gamma = 18 in this study) can be imposed on the sample. The deformation conditions are well-defined because of the local deformation mode, which is simple shear. Due to the monoclinic sample symmetry one half of the pole figure is needed in order to obtain the complete texture information, which is more than is needed e.g. by extrusion or rolling. Therefore, texture analysis might be more sensitive with respect to texture components. Furthermore, torsion deformation is characterized by being inhomogeneous in terms of the amount of shear strain and shear strain rate along the sample radius. The shear strain gradient makes the analysis of different stages of deformation on the same sample (i.e. under the same deformation conditions) possible. Another characteristic being special for torsion is that samples change their length, although no axial stress is applied. This effect is known as Swift effect and will be analyzed in detail. The deformation, microstructure and texture development subject to the shear strain are studied by different techniques (Electron Back-Scatter and High Energy Synchrotron Radiation). Beside the development of microstructure and texture with shear strain, the effect of an initial texture as well as the deformation temperature on the development of texture and microstructure constitute an important part of this study. Therefore, samples with three different initial textures were deformed in the temperature range T = 700K – 1300K. The development of the microstructure is characterized by two different regimes depending on the deformation temperature T. For T up to 1000K, continuous dynamic recrystallization (CDRX) takes place. This mechanism leads to the deformation-induced dislocations forming low angle grain boundaries (LAGBs) or being incorporated into them and the successive transformation of these boundaries into high angle grain boundaries (HAGBs) by a further increase of their misorientation. The predictions of this model were compared with the experimental results. The shear stress – shear strain curves are characterized by a peak at low strains, which is followed by softening and a steady state at high strains. This condition is fulfilled for a number of samples, but especially <111> oriented samples do not show a softening stage at low temperatures. Grain refinement takes place for all samples and the average grain size decreases with temperature. The predicted LAGB decrease is in best agreement with the experiments at the lowest temperatures (T = 700K and 800K). Deviations from the model can be explained by the temperature dependence of the grain boundary mobility. For temperatures T > 1000K, discontinuous dynamic recrystallization (DDRX) occurs, by which new grains form by nucleation and subsequent growth. The texture is characterized by two components, {100}<100> (cube, C) and {110}<100> (Goss, G). The intensity of G increases with temperature, while that of C decreases independent of the initial orientation. Both components have their maximum deviated about the 1 axis. The deviation is larger for grains containing the C component and decreases with temperature. Grains containing the G component have the smaller deviation, which decreases with temperature and strain. Texture simulations based on the full constraint Taylor model under the assumption of {110}<100> and {110}<110> slip were done with the experimental <110> and <111> fibres as well as a theoretical <100> fibre and a {100}<100> single orientation (ideal as well as rotated about the torsion axis). The G component is predicted by the simulations and is therefore a deformation texture. However the C component does not appear in the simulation. It therefore must originate by different mechanisms. For the non-<100> oriented samples, possibly nucleation is responsible for the formation of C oriented nuclei. Simulations with single orientations lead to the conclusion, that the ideal C orientation rotates about the 1 axis, while other C orientations, which are rotated about the torsion axis, increasingly converge towards the G component with strain. A single G orientation on the other hand is stable against such a rotation and is therefore the most likely steady state texture. Based on these results it is proposed, that ideally C oriented nuclei rotate until an orientation is reached into which they grow. These new grains are further rotated up to a critical angle, at which a part of them disappears either by adjacent grains or new C oriented nuclei. The recrystallization texture for T > 1000K is most likely the C component as well. Torsional creep of NiAl is characterized by a stress exponent, which depends on temperature and an activation energy, which is stress dependent. A model incorporating both dependencies is proposed and applied to the creep data. It is shown that these equations are able to describe the experimental findings. Thus creep of NiAl based on this model is dominated by non-diffusional processes such as cross slip of <100> screw dislocations for T  1000K. For T > 1000K the stress exponent and the activation energy are in a region, which according to previous reports is rather dominated by dislocation-climb controlled creep. The Swift effect, due to which samples change their axial dimension during torsion without applied axial stress, is observed for NiAl. It is strongly related to the texture development and in the case of NiAl the C component is identified as being responsible for shortening, whereas the G component leads to lengthening as long as it is not aligned with the shear system. Both tendencies can be explained based on the active slip systems. Simulations fail to predict the experimental observation, because the C component is not present. HESR and EBSD were compared with respect to local texture measurements. It was concluded depending on the average grain size HESR has an advantage in terms of grain statistics. For DDRX samples however, both methods are limited. Local texture inhomogeneities can be better detected using EBSD, whereas for an overall local texture information HESR is better suited.

Self-organization of regular surface pattern under ion beam erosion was described in detail by Navez in 1962. Several years later in 1986 Bradley and Harper (BH) published the first self-consistent theory on this phenomenon based on the competition of surface roughening described by Sigmund's sputter theory and surface smoothing by Mullins-Herring diffusion. Many papers that followed BH theory introduced other processes responsible for the surface patterning e.g. viscous flow, redeposition, phase separation, preferential sputtering, etc. The present understanding is still not sufficient to specify the dominant driving forces responsible for self-organization. 3D atomistic simulations can improve the understanding by reproducing the pattern formation with the detailed microscopic description of the driving forces. 2D simulations published so far can contribute to this understanding only partially. A novel program package for 3D atomistic simulations called TRIDER (TRansport of Ions in matter with DEfect Relaxation), which unifies full collision cascade simulation with atomistic relaxation processes, has been developed. The collision cascades are provided by simulations based on the Binary Collision Approximation, and the relaxation processes are simulated with the 3D lattice kinetic Monte-Carlo method. This allows, without any phenomenological model, a full 3D atomistic description on experimental spatiotemporal scales. Recently discussed new mechanisms of surface patterning like ballistic mass drift or the dependence of the local morphology on sputtering yield are inherently included in our atomistic approach. The atomistic 3D simulations do not depend so much on experimental assumptions like reported 2D simulations or continuum theories. The 3D computer experiments can even be considered as 'cleanest' possible experiments for checking continuum theories. This work aims mainly at the methodology of a novel atomistic approach, showing that: (i) In general, sputtering is not the dominant driving force responsible for the ripple formation. Processes like bulk and surface defect kinetics dominate the surface morphology evolution. Only at grazing incidence the sputtering has been found to be a direct cause of the ripple formation. Bradley and Harper theory fails in explaining the ripple dynamics because it is based on the second-order-effect 'sputtering'. However, taking into account the new mechanisms, a 'Bradley-Harper equation' with redefined parameters can be derived, which describes pattern formation satisfactorily. (ii) Kinetics of (bulk) defects has been revealed as the dominating driving force of pattern formation. Constantly created defects within the collision cascade, are responsible for local surface topography fluctuation and cause surface mass currents. The mass currents smooth the surface at normal and close to normal ion incidence angles, while ripples appear first at incidence angles larger than 40°. The evolution of bimetallic interfaces under ion irradiation is another application of TRIDER described in this thesis. The collisional mixing is in competition with diffusion and phase separation. The irradiation with He ions is studied for two extreme cases of bimetals: (i) Irradiation of interfaces formed by immiscible elements, here Al and Pb. Ballistic interface mixing is accompanied by phase separation. Al and Pb nanoclusters show a self-ordering (banding) parallel to the interface. (ii) Irradiation of interfaces by intermetallics forming species, here Pt and Co. Well-ordered layers of phases of intermetallics appear in the sequence Pt/Pt3Co/PtCo/PtCo3/Co. The TRIDER program package has been proven to be an appropriate technique providing a complete picture of mixing mechanisms.