Universität Stuttgart
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Item Open Access Vortex-Kern-Korrelation in gekoppelten Systemen(2014) Jüllig, Patrick; Schütz, Gisela (Prof. Dr.)In der vorliegenden Arbeit wurden strukturierte ferromagnetische Dreischichtsysteme zum einen auf ihre statische in-plane- sowie out-of-plane-Magnetisierungsverteilung als auch auf deren dynamisches Verhalten hin untersucht. Die sowohl quadratischen als auch kreisförmigen Strukturen bestanden aus zwei ferromagnetischen Lagen mit einer Dicke von jeweils 50nm, welche durch eine nicht magnetische Cu-Zwischenschicht getrennt waren. Die Dicke dieser Zwischenschicht variierte schrittweise von t(Cu)=3nm bis 15nm. Als Magnetmaterialien kamen für die untere Schicht Kobalt (Co) und für die obere Schicht das magnetisch isotrope Permalloy (Ni80Fe20) zum Einsatz. Die lateralen Abmessungen sowie das Aspektverhältnis der beiden Einzelschichten wurden so gewählt, dass der Vortexzustand die stabile Domänenkonfiguration ist. Somit resultierten zwei vertikal übereinander angeordnete Vortexkonfigurationen, sodass deren Wechselwirkung sowohl im statischen als auch im dynamischen Fall untersucht werden konnte. Aufgrund der gewählten Cu-Schichtdicke von mindestens 3nm war gewährleistet, dass die Kopplung der in-plane-Schichtmagnetisierung hauptsächlich durch die elektrostatische Streufeldenergie beeinflusst wurde und somit der Beitrag der Oszillatorischen Zwischenschichtaustauschwechselwirkung vernachlässigt werden konnte. Im Falle zweier vertikal übereinander angeordneter Vortexstrukturen kann man bezüglich der Zirkulation C (beschreibt die Orientierung der in-plane-Magnetisierung) und der Polarisation P (beschreibt die Orientierung der out-of-plane-Komponente des Vortexkerns) unter Berücksichtigung der Symmetrie vier verschiedene Konfigurationen voneinander unterscheiden: Die beiden Fälle, bei denen C und P jeweils bzw. orientiert sind, sowie die beiden Fälle, bei denen lediglich C oder P parallel ausgerichtet ist. Der erste Schritt dieser Arbeit bestand in der Probenpräparation. Als Strukturierungsverfahren kamen zum einen das Ionenstrahlätzen und zum anderen die Elektronenstrahllithographie zum Einsatz. Anhand von Röntgenbeugungsexperimenten konnte herausgefunden werden, dass beide Schichtmaterialien, sowohl das Permalloy als auch das Kobalt, eine polykristalline, fasertexturierte Schichtstruktur mit einer fcc-Gitterstruktur aufwiesen. Diese Erkenntnisse waren vor allem für die korrekte Parameterwahl für die nachfolgend durchgeführten mikromagnetischen Simulationen von großer Bedeutung. Messungen der Oberflächenrauigkeiten mittels des AFM ließen darauf schließen, dass neben dem Beitrag der Streufeldenergie ebenso korrelierte bzw. unkorrelierte Zwischenschichtrauigkeiten zur gegenseitigen Ausrichtung der in-plane-Schichtmagnetisierungen beitrugen. Mit Hilfe von SQUID-Messungen bei T=40K an unstrukturierten Co/Cu/Py-Dreischichtsystemen konnte nachgewiesen werden, dass erst für Proben mit Cu-Schichtdicken ab t(Cu)=2,0nm beide ferromagnetische Materialien chemisch voneinander getrennt vorlagen und keine direkte ferromagnetische Kopplung aufgrund von sogenannten Pinholes auftrat. Somit konnte geschlussfolgert werden, dass erst ab einer Dicke von t(Cu) größer gleich 2,0nm eine vollständig geschlossene Cu-Schicht vorlag. Die ersten statischen in-plane-Messungen am STXM zeigten, dass Proben, welche im as-sputtered Zustand eine undefinierte metastabile Mehrdomänenkonfigurationen aufwiesen, durch einen Entmagnetisierungsprozess in den stabilen Vortexzustand überführt werden konnten. Neben antiparallel gekoppelten Systemen bezüglich der Zirkulation C wurden mit einer ähnlich hohen Wahrscheinlichkeit Proben mit einer parallelen Ausrichtung der in-plane-Magnetisierung gefunden. Dies zeigte, dass die Kopplung der Schichtmagnetisierungen nicht allein durch die Streufelder realisiert wurde, sondern ein weiterer Beitrag hinzukam, dessen Ursache mit hoher Wahrscheinlichkeit in den Zwischenschichtrauigkeiten zu finden war. Statische mikromagnetische Simulationen an quadratischen Co/Spalt/Py-Strukturelementen haben gezeigt, dass die in-plane-Magnetisierungsverteilung der Systeme mit C=parallel eine merklich verzerrte Landaustruktur aufwies. Zudem lag bei Konfigurationen mit P=antiparallel ein lateraler Shift bezüglich der Gleichgewichtspositionen der Kerne vor, was aufgrund der Interaktion der out-of-plane-Streufelder zu erwarten war. Dies spiegelte sich auch in der Energiebetrachtung wieder, wobei die beiden Systeme mit der Konfiguration C=parallel deutlich höhere Gesamtenergien aufwiesen als diejenigen mit C=antiparallel. Allgemein lagen im Falle von parallelen Kernpolarisationen die Energiewerte etwas niedriger als bei antiparallel ausgerichteten Kernen. Die dynamische Anregung der ferromagnetischen Schichtsysteme wurde experimentell mittels eines in-plane-Magnetfeldpulses realisiert, welcher durch die lineare Stripline generiert wurde. Die Pulsdauer betrug je nach Element 0,5 bis 1,6ns, und bezüglich der Pulsamplitude mussten Feldstärken von B(Puls)=3,1mT bis zu 6,0mT angelegt werden, um eine Gyrationsbewegung beobachten zu können.Item Open Access Analytical and numerical investigations of form-finding methods for tensegrity structures(2007) Gomez Estrada, Giovani; Bungartz, Hans-Joachim (Prof. Dr.)The analysis of statically indeterminate structures requires the calculation of an initial equilibrium geometry. Tensegrity structures are one of such statically indeterminate structures, with the additional constraint of holding their equilibrium configuration with the action of internal forces and without any anchorage point or external forces. The only source of balance is the state of self-stress held among tensile and compression forces. Tensegrity structures are thus statically indeterminate structures in a stable state of self-stressed self-equilibrium. The basic problem with the modelling of statically indeterminate structures is that there is no unique solution for the forces or geometry that equilibrate a structure. This is where form-finding comes into play. The process of determining their three-dimensional equilibrium shape is commonly called form-finding. This dissertation presents two investigations, one analytical and one numerical on the form-finding of tensegrity structures. Both are in fact complementary. The main results from these investigations appear in [77, 78, 79, 80]. The analytical form-finding for a class of highly symmetric structures with cylindrical shape is first presented, while the numerical procedure for general structures is given in the second part. A thorough analysis of tensegrity cylinders, e.g., the triplex and the quadruplex, is presented in analytical form. Moreover, the numerical procedure here presented is able to reproduce the results obtained with other form-finding methods with great accuracy. The versatility of the novel numerical form-finding procedure is nonetheless demonstrated by solving not only cylindrical and spherical but also new tensegrity structures.Item Open Access Nitriding of iron-based alloys : residual stresses and internal strain fields(2007) Vives Díaz, Nicolás; Mittemeijer, Eric (Prof. Dr. Ir.)Different iron-chromium alloys (4, 8, 13 and 20 wt.% Cr) were nitrided in NH3/H2 gas mixtures at 580 ºC. The nitrided microstructure was investigated by X-ray diffraction, light microscopy, hardness measurements and scanning electron microscopy. Composition depth-profiles of the nitrided zone were determined by electron probe microanalysis. Various microstructures develop, depending on the nitriding conditions and the alloy composition (chromium content). The initial development of coherent, sub-microscopical CrN nitrides leads to a state of hydrostatic stress allowing the uptake of excess nitrogen dissolved in the ferrite matrix. It is shown that the outcome of the subsequent discontinuous coarsening process, which leads to a lamellar microstructure, has a close relation to the nitrogen supersaturation. As a result, the occurrence of a distinct gradient in hardness across the nitrided zone can be understood. Residual stress-depth profiles of the nitrided specimens were measured using the (X-ray) diffraction sin^2 (psi) method in combination with cumulative sublayer removals and correction for corresponding stress relaxations. Unusual, nonmonotonous changes of stress with depth could be related to the microstructure of the nitrided zone. A model description of the evolution of the residual stress as function of depth and nitriding time is given. Specimens of Fe-2.23 at.% V alloy were nitrided in a NH3/H2 gas mixture at 580 ºC. The nitrided microstructure was investigated by X-ray diffraction, and (conventional and high resolution) transmission electron microscopy. For specimens homogeneously nitrided during relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the Fe-200 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the nitride platelets are represented by a single b.c.t. lattice, whereas the remaining part of the ferrite is described by a b.c.c. lattice. Analysis of the microstructure of the nitrided specimens using high resolution transmission electron microscopy investigations confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 ºC) after nitriding led to (moderate) coarsening of the nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes that the VN platelets appear segmented in the diffraction contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequence of the transition from coherent to incoherent diffraction of the nitride platelets with reference to the surrounding ferrite matrix.Item Open Access The strength limits of ultra-thin copper films(2007) Wiederhirn, Guillaume; Arzt, Eduard (Prof. Dr.)Elucidating size effects in ultra-thin films is essential to ensure the performance and reliability of MEMS and electronic devices. In this dissertation, the influence of a capping layer on the mechanical behavior of copper (Cu) films was analyzed. Passivation is expected to shut down surface diffusion and thus to alter the contributions of dislocation- and diffusion-based plasticity in thin films. Experiments were carried out on 25 nm to 2 µm thick Cu films magnetron-sputtered onto amorphous-silicon nitride coated silicon (111) substrates. These films were capped with 10 nm of aluminum oxide or silicon nitride passivation without breaking vacuum either directly after Cu deposition or after a 500 °C anneal. The evolution of thermal stresses in these films was investigated mainly by the substrate curvature method betweeen -160 °C and 500 °C. Negligible differences were detected for the silicon nitride vs. the aluminum oxide passivated Cu films. The processing parameters associated with the passivation deposition also had no noticeable effect on the stress-temperature behavior of the Cu. However, the thermomechanical behavior of passivated Cu films strongly depended on the Cu film thickness. For films in the micrometer range, the influence of the passivation layer was not significant, which suggests that the Cu deformed mainly by dislocation plasticity. However, diffusional creep plays an increasing role with decreasing film thickness since it becomes increasingly difficult to nucleate dislocations in smaller grains. Size effects were investigated by plotting the stress at room temperature after thermal cycling as a function of the inverse film thickness. Between 2 µm and 200 nm, the room temperature stress was inversely proportional to the film thickness. The passivation exerted a strong effect on Cu films thinner than 100 nm by effectively shutting down surface diffusion mechanisms. Since dislocation processes were also shut off in these ultra-thin films, they exhibited purely elastic behavior in the measured temperature range. Their lack of plasticity was confirmed by in-situ TEM analysis, which revealed the presence of sessile parallel glide dislocations during thermal cycling. The stress plateau reported for films thinner than 100 nm was attributed to the fact that the thermal strain applied was insufficient to induce yielding. The highest stress value of 1.7 GPa measured at -150 °C is therefore a lower limit for the actual flow stress since even at this high stress the films remained elastic.Item Open Access Nitriding of iron-based binary and ternary alloys : microstructural development during nitride precipitation(2011) Meka, Sai Ramudu; Mittemeijer, E. J. (Prof. Dr. Ir.)In der vorliegenden Arbeit wurden die beim Gasnitrieren von ferritischen binären und ternären Eisenlegierungen entstehenden Nitride der jeweiligen Legierungselemente untersucht. Zusätzlich wurde der Einfluss von substitutionell gelöstem Al auf die Keimbildung und das Wachstum von Eisennitrid, γ′, näher betrachtet. Beim Nitrieren einer Fe-4.65at.%Al Legierung wurde erstmalig das interessante Phänomen des Entstehens und des Verschwindens einer hohen Zahl von Mikrorissen in der Nitrierzone, sowie die Bildung einer ausscheidungsfreien Zone entlang der Korngrenzen beobachtet. Aufgrund der Volumen-Fehlpassung zwischen der thermodynamisch stabilen, hexagonalen AlN-Modifikation und der ferritischen Matrix, ist deren Ausscheidung in rekristallisiertem Ferrit erschwert. Die Folge ist ein Wettbewerb zwischen der Bildung von gasförmigem Stickstoff an den Korngrenzen und der Ausscheidung von hexagonalem AlN. Dies führt zur Entstehung von Mikrorissen entlang der Korngrenzen und der Entstehung einer von AlN-Ausscheidungen freien Zone nahe der Korngrenzen. Während des fortschreitenden Nitrierens werden die ursprünglich nur teilweise nitrierten Körner vollständig nitriert und die Mikrorisse verschwinden. Dieses Phänomen kann in kalt gewalzten Proben nicht auftreten, da hier die Bildung von gasförmigem N2 durch die vergleichsweise leichte Ausscheidung der metastabilen, kubischen AlN-Modifikation an Versetzungen verhindert wird. Im Unterschied zu nitriertem Reineisen, wo sich eine γ′-Schicht an der Oberfläche ausbildet, entstehen beim Nitrieren einer ferritischen Fe-4.65at.%Al Legierung γ′-Platten mit einer ungewöhnlichen Morphologie an der Oberfläche, welche tief in das darunter liegende Material eindringen. In der Diffusionszone treten nanoskalige γ′- und metastabile, kubische AlN-Ausscheidungen auf, welche eine Nishiyama-Wassermann-, beziehungsweise eine Bain-Orientierungsbeziehung zur Ferritmatrix aufweisen. Die γ′-Platten haben eine hohe Stapelfehlerdichte und enthalten feine ε-Eisennitrid Ausscheidungen, obwohl die Bildung von ε-Eisennitrid unter den angewandten Nitrierbedingungen nicht erwartet wird. Mit geeigneten Nitrierexperimenten konnte gezeigt werden, dass die ungewöhnliche Mikrostruktur eine Folge der vernachlässigbaren Al-Löslichkeit in γ′ und der gehemmten Ausscheidung der thermodynamisch stabilen AlN-Modifikation in Ferrit ist. Beim Nitrieren von dicken (1 mm) rekristallisierten Fe-2at.%Si Proben wurde unerwartet ein ideal schwaches Nitrierverhalten beobachtet. Dies kann dem Umstand zugeschrieben werden, dass die Ausscheidung von Siliziumnitrid erst beginnt, nachdem ein gewisser Grad an Stickstoffübersättigung über die ganze Dicke der Probe erreicht wurde. Siliziumnitrid-Ausscheidungen bilden sich innerhalb der Ferritkörner und entlang der Korngrenzen. Sie sind amorph und haben eine stöchiometrische Zusammensetzung analog zu Si3N4. Die amorphe Natur der kleinen Ausscheidungen hat thermodynamische Gründe. Die Nitridbildung geschieht wegen der sehr großen Volumen-Fehlpassung zwischen Ausscheidung und Matrix sehr langsam. Es tritt ein ungewöhnlicher, nichtmonotoner Härteanstieg mit zunehmender Nitrierzeit auf, welcher der anfänglich völlig elastischen Akkommodation der Ausscheidung/Matrix-Fehlpassung zugeschrieben wird. Die Stickstoffaufnahmerate nimmt mit fortschreitendem Nitrieren als Folge einer „self-catalysis“ zu. Es wurde darauf hingewiesen, dass die Ausscheidung von amorphem Siliziumnitrid entlang der Korngrenzen in der Praxis angewandt werden kann, um Kornwachstum bei der Produktion von kornorientiertem Elektrostahl zu verhindern. Für Fe-Ti-Cr Legierungen wurde der Einfluss eines konstanten Legierungselementgehalt (Ti+Cr) von 0.3 at.%, jedoch mit variierendem Ti/Cr Verhältnis, untersucht. Die Bildung der thermodynamisch stabilen TiN- und CrN-Nitride wurden beim Nitrieren nicht beobachtet. Stattdessen entwickelten sich in der Nitrierzone extrem feine, metastabile Ti1-xCrxN Mischnitride mit plättchenförmiger Morphologie (Länge ≤ 30 nm, Dicke ≤ 3 nm) und kubischer (NaCl-Typ) Kristallstruktur. Die durch die Fehlpassung der Nitridplättchen hervorgerufenen Spannungen in der ferritischen Matrix nehmen mit zunehmendem Ti/Cr Verhältnis zu. Als Konsequenz entsteht, insbesondere für das größte Ti/Cr-Verhältnis, eine tetragonal verzerrte Ferritmatrix um die Ausscheidungen. Die Stickstoffaufnahme wurde quantitativ bestimmt indem Stickstoff-Absorptions-Isothermen aufgenommen wurden. Es zeigte sich, dass die Menge des so genannten Exzess-Stickstoffs, welcher in der Matrix gelöst und an der Grenzfläche zwischen Matrix und Nitrid-Aussscheidung adsorbiert ist, mit größer werdendem Ti/Cr Verhältnis deutlich zunimmt. Die Ergebnisse wurden bezüglich der Abhängigkeit der Fehlpassung vom Ti/Cr Verhältnis und der größeren chemischen Affinität von Ti zu N gegenüber Cr zu N diskutiert.Item Open Access Phase transformations of the NbCr2 and HfCr2 Laves phases(2010) Aufrecht, Jochen Marc; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The polytypic transformations of the Laves phases in the Nb-Cr and the Hf-Cr system have been investigated by means of Differential thermal analysis (DTA), X-ray powder diffraction (XRPD), neutron diffraction (ND) and high-resolution electron microscopy (HRTEM). The interrelation between specific layer-stacking irregularities occuring in these phases and the polytypic phase transformations have been established. For Hf-Cr, a kinetical analysis based on XRPD data has been performed and the effective activation energy of the C14-to-C15 transformation has been obtained. For both systems, the constitution has been reinvestigated and in case of Nb-Cr, the absence of a stable hexagonal Laves-phase modification has been found.Item Open Access Microstructural and film thickness effects on the thermomechanical behavior of thin Au films(2006) Sauter, Linda; Arzt, Eduard (Prof. Dr. phil.)The thermomechanical behavior of thin metal films depends strongly on the film thickness, the film microstructure and the surface state. The latter becomes important at elevated temperatures, where diffusion controls the stress evolution. Au provides the unique opportunity to investigate this effect because of its chemical inertness. Au films between 57 and 1738 nm with two different microstructures: (i) typically columnar grained films with grain sizes on the order of the film thickness and (ii) atypical films with non-columnar grains exhibiting thermally stable fine grain sizes (100 to 200 nm) were sputter-deposited and examined. Thus, film thickness and grain size effects on the mechanical properties could be separated, which is normally not possible and has been hardly studied. Room temperature plasticity was accomplished by dislocation glide and, as expected, the finer grained films exhibited more hardening and higher total stresses. The smallest dimension was found to determine the strength of the material. At elevated temperatures, all films relaxed by diffusional creep. For columnar Au films, parallel glide dislocations were observed the first time by transmission electron microscopy. Consequently, the thermomechanical behavior of these films was discussed in light of constrained diffusional creep. Non-columnar films could additionally relax local compressive stresses by hillock formation. The requirements to form hillocks were analyzed with respect to the unusual microstructure. The network of interconnected horizontal grain boundaries providing for the necessary long-range diffusive material transport was found to be the major precondition.Item Open Access Grain growth and texture evolution in copper thin films(2010) Sonnweber-Ribic, Petra; Arzt, Eduard (Prof. Dr. phil)An improved basic understanding of mechanisms causing grain growth and texture evolution in Cu thin films contains the potential to improve performance and reliability of components and devices. In this work, the influence of film thickness, strain and temperature on grain growth and texture evolution in Cu thin films was investigated. By varying the parameters, information about the underlying mechanisms were revealed. The 0.5 to 10 micrometer thick Cu films were deposited on 125 micrometer thick polyimide substrates (Kapton®, DuPont) using a UHV magnetron sputtering system. For detailed observation of grain growth and texture evolution an EBSD-based in situ testing appliance was constructed. This system allowed the simultaneous observation of grain growth and texture evolution, giving new insight into growth kinetics and details of grain growth. In a first step, Cu thin films of thicknesses in between 0.5 and 10 micrometer were deposited on polymer substrates and annealed at 330°C for 30 min. Their resulting texture and microstructure were investigated by EBSD. A texture transition from (111) to (100) was observed at film thicknesses between 3 and 5 micrometer. The experimental findings were explained by the texture evolution model of Thompson and Carel. A significant observation which cannot be explained by a purely energetic argument is the broad texture transition. In order to get more information about the critical role of strain energy, uniaxial tensile tests were carried out on 3 micrometer thick films. In contrast to theoretical predictions, various tensile tests revealed no influence of strain on grain growth behaviour. Neither at room temperature nor at elevated temperatures, further (100) grain growth was observed. In a next step, the abnormal growth of individual (100) oriented grains was recorded for more than 24 hours at temperatures between 90 and 118°C. Annealing was carried out inside a Leo 1530-VP SEM equipped with a heating facility. Detailed analysis of grain growth and estimates of the possibly acting driving forces indicated that the reduction of dislocation density played an important role for abnormal grain growth. A further hint for the critical importance of defect density was given by the HWHM of the (100) texture fraction. Nevertheless, it was not clear why this driving force favours the growth of (100) oriented grains. A possible answer could be given by the strain energy release maximization (SERM) model developed by Lee. In addition, when analysing the activation energy for grain growth, they were found to possess a higher grain boundary mobility, supporting the preferred growth of (100) oriented grains. A new texture map, considering dislocation density as driving force, was constructed. Assuming dislocation density to play a significant role for grain growth and texture evolution in Cu thin films, the influence of deposition parameters is pointed out.Item Open Access Synthesis and characterization of carbon nanotube reinforced copper thin films(2006) Otto, Cornelia; Arzt, Eduard (Prof. Dr.)Two model composites of copper and carbon nanotubes were fabricated by very different deposition methods. Copper electrodeposition in a plating bath containing nanotubes created a 3D matrix of randomly oriented CNTs within a thick, 20 micron Cu film. In contrast, sandwiching a layer of well-separated nanotubes between two sub-micron sputtered Cu layers produced a 2D-composite with nanotubes lying parallel to the substrate surface. These composites, which were mechanically tested using various techniques, proved to be well suited to explore the nature of the CNT/Cu matrix interface. Columns approximately 600 nm in diameter and 1.4 microns high were cut from the sputter-deposited composite and microcompression tested in a nanoindenter. No influence of the presence of nanotubes on the stress-strain-curves was observed, which was attributed to the low nanotube content. On the other hand, microscopic analysis showed an influence of the nanotube on the copper immediately surrounding it, resulting in funnel-like depressions on the column surface. In addition to deformation by slip, twinning was observed in some columns, which has never before been reported in the literature for micron-sized columns. Macroscopic tensile tests were performed on the electrodeposited films and the samples with the highest carbon content showed an increase in toughness of over 100% with respect to the CNT-free control samples produced by the same method. Finally, short copper electrodepositions into carbon nanotube carpets revealed large regions with conformally coated nanotubes. Until now, it was assumed that copper would not wet the nanotubes and that the interfacial strength between copper and CNTs would be low, since copper does not form a carbide. However, these experiments all revealed clear evidence of adhesion exceeding the copper shear strength. To our knowledge, this is the first time such strong adhesion was demonstrated between a nanotube and a metal matrix. We attribute this unexpected, but highly desirable adhesion and wetting behaviour to the defect structure in the nanotubes used. Most of the experiments were done with nitrogen-doped carbon nanotubes, which are known to be rich in defects. The nanotube carpets used in the last experiment were not doped but had a high defect density due to the synthesis method used. As a good adhesion between fiber and matrix is a prerequisite for the successful use of carbon nanotubes in metal matrix composites, these results are very encouraging. The composites and methods presented here provide a foundation for further studies needed to understand the nanotube-metal interaction in more detail and thus ultimately for successful metal-carbon nanotube composites.Item Open Access Nitriding behavior of Ni and Ni-based binary alloys(2015) Fonovic, Matej; Mittemeijer, Eric J. (Prof. Dr. Ir.)Gaseous nitriding is a prominent thermochemical surface treatment process which can improve various properties of metallic materials such as mechanical, tribological and/or corrosion properties. This process is predominantly performed by applying NH3+H2 containing gas atmospheres serving as the nitrogen donating medium at temperatures between 673 K and 873 K (400 °C and 600 °C). NH3 decomposes at the surface of the metallic specimen and nitrogen diffuses into the surface adjacent region of the specimen whereas hydrogen remains in the gas atmosphere. One of the most important parameters characterizing a gaseous nitriding process is the so-called nitriding potential (rN) which determines the chemical potential of nitrogen provided by the gas phase. The nitriding potential is defined as where and are the partial pressures of the NH3 and H2 in the nitriding atmosphere. In contrast with nitriding of alpha-Fe where the nitriding potential is usually in the range between 0.01 and 1 atm-1/2, nitriding of Ni and Ni-based alloys requires employing nitriding potentials higher than 100 atm-1/2 and even up to infinity (nitriding in pure NH3 atmosphere). This behavior is compatible with decreased thermodynamic stability of the 3d-metal nitrides with increasing atomic number. Depending on the nitriding conditions (temperature, nitriding potential and treatment time), different phases are formed at the surface of the Ni-based alloys. By applying very high nitriding potential, formation of hexagonal Ni3N at the surface of the specimen (known as external nitriding) leads to the development of a compound layer, which may improve tribological properties. Underneath the Ni3N compound layer, two possibilities exist: (i) alloying element precipitation within the nitrided zone (known as internal nitriding) and/or (ii) development of metastable and precipitate-free microstructure known as expanded austenite or S-phase, which can enhance surface hardness, fatigue properties and corrosion properties.Nitriding of multicomponent Ni-based alloys is usually applied in the industry. Nevertheless, the understanding of nitriding is mostly based on phenomenological research and experience. Thereby there is still absence of complete understanding of nitriding of Ni-based alloys, which requires further detailed investigations. Since studying the nitrided multicomponent alloys is complicated, in this thesis fundamental investigations were performed on pure nickel and binary Ni-based model alloys.This thesis focuses on the nitriding behavior of pure nickel, which will result with an thermodynamic evaluation of the Ni-N system. Furthermore, deeper insights in the nitriding behavior of the binary Ni-based alloys is obtained upon nitriding Ni-4 wt.% Ti and Ni-2 wt.% Ti (Ni-5 at.% Ti and Ni-2.5 at.% Ti) alloys. Thereby, the development of large residual macrostresses parallel to the surface of the specimen is related with the N concentration gradient in the nitrided zone.
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