Universität Stuttgart

Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1

Browse

Search Results

Now showing 1 - 10 of 30

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.
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.
Open Access
Microstructural changes and intermetallic compound formation in metallic bilayers
(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Rossi, Paul J.; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
This thesis investigates interdiffusion and intermetallic compound (IMC) formation, as well as their effects on the microstructure, in metallic thin-film bilayers. The investigation focuses on bilayers based on the Ag-Sn and Ag-In binary systems, which are technologically important as basis for lead free solders. Due to the enhanced diffusional mechanisms in these systems, diffusion occurs readily even at room and low temperatures. The proceeding interdiffusion eventually leads to IMC formation in the bilayers, allowing for the investigation of the kinetics of IMC formation and the associated microstructural changes at room and low temperatures. The combination of the properties special to thin films with the diffusional mechanisms in the binary Ag-Sn and Ag-In systems leads to interesting effects, such as the dependence of IMC formation on the stacking sequence in the bilayers. The obtained experimental results for both systems could be explained using thermodynamic and kinetic models. Experimental characterization of the bilayers mainly relied on X-ray diffraction (XRD) and electron microscopy. In order to investigate the effect of the deposition process on IMC formation and the microstructure of the bilayers, different physical vapor deposition (PVD) techniques, especially thermal evaporation and magnetron sputtering, were used for the preparation of the bilayers. During investigation of the Ag-Sn system it was found that ambiguity exists among the published crystal structures of the Ag3Sn IMC. Therefore, the crystal structure of Ag3Sn has been reinvestigated using high-resolution XRD in connection with Rietveld refinements.
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.
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.
Open Access
Design of nanostructured vanadium pentoxide scaffolds inspired by natural cuttlebone
(2018) Knöller, Andrea; Bill, Joachim (Prof. Dr.)
Metadata only
Internal precipitation of nitrides in iron-based alloys
(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Steiner, Tobias; Mittemeijer, Eric J. (Prof. Dr. Ir.)
X‐ray diffraction has become a standard method of microstructural analysis. However, in systems with a complex microstructure, the interpretation of the measured diffractograms may not be very straightforward. The influence of the precipitation of fine alloying element nitrides and the changes in precipitation morphology that occur upon continued nitriding on the shape and position of XRD peaks have been identified. The thus obtained quantitative model has been applied to a variety of precipitation systems and in general good agreement of predicted values and experimental results is found. The precipitation of finely distributed alloying element nitrides is the main strengthening mechanism in the diffusion zone of nitrided parts. Various alloying elements having an affinity for N show considerably different nitriding behavior. Cr shows a strong N‐affinity and therefore readily precipitates in the presence of N. However, Mo has a weak strength of interaction with N and reacts only slowly. In order to better understand the nitriding behavior of nitrided steels containing both Cr and Mo, the nitriding behavior of ternary Fe‐Cr‐Mo model alloys is investigated. The complex precipitation sequence of ternary mixed Cr-Mo-nitrides has been identified and the role of the Cr/Mo-ratio of the alloy is exposed. X‐ray diffraction has become a standard method of microstructural analysis. However, in systems with a complex microstructure, the interpretation of the measured diffractograms may not be very straightforward. The influence of the precipitation of fine alloying element nitrides and the changes in precipitation morphology that occur upon continued nitriding on the shape and position of XRD peaks have been identified. The thus obtained quantitative model has been applied to a variety of precipitation systems and in general good agreement of predicted values and experimental results is found.
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.
Open Access
Template controlled mineralization of functional ZnO thin films
(2017) Blumenstein, Nina; Bill, Joachim (Prof. Dr.)
In this thesis, the influence of different organic templates on the bioinspired deposition of ZnO thin films is investigated. Depending on the polarity of the templates, the growth and the properties of the films can be influenced. On a non-polar template, film growth is inhibited whereas homogeneous films grow on polar templates. Additionally, it was shown that on a template with high polarity a crystallographic texture is observed. This leads to a macroscopically measurable piezoelectric response of these samples. In the last part of this work, the incorporation of Al, Ga and In into the ZnO films was investigated. Measurements showed a blue shift of the UV photoluminescence emission and an improved electrical conductivity with increasing doping content.
Open Access
Interaction of carbon and nitrogen in iron
(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Göhring, Holger; Mittemeijer, Eric Jan (Prof. Dr. Ir.)

Universität Stuttgart

Browse

Filters

Settings

Sort By

Results per page

Search Results