Made available in DSpace on 2017-07-21T20:43:51Z (GMT). No. of bitstreams: 1 Milene Yumi Maeda.pdf: 4409513 bytes, checksum: c08056fcd59620f956225201d8330824 (MD5) Previous issue date: 2017-02-24 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Commercially pure aluminum, copper and silver samples were rolled at room and cryogenic temperatures until approximately 99% of thickness total reduction, causing deformation (ε) between 3.93 and 4.61 Although not in balance state, the metals tend to have more defects density when cryo rolled, especially higher dislocation density, evidenced by calculations based on X-ray data for copper and silver. Higher defects density implies superior hardness, tensile strength limit and yield strength, but smaller elongation. There was evidence of stacking fault energy (SFE) influence in the process, evaluating hardness and properties obtained through tensile tests of the materials. The cryogenic temperature (CT) and room temperature (RT) rolled samples were evaluated by hardness tests, tensile tests, scanning electron microscopy (SEM) and X-ray diffraction (XRD), which indicate influence of stacking fault energy (SFE) on process. The hardness of all the materials tend to drop when they are kept at RT after cryo rolling and bigger larger hardness decrease was observed for silver, which one has the lowest SFE and slightest hardness decreased was noticed for aluminum, which has high SFE. There is evidence that cryo rolling is more attractive for low SFE materials after ageing at RT, as long as silver presented simultaneous increase in higher tensile strength of about 53% and 29% gain of elongation when compared to the same one rolled at RT. Elongation gain of silver can be associated to static recrystallization, as evidenced contrasting silver’s tensile charts after ageing and recrystallized silver. In turn, copper presented 15% of strength limit increase and just 5% elongation, whereas aluminum had both strength limit and elongation reduced. / Amostras de alumínio, cobre e prata comercialmente puros foram laminadas à temperatura ambiente (TA) e criogênica (TC) até aproximadamente 99% de redução total de espessura, causando deformações (ε) entre 3,93 e 4,61. Embora não seja em estado de equilíbrio, os metais tendem a possuir maior densidade de defeitos quando laminados criogenicamente, sobretudo maior densidade de discordâncias, evidenciado pelos cálculos baseados nos dados obtidos através difração de raios-X para cobre e prata. Uma quantidade maior de defeitos implica em maiores dureza e limites de escoamento e resistência, mas menor alongamento. Houve indícios da influência da energia de falha de empilhamento (EFE) no processo, avaliando-se a dureza e as propriedades obtidas através dos ensaios de tração dos materiais. A dureza de todos tende a cair quando mantidos em TA após a laminação criogênica e observou-se uma maior queda de dureza para a prata, que tem baixa EFE e uma menor queda de dureza para o alumínio, que tem elevada EFE. Há indicativos de que a laminação criogênica é mais vantajosa para metais de baixa EFE após envelhecimento em TA, visto que a prata apresentou um aumento simultâneo de limite de resistência de aproximadamente 53% e um ganho de 29% de alongamento quando comparado à mesma laminada em TA. O aumento de alongamento da prata pode ser associado à recristalização estática da mesma, como pode ser evidenciado comparando-se os gráficos de tração da prata após envelhecimento com a prata recristalizada. O cobre, por sua vez, apresentou um aumento de 15% do limite de resistência e apenas 5% de alongamento, enquanto o alumínio apresentou redução tanto do limite de resistência quanto de alongamento.

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2018-04-26T13:22:22Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Marcel Tadshi Izumi.pdf: 4222698 bytes, checksum: e7c749993223b983b4e37d54985610de (MD5) / Made available in DSpace on 2018-04-26T13:22:22Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Marcel Tadshi Izumi.pdf: 4222698 bytes, checksum: e7c749993223b983b4e37d54985610de (MD5) Previous issue date: 2018-02-22 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Três metais CFC comercialmente puros (alumínio, cobre e prata) foram deformados por ensaios de tração uniaxial e caracterizados por difração de raios X in situ, utilizando uma fonte síncrotron, em temperatura ambiente (293K) e criogênica (77K). A supressão parcial da recuperação dinâmica decorrente do processamento criogênico permite melhorias nas propriedades mecânicas, tais como ductilidade e resistência. Esta supressão resulta em um aumento na densidade de defeitos internos dos metais durante a deformação, promovendo um refino microestrutural e aumento da microdeformação. O refino microestrutural é manifestado pela evolução de dimples na superfície de fratura e pela redução do tamanho médio de cristalitos. Todos os metais apresentaram maior resistência mecânica em temperaturas criogênicas, entretanto somente o cobre e a prata apresentaram aumento de ductilidade. Esse comportamento é atribuído à menor energia de defeito de empilhamento destes metais em comparação com o alumínio. / Three FCC commercially pure metals (aluminum, copper and silver) were deformed by uniaxial tensile tests and were characterized by in situ X-ray diffraction, using a synchrotron source, at room (293K) and cryogenic (77K) temperatures. The partial suppression of dynamic recovery due to cryogenic processing allows an improvement in mechanical properties, such as ductility and strength. This suppression results in an increase in the internal defects density of metals during the strain, promoting microstructural refining and increase of microstrain. The microstructural refinement is manifested by dimples evolution on the fracture surface and reduction of average crystallite size. All metals present higher mechanical strength at cryogenic temperature, nevertheless the ductility only was increased in copper and silver. This behavior is attributed to lower stacking fault energy of these metals in comparison with aluminum.

Plastic deformation in crystalline materials largely depends on the properties of dislocations, in particular their mobility. While continuum description of deformation of a crystalline metal can be made reasonably well by considering the elastic properties of dislocations and neglecting the core, crystallographic aspects of dislocation motion require precise understanding of the core effects. The concept of the generalized stacking fault (GSF) energy was proposed as means to describe this. GSF energy, a fundamental property of a given material, can be determined using first principles total energy calculations. In this thesis, we use GSF to understand some of the intriguing mechanical responses recently observed in some metallic systems. First, we examine the structures and stacking fault energies in Mg-Zn-Y alloy system. This system is unique in the sense that trace additions of Zn and or Y result in long period stacking sequences such as 6l and 14l, as reported in recent literature. Further, these alloys exhibit extraordinary mechanical properties. We attempt to rationalize these experimental observations through first principles calculations of energies of periodic structures with different stacking sequences and stacking faults. For pure Mg, we find that the 6-layer structure with the ABACAB stacking is most stable after the lowest energy hcp structure with ABAB stacking. Charge density analysis shows that the 2l and 6l structures are electronically similar, which might be a cause for better stability of 6l structure over a 4l sequence or other periodic structures. Addition of 2 atomic% Y leads to stabilization of the structure to 6l sequence whereas the addition of 2 atomic% Zn makes the 6l energetically comparable to that of the hcp. Stacking fault (SF) on the basal plane of 6l structure is higher in energy than that of the hcp 2l Mg, which further increases upon Y doping and decreases significantly with Zn doping. SF energy surface for the prismatic slip indicates dissociation of dislocations in alloys with a 6l structure. Thus, in an Mg-Zn-Y alloy, Y stabilizes the long periodicity, while Zn doping provides a synergistic effect in improving the mechanical properties alongwith strengthening due to long periodic phases. Our investigation of surface properties and magnetism in Ni revealed that, the universal binding energy relation (UBER) derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleavage cracked surfaces are allowed to relax through atomic displacements. We find that two characteristic length-scales are involved in the cleavage of a crystal accompanied by structural relaxation at the cleaved surface. Based on that, we suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of relaxed surfaces upon cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in fcc, bcc, and hcp metals, as well as covalently bonded materials. We also found that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occuring at higher displacements. We have attempted understanding these ideas through images obtained from electronic densities and eigen states. Our work should be useful in providing inputs to multi-scale simulations of fracture in materials. The third phase of the work reports the stacking fault energy and twinning in Ni with a particular emphasis on the size effect. Experimental and computational research on Nan crystalline metals (mostly on Ni) indicates unique facets of dislocation activity (extended partial dislocations) and modes of deformation (twinning). In order to capture the intrinsic scaling eject in the nano-regime, it is imperative to account for the complex electronic structure of the metal in question. The stacking fault (SF) and twinning fault (TF) energies in nano-thin elm of Ni with 7, 13, 19, and 25 layers of (111) planes were determined using rest-principles density functional theory (DFT) total energy calculations. Generalized planar fault (GPF) energy curves of the nano-thin alms show higher extreme vis-a-vis the bulk, indicating that creation of SFs in nano-Ni is relatively difficult. In contrast, the ratios of energy barriers relevant to nucleation of dislocations and twinning support the observed enhanced tendency for extended partial dislocation formation and twinning in the nano-thin films in comparison with bulk. Our results should be useful in understanding deformation behavior of nano-structured Ni-based alloys used as advanced structural materials.

Abstract Austenitic high-Mn (15–30 wt.%) based twinning-induced plasticity (TWIP) steels provide great potential in applications for structural components in the automotive industry, owing to their excellent tensile strength-ductility property combination. In certain cases, these steels might also substitute austenitic Cr-Ni stainless steels. The aim of this present work is to investigate the high-temperature flow resistance, recrystallisation and the evolution of microstructure of high-Mn steels by compression testing on a Gleeble simulator. The influence of Al alloying (0–8 wt.%) in the hot rolling temperature range (800°C–1100°C) is studied in particular, but also some observations are made regarding the influence of Cr alloying. Microstructures are examined in optical and electron microscopes. The results are compared with corresponding properties of carbon and austenitic stainless steels. In addition, the mechanical properties are studied briefly, using tension tests over the temperature range from -80°C to 200°C. Finally, a preliminary study is conducted on the corrosion behaviour of TWIP steels in two media, using the potentiodynamic polarization technique. The results show that the flow stress level of high-Mn TWIP steels is considerably higher than that of low-carbon steels and depends on the Al concentration up to 6 wt.%, while the structure is fully austenitic at hot rolling temperatures. At higher Al contents, the flow stress level is reduced, due to the presence of ferrite. The static recrystallisation kinetics is slower compared to that of carbon steels, but it is faster than is typical of Nb-microalloyed or austenitic stainless steels. The high Mn content is one reason for high flow stress as well as for slow softening. Al plays a minor role only; but in the case of austenitic-ferritic structure, the softening of the ferrite phase occurs very rapidly, contributing to overall faster softening. The high Mn content also retards considerably the onset of dynamic recrystallisation, but the influence of Al is minor. Similarly, the contribution of Cr to the hot deformation resistance and static and dynamic recrystallisation, is insignificant. The grain size effectively becomes refined by the dynamic and static recrystallisation processes. The tensile testing of TWIP steels revealed that the Al alloying and temperature have drastic effects on the yield strength, tensile strength and elongation. The higher Al raises the yield strength because of the solid solution strengthening. However, Al tends to increase the stacking fault energy that affects strongly the deformation mechanism. In small concentrations, Al suppresses martensite formation and enhances deformation twinning, leading to high tensile strength and good ductility. However, with an increasing temperature, SFE increases, and consequently, the density of deformation twins decreases and mechanical properties are impaired. Corrosion testing indicated that Al alloying improves the corrosion resistance of high-Mn TWIP steels. The addition of Cr is a further benefit for the passivation of these steels. The passive film that formed on 8wt.% Al-6wt.%Cr steel was found to be even more stable than that on Type 304 steel in 5–50% HNO3 solutions. A prolonged pre-treatment of the steel in the anodic passive regime created a thick, protective and stable passive film that enhanced the corrosion resistance also in 3.5% NaCl solution.

This Master’s thesis is focused on theoretical study of the high entropy alloy CoCrNi using ab initio calculations. The focus was on the effect of short range order on the relative stability of FCC and HCP structures and the value of stacking fault energy.The results show increase of stability in both types of structures wtih decreasing number of Cr-Cr nearest neighbours. The effect of the number of Cr-Cr nearest neighbours on the stacking fault energy previously shown in literature was not observed. However the strong dependency was found on the change of short range order caused by the shift of (1 1 1) planes after the transformation from the FCC to HCP structure. The effect of interstitial atoms C a N was also studied. Both these interstitials stabilise FCC structure and thus cause the increase of stacking fault energy. Both interstitials prefer octahedral positions with higher amount of Cr in their nearest neighbour shell.

Brønsted and Lewis acid sites located within microporous solids catalyze a variety of chemical transformations of oxygenates and hydrocarbons. Such reactions occur in condensed phases in envisioned biomass and shale gas upgrading routes, motivating deeper fundamental understanding of the reactivity-determining interactions among active sites, reactants, and solvents. The crystalline structures of zeolites, which consist of SiO₄ tetrahedra with isomorphously-substituted M⁴⁺ (e.g., Sn⁴⁺, Ti⁴⁺) as Lewis acid sites, or Al³⁺ with charge-compensating extraframework H⁺ as Brønsted acid sites, provide a reasonably well-defined platform to study these interactions within confining voids of molecular dimension. In this work, gas-phase probe reactions that afford independent control of solvent coverages are developed and used to interpret measured rate data in terms of rate and equilibrium constants for elementary steps, which reflect the structure and stability of kinetically relevant transition states and reactive intermediates. The foundational role of quantitative kinetic information enables building molecular insights into the mechanistic and active site requirements of catalytic reactions, when combined with complementary tools including synthetic approaches to prepare active sites and surrounding environments of diverse and intended structure, quantitative methods to characterize and titrate active sites and functional groups in confining environments, and theoretical modeling of putative active site structures and plausible reaction coordinates.<br><div><br></div><div>Bimolecular ethanol dehydration to diethyl ether was developed as a gas-phase catalytic probe reaction for Lewis acid zeolites. A detailed mechanistic understanding of the identities of reactive intermediates and transition states on Sn-Beta zeolites was constructed by combining experimental kinetic measurements with density functional theory treatments. Microkinetic modeling demonstrated that Sn active site configurations undergo equilibrated interconversion during catalysis (404 K, 0.5–35 kPa C₂H₅OH, 0.1–50 kPa H₂O) from hydrolyzed-open configurations ((HO)-Sn-(OSi≡)₃---HO-Si) to predominantly closed configurations (Sn-(OSi≡)₄), and identified the most abundant productive (ethanol-ethanol dimer) and inhibitory (ethanol-water dimer) reactive intermediates and kinetically relevant transition state (S_N2 at closed sites). Mechanism-based interpretations of bimolecular ethanol dehydration turnover rates (per Lewis acidic Sn, quantified by CD₃CN IR) enabled measuring chemically significant differences between samples synthesized to contain high or low densities of residual Si-OH defects (quantified by CD₃CN IR) within microporous environments that confine Sn active sites. Hydrogen-bonding interactions with Si-OH groups located in the vicinity of Sn active sites in high-defect Sn-Beta zeolites stabilize both reactive and inhibitory intermediates, leading to differences in reactivity within polar and non-polar micropores that reflect solely the different coverages of intermediates at active sites. The ability of confining microporous voids to discriminate among reactive intermediates and transition states on the basis of polarity thus provides a strategy to mitigate inhibition by water and to influence turnover rates by designing secondary environments of different polarity via synthetic and post-synthetic techniques. </div><div><br></div><div>Despite the expectation from theory that Sn active sites adopt the same closed configurations after high-temperature (823 K) oxidation treatments, distinct Sn sites can be experimentally identified and quantified by the ν(C≡N) infrared peaks of coordinated CD₃CN molecules, and a subset of these sites are correlated with first-order rate constants of aqueous-phase glucose-fructose isomerization (373 K). In contrast, <i>in situ</i> titration of active sites by pyridine during gas-phase ethanol dehydration catalysis (404 K) on a suite of Sn-zeolites of different topology (Beta, MFI, BEC) quantified the dominant active site to correspond to a different subset of Sn sites than those dominant in glucose-fructose isomerization. An extensive series of synthetic and post-synthetic routes to prepare Sn-zeolites containing Sn sites hosted within diverse local coordination environments identified a subset of Sn sites located in defective environments such as grain boundaries, which are more pronounced in Beta crystallites comprised of intergrowths of two polymorphs than in zeolite frameworks with un-faulted crystal structures. Sn sites in such environments adopt defect-open configurations ((HO)-Sn-(OSi≡)₃) with proximal Si-OH groups that do not permit condensation to closed configurations, which resolves debated spectroscopic assignments to hydrolyzed-open site configurations. Defect-open Sn sites are dominant in glucose-fructose isomerization because their proximal Si-OH groups stabilize kinetically relevant hydride shift transition states, while closed framework Sn sites are dominant in alcohol dehydration because they stabilize S_N2 transition states via Sn site opening in the kinetically relevant step and re-closing as part of the catalytic cycle. The structural diversity of real zeolite materials, whose defects distinguish them from idealized crystal structures and allows hosting Lewis acid sites with distinct local configurations, endows them with the ability to effectively catalyze a broad range of oxygenate reactions.</div><div><br></div><div>During aqueous-phase catalysis, high extra-crystalline water chemical potentials lead to intra-pore stabilization of H₂O molecules, clusters, and extended hydrogen-bonded networks that interact with adsorbed intermediates and transition states at Lewis acid sites. Glucose-fructose isomerization turnover rates (373 K, per defect-open Sn, quantified by CD₃CN IR) are higher when Sn sites are confined within low-defect, non-polar zeolite frameworks that effectively prevent extended water networks from forming; however, increasing exposure to hot (373 K) liquid water generates Si-OH groups via hydrolysis of siloxane bridges and leads to lower turnover rates commensurate with those of high-defect, polar frameworks. Detailed kinetic, spectroscopic, and theoretical studies of polar and non-polar titanosilicate zeolite analogs indicate that extended water networks entropically destabilize glucose-fructose isomerization transition states relative to their bound precursors, rather than influence the competitive adsorption of water and glucose at active sites. Infrared spectra support the stabilization of extended hydrogen-bonded water networks by Si-OH defects located within Si- and Ti-Beta zeolites, consistent with ab initio molecular dynamics simulations that predict formation of distinct thermodynamically stable clustered and extended water phases within Beta zeolites depending on the external water chemical potential and the nature of their chemical functionality (closed vs. hydrolyzed-open Lewis acid site, or silanol nest defect). The structure of water confined within microporous solids is determined by the type and density of intracrystalline polar binding sites, leading to higher reactivity in aqueous media when hydrogen-bonded networks are excluded from hydrophobic micropores.</div><div><br></div><div>Aluminosilicate zeolites adsorb water to form (H₃O⁺)(H₂O)_n clusters that mediate liquid-phase Brønsted acid catalysis, but their relative contributions to the solvation of reactive intermediates and transition states remain unclear. Bimolecular ethanol dehydration turnover rates (per H⁺, quantified by NH₃ temperature-programmed desorption and <i>in situ</i> titrations with 2,6-di-<i>tert</i>-butylpyridine) and transmission infrared spectra measured on Brønsted acid zeolites under conditions approaching intrapore H₂O condensation (373 K, 0.02–75 kPa H₂O) reveal the formation of clustered, solvated (C₂H₅OH)(H⁺)(H₂O)_n intermediates, which are stabilized to greater extents than bimolecular dehydration transition states by extended hydrogen-bonded water networks. Turnover rates deviate sharply below those predicted by kinetic regimes in the absence of extended condensed water networks because non-ideal thermodynamic formalisms are required to account for the different solvation of transition states and MARI. The condensation of liquid-like phases within micropores that stabilize reaction intermediates and transition states to different extents is a general phenomenon for Brønsted acid-catalyzed alcohol dehydration within zeolites of different topology (CHA, AEI, TON, FAU), which governs the initial formation and structure of clustered hydronium-reactant and water-protonated transition state complexes. Systematic control of liquid-phase structures within confined spaces by gas-phase measurements around the point of intrapore condensation enables more detailed mechanistic and structural insights than those afforded by either kinetic measurements in the liquid phase, or structural characterizations of aqueous systems in the absence of reactants.</div>

Hoch manganhaltige Eisenbasislegierungen sind bei Raumtemperatur austenitisch und antiferromagnetisch (afm). Dabei besteht die Besonderheit, dass sich durch Legierung die afm Übergangstemperatur (Néeltemperatur) so einstellen lässt, dass sie nahe Raumtemperatur liegt. FeMn-Basislegierungen zeigen in Abhängigkeit von der Zusammensetzung Transformation Induced Plasticity (TRIP) und/oder Twinning Induced Plasticity (TWIP), d.h. die niedrige Stapelfehlerenergie dieser Legierungen führt zu verformungsinduzierter, metastabiler Phasenbildung (TRIP) bzw. zur Bildung von Verformungszwillingen (TWIP) und dadurch zu außerordentlich hoher Duktilität bei gleichzeitig hoher Verfestigung. Darüber hinaus haben FeMn-Basislegierungen einen ausgeprägten Magnetovolumeneffekt und magnetoelastischen Effekt durch magnetische Ordnung. Daher sind die untersuchten FeMnNiCr-Basislegierungen auch prototypisch für afm Élinvarlegierungen. Da Élinvar jedoch für invariable Elastizität steht, bedingt eine Anwendung als temperaturkompensierte Konstantmodullegierungen die Glättung der ausgeprägten magnetischen Anomalien, die industriell noch in keiner Anwendung realisiert wurde. Der Vorteil dies für eine Anwendung zu erreichen, läge in der Unempfindlichkeit feinmechanischer Bauelemente, gegenüber magnetischen Feldern, die bei den industriell verfügbaren ferromagnetischen Élinvarlegierungen nicht gewährleistet ist. Mit Bezug zu feinmechanischen Schwingsystemen spielen dabei neben der Einstellung der magnetoelastischen Eigenschaften die Prozessierbarkeit, Kaltumformbarkeit und Festigkeit sowie deren wechselseitige Beeinflussung eine große Rolle. Die vorliegende Arbeit befasst sich daher mit der Anwendbarkeit der untersuchten FeMnNiCr-Legierungen. Dabei wurden grundlegende Untersuchungen zur Plastizität durchgeführt, die die mechanische Zwillingsbildung in diesen Legierungen charakterisiert und ein Modell der mechanischen Zwillingsbildung bei kleinen plastischen Dehnungen vorschlägt, das eine Abschätzung der Stapelfehlerenergie erlaubt. Die Untersuchung des Antiferromagnetismus umgeformter Proben zeigt das Auftreten thermoremanenter Magnetisierung (TRM), deren Größe mit dem Umformgrad der untersuchten Proben skaliert. Sie wird den durch Umformdefekte erzeugten unkompensierten Momenten in der afm Spinstruktur zugeschrieben. Diese werden durch eine magnetische Feldkühlung magnetisiert und koppeln durch Austauschwechselwirkung an die umgebende antiferromagnetische Matrix unterhalb der Néeltemperatur. Das komplexe thermomagnetische Verhalten der unkompensierten Momente wird experimentell beschrieben und phänomenologisch gedeutet. Die Weiterentwicklung und Bewertung technischer, ausscheidbarer FeMnNiCrBe- und FeMnNiCr(Ti, Al)-Legierungen wird mit Bezug zu den grundlegenden Untersuchungen dargestellt. Es wird gezeigt, dass die neu entwickelten ausscheidbaren FeMnNiCr(Ti, Al)-Legierungen eine vielversprechende Ausgangsbasis darstellen, afm Élinvarlegierungen technisch umzusetzen. / High manganese iron-base alloys are austenitic and antiferromagnetic (afm) at room temperature. By further alloying it is possible to tune the afm transition temperature (Néel temperature) near room temperature. FeMn-base alloys show extraordinary strain hardening as well as ductility because of Transformation Induced Plasticity (TRIP) and/or Twinning Induced Plasticty (TWIP), i.e. in dependence on composition the generally low stacking fault energy in these alloys allows for the mechanically induced formation of metastable phases (TRIP) or deformation twinning (TWIP). Furthermore, magnetic order causes distinct magnetovolume and magnetoelastic effects in these afm FeMn-base alloys. The investigated FeMnNiCr-base alloys are therefore prototypic for afm Élinvar alloys. However, as Élinvar is meant for invariant elasticity, an application as temperature compensated alloy with constant elastic modulus requires the smoothing of the pronounced magnetic anomalies, that is not industrially available yet. The advantage of afm Élinvar alloys in precision mechanics applications, would be their impassiveness with respect to magnetic fields that is not achievable by their ferromagnetic counterparts. For precision components like mechanic oscillators not only the tuning of the magnetoelastic properties but also the processing, cold formability and mechanical properties as well as their interplay have strong influence. Therefore this work addresses the applicability of the studied FeMnNiCr alloys. Elementary investigations on plasticity characterise the occurrence of TWIP in these alloys and propose a modell for deformation twinning at low plastic strains that allows for an estimation of the stacking fault energy. The investigations on the antiferromagnetism of deformed samples show the appearance of thermoremanent magnetisation (TRM). Its magnitude scales with the degree of deformation. The TRM is therefore attributed to uncompensated moments in the afm spin structure due to deformation induced defects. These are magnetised by a magnetic field cooling and couple to the afm matrix by exchange interaction below the Néel temperature. The complex thermomagnetic behaviour of the uncompensated moments is experimentally described and phenomenologically explained. The further development and assessment of engineering-grade pecipitable FeMnNiCrBe and FeMnNiCr(Ti, Al) alloys is presented in relation to the aforementioned elementary investigations. It is shown that the newly developped precipitable FeMnNiCr(Ti, Al) alloys are good candidates for afm Élinvar alloys in application.

Les aciers TWIP se déforment par maclage et par glissement de dislocations, avec pour conséquence de forts taux d’écrouissage. Les mécanismes de déformation sont contrôlés par l’énergie de faute d’empilement (EFE). Un modèle de prévision de l’EFE et une régression de TNéel (transition antiferro/paramagnétique) de l’austénite sont proposés pour les systèmes FeMnXC (X = Cu, Cr, Al, Si et Ti). Les nuances FeMnCuC étudiées ont une EFE plus faible que la nuance de référence Fe22Mn0,6C. La formation de martensite [epsilon]?se substitue au maclage, sans dégradation des caractéristiques mécaniques en traction. La contrainte d'écoulement diminue avec la teneur en carbone et la formation de martensite [alpha]' aux plus basses EFE réduit l'allongement à rupture. La substitution d'une partie du manganèse par du cuivre permet un gain de 20% sur le module d'Young à température ambiante, en abaissant TNéel en dessous de 0ºC. La précipitation intragranulaire de carbures de vanadium augmente la limite d’élasticité mais n’influence pas le taux d’écrouissage. Aucune interaction entre précipités et macles n'a été observée en microscopie. Les calculs de cohérence et les mesures au MET montrent que les carbures ont une relation d'orientation avec l'austénite et sont semi-cohérents avec une faible cohérence résiduelle. Les contraintes induites ne semblent pas suffisantes pour piéger de grandes quantités d'hydrogène. Les alliages FeMnC + TiC présentent un fort durcissement par effet composite en début de déformation, tandis que l'écrouissage par effet TWIP n'est pas modifié par la présence des particules TiC. Cependant, le clivage des précipités primaires de grande taille réduit l'allongement à rupture / TWIP steels deformation occurs by twinning and by dislocations gliding which leads to high a strain hardening. The deformation mechanisms are controlled by the stacking fault energy (SFE). A model for the prediction of the SFE and a law for TNéel (antiferro to paramagnetic transition) for austenite are proposed in FeMnXC systems (X = Cu, Cr, Al, Si et Ti). The studied FeMnCuC grades have a lower SFE than the Fe22Mn0,6C reference. The formation of [epsilon]-martensite replaces twinning without any deterioration of the mechanical properties. The flow stress decreases with the carbon content and the formation of [alpha]'-martensite at the lowest SFEs reduces the elongation to fracture. Substituting a part of the manganese content by copper leads to a 20% increase of the Young's Modulus at room temperature by decreasing TNéel below 0ºC. The precipitation of intragranular vanadium carbide increases the yield stress but does not influence the strain hardening rate. No interaction between precipitates and twins has been observed by microscopy. The coherency calculations and the TEM observations show that the carbides have an orientation relation with the austenite and are semi-coherent with a low residual coherency. The resulting stresses do not seem to be high enough to trap large quantities of hydrogen. The FeMnC + TiC alloys exhibit a strong hardening by composite effect at the beginning of deformation, while the strain hardening due to TWIP effect is not modified by the presence of the TiC particles. Meanwhile, cleavage occurs in the largest primary precipitates, which reduces the elongation to fracture

In dieser Dissertation wird die spektrale und örtliche Verteilung der Lumineszenz von Heterostrukturen in selbstorganisierten Nanodrähten (ND) mit Hilfe von Kathodolumineszenz-Spektroskopie (KL) im Rasterelektronenmikroskop untersucht. Diese Methode wird ergänzt durch Messungen der kontinuierlichen und zeitaufgelösten Mikro-Photolumineszenz. Drei verschiedene Strukturen werden behandelt: (i) GaAs-ND bestehend aus Segmenten der Wurtzit (WZ) bzw. Zinkblende (ZB) Kristallstrukturen, (ii) auf GaN-ND überwachsene GaN-Mikrokristalle und (iii) (In,Ga)N Einschlüsse in GaN-ND. Die gemischte Kristallstruktur der GaAs-ND führt zu komplexen Emissionsspektren. Dabei wird entweder ausschließlich Lumineszenz bei Energien unterhalb der ZB Bandlücke, oder aber zusätzlich bei höheren Energien, gemessen. Diese Differenz wird durch unterschiedliche Dicken der ZB und WZ Segmente erklärt. Messungen bei Raumtemperatur zeigen, dass die Bandlücke von WZ-GaAs mindestens 55 meV größer als die von ZB-GaAs ist. Die Lumineszenz-Spektren der GaN-Mikrokristalle enthalten verschiedene Emissionslinien, die auf Stapelfehler (SF) zurückzuführen sind. SF sind ZB Quantentöpfe verschiedener Dicke in einem WZ-Kristall und es wird gezeigt, dass ihre Emissionsenergie durch die spontane Polarisation bestimmt wird. Aus einer detaillierten statistischen Analyse der Emissionsenergien der verschiedenen SF-Typen werden Emissionsenergien von 3.42, 3.35 und 3.29 eV für die intrinsischen (I1 und I2) sowie für extrinsische SF ermittelt. Aus den entsprechenden Energiedifferenzen wird -0.022C/m² als experimenteller Wert für die spontane Polarisation von GaN bestimmt. Die Bedeutung sowohl der piezoelektrischen Polarisation als auch die der Lokalisierung von Ladungsträgern wird für (In,Ga)N-Einschlüsse in GaN-ND gezeigt. Hierbei spielt nicht nur die Lokalisierung von Exzitonen, sondern auch die individueller Elektronen und Löcher an unterschiedlichen Potentialminima eine Rolle. / In this thesis, the spectral and spatial luminescence distribution of heterostructures in self-induced nanowires (NWs) is investigated by cathodoluminescence spectroscopy in a scanning electron microscope. This method is complemented by data from both continuous and time-resolved micro-photoluminescence measurements. Three different structures are considered: (i) GaAs NWs containing segments of the wurtzite (WZ) and zincblende (ZB) polytypes, (ii) GaN microcrystals overgrown on GaN NWs, and (iii) (In,Ga)N insertions embedded in GaN NWs. The polytypism of GaAs NWs results in complex emission spectra. The observation of luminescence either exclusively at energies below the ZB band gap or also at higher energies is explained by differences in the distribution of ZB and WZ segment thicknesses. Measurements at room temperature suggest that the band gap of WZ GaAs is at least 55 meV larger than that of the ZB phase. The luminescence spectra of the GaN microcrystals contain distinct emission lines associated with stacking faults (SFs). SFs essentially constitute ZB quantum wells of varying thickness in a WZ matrix and it is shown that their emission energy is dominated by the spontaneous polarization. Through a detailed statistical analysis of the emission energies of the different SF types, emission energies of 3.42, 3.35 and 3.29 eV are determined for the intrinsic (I1 and I2) as well as the extrinsic SFs, respectively. From the corresponding energy differences, an experimental value of -0.022C/m² is derived for the spontaneous polarization of GaN. The importance of both carrier localization and the quantum confined Stark effect induced by the piezoelectric polarization is shown for the luminescence of (In,Ga)N insertions in GaN NWs. Not only localized excitons, but also electrons and holes individually localized at different potential minima contribute to the observed emission.

Durant cette thèse, on s’intéresse à l’application et au développement d’une théorie de mécanique des champs de dislocations et de désinclinaisons pour modéliser de façon continue les structures de cœur des dislocations et des joints de grains ainsi que leurs interactions. Le vecteur de Burgers/Frank des dislocations/désinclinaisons est régularisé par l’introduction d’un tenseur densité de dislocations/désinclinaisons. A ces densités de défauts sont associées des déformations et des courbures élastiques et plastiques incompatibles responsables de champs de contraintes et de moments de contraintes internes. Le mouvement des défauts produit de la plasticité et est pris en compte par des équations de transport qui font intervenir des forces motrices agissant sur les densités de défauts. Dans un premier temps, les désinclinaisons sont ignorées et nous appliquons la théorie de champ de dislocations seule pour étudier les structures de cœur de dislocations planaires en comparaison avec le modèle de Peierls-Nabarro. La relaxation d’une structure de cœur de dislocation coin initiale arbitraire révèle un étalement infini des densités de dislocations sous l’action de leur propre champ de contrainte interne. Pour stopper cette relaxation infinie, nous proposons d’ajouter une énergie de misfit dans notre modèle. Cette dernière donne lieu à une contrainte de rappel qui s’oppose à l’étalement des cœurs de dislocations et permet d’obtenir des configurations équilibrées. On retrouve la solution de Peierls-Nabarro si on utilise un potentiel sinusoïdal pour l’énergie. Nous substituons ensuite ce potentiel par des énergies de fautes d’empilement généralisées obtenues à partir de simulations atomistiques pour modéliser la dissociation des dislocations et leur mouvement dans le zirconium et le titane. Dans un deuxième temps, nous considérons la théorie complète et nous développons des lois d’élasticité constitutives qui sont propres aux défauts cristallins. Nous proposons qu’en plus des tenseurs élastiques habituels, des tenseurs d’élasticité additionnels existent au niveau du cœur des défauts et relient respectivement les contraintes aux courbures et les moments de contraintes aux déformations. Ces tenseurs sont de nature non locale par définition à cause des relations cinématiques entre déformations et courbures. Ils sont non nuls au niveau des cœurs des défauts où les hétérogénéités de déformations et de courbures sont fortes et deviennent nuls loin des défauts par centrosymétrie. On applique ces nouvelles lois d’élasticité à des distributions de dislocations et de désinclinaisons. On montre que les termes non locaux donnent lieu à des contraintes/moments de contraintes de rappel qui s’opposent aux parties locales. Dans le cas de la dislocation coin, on montre que sa représentation avec un dipôle de désinclinaison coin permet d’obtenir une configuration équilibrée sans l’ajout d’énergie de misfit. On étudie ensuite les interactions élastiques entre dislocations et joints de grains / In this contribution, we apply and develop a mechanical theory of dislocation and disclination fields, to model in a continuous way the core structure of dislocations and grain boundaries, as well as their interactions. The Burgers/Frank vector of dislocations/disclinations is regularized by the introduction of dislocation/disclination density tensors. Incompatible elastic and plastic strains and curvatures are associated to these defect densities and they lead to internal stress and couple stress fields. The motion of defects yields plasticity. It is accounted for by transport equations, where driving forces act on the defect densities. First, we overlook disclinations and we apply the pure dislocation model to investigate the structure of planar dislocation cores, in comparison with the Peierls-Nabarro model. The self-relaxation of an initially arbitrary core structure of an edge dislocation reveals that an infinite spreading of the dislocation density occurs under its own stress field. To stop this endless relaxation, we propose to add a misfit energy in our model. The latter yields a restoring stress that opposes to the spreading of dislocation cores and allows predicting equilibrium core structures. We retrieve the Peierls-Nabarro solution when we use a sinusoidal potential for the misfit energy. We then substitute this sinusoidal potential for generalized stacking fault energies as obtained from atomistic simulations, in order to model the dissociation and motion of dislocations in zirconium and titanium. Second, we consider the full theory and we develop elastic constitutive laws that are specific to crystal defects. We propose that in addition to standard elasticmoduli tensors, additional elastic tensors exist in the core regions of defects and relate respectively stresses to curvatures and couple stresses to strains. These tensors are nonlocal by definition due to kinematic relations between strains and curvatures. They are non-zero in the core of defects, where strong heterogeneities of strains and curvatures occur, and they become progressively null far from the defects due to centrosymmetry. We apply these new elastic laws to distributions of dislocations and disclinations. We show that the nonlocal elastic tensors lead to restoring stresses and couple stresses that oppose to their local parts. In the framework of edge dislocations, we show that the representation using dipoles of wedge disclination cores allows predicting equilibrium structures without adding a misfit energy. We then investigate elastic interactions between dislocations and grain boundaries