Ce travail est dédié à la compréhension du comportement mécanique des verres silicates à micro-échelle. Il est consacré à l'étude de la densification permanente des verres silicates et aux changements structuraux induits par différents méthodes. D'abord la densification par presse « Belt » en fonction de la température et de la pression est étudiée. Une inhomogénéité macroscopique à l'échelle de l'échantillon dépendant de la température et de la pression est observée. Ces observations sont interprétées par une diminution de la viscosité du verre avec la pression. Ensuite, une étude dans le domaine plastique du verre de silice par différents techniques « hautes pressions » est réalisée. Des modifications structurales sont observées durant la compression, en particulier une diminution de l'angle Si-O-Si intertétraèdre et une augmentation des petits cycles à 3 trétraèdres. De plus, il est montré que dans une compression hydrostatique la limite élasto-plastique d'un verre de silice prédensifiée ne dépend pas du chemin de densification. Le comportement du verre de silice prédensifié puis indenté évolue d'un verre anormal, vers un verre normal à mesure que la densité augmente. Enfin, l'indentation d'un verre silico-sodo-calcique sous indentation Vickers montre que la cartographie de la densification est similaire pour les deux charges (1 et 2 Kg) et que la distribution de la densification n'est pas affectée par la préparation des échantillons. Ces résultats sont comparés avec des résultats numériques obtenus par modélisation par éléments finis / This work was dedicated to the comprehension of mechanical behavior of silica glass at micro scale. It is focused on the study of the permanent densification of silica glass and structural changes induced by different methods. First, “Belt” densification is studied as a function of temperature and pressure. A macroscopic inhomogeneity at the sample scale was observed and it was dependent upon temperature. These observations were due to glass viscosity decrease with pressure. The plastic domain of silica glass with different “high pressure” techniques was analyzed. Structural modifications were observed during compression especially as Si-O-Si intertetrahedral angle decrease and an increase of small rings. Furthermore, it is demonstrated that during a hydrostatic compression, the elasto-plastic limit of predensified silica glass does not depend on densification path. We observed that the behavior of the predensified and indented silica glass evolves from normal to abnormal glass when density increases. Finally, the Vickers indentation of sodo-lime-silicate glass shows that densification cartography is similar for the two charges (1 and 2 Kg) and that densification distribution was not affected by sample preparation method. These results were compared with numerical calculations obtained by finite element modeling

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This thesis is focused on processing of pure magnesium by high pressure torsion method (HPT). This process belongs to the group of intensive plastic deformation methods (SPD). SPD methods are in the centre of scientific interest for several decades. Theoretical part of this thesis puts an effort to summarize basic knowledge and principles of SPD methods with extra focus on method HPT. As theoretical part continues magnesium as technical material is presented. Influence of SPD on use and properties of pure magnesium is then presented. This trend is further developed in effort to describe the effect of individual HPT process variables on the properties of pure magnesium and its alloys. Focus of practical part of this thesis is in influence of number of revolutions. Samples were processed at 1/8, 1/4, 1/2, 1, 4 and 8 turns at room temperature. Speed of process was 1rpm and applied pressure was 6 GPa. The structure of commercially pure magnesium prepared by casting and moulding were observed with focus on differences caused by input material. The structure was observed by both light microscopy and back scattered electron diffraction (EBSD), focusing on structure development, grain size and grain orientation. Compared to other works on similar topic, the emphasis here is on observing the microhardness on the vertical edge of the sample. The hardness shows a steep increase right after 1/8 of a turn. With increasing number of turns gradual homogenization of microhardness is presented accompanied by slight decrease in microhardness. No trend in microhardness relative to the distance from anvil has occurred. Structure observed with EBSD shows a bimodal character with larger grains oriented in the same direction. The three-point bending test didn’t end up as expected, and the approach to evaluation of magnesium-based HPT needs to be re-evaluated for future work. A three-point bending test was designed for the initial assessment of the basic mechanical properties of the material.

This thesis dealt with thermal stability of magnesium alloy AZ91 prepared by severe plastic deformation, which leeds to fine grained structure. This structure is characterised by its inherent instability and this thesis tries to find out the value of critical temperature and rate of this instability, which manifests as grain coarsening.

This thesis deals with mechanical and fatigue properties of magnesium alloy AZ91 processed by EX-ECAP method. This method involves the application of extrusion followed by equal-channel angular pressing. To obtain basic mechanical characteristics, the tensile test were carried out at room and elevated temperatures. Further the fatigue tests were performed and obtained data were used to compile S-N curve. The work also includes metallographics analysis of microstructure and fractographic analysis of fracture surfaces of tensile and fatigue test specimens.

De récents travaux consacrés à l’étude des propriétés des matériaux aux petites échelles ont souligné des différences exceptionnelles dans le comportement mécanique des nano-objets par rapport aux matériaux massifs. Dans le cas du silicium, une transition fragile-ductile à température ambiante a été observée lorsque la taille des échantillons est réduite. Cependant, les défauts et les mécanismes à l’origine de ce changement de comportement n’ont pas été clairement identifiés. Ce travail repose sur l’étude post mortem de nanopiliers déformés, en utilisant différentes techniques de microscopie électronique. Les nanopiliers étudiés ont été préparés par gravure plasma et déformés en compression à température ambiante. Les résultats obtenus durant cette thèse, confirment la différence de comportement des nano-objets par rapport au matériau massif. Par ailleurs, une grande variété de défauts produits lors de la compression a été observée. L’orientation cristallographique de l’axe de sollicitation semble avoir un impact important sur les mécanismes à l’origine du comportement ductile observé. La comparaison entre images HRTEM expérimentales et simulées témoigne de la propagation simultanée de dislocations partielles et parfaites dans les plans {111}. De plus, des événements plastiques ont également été observés dans des plans {115}. Divers mécanismes de déformation possibles impliqués lors de la compression des piliers sont décrits à partir des observations microscopiques. Un modèle tenant compte de l’influence sur la mobilité des dislocations des interactions entre systèmes de glissement est proposé afin d’expliquer la transition fragile-ductile observé aux petites échelles / Several recent works devoted to the study of the properties of materials at small scales have revealed exceptional differences in the mechanical behavior of nano-objects as compared to bulk material. In the case of silicon, a brittle-ductile transition at room temperature has been observed when the sample size decreases. However, the extended defects and mechanisms behind this behavioral change have not been clearly identified. This work is based on the post mortem study of deformed nanopillars, using different electron microscopy techniques. The studied nano-pillars of 100 nm in diameter were prepared by plasma etching (RIE) and deformed in compression at room temperature. The results obtained during this thesis confirm the difference in the behavior of nano-objects compared to bulk material. Moreover, a large variety of defects produced during plastic deformation has been observed. The crystallographic orientation of the deformation axis seems to have a significant impact on the mechanisms behind the observed ductile behavior. The comparison between experimental and simulated HRTEM images notably evidences the simultaneous propagation of partial and perfect dislocations in {111} planes. In addition, unexpected plastic events have also been observed in {115} planes. Various possible deformation mechanisms involved during the nano-compression of the pillars are described, based on the microscopic observations. Finally, a model considering the influence of interactions between various activated systems on the mobility of dislocations is proposed to explain the brittle-ductile transition observed at small scales in silicon

In the presented thesis we study the changes in distribution of the size of vacancy clusters in metals processed by severe plastic deformation, so called ul- tra fine grained materials. We use a modern non-destructive method of positron annihilation spectroscopy, which is one of the few methods that allow us to inves- tigate point defects like vacancies with sizes of a few ˚A. The obtained spectra of positrons annihilating in the samples enable us to determine changes of vacancy cluster sizes depending on temperature or severity of the deformation applied on the samples. 1

Title: Phase transformations in ultra-fine grained titanium alloys Author: Kristína Bartha Department: Department of Physics of Materials Supervisor of the doctoral thesis: PhDr. RNDr. Josef Stráský, Ph.D., Department of Physics of Materials Abstract: Ti15Mo alloy in a metastable β solution treated condition was processed by high pressure torsion (HPT) and equal channel angular pressing (ECAP). The microstructure after HPT is severely deformed and ultra-fine grained, while ECAP deformation results in rather coarse-grained structure with shear bands containing high density of lattice defects. Two types of thermal treatments - isothermal annealing and linear heating - were carried out for the solution treated condition and both deformed materials. Wide spectrum of experimental techniques was employed to elucidate the differences in phase transformations, especially in α phase precipitation, occurring in deformed and non-deformed material upon thermal treatment. It was shown that the α phase precipitation is accelerated in the deformed materials due to a high density of lattice defects, which provide a dense net of preferred sites for nucleation and also fast diffusion paths necessary for accelerated growth. The enhanced precipitation of the α phase in deformed materials also affects the stability of the ω...

in english For their positive influence on mechanical and physical properties of the material, methods of severe plastic deformation are popular for quite some time today. Rotary swaging is one of them. With its simplicity and productivity, it has the potential for industrial use. It is a radial swaging of rods or tubes, which results in decreasing of their diameter. Influence of this method is researched on extruded rods made of magnesium alloy AZ31. Experiments are made on five samples of different degree of swaging (different diameter). This allows to research gradual evolution of properties during the swaging. Results show significant positive influence on grain size (and microstructure in general) of originally extruded rod. During swaging there is an evolution of material texture and increase in strength.

Da in der Natur die Festigkeit der Stoffe bzw. Werkstoffe mit deren Massendichte korreliert [1], bieten sich dem Werkstoffingenieur zwei Möglichkeiten das genannte Ziel zu erreichen: Entweder er reduziert die effektive Dichte bereits sehr fester Werkstoffe durch konstruktive bzw. geometrische Optimierungen, oder es gelingt sehr leichte Werkstoffe mit deutlich gesteigerter Festigkeit herzustellen. Die erstgenannte Verfahrensweise stellt zu großen Teilen ein konstruktives bzw. fertigungstechnisches Problem dar. Von werkstoffwissenschaftlichem Interesse ist deshalb nur die zweite Möglichkeit. Dabei sollen sämtliche derzeit bekannte festigkeitssteigernde Faktoren und möglicherweise auch deren Synergien genutzt werden um einen hochfesten Leichtbauwerkstoff herzustellen. Dazu muss gleichzeitig ein neuartiges Hochumformverfahren für Leichtmetallverbundwerkstoffe erarbeitet werden, das diesen Anforderungen entspricht und eine dafür geeignete Werkstoffkombination gefunden werden. Konventionelle Verfahren zur Hochumformung erlauben häufig nur unter erheblichem Mehraufwand die Verarbeitung von Verbundwerkstoffen, weshalb die Hochumformung von Leichtmetallverbundwerkstoffen zur Festigkeitssteigerung in der Literatur praktisch keine Rolle spielt. Deshalb soll in dieser Arbeit das Umformverfahren Rundkneten zur Anwendung kommen, das die Hochumformung auch sehr heterogener Werkstoffe erlaubt. Darüber hinaus wird eine zusätzliche positive Wirkung auf die Festigkeit durch eingebaute Grenzflächen auf den Gesamtverbund erwartet. Wie sich im Laufe der Arbeit heraus stellte, eignet sich das verwendete Verfahren nicht ausschließlich zur Festigkeitssteigerung von Verbundwerkstoffen. Durch die sehr regelmäßige und fraktale Anordnung der Komponenten im Gesamtverbund ergaben sich auch einige Anknüpfungspunkte, die weit über die Eignung im Sinne eines Leichtbauwerkstoffes hinaus gehen. Aus diesem Grund liegt der Schwerpunkt der Arbeit zwar auf der mechanischen Charakterisierung der hergestellten Verbunde, in Kapitel 6 werden aber auch weitere Nutzungsmöglichkeiten diskutiert. Die gewählte Materialkombination Titan-Aluminium ist als Beispiel zu verstehen. Prinzipiell ist das vorgestellte Verfahren auf viele weitere Materialkombinationen anwendbar, solange grundlegende umformtechnische Regeln beachtet werden. [1] Ashby, M. F.: Materials Selection in Mechanical Design. Heidelberg: Spektrum Akademischer Verlag, 2006. 648 S.