Ultrafeinkörnige metallische Werkstoffe haben verstärkt wissenschaftliche Bedeutung erlangt. Um dieser neuartigen Werkstoffklasse über die grundlagenorientierte Forschung hinaus einen Einsatz in technischen Anwendungen zu ermöglichen, ist es notwendig, deren Verhalten unter verschiedenen einsatzrelevanten Belastungsbedingungen vorhersagen zu können. In der vorliegenden Arbeit wird das Schädigungsverhalten einer ultrafeinkörnigen Aluminiumlegierung in den Bereichen der hochzyklischen (HCF) und niedrigzyklischen (LCF) Ermüdung sowie des Rissfortschritts untersucht. Im Mittelpunkt steht dabei die Identifikation der mikrostrukturell wirksamen Mechanismen bei der Entstehung und Ausbreitung von Ermüdungsrissen. Es werden ein homogen ultrafeinkörniger und ein bimodaler Zustand sowie verschiedene duktilitätsoptimierte Zustände betrachtet und systematisch der Einfluss der Korngröße, der Korngrößenverteilung, der Ausscheidungscharakteristik sowie der Festigkeit und Duktilität auf das Ermüdungs- und Rissfortschrittsverhalten ermittelt. Die Untersuchungen zeigen, dass das Schädigungsverhalten der ultrafeinkörnigen Aluminiumlegierung insbesondere durch die Korngröße und Korngrößenverteilung sowie den Kohärenzgrad der festigkeitssteigernden Ausscheidungen beeinflusst wird.

Industrial products of high-strength Al-alloys are currently manufactured by thermo-mechanical processes, which are only applicable in the integrated plants requiring high investment cost. Moreover, reduction of the average grain size not less than 10 &mu / m and re-adjustment of process parameters for each alloy type is evaluated as disadvantage. Therefore, recently there have been many research studies for development of alternative manufacturing techniques for aluminum alloys. Research activities have shown that it is possible to improve the strength of Al-alloys remarkably by severe plastic deformation which results in ultra-fine grain size. This study aims to design and manufacture the laboratory scale set-ups for severe plastic deformation of aluminum alloys, and to characterize the severely deformed samples. The stages of the study are summarized below: First, for optimization of die design and investigation of parameters affecting the deformation finite element modeling simulations were performed. The effects of process parameters (die geometry, friction coefficient) and material properties (strain hardening, strain-rate sensitivity) were investigated. Next, Equal Channel Angular Pressing (ECAP) system that can severely deform the rod shaped samples were designed and manufactured. The variations in the microstructure and mechanical properties of 2024 Al-alloy rods deformed by ECAP were investigated. Finally, based on the experience gained, a Dissimilar Channel Angular Pressing (DCAP) system for severe plastic deformation of flat products was designed and manufactured / then, 6061 Al-alloy strips were deformed. By performing hardness and tension tests on the strips that were deformed by various passes, the capability of the DCAP set-up for production of ultra-fine grain sized high-strength aluminum flat samples were investigated.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This investigated the use of equiangular channel press (ECAP) in samples of pure niobium at room temperature up to 14 passes, using the Bc route, with a matrix with intersection angle between the channels of θ = 90º and 120º, with and curvature angle Ψ = 0°. During ECAP (equal channel angular pressing) deformation a bar is pressed through a rigid matrix consisting of two channels of the same cross-section, which intersect at an angle θ. The microstructure of the deformed samples has been analyzed with the aid of optic microscopy, scanning electron microscopy and Vickers Hardness (HV). With the deformation of pure niobium through ECAP, it was possible to refine the structure and increase the hardness due to the increase of the number of passes and the type of matrix. The results indicated that deformation of the niobium with a pass in the matrices of 90 ° and 120 ° was enough to produce the refinement of the microstructure and practically double the hardness values in relation to the material without deformation. The maximum microstructural refinement, steady state, occurred with 8 passes in the 120 ° matrix and 6 passes in the 90 ° matrix, indicating a higher deformation severity of the last matrix. The microstructures analyzed by scanning electron and optical microscopes revealed the microstructural changes with the increase of the number of passes and the type of matrix. / Este trabalho investigou o uso da prensagem em canais equiangulares (ECAP) em amostras de nióbio puro na temperatura ambiente até 14 passes, utilizando a rota Bc, com matrizes contendo ângulo de intersecção entre os canais de θ = 90º e 120°. Durante a deformação ECAP uma barra é pressionada através de uma matriz rígida consistindo de dois canais de mesma seção transversal, os quais se interceptam a um ângulo θ. A microestrutura das amostras deformadas foi analisada com auxílio de microscopia óptica, microscopia eletrônica de varredura e dureza Vickers (HV). Com a deformação do nióbio puro através do ECAP foi possível refinar a estrutura e aumentar a dureza em função do aumento do número de passes e do tipo de matriz. Os resultados indicaram que deformação do nióbio com um passe, nas matrizes de 90º e 120°, foi suficiente para produzir o refinamento da microestrutura e praticamente dobrar os valores de dureza em relação ao material sem deformação. O refinamento microestrutural máximo, estado estacionário, ocorreu com 8 passes na matriz de 120° e 6 passes na matriz de 90°, indicando maior severidade de deformação da última matriz. As microestruturas analisadas por microscopias ótica e eletrônica de varredura revelaram as mudanças microestruturais com o aumento do número de passes e com o tipo de matriz. / São Cristóvão, SE

Magnesium alloys have poor formability at room temperature, due to a limited number of slip systems owing to the hexagonal closed packed structure of magnesium. One possibility to increase the formability of magnesium alloys is to refine the grain size. A fine grain magnesium alloy shows high strength and high ductility at room temperature, hence an improved formability. In addition to grain refinement, the formability of Mg alloys can be improved by controlling crystallographic texture. Severe plastic deformation (SPD) processes namely, equal channel angular pressing (ECAP) and multi-axial forging (MAF) have led to improvement in room temperature mechanical property of magnesium alloys. Further, it has been reported that by adding rare earth elements, room temperature ductility is enhanced to nearly 30%. The increase in property is attributed to crystallographic texture. Many rare earth elements have been added to magnesium alloys and new alloy systems have been developed. Amongst these elements, Ce addition has been shown to enhance the tensile ductility in rolled sheets at room temperature by causing homogeneous deformation. It has been observed that processing of rare-earth containing alloys below 300°C is difficult. Processing at higher temperatures leads to grain growth which ultimately leads to low strength at room temperature. The present thesis is an attempt to combine the effect SPD and rare earth addition, and to examine the overall effect on microstructure and texture, hence on room temperature mechanical properties. In this thesis, Mg-0.2%Ce alloy has been studied with regard to the two SPD processes, namely, ECAP and MAF. The thesis has been divided into six chapters. Chapter 1 is dedicated to introduction and literature review pertaining to different severe plastic deformation processes as applied to different Mg alloys. Chapter 2 includes the details of experimental techniques and characterization procedures, which are commonly employed for the entire work. Chapter 3 addresses the effect of ECAP on the evolution of texture and microstructure in Mg-0.2%Ce alloy. ECAP has been carried out on two different initial microstructure and texture in the starting condition, namely forged and extruded. ECAP has been successfully carried out for the forged billets at 250°C while cracks get developed in the extruded billet when ECAP was done at 250°C. The difference in the deformation behaviour of the two alloys has been explained on the basis of the crystallographic texture of the initial materials. The microstructure of the ECAP materials indicates the occurrence of recrystallization. The recrystallization mechanism is identified as “continuous dynamic recovery and recrystallization” (CDRR) and is characterized by a rotation of the deformed grains by ~30⁰ along c-axis. The yield strengths and ductility of the two ECAP materials have been found quite close. However, there is a difference in the yield strength as well as ductility values when the materials were tested under compression. The extruded billet has the tension compression asymmetry ~1.7 while the forged material has the asymmetry as ~2.2. After ECAP, the yield asymmetry reduces to ~1 for initially extruded billet, while for the initially forged billet the yield asymmetry value reduces to ~1.9. In chapter 4, the evolution of microstructure and texture was examined using another severe plastic deformation technique, namely multi axial forging (MAF). In this process, the material was plastically deformed by plane strain compression subsequently along all three axes. In this case also two different initial microstructures and texture were studied, namely the material in as cast condition and the extruded material. The choice of initial materials in this case was done in order to examine the effect of different initial grain size in addition to different textures. By this method, the alloy Mg-0.2%Ce could be deformed without fracture at a minimum temperature of 350⁰C leading to fine grain size (~3.5 µm) and a weak texture. Grain refinement was more in the initial cast billets compared to the initial extruded billet after processing. The mechanism of grain refinement has been identified as twin assisted dynamic recrystallization (TDRX) and CDRR type. The mechanical properties under tension as well as under compression were also evaluated in the present case. The initially extruded billet has shown low tension compression asymmetry (~1.2) than cast billet (~1.9), after MAF. Chapter 5 addresses the exclusive effect of texture on room temperature tensile properties of the alloy. Different textures were the outcomes of ECAP and MAF processes. In this case, in order to obtain an exact role of texture, a third of deformation mode, rolling, was also introduced. All the processed materials were annealed to obtain similar grain size but different texture. A similar strength and ductility for ECAP and MAF, where the textures were qualitatively very different, was attributed to the fact that texture of both the ECAP and MAF processed materials, was away from the ideal end orientation for tensile tests. In chapter 7, the final outcomes of the thesis have been summarized and scope for the future work has been presented.

This thesis deals with complex analysis of fine-grained magnesium alloy AZ91 prepared by ECAP process. Mechanical properties of investigated alloy in different states at various external conditions are compared. The structure of this material is inherently unstable therefore changes on microstructural and sub-microstructural level occur during thermal exposure and/or mechanical loading. These changes are analysed and quantified for investigated alloy in selected states in this thesis.

Les effets du pressage à canaux égaux (ECAP), un procédé de déformation plastique sévère, ont été examinés dans un alliage Al-2.5Cu-1.5Mg (pourcentage en masse) prône à être durci par traitement thermique et précipitant dans la région α + S. Une multitude de techniques microscopiques, calorimétriques et analytiques ont été utilisés pour caractériser et quantifier les microstructures, incluant la diffraction Kikuchi, la microscopie électronique en transmission, la calorimétrie différentielle à balayage et la sonde atomique tomographique. Quatre différents traitements thermiques initiaux ont été réalisés pour créer quatre microstructures différentes, contenant soit aucun précipités, des clusters Cu-Mg ou/et des composés intermétalliques Al2CuMg. Chaque spécimen a été soumis au procédé ECAP à température ambiante et les effets correspondants sur la microstructure et les propriétés mécaniques ont été analysés. Des expériences en compression pour de petite déformation (inférieures à 7%) ont aussi été entreprises sur les échantillons trempés pour étudier les effets de la compression sur la formation des clusters. Après la trempe et la compression, des clusters Cu-Mg ont été trouvés dans la matrice et il a été élucidé que la formation des clusters était déclenchée par la compression. La fraction volumique des clusters est corrélée directement par la déformation appliquée : plus la déformation est importante, plus la fraction volumique des clusters est importante. Après ECAP, la microstructure est constituée de longues bandes nanocristallines séparée par de gros grains non-déformés pour les échantillons contenant seulement des clusters avant la déformation, tandis que la présence de phase S, avant ECAP, conduit à des microstructures constituées de zones à gros grains et de zones à grains raffinés, distribués d’une façon homogène à travers les échantillons. Bien que les spécimens présentaient clairement des microstructures différentes après ECAP, impliquant que différents mécanismes de renforcement entre en jeux, la limite élastique se situait au-delà de 500 MPa. La limite élastique des échantillons fabriqués par ECAP a été modélisée en superposant les différents mécanismes de renforcement et en saisissant les paramètres microstructurels venant de la caractérisation dans le modèle. Il a été démontré qu’une très bonne corrélation existait entre les limites élastiques provenant du modèle et celles expérimentales. / The effects of equal channel angular pressing (ECAP), a severe plastic deformation (SPD) technique, were investigated in an age hardenable Al-2.5Cu-1.5Mg (weight percent (wt.%)) alloy precipitating in the α + S phase field. A variety of microscopy, calorimetry and analytical techniques were employed to characterize and quantify the microstructure, including transmission kikuchi diffraction (TKD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and atom probe tomography (APT). Four different initial heat-treatments were conducted to achieve four different microstructures, containing either no precipitates, Cu-Mg clusters or/and Al2CuMg intermetallics. Each specimen was subjected to ECAP at room temperature and the related effects on the microstructure and mechanical properties were analysed. Compression experiments for small strains (less than 7%) were also undertaken on the as-quenched samples to investigate the effects of compression on the formation of clusters.After quenching and compression, Cu-Mg clusters were found in the matrix and it was elucidated that the formation of clusters was triggered by pressing. The volume fraction of clusters was found to be correlated to the strain applied: the higher the strain, the higher the volume fraction.After ECAP, the microstructure was constituted of long nanocrystalline bands separated by large undeformed grains for the samples containing only clusters before deformation, while the presence of S phase, prior to ECAP, lead to microstructures constituted of both coarse and refined zones distributed homogeneously throughout the sample. Although the samples presented clearly different microstructures after ECAP, implying that different strengthening mechanisms were active, the yield strength was found to lie above 500 MPa. The yield strength of the ECAP processed samples was modelled by superposing the different strengthening mechanisms altogether and by inputting the microstructural parameters coming from characterisation in the model. It was demonstrated that a very good correlation existed between the modelled and experimental yield strength values.

FCC組織を有する純金属と合金において強ひずみ加工法の1つである側方押出し加工（ECAP）法を用いて超微細結晶材を作製し，ECAPの各段階における力学特性と微細組織を調査した．微細組織形成過程におけるセル壁の形成から結晶粒界の形成に着目し，積層欠陥エネルギーと固溶原子による固溶強化の効果の観点から，強ひずみ加工における加工硬化ステージの推移と微細組織形成の関係を議論した. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

ヘテロ構造組織を有する鉄・非鉄金属の組織形態に注目して,微視的構造やその挙動が巨視的現象(変形特性,成形性)として発現するメカニズムを解明することを目的とした.鉄系金属ではDual Phase型高張力鋼のスプリングバック現象のひずみ速度依存性,非鉄系金属では半凝固鋳造法と強ひずみ加工を組み合わせた亜共晶アルミニウム合金の衝撃後方押出し成形性に注目した.これらの検討によりいずれの試料においても,結晶粒界よりもスケールの大きいヘテロ構造に由来した変形機構が支配的であることが明らかになった. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University