京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19893号 / 工博第4209号 / 新制||工||1651(附属図書館) / 32970 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 松原 英一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Foi estudado, através de ensaio de tração, o Nb policristalino com teor de hidrogênio de 0 a 50 partes por milhão em peso (ppm-p) nas temperaturas de 223, 273 e 293 K. Os ensaios de tração a velocidades constantes foram realizados com taxas iniciais de deformaçao de 4,2 X 10-5 e 42 X 10-5 s-1 , e os parâmetros de ductilidade e resistência mecânica foram analisados em função da concentração de hidrogênio. Foram também obtidos o coeficiente de sensitividade a taxa de deformação (m) e o volume de ativação (V) através de ensaios de tração, alternando-se abruptamente a taxa de deformação entre os valores acima mencionados, em sucessivos pontos da curva tensão-deformação. Para o cálculo destes dois últimos parâmetros foi proposta uma análise detalhada, considerando-se os efeitos elásticos e o encruamento durante a deformação plástica uniforme. Todo o estudo foi acompanhado por análise fratográfica Que permitiu verificar três comportamentos de ruptura da liga Nb-H, dependendo do teor de hidrogênio e da temperatura. A 223K foi observado que há uma forte redução de ductilidade do Nb para teores de hidrogênio até 10 ppm-p, sendo que para este teor o seu comportamento foi totalmente frágil / Tensile testing at 223, 273 and 293 K was carried out on polycrystalline Nb dopped from 0 to 50 parts per million in weight (ppm-wt) of Hydrogen. The tensile testing at constant velocity was done at 4,2 X 10 -5 and 42 X 10-5 s-1 of initial strain rate, and the ductility and strength parameters was analysed as a function of the hydrogen content. It was also obtained the strain rate sensitivity (m) and the activation volume (V), from tensile testing, cycling between the two above specified strain rates, at several points of the stress-strain curve. For the calculation of this two last parameters it was proposed a detailed analysis, considering the elastic effect and the work hardening during the uniform plastic deformation. All these studies was followed by fratographic analysis that alowed the identification of three rupture behavior for the Nb-H alloy, depending of the temperature and the hydrogen content. At 223 K, it was observed that there is a strong embrittlement of Nb for hydrogen content up to 10 ppm-wt, and for this value the behavior was completely brittle

With the recent development of in situ transmission electron microscopy (TEM) characterization techniques, the real time study of property-structure correlations in nanomaterials becomes possible. This dissertation reports the direct observations of deformation behavior of Al2O3-ZrO2-MgAl2O4 (AZM) bulk ceramic nanocomposites, strengthening mechanism of twins in YBa2Cu3O7-x (YBCO) thin film, work hardening event in nanocrystalline nickel and deformation of 2wt% Al doped ZnO (AZO) thin film with nanorod structures using the in situ TEM nanoindentation tool. The combined in situ movies with quantitative loading-unloading curves reveal the deformation mechanism of the above nanomaterial systems. At room temperature, in situ dynamic deformation studies show that the AZM nanocomposites undergo the deformation mainly through the grain-boundary sliding and rotation of small grains, i.e., ZrO2 grains, and some of the large grains, i.e., MgAl2O4 grains. We observed both plastic and elastic deformations in different sample regions in these multi-phase ceramic nanocomposites at room temperature. Both ex situ (conventional) and in situ nanoindentation were conducted to reveal the deformation of YBCO films from the directions perpendicular and parallel to the twin interfaces. Hardness measured perpendicular to twin interfaces is ~50% and 40% higher than that measured parallel to twin interfaces, by ex situ and in situ, respectively. By using an in situ nanoindentation tool inside TEM, dynamic work hardening event in nanocrystalline nickel was directly observed. During stain hardening stage, abundant Lomer-Cottrell (L-C) locks formed both within nanograins and against twin boundaries. Two major mechanisms were identified during interactions between L-C locks and twin boundaries. Quantitative nanoindentation experiments recorded during in situ experiments show an increase of yield strength from 1.64 to 2.29 GPa during multiple loading-unloading cycles. In situ TEM nanoindentation has been conducted to explore the size dependent deformation behavior of two different types (type I: ~ 0.51 of width/length ratio and type II: ~ 088 ratio) of AZO nanorods. During the indentation on type I nanord structure, annihilation of defects has been observed which is caused by limitation of the defect activities by relatively small size of the width. On the other hand, type II nanorod shows dislocation activities which enhanced the grain rotation under the external force applied on more isotropic direction through type II nanorod.

<p>Commercial alloys are continuously developed to improve their performance. Therefore it is important to develop new optimisation software, which could be used in development of new materials or in materials selection. In this study the mechanical properties which are important in materials selection in mechanical design are investigated. Two types of materials are analysed, aluminium alloys and stainless steels but focus will be on the aluminium alloys.</p><p>Thermodynamic analysis has been used to evaluate the effect of the microstructure. Solid solution hardening has been successfully modelled for both aluminium alloys and stainless steels and follows the theories by Labusch and Nabarro. The precipitation hardening is most dominant for the hardenable aluminium alloys, but the non-hardenable alloys also increase their strength from precipitation hardening. The non-hardenable alloys are divided into tempers, which differ in the amount of strain hardening. This has also been modelled successfully.</p><p>Combining these fundamental results with multiple regression, models for mechanical properties have been created. Separate models are developed for wrought aluminium alloys and stainless steels. For the aluminium alloys this includes the solid solution hardening and the precipitation hardening. For the stainless steels the thickness, nitrogen content and ferrite content are included together with the solid solution hardening.</p>

Foi estudado, através de ensaio de tração, o Nb policristalino com teor de hidrogênio de 0 a 50 partes por milhão em peso (ppm-p) nas temperaturas de 223, 273 e 293 K. Os ensaios de tração a velocidades constantes foram realizados com taxas iniciais de deformaçao de 4,2 X 10-5 e 42 X 10-5 s-1 , e os parâmetros de ductilidade e resistência mecânica foram analisados em função da concentração de hidrogênio. Foram também obtidos o coeficiente de sensitividade a taxa de deformação (m) e o volume de ativação (V) através de ensaios de tração, alternando-se abruptamente a taxa de deformação entre os valores acima mencionados, em sucessivos pontos da curva tensão-deformação. Para o cálculo destes dois últimos parâmetros foi proposta uma análise detalhada, considerando-se os efeitos elásticos e o encruamento durante a deformação plástica uniforme. Todo o estudo foi acompanhado por análise fratográfica Que permitiu verificar três comportamentos de ruptura da liga Nb-H, dependendo do teor de hidrogênio e da temperatura. A 223K foi observado que há uma forte redução de ductilidade do Nb para teores de hidrogênio até 10 ppm-p, sendo que para este teor o seu comportamento foi totalmente frágil / Tensile testing at 223, 273 and 293 K was carried out on polycrystalline Nb dopped from 0 to 50 parts per million in weight (ppm-wt) of Hydrogen. The tensile testing at constant velocity was done at 4,2 X 10 -5 and 42 X 10-5 s-1 of initial strain rate, and the ductility and strength parameters was analysed as a function of the hydrogen content. It was also obtained the strain rate sensitivity (m) and the activation volume (V), from tensile testing, cycling between the two above specified strain rates, at several points of the stress-strain curve. For the calculation of this two last parameters it was proposed a detailed analysis, considering the elastic effect and the work hardening during the uniform plastic deformation. All these studies was followed by fratographic analysis that alowed the identification of three rupture behavior for the Nb-H alloy, depending of the temperature and the hydrogen content. At 223 K, it was observed that there is a strong embrittlement of Nb for hydrogen content up to 10 ppm-wt, and for this value the behavior was completely brittle

L'emboutissage est une technique de mise en forme des tôles métalliques. L'emboutissage est généralement suivi par un phénomène de retour élastique de la tôle emboutie. Le phénomène de retour élastique correspond à une modification de la géométrie de la tôle après enlèvement de la charge d'emboutissage. L'écrouissage du matériau est l'un des facteurs les plus importants qui influence ce phénomène. Dans cette thèse, l'écrouissage de différentes tôles dédiées à l'emboutissage a été étudié en utilisant la technique d'indentation instrumentée. Cette technique permet d'obtenir la loi d'écrouissage d'un matériau à partir de la mesure de l'évolution de l'enfoncement d'une bille dans le matériau testé en fonction de l'effort appliqué sur cette bille. Trois aspects essentiels de l'écrouissage ont été étudiés : La variation de la loi d'écrouissage dans l'épaisseur de la tôle, le niveau d'écrouissage atteint par la tôle après déformation plastique et le type d'écrouissage. Le test d'indentation instrumentée a permis de caractériser chacun de ces aspects influençant le retour élastique. Cet outil peut ainsi être utile dans une démarche d'amélioration de la prédiction du phénomène de retour élastique en emboutissage. / Stamping is a forming technique of the metal sheets. Stamping is generally followed by a springback phenomenon of the stamped sheet metal. The phenomenon of springback corresponds to a modification of the geometry of the sheet after removal of the load. One of the most important factors influencing springback is the work-hardening of the material. In this thesis, the work-hardening of different sheet metals used for stamping process was investigated using the instrumented indentation technique. This technique allows obtaining the work-hardening law of a material from the measurement of the evolution of the penetration depth of a ball in the tested material as a function of the applied force on the ball. Three major aspects of the work-hardening were studied: The variation of the work-hardening law in the thickness of the sheet, the level of work-hardening obtained after plastic deformation and the type of the work-hardening. The instrumented indentation test allows characterizing each of these aspects influencing springback. This tool can thus be useful in a process of improving the prediction of the springback phenomenon in stamping.

Commercial alloys are continuously developed to improve their performance. Therefore it is important to develop new optimisation software, which could be used in development of new materials or in materials selection. In this study the mechanical properties which are important in materials selection in mechanical design are investigated. Two types of materials are analysed, aluminium alloys and stainless steels but focus will be on the aluminium alloys. Thermodynamic analysis has been used to evaluate the effect of the microstructure. Solid solution hardening has been successfully modelled for both aluminium alloys and stainless steels and follows the theories by Labusch and Nabarro. The precipitation hardening is most dominant for the hardenable aluminium alloys, but the non-hardenable alloys also increase their strength from precipitation hardening. The non-hardenable alloys are divided into tempers, which differ in the amount of strain hardening. This has also been modelled successfully. Combining these fundamental results with multiple regression, models for mechanical properties have been created. Separate models are developed for wrought aluminium alloys and stainless steels. For the aluminium alloys this includes the solid solution hardening and the precipitation hardening. For the stainless steels the thickness, nitrogen content and ferrite content are included together with the solid solution hardening. / QC 20101122

La déformation plastique du alpha-titane est fortement anisotrope. Elle met en jeu des familles de systèmes de glissement aux propriétés diverses et différents types de macles. Dans cette étude, des essais de traction sur des échantillons de alpha-titane de pureté commerciale sont couplés avec des mesures d'émission acoustique et d'extensométrie locale à haute résolution. Ces essais révèlent la présence d'un puits sur la courbe d'évolution du taux d'écrouissage. Un effet inverse de la vitesse de déformation sur la profondeur de ce puits est trouvé selon que les échantillons sont déformés suivant le sens long ou le sens travers de la tôle laminée initiale. Des analyses statistiques des lignes de glissement montrent une prédominance initiale du glissement prismatique, particulièrement prononcée dans les échantillons prélevés suivant le sens long. Une diminution de l'activité relative du glissement prismatique est observée au cours de la déformation des deux types d'éprouvettes. Les fractions volumiques de macles sont plus élevées dans les essais réalisés en sens travers mais restent néanmoins très faibles (< 5 %), en particulier au niveau du puits (< 2 %). Ces résultats fournissent une base physique pour l'élaboration d'un modèle capable d'expliquer ce comportement particulier de l'écrouissage. Le modèle s'appuie sur un schéma auto-cohérent en élastoviscoplasticité, basé sur la méthode des champs translatés et utilisant une linéarisation affine de la relation constitutive viscoplastique. Le modèle considère la plasticité cristalline et traite séparément la densité de dislocations mobiles et la vitesse moyenne des dislocations. Il suppose une plus faible sensibilité à la vitesse de déformation ainsi qu'une multiplication plus rapide des dislocations sur les systèmes prismatiques. A partir de ces différentes hypothèses, les courbes de traction sont correctement reproduites et des estimations raisonnables des coefficients de Lankford, de l'activité relative du glissement prismatique et de l'évolution des textures sont obtenues. Plus important encore, l'effet inverse de la vitesse de déformation sur la profondeur du puits du taux d'écrouissage selon l'orientation de l'axe de traction est retrouvé de manière qualitative, ce qui permet d'avancer une explication aux phénomènes observés. Par ailleurs, les mesures d'émission acoustique et d'extensométrie locale à haute résolution permettent d'analyser le caractère intermittent et ondulatoire du alpha-titane à une échelle mésoscopique. Ces données sont confrontées aux prédictions du modèle actuel et serviront de base pour le développement futur d’un modèle plus complexe / The plasticity of alpha-titanium is strongly anisotropic. It involves slip systems families with various properties and different kinds of twins. In this study, tensile tests on commercially pur alpha-titanium samples are coupled with acoustic emission and high-resolution extensometry measurements. These tests show the presence of a well on the strain dependence of the work hardening. An opposite strain rate effect on the well depth is found whether specimens are elongated along the rolling or the transverse direction of the initially laminated sheet. Slip lines analysis reveals an initial predominance of prismatic slip, particularly pronounced in specimens strained along the rolling direction. The relative activity of prismatic slip is then observed to decrease with the deformation of both kinds of samples. The twin volume fractions are higher in the tests performed in the transverse direction but still remain very low (< 5 %), especially around the well (< 2 %). These results provide grounds for elaboration of a model capable of explaining such peculiar work hardening behavior. The model relies on a self-consistent scheme in elastoviscoplasticity, based on the translated field method and an affine linearization of the viscoplastic flow rule. The model considers crystal plasticity and deals separately with mobile dislocation density and dislocation velocity. It assumes lower strain rate sensitivity as well as higher dislocation multiplication rate for prismatic systems. Based on these assumptions, the model reproduces correctly the stress-strain curves and gives sound estimates of Lankford coefficients, prismatic slip activity and textures evolution. Most importantly, the opposite effect of strain rate on the well depth with regard to the orientation of the tensile axis is qualitatively retrieved, which allows putting forward an explanation of the observed phenomena. Besides, acoustic emission and high-resolution extensometry measurements allow analyzing the intermittent and wave nature of alpha-titanium at a mesoscopic scale. These data are confronted with the predictions of the present model and will be used as grounds for the future development of a more complex model

Tensile deformation characteristics of cast aluminum alloys have been investigated extensively. Cast Mg alloys have remained mostly neglected by researchers, despite their potential for weight savings. This present study is motivated by this gap in the literature and consists of two stages; in Stage 1, analysis of tensile data gathered from literature were reanalyzed, and in Stage 2, data generated from tensile testing of 60 specimens of AZ91 Mg alloy castings in both T4 and T6 conditions were analyzed to characterize work hardening behavior. In Stage 1, more than 1600 data were collected from the literature for various Mg alloy families. After plotting these data in yield strength-elongation charts, highest points were identified and interpreted as the maximum ductility, i.e., ductility potential (eFmax). The trend in maximum points indicated a linear relationship with yield strength (σY), expressed as; eF(max) = 41.8 - 0.106σY (1) This ductility potential equation can be used as a metric to compare elongation obtained from tensile specimens to measure the structural quality of Mg alloy castings. Moreover, results indicated that ductility potential was not affected by heat treatment, grain size (within 30-120 μm), casting geometry, size, the type of casting process nor chemical composition. In Phase 2, AZ91 cast Mg alloy specimens in T4 and T6 conditions were tested in tension to obtain stress-strain data for each specimen. Fits of four constitutive equations, namely, the Hollomon, Voce, Ludwik and Swift, to true stress-true plastic strain data in the elastoplastic region were characterized for the specimens with highest elongation values for T4 and T6 specimens. The coefficient of determination, R2, values for all equations were in excess of 0.99, suggesting that all four equations provide excellent fits to tensile data in both conditions. The change in work hardening rate with true stress was investigated for all specimens by using Kocks-Mecking (KM) plots. It was determined that work hardening behavior of Mg alloy castings in T4 and T6 is distinctly different. In T4 specimens, there is a plateau in work hardening rate at approximately E/25 which was observed in all specimens. The presence of this plateau is consistent with results given in the literature for pure Mg. However, this plateau was not observed in any of the T6 specimens. The reasons for the absence of the plateau in T6 specimens are unknown at this time. In both T4 and T6 specimens, the KM work hardening model in which work hardening rate changes linearly with true stress was found to be applicable. This is the first time that KM model was found to be valid for Mg alloys. Moreover in all specimens, there was a sudden drop in work hardening rate just prior to final fracture. This drop was first hypothesized to be due to structural defects in specimens, which was subsequently validated via fractography. Structural defects were found in all specimens whose fracture surfaces were investigated, indicating low to medium levels of quality. The quality index method, originally developed for cast aluminum alloys as the ratio of elongation to ductility potential, was found not to be applicable to Mg alloys, at least in its original form. This is due to the fact that work hardening behavior of cast aluminum alloys follows the KM model and there is no plateau where work hardening rate is constant. Hence the work hardening behavior of cast aluminum alloys and AZ91 specimens in T6 condition was similar. However the plateau of constant work hardening rate had a strong effect on elongation in T4 specimens. Therefore quality index analysis, which is supposed to be independent of alloy condition, did show that T4 and T6 specimens had different quality index levels. This finding contradicted the result from Stage 1 that aging has no effect on ductility potential. However because of the presence of structural defects in all specimens, quality index levels were low (0.30-0.45). Therefore it is unclear at this point whether the work hardening behavior of T4 and T6 specimens would still be different if elongation values were in the proximity of the ductility potential line. More research is needed to characterize work hardening behavior of cast Mg alloys in the absence of major structural defects and also address other questions raised in this study.

Although it offers a wide variety of interesting properties such as fire resistance or durability, stainless steel has been used in limited amount in structures. It is a known fact that the design rules don't properly account for the additional benefits of stainless steel properties and are largely based on the specifications for carbon steel. Indeed, a number of similarities exist between stainless steel and ordinary carbon steel but there is sufficient differences to afford a specific treatment in design standards. And since stainless steel is an expensive material, it is important to accurately predict the resistance of structural members. The present research work is dedicated to the study of cold-formed stainless steel profiles. It actually follows the life of a stainless steel construction element and falls on three fundamental topics: the material behavior, the through-thickness residual stress distribution and mechanical enhancement due to the cold-forming process and, last, the strength of concentrically compressed thin-walled columns. Firstly, several constitutive models are characterized such as Teodosiu-Hu's micro-structural based hardening model, capable of predicting the behavior of the studied stainless steel grade submitted to biaxial loading causing plastic strain. This model accounts for the nonlinear hardening behavior, the anisotropy, the Bauschinger effect and more complex behavior such as the observed work-hardening stagnation under reversed deformation at large strains. For this purpose, a collection of tests is carried out including multiaxial tests such as tensile-shear tests and successive simple shear tests and plane-strain tests. Secondly, the effects of the forming process on the mechanical properties are studied. To begin with, on the basis of the constitutive models developed previously, an analytical method that calculates the biaxial residual stress distribution in the walls and in the corners of cold-formed profiles is established. Based on the conclusions drawn from this theoretical analysis, a new formula for the evaluation of the actual mechanical properties is established. This formula is not restricted to a single alloy or type of cross-section. Current design standards are then used to calculate the strength of lipped-channel section columns failing by combined distortional and overall flexural-torsional buckling and the results are compared to tests. Indeed, full-scale tests on cold-formed stainless steel lipped channel section columns were achieved in the Structures Laboratory of the University of Liège. And, once verified against the test results, finite element models were used to generate additional results when necessary. The author then presents a new Direct Strength Method taking into account this phenomenon. Finally, a wide amount of reference results are gathered from the literature, without limiting oneself to any kind of cross-section or stainless steel grade. This database is used to propose an improved formulation for the design of stainless steel thin-walled section columns failing by distortion, local or combination of local and overall buckling in the low slenderness range.