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京都大学 / 新制・課程博士 / 博士(工学) / 甲第24614号 / 工博第5120号 / 新制||工||1979(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 邑瀬 邦明, 教授 宇田 哲也, 教授 作花 哲夫 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

<p>Solidification of Nb-microalloyed HSLA steels may result in the precipitation of niobium carbonitrides, which is hardly surprising in view of their extreme thermodynamic stability. Recently, it was proposed in literature that coarse Nb-rich particles found along the centerline of continuously cast HSLA steels originated from ferroniobium additions during ladle metallurgy. In particular, it was hypothesized that thermally stable phases formed during manufacturing of ferroniobium were released into the melt once the ferroniobium had partially fused. In this contribution, Scheil–Gulliver formalism is employed to predict the phase portrait of ferroniobium in an attempt to simulate the manufacturing process of ferroniobium.</p> <p>To corroborate the predictions, the microstructure of ferroniobium is characterized to determine if thermally stable particles exist in ferroniobium. Further, a model is developed to predict the dissolution rate of thermally stable phases that were observed in ferroniobium as well as in the centerline region of as-cast HSLA steel. Finally, a sample near the centreline region of a Nb-microalloyed HSLA steel is characterized and centreline compositions are measured. Based on experimental evidences, an alternative explanation to the origin of thermally stable particles found near the centreline of HSLA steels is proposed.</p> / Master of Materials Science and Engineering (MMatSE)

Yes / Diffusion brazing has gained much popularity as a technique capable of joining dissimilar lightweight metal alloys and has the potential for a wide range of applications in aerospace and transportation industries, where microstructural changes that will determine the mechanical and chemical properties of the final joint must be controlled. This study explores the effect of Al2O3 nanoparticles on the mechanical and microstructural properties of diffusion brazed magnesium (AZ31) and aluminium (Al-1100) joints. The results showed that the addition of Al2O3 nanoparticle to the electrodeposited Cu coating increased the volume of eutectic liquid formed at the interface which caused a change to the bonding mechanism and accelerated the bonding process. When the Cu/Al2O3 nanocomposite coatings were used as the interlayer, a maximum bond strength of 46 MPa was achieved after 2 min bonding time while samples bonded using pure-Cu interlayers achieved maximum strength after 10 min bonding time. Chemical analysis of the bond region confirmed that when short bonding times are used, the intermetallic compounds formed at the interface are limited to the compounds consumed in the eutectic reaction.

Yes / This study's objective was to conduct a comparative analysis and characterization of the microstructural evolution within the weld nugget for joints of AA5083-H112 and AA6061-T6 produced by friction stir spot welding (FSSW) and modified friction stir clinching (MFSC) processes. The mechanical performance of the welded joints was assessed in shear using a single lap joint. The microstructural study identified significant variation in joint microstructure and material flow due to the differences in the tool geometry and methodology used for the welding processes. The results show that the use of modified friction clinching process improves the welded joint by eliminating keyholes/hook defects leading to the formation of high-strength joints. Mechanical characterization of the welded joints indicated that the shear strength increased significantly from 78.69 MPa for the conventional FSSW to 131 MPa for the MFSC process.

Yes / Degrading infrastructure and applications of structural demolition create tremendous amounts of construction and demolition waste (CDW) all around the world. To address this issue in an effective way, recycling CDW in a most appropriate way has become a global concern in recent years. To this end, this study focused on the valorization of CDW-based materials such as tile, bricks, glass, and concrete in the development of geopolymer mortars. CDWs were first collected from demolition zone and then subjected to crushing-milling operations. To investigate the influence of slag (S) addition to the mixtures, 20% S substituted mixture designs were also made. Fine recycled concrete aggregates (FRCA) obtained from crushing and sieving of the waste concrete were used as the aggregate. A series of mixtures were designed using different proportions of three distinct alkali activators such as sodium hydroxide (NaOH), sodium silicate (Na2SiO3), and calcium hydroxide (CH; Ca(OH)2). To improve their applicability, the mixtures were left to cure at room temperature rather than the heat curing which is frequently applied in the literature. After 28 days ambient curing, the 100% CDW-based geopolymer mortar activated with three different activators reached a compressive strength of 31.6 MPa, whereas the 20% S substituted geopolymer mortar showed a 51.9 MPa compressive strength. While the geopolymer mortars activated with only NaOH exhibited poor performance, it was found that the use of Na2SiO3 and CH improved the mechanical performance. Main geopolymerization products were related to NASH (Sodium alumino-silicate hydrate), CASH (Calcium alumino-silicate hydrate), and C(N)ASH gel formations. Results demonstrated that mixed CDWs can be employed in the manufacturing geopolymers, making them potential alternatives to Portland cement (PC)-based systems by being eco-friendly, energy-efficient, and comparable in compressive strength. / This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 894100.

The purpose of this study was to characterize the craze growth behavior of polycarbonate (PC) as a function of stress level, model the residual mechanical properties of PC at various craze levels and strain rates, and determine if the total surface area of crazing is the sole factor in residual properties or if the crazing stress plays a role. To obtain these goals, a new in-situ reflective imaging technique was developed to quantify the craze severity in transparent polymers. To accomplish the goal of craze growth rate characterization, polycarbonate samples were placed under a creep load in a constant temperature, constant humidity environment. Using the new technique, the relative craze density was measured as a function of time under load at stresses of 40, 45, and 50 MPa. The craze growth rates were found to increase exponentially with stress level, and the times to 1% relative craze density were found to decrease exponentially with stress level. One exception to this behavior was found at a crazing stress of 50 MPa at which over half of the samples tested experienced delayed necking, indicating competitive mechanisms of crazing and shear yielding. The draw stress was found to be a lower bound below which delayed necking will not occur in a reasonable time frame. The yield stress, elastic modulus, failure stress, and ductility were correlated to crazing stress, relative craze density, and strain rate using a Design of Experiments (DOE) approach. The yield stress was found to correlate only to the strain rate, appearing to be unaffected by the presence of crazes. No correlation was found between the elastic modulus and the experimental factors. The failure stress was found to decrease with an increase in relative craze density from 0 to 1%, increase with an increase in crazing stress from 40 to 45 MPa, and correlate to the interaction between the crazing stress and the strain rate. The ductility of polycarbonate was found to decrease significantly with an increase in relative craze density, a decrease in crazing stress, and an increase in strain rate. The craze microstructure was correlated to the magnitude of stress during craze formation. The area of a typical craze formed at 40 MPa was measured to be more than 2.5 times larger than the area of a typical craze formed at 45 MPa. The fewer, but larger, crazes formed at the lower stress level were found to decrease the failure strength and ductility of polycarbonate more severely than the large number of smaller crazes formed at the higher stress level. / Ph. D.

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2009-2010 / Les propriétés mécaniques de la soie sont supérieures à toute autre protéine fibreuse et n'ont jamais été surpassées par aucun matériau synthétique. Une meilleure compréhension de la structure moléculaire de la soie et des changements qui s'opèrent pendant, avant et après le filage est requise pour maîtriser sa conception. La spectroscopic infrarouge fut utilisée pour étudier la structure moléculaire des fibres microscopiques de soie. Afin d'éviter les problèmes liés à l'acquisition de spectres par transmission, la réflexion totale atténuée (ATR) a été utilisée. Avec un nouveau montage permetant la rotation de l'échantillon, l'orientation et la conformation d'une seule fibre de soie furent quantifiées pour la première fois par spectroscopic infrarouge. Des simulations spectrales ont été effectuées pour trouver les conditions expérimentales minimisant ces effets optiques nuisibles à l'interprétation des spectres. Les constantes optiques de la soie native du ver à soie ont été déterminées pour la première fois afin d'exécuter des simulations spectrales avec des paramètres variables. Ses simulations ont également permis d'estimer l'amplitude des effets optiques observés pour tout autre système.