Nanoindentation of Bridgman YBCO samples

In this study, the mechanical properties of YBa2Cu3O7���x, obtained by the Bridgman technique, were examined using a Berkovich tip indenter on the basal plane (0 0 1). Intrinsic hardness was measured by nanoindentation tests and corrected using the Nix and Gao model for this material. Furthermore, Vickers hardness tests were performed, in order to determine the possible size effect on these measurements. The results showed an underestimation of the hardness value when the tests were performed with large loads. Moreover, the elastic modulus of the Bridgman samples was 128 �� 5 GPa. Different residual imprints were visualised by atomic force microscopy and a focused ion beam, in order to observe superficial and internal fracturing. Mechanical properties presented a considerable reduction at the interface. This effect could be attributed to internal stress generated during the texturing process. In order to corroborate this hypothesis, an observation using transmission electron microscopy was performed.

Flow and fracture behaviour of FV535 steel at different triaxialities, strain rates and temperatures

The new generation jet engines operate at highly demanding working conditions. Such conditions need very precise design which implies an exhaustive study of the engine materials and behaviour in their extreme working conditions. With this purpose, this work intends to describe a numerically-based calibration of the widely-used Johnson���Cook fracture model, as well as its validation through high temperature ballistic impact tests. To do so, a widely-used turbine casing material is studied. This material is the Firth Vickers 535 martensitic stainless steel. Quasi-static tensile tests at various temperatures in a universal testing machine, as well as dynamic tests in a Split Hopkinson Pressure Bar, are carried out at different triaxialities. Using ABAQUS/Standard and LS-DYNA numerical codes, experimental data are matched. This method allows the researcher to obtain critical data of equivalent plastic strain and triaxility, which allows for more precise calibration of the Johnson���Cook fracture model. Such enhancement allows study of the fracture behaviour of the material across its usage temperature range.

Comportamiento mec��nico de la aleaci��n W+1%Y2O3 en funci��n de la atm��sfera y la temperatura

Mediante ensayos de flexi��n en tres puntos se compara y eval��a el comportamiento mec��nico de la aleaci��n W- 1wt%Y2O3 con el W puro fabricados ambos mediante HIP. Se ha obtenido la tenacidad de fractura, la resistencia a flexi��n y el l��mite el��stico en atm��sfera oxidante y de vac��o en un intervalo de temperaturas comprendido entre -196 ��C, ensayos de inmersi��n en nitr��geno l��quido, y 1200 ��C. Previamente, se ha medido la densidad, la dureza mediante ensayos Vickers y el m��dulo de elasticidad din��mico de los materiales. Adem��s, la dureza y el m��dulo de elasticidad se han comparado con los obtenidos mediante ensayos instrumentados de nanoindentaci��n. Finalmente se ha realizado un peque��o estudio de las superficies de fractura de las muestras ensayadas mediante microscop��a electr��nica de barrido para poder relacionar el modo de rotura de los materiales y las propiedades mec��nicas macrosc��picas con los micromecanismos de fallo involucrados en funci��n de la temperatura.

Microestructura y propiedades mec��nicas del material masivo superconductor YBCO a 300 Y 77 K.

En este trabajo se realiza una caracterizaci��n mec��nica y microestructural del material masivo superconductor YBCO. El material ha sido procesado mediante dos t��cnicas distintas, Top-Seeding Melt Growth (TSMG) y Bridgman, y este estudio profundiza en el efecto de la microestructura, el m��todo de procesado y la temperatura de ensayo en el comportamiento mec��nico de material. Con el fin de conseguir un amplio conocimiento de sus propiedades mec��nicas se han realizado ensayos de resistencia a flexi��n, tenacidad de fractura y dureza Vickers a 300 y 77 K. Asimismo, se llevaron a cabo ensayos de nanoindentaci��n y el tama��o cr��tico del defecto semiel��ptico. Los resultados obtenidos muestran que el comportamiento mec��nico de los dos materiales est�� controlado por defectos y grietas, introducidas durante el procesado. Tambi��n se ha encontrado un buen acuerdo entre el tama��o del defecto critico detectado experimentalmente con los valores obtenidos mediante de an��lisis de mec��nica de fractura.

Laser shock peening without absorbent coating (LSPwC) effect on 3D surface topography and mechanical properties of 6082-T651 Al alloy

The influence of nanosecond laser pulses applied by laser shock peening without absorbent coating (LSPwC) with a Q-switched Nd:YAG laser operating at a wavelength of �� = 1064 nm on 6082-T651 Al alloy has been investigated. The first portion of the present study assesses laser shock peening effect at two pulse densities on three-dimensional (3D) surface topography characteristics. In the second part of the study, the peening effect on surface texture orientation and micro-structure modification, i.e. the effect of surface craters due to plasma and shock waves, were investigated in both longitudinal (L) and transverse (T) directions of the laser-beam movement. In the final portion of the study, the changes of mechanical properties were evaluated with a residual stress profile and Vickers micro-hardness through depth variation in the near surface layer, whereas factorial design with a response surface methodology (RSM) was applied. The surface topographic and micro-structural effect of laser shock peening were characterised with optical microscopy, InfiniteFocus�� microscopy and scanning electron microscopy (SEM). Residual stress evaluation based on a hole-drilling integral method confirmed higher compression at the near surface layer (33 ��m) in the transverse direction (��min) of laser-beam movement, i.e. ��� 407 �� 81 MPa and ��� 346 �� 124 MPa, after 900 and 2500 pulses/cm2, respectively. Moreover, RSM analysis of micro-hardness through depth distribution confirmed an increase at both pulse densities, whereas LSPwC-generated shock waves showed the impact effect of up to 800 ��m below the surface. Furthermore, ANOVA results confirmed the insignificant influence of LSPwC treatment direction on micro-hardness distribution indicating essentially homogeneous conditions, in both L and T directions.

Influencia de la adici��n de nano s��lice y nano al��mina en morteros de cemento de uso agro-ganadero

El objetivo de este trabajo es determinar la influencia de la incorporaci��n de nanoSiO2 y nanoAl2O3 sobre las propiedades macroestructurales de resistencia, dureza superficial y abrasi��n en morteros de cemento. Para ello se ha determinado la resistencia a compresi��n, la dureza Vickers y el coeficiente de Los Angeles en cuatro dosificaciones de mortero de cemento CEM I 52,5R, con una relaci��n agua/material cementante de 0.47. Las dosificaciones ensayadas fueron la de un mortero normalizado a la que se a��ade un 5% de nSi, un 5% de nAl y una adici��n mezcla de 2,5% de nSi y 2,5% de nAl. Se utilizo un mortero normalizado sin adici��n como control. Los resultados confirman que, aunque no hay diferencias significativas en las resistencias a compresi��n, hay un aumento considerable de la dureza superficial Vickers en el mortero con un 5% de la adici��n de nSi. Esto es debido a una densificaci��n de la matriz por un refinamiento de la estructura porosa y una reducci��n del tama��o de poro cr��tico. Sin embargo los resultados de desgaste no presentan diferencias significativas entre los morteros estudiados. Sin embargo, los coeficientes de Los ��ngeles est��n alrededor de 8,85, lo que les clasifica como materiales con buen comportamiento al desgaste.

Comportamiento mec��nico de nuevas aleaciones de wolframio en funci��n de la temperatura

La presente memoria de tesis tiene como objetivo principal la caracterizaci��n mec��nica en funci��n de la temperatura de nueve aleaciones de wolframio con contenidos diferentes en titanio, vanadio, itria y lantana. Las aleaciones estudiadas son las siguientes: W-0.5%Y2O3, W-2%Ti, W-2% Ti-0.5% Y2O3, W-4% Ti-0.5% Y2O3, W-2%V, W- 2%Vmix, W-4%V, W-1%La2O3 and W-4%V-1%La2O3. Todos ellos, adem��s del wolframio puro se fabrican mediante compresi��n isost��tica en caliente (HIP) y son suministradas por la Universidad Carlos III de Madrid. La investigaci��n se desarrolla a trav��s de un estudio sistem��tico basado en ensayos f��sicos y mec��nicos, as�� como el an��lisis post mortem de las muestras ensayadas. Para realizar dicha caracterizaci��n mec��nica se aplican diferentes ensayos mec��nicos, la mayor��a de ellos realizados en el intervalo de temperatura de 25 a 1000 �� C. Los ensayos de caracterizaci��n que se llevan a cabo son: ��� Densidad ��� Dureza Vicker ��� M��dulo de elasticidad y su evoluci��n con la temperatura ��� L��mite el��stico o resistencia a la flexi��n m��xima, y su evoluci��n con la temperatura ��� Resistencia a la fractura y su comportamiento con la temperatura. ��� An��lisis microestructural ��� An��lisis fractogr��fico ��� An��lisis de la relaci��n microestructura-comportamiento macrosc��pico. El estudio comienza con una introducci��n acerca de los sistemas en los que estos materiales son candidatos para su aplicaci��n, para comprender las condiciones a las que los materiales ser��n expuestos. En este caso, el componente que determina las condiciones es el Divertor del reactor de energ��a de fusi��n por confinamiento magn��tico. Parece obvio que su uso en los componentes del reactor de fusi��n, m��s exactamente como materiales de cara al plasma (Plasma Facing Components o PFC), hace que estas aleaciones trabajen bajo condiciones de irradiaci��n de neutrones. Adem��s, el hecho de que sean materiales nuevos hace necesario un estudio previo de las caracter��sticas b��sicas que garantice los requisitos m��nimos antes de realizar un estudio m��s complejo. Esto constituye la principal motivaci��n de la presente investigaci��n. La actual crisis energ��tica ha llevado a aunar esfuerzos en el desarrollo de nuevos materiales, t��cnicas y dispositivos para la aplicaci��n en la industria de la energ��a nuclear. El desarrollo de las t��cnicas de producci��n de aleaciones de wolframio, con un punto de fusi��n muy alto, requiere el uso de precursores de sinterizado para lograr densificaciones m��s altas y por lo tanto mejores propiedades mec��nicas. Este es el prop��sito de la adici��n de titanio y vanadio en estas aleaciones. Sin embargo, uno de los principales problemas de la utilizaci��n de wolframio como material estructural es su alta temperatura de transici��n d��ctil-fr��gil. Esta temperatura es caracter��stica de materiales met��licos con estructura c��bica centrada en el cuerpo y depende de varios factores metal��rgicos. El proceso de recristalizaci��n aumenta esta temperatura de transici��n. Los PFC tienen temperaturas muy altas de servicio, lo que facilita la recristalizaci��n del metal. Con el fin de retrasar este proceso, se dispersan part��culas insolubles en el material permitiendo temperaturas de servicio m��s altas. Hasta ahora se ha utilizado ��xidos de torio, lantano e itrio como part��culas dispersas. Para entender c��mo los contenidos en algunos elementos y part��culas de ��xido afectan a las propiedades de wolframio se estudian las aleaciones binarias de wolframio en comparaci��n con el wolframio puro. A su vez estas aleaciones binarias se utilizan como material de referencia para entender el comportamiento de las aleaciones ternarias. Dada la estrecha relaci��n entre las propiedades del material, la estructura y proceso de fabricaci��n, el estudio se completa con un an��lisis fractogr��fico y microgr��fico. El an��lisis fractogr��fico puede mostrar los mecanismos que est��n implicados en el proceso de fractura del material. Por otro lado, el estudio microgr��fico ayudar�� a entender este comportamiento a trav��s de la identificaci��n de las posibles fases presentes. La medida del tama��o de grano es una parte de la caracterizaci��n microestructural. En esta investigaci��n, la medida del tama��o de grano se llev�� a cabo por ataque qu��mico selectivo para revelar el l��mite de grano en las muestras preparadas. Posteriormente las micrograf��as fueron sometidas a tratamiento y an��lisis de im��genes. El documento termina con una discusi��n de los resultados y la compilaci��n de las conclusiones m��s importantes que se alcanzan despu��s del estudio. Actualmente, el desarrollo de nuevos materiales para aplicaci��n en los componentes de cara al plasma contin��a. El estudio de estos materiales ayudar�� a completar una base de datos de caracter��sticas que permita hacer una selecci��n de ellos m��s fiable. The main goal of this dissertation is the mechanical characterization as a function of temperature of nine tungsten alloys containing different amounts of titanium, vanadium and yttrium and lanthanum oxide. The alloys under study were the following ones: W-0.5%Y2O3, W-2%Ti, W-2% Ti-0.5% Y2O3, W-4% Ti-0.5% Y2O3, W-2%V, W- 2%Vmix, W-4%V, W-1%La2O3 and W-4%V-1%La2O3. All of them, besides pure tungsten, were manufactured using a Hot Isostatic Pressing (HIP) process and they were supplied by the Universidad Carlos III de Madrid. The research was carried out through a systematic study based on physical and mechanical tests as well as the post mortem analysis of tested samples. Diverse mechanical tests were applied to perform this characterization; most of them were conducted at temperatures in the range 25-1000 ��C. The following characterization tests were performed: ��� Density ��� Vickers hardness ��� Elastic modulus ��� Yield strength or ultimate bending strength, and their evolution with temperature ��� Fracture toughness and its temperature behavior ��� Microstructural analysis ��� Fractographical analysis ��� Microstructure-macroscopic relationship analysis This study begins with an introduction regarding the systems where these materials could be applied, in order to establish and understand their service conditions. In this case, the component that defines the conditions is the Divertor of magnetic-confinement fusion reactors. It seems obvious that their use as fusion reactor components, more exactly as plasma facing components (PFCs), makes these alloys work under conditions of neutron irradiation. In addition to this, the fact that they are novel materials demands a preliminary study of the basic characteristics which will guarantee their minimum requirements prior to a more complex study. This constitutes the motivation of the present research. The current energy crisis has driven to join forces so as to develop new materials, techniques and devices for their application in the nuclear energy industry. The development of production techniques for tungsten-based alloys, with a very high melting point, requires the use of precursors for sintering to achieve higher densifications and, accordingly, better mechanical properties. This is the purpose of the addition of titanium and vanadium to these alloys. Nevertheless, one of the main problems of using tungsten as structural material is its high ductile-brittle transition temperature. This temperature is characteristic of metallic materials with body centered cubic structure and depends on several metallurgical factors. The recrystallization process increases their transition temperature. Since PFCs have a very high service temperature, this facilitates the metal recrystallization. In order to inhibit this process, insoluble particles are dispersed in the material allowing higher service temperatures. So far, oxides of thorium, lanthanum and yttrium have been used as dispersed particles. Tungsten binary alloys are studied in comparison with pure tungsten to understand how the contents of some elements and oxide particles affect tungsten properties. In turn, these binary alloys are used as reference materials to understand the behavior of ternary alloys. Given the close relationship between the material properties, structure and manufacturing process, this research is completed with a fractographical and micrographic analysis. The fractographical analysis is aimed to show the mechanisms that are involved in the process of the material fracture. Besides, the micrographic study will help to understand this behavior through the identification of present phases. The grain size measurement is a crucial part of the microstructural characterization. In this work, the measurement of grain size was carried out by chemical selective etching to reveal the boundary grain on prepared samples. Afterwards, micrographs were subjected to both treatment and image analysis. The dissertation ends with a discussion of results and the compilation of the most important conclusions reached through this work. The development of new materials for plasma facing components application is still under study. The analysis of these materials will help to complete a database of the features that will allow a more reliable materials selection.

Mechanical behavior of 2G REBCO HTS at 77 and 300 K

El gran esfuerzo realizado durante la ��ltima d��cada con el fin de integrar los diferentes materiales superconductores en el campo de los sistemas el��ctricos y en otras aplicaciones tecnol��gicas ha dado lugar a un campo de investigaci��n amplio y prometedor. El comportamiento el��ctrico de los Superconductores de Alta Temperatura (SAT) cr��tica (masivo y cintas) depende de diferentes par��metros desde su fabricaci��n hasta la aplicaci��n final con imanes o cables. Sin embargo, las aplicaciones pr��cticas de estos materiales est��n fuertemente vinculadas con su comportamiento mec��nico tanto a temperatura ambiente (manipulaci��n durante fabricaci��n o instalaci��n) como a temperaturas criog��nicas (condiciones de servicio). En esta tesis se ha estudiado el comportamiento mec��nico de materiales masivos y cintas de alta temperatura cr��tica a 300 y 77 K (utilizando nitr��geno l��quido). Se han obtenido la resistencia en flexi��n, la tenacidad de fractura y la resistencia a tracci��n a la temperatura de servicio y a 300 K. Adicionalmente, se ha medido la dureza mediante el ensayo Vickers y nanoindentaci��n. El m��dulo Young se midi�� mediante tres m��todos diferentes: 1) nanoindentaci��n, 2) ensayos de flexi��n en tres puntos y 3) resonancia vibracional mediante grindosonic. Para cada condici��n de ensayo, se han analizado detalladamente las superficies de fractura y los micromecanismos de fallo. Las propiedades mec��nicas de los materiales se han comparado con el fin de entender la influencia de las t��cnicas de procesado y de las caracter��sticas microestructurales de los monocristales en su comportamiento mec��nico. Se ha estudiado el comportamiento electromec��nico de cintas comerciales superconductoras de YBCO mediante ensayos de tracci��n y fatiga a 77 y 300 K. El campo completo de deformaciones en la superficie del material se ha obtenido utilizando Correlaci��n Digital de Im��genes (DIC, por sus siglas en ingl��s) a 300 K. Adem��s, se realizaron ensayos de fragmentaci��n in situ dentro de un microscopio electr��nico con el fin de estudiar la fractura de la capa superconductora y determinar la resistencia a cortante de la intercara entre el substrato y la capa cer��mica. Se ha conseguido ver el proceso de la fragmentaci��n aplicando tensi��n axial y finalmente, se han implementado simulaciones mediante elementos finitos para reproducir la delaminaci��n y el fen��meno de la fragmentaci��n. Por ��ltimo, se han preparado uniones soldadas entre las capas de cobre de dos cintas superconductoras. Se ha medido la resistencia el��ctrica de las uniones con el fin de evaluar el metal de soldadura y el proceso. Asimismo, se ha llevado a cabo la caracterizaci��n mec��nica de las uniones mediante ensayos "single lap shear" a 300 y 77 K. El efecto del campo magn��tico se ha estudiado aplicando campo externo hasta 1 T perpendicular o paralelo a la cinta-uni��n a la temperatura de servicio (77 K). Finalmente, la distribuci��n de tensiones en cada una de las capas de la cinta se estudi�� mediante simulaciones de elementos finitos, teniendo en cuenta las capas de la cinta mec��nicamente m��s representativas (Cu-Hastelloy-Cu) que influyen en su comportamiento mec��nico. The strong effort that has been made in the last years to integrate the different superconducting materials in the field of electrical power systems and other technological applications led to a wide and promising research field. The electrical behavior of High Temperature Superconducting (HTS) materials (bulk and coated conductors) depends on different parameters since their processing until their final application as magnets or cables. However, practical applications of such materials are strongly related with their mechanical performance at room temperature (handling) as well as at cryogenic temperatures (service conditions). In this thesis, the mechanical behavior of HTS bulk and coated conductors was investigated at 300 and 77 K (by immersion in liquid nitrogen). The flexural strength, the fracture toughness and the tensile strength were obtained at service temperature as well as at 300 K. Furthermore, their hardness was determined by Vickers measurements and nanoindentation and the Young's modulus was measured by three different techniques: 1) nanoindentation, 2) three-point bending tests and 3) vibrational resonance with a grindosonic device. The fracture and deformation micromechanics have been also carefully analyzed for each testing condition. The comparison between the studied materials has been performed in order to understand the influence of the main sintering methods and the microstructural characteristics of the single grains on the macroscopic mechanical behavior. The electromechanical behavior of commercial YBCO coated conductors was studied. The mechanical behavior of the tapes was studied under tensile and fatigue tests at 77 and 300 K. The complete strain field on the surface of the sample was obtained by applying Digital Image Correlation (DIC) at 300 K. Addionally, in situ fragmentation tests inside a Scanning Electron Microscope (SEM) were carried out in order to study the fragmentation of the superconducting layer and determine the interfacial shear strength between substrate and ceramic layer. The fragmentation process upon loading of the YBCO layer has been observed and finally, Finite Element Simulations were employed to reproduce delamination and fragmentation phenomena. Finally, joints between the stabilizing Cu sides of two coated conductors have been prepared. The electrical resistivity of the joints was measured for the purpose of qualifying the soldering material and evaluating the soldering process. Additionally, mechanical characterization under single lap shear tests at 300 and 77 K has been carried out. The effect of the applied magnetic field has been studied by applying external magnetic field up to 1 T perpendicular and parallel to the tape-joint at service temperature (77 K). Finally, finite element simulations were employed to study the distribution of the stresses in earch layer, taking into account the three mechanically relevant layers of the coated conductor (Cu-Hastelloy-Cu) that affect its mechanical behavior

Mechanical characterisation of tungsten-1wt.% yttrium oxide as a function of temperature and atmosphere

This study evaluates the mechanical behaviour of an Y2O3-dispersed tungsten (W) alloy and compares it to a pure W reference material. Both materials were processed via mechanical alloying (MA) and subsequent hot isostatic pressing (HIP). We performed non-standard three-point bending (TPB) tests in both an oxidising atmosphere and vacuum across a temperature range from 77 K, obtained via immersion in liquid nitrogen, to 1473 K to determine the mechanical strength, yield strength and fracture toughness. This research aims to evaluate how the mechanical behaviour of the alloy is affected by oxides formed within the material at high temperatures, primarily from 873 K, when the materials undergo a massive thermal degradation. The results indicate that the alloy is brittle to a high temperature (1473 K) under both atmospheres and that the mechanical properties degrade significantly above 873 K. We also used Vickers microhardness tests and the dynamic modulus by impulse excitation technique (IET) to determine the elastic modulus at room temperature. Moreover, we performed nanoindentation tests to determine the effect of size on the hardness and elastic modulus; however, no significant differences were found. Additionally, we calculated the relative density of the samples to assess the porosity of the alloy. Finally, we analysed the microstructure and fracture surfaces of the tested materials via field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). In this way, the relationship between the macroscopic mechanical properties and micromechanisms of failure could be determined based on the temperature and oxides formed

Mechanical properties up to 1900 K of Al2O3/Er3Al5O12/ZrO2 eutectic ceramics grown by the laser floating zone method

Directionally solidified Al2O3���Er3Al5O12���ZrO2 eutectic rods were processed using the laser floating zone method at growth rates of 25, 350and 750 mm/h to obtain microstructures with different domain size. The mechanical properties were investigated as a function of the processing rate. The hardness, 15.6 GPa, and the fracture toughness, 4 MPa m1/2, obtained from Vickers indentation at room temperature were practically independent of the size of the eutectic phases. However, the flexural strength increased as the domain size decreased, reaching outstanding strength values close to 3 GPa in the samples grown at 750 mm/h. A high retention of the flexural strength was observed up to 1500 K in the materials processed at 25 and 350 mm/h, while superplastic behaviour was observed at 1700 K in the eutectic rods solidified at the highest rate of 750 mm/h

Effect of nano-Si2O and nano-Al2O3 on cement mortars for use in agriculture and livestock production

The effect of nano-silica, nano-alumina and binary combinations on surface hardness, resistance to abrasion and freeze-thaw cycle resistance in cement mortars was investigated. The Vickers hardness, the Los Angeles coefficient (LA) and the loss of mass in each of the freeze���thaw cycles to which the samples were subjected were measured. Four cement mortars CEM I 52.5R were prepared, one as control, and the other three with the additions: 5% nano-Si, 5% nano-Al and mix 2.5% n-Si and 2.5% n-Al. Mortars were tested at 7, 28 and 90 d of curing to determine compression strength, total porosity and pore distribution by mercury intrusion porosimetry (MIP) and the relationship between the CSH gel and Portlandite total by thermal gravimetric analysis (TGA). The capillary suction coefficient and an analysis by a scanning electron microscope (SEM) was made. There was a large increase in Vickers surface hardness for 5% n-Si mortar and a slight increase in resistance to abrasion. No significant difference was found between the mortars with nano-particles, whose LA was about 10.8, classifying them as materials with good resistance to abrasion. The microstructure shows that the addition of n-Si in mortars refines their porous matrix, increases the amount of hydrated gels and generates significant changes in both Portlandite and Ettringite. This produced a significant improvement in freeze���thaw cycle resistance. The effect of n-Al on mortar was null or negative with respect to freeze���thaw cycle resistance.