Ingeniero Civil Mecánico / En el contexto de un estudio de creep en cobre reforzado con carburo de titanio nanométrico producido por pulvimetalurgia, es necesario conocer el valor del aporte del refinamiento de grano al esfuerzo de fluencia a temperatura ambiente. Durante tal estudio se han observado discrepancias entre lo predicho en la teoría y lo observado en el laboratorio para el aporte del tamaño de grano a una escala nanométrica. El endurecimiento por refinamiento de grano se modela a través de una ley de potencia, donde el aporte del tamaño de grano es igual a una constante por el tamaño de grano elevado a la menos un exponente. Tanto la constante como el exponente de la ley de potencia generan diferencias. Por esta razón, se busca estudiar el caso de cobre puro a temperatura ambiente, con tamaño de grano nanométrico, replicando el proceso de manufactura de pulvimetalurgia. El objetivo principal de este trabajo es encontrar la relación que existe entre el tamaño medio de grano y el límite de fluencia del cobre a temperatura ambiente. Para alcanzar el objetivo principal, se plantean los objetivos específicos de encontrar la constante y el exponente de la ley de potencia, junto con identificar el mecanismo de interacción dislocaciones-límite de grano y comparar los resultados obtenidos con la bibliografía. Para lograr los objetivos específicos, la metodología corresponde a realizar una extensiva revisión bibliográfica, manufacturar cinco probetas con distintos tamaños de grano nanométrico y evaluar el esfuerzo de fluencia de cada una de ellas. Se debe determinar el tiempo de molienda de los polvos de cobre para manufacturar las probetas, extruirlas en caliente y finalmente recocerlas para lograr distintos tamaños de grano, con densidad de dislocaciones constante. Las probetas se analizan con SEM para verificar el tamaño de grano y defectos de manufactura, DRX para evaluar el tamaño de cristalita y microdeformación, y con la máquina de ensayos de compresión para determinar el límite de fluencia. Los principales resultados corresponden a que el exponente es igual a 1, coincidiendo con el modelo de Pande y Masumura. La constante a temperatura ambiente es de 2,6Gb, la cual está influenciada por los defectos de manufactura del proceso de pulvimetalurgia, como poros y contaminación con la cápsula de extrusión. El mecanismo predominante es el de apilamiento de dislocaciones, el cual, dado el tamaño nanométrico de los granos, poseen una menor acción en el grano contiguo.

This thesis is a study of the size effect. Improvements on both theoretical work and experimental design are involved in this thesis. The theoretical section focuses on the grain size effect, while the experimental section is related to the micro-foil bending test. Both classic experimental data and theories for the Hall-Petch relationship are reviewed comprehensively. The fitting of the datasets show that the inverse square-root dependence and simple inverse expressions are equally good. The fully Bayesian analysis strongly suggests that the latter is correct. Since the physical mechanism underlying the simple inverse dependence is a general size effect, the precise description of the Hall-Petch effect is that it is a manifestation of the general size effect, instead of having its own special character. Improvements on the classic Stolken and Evans' micro-foil bending experiments are also carried out in this thesis. The smart design of the new equipment eliminates the big risk of error in the classic experiment. By using the new device, precise datasets from the elastic region through the yield point and to high plastic strain area can be obtained. The initial results correspond well with the old published data.

No description available.

As a candidate for dynamic electric contacts, Nano-twinned copper has intrinsic conductivity and enhanced fretting resistance. To better understand its general mechanical behavior, we conduct molecular dynamics simulation studies to investigate responses of nano-twinned copper to stress and to one-directional and two-directional sliding processes, in comparison with single crystal and nano-grained model systems. Obtained results suggest that the twin boundary blocks dislocation movement more effectively and the degree of emitting dislocations under stress is considerably lower than that of grain boundary. The inverse H-P relation only occurring in nano-grained materials does not necessarily result from grain boundary sliding. Under sliding conditions, dislocations are easier to be generated in the single crystal system. During the two-directional sliding process, Bauschinger effect is observed in the single crystal and nano-twinned systems, while the situation is opposite for the nano-grained system. The nano-twinned Cu shows the least dislocation accumulation during two-directional sliding. / Materials Engineering

No description available.

Submitted by Gabriela Lopes (gmachadolopesufpel@gmail.com) on 2016-10-03T17:35:05Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Dissertacao_Taiane_Viana.pdf: 904492 bytes, checksum: 30037558592bcb3fdf60ab01d643c574 (MD5) / Approved for entry into archive by Aline Batista (alinehb.ufpel@gmail.com) on 2016-10-06T20:46:17Z (GMT) No. of bitstreams: 2 Dissertacao_Taiane_Viana.pdf: 904492 bytes, checksum: 30037558592bcb3fdf60ab01d643c574 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2016-10-06T20:46:17Z (GMT). No. of bitstreams: 2 Dissertacao_Taiane_Viana.pdf: 904492 bytes, checksum: 30037558592bcb3fdf60ab01d643c574 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2013-12-04 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / O cultivo do trigo no Brasil é marcado pelo elevado número de doenças capazes de causar danos à produção. Entre as principais doenças que afetam o trigo está a giberela ou fusariose da espiga, causada pelo agente etiológico Gibberella zeae (Schw.) Petch, (anamorfo Fusarium graminearum Schwabe) que causa impacto negativos não somente à produtividade mas também a qualidade dos grãos produzidos. As condições ambientais para que a infecção ocorra carac terizam-se com temperaturas de 20-25°C e duração do molhamento foliar de, no mínimo, 48 horas consecutivas. O controle químico é realizado via aplicação de fungicidas a partir do inicio da floração estendendo-se até o final do florescimento, sendo que este tipo de controle caracteriza-se pela sua menor eficiência quando comparado com o controle de doenças foliares. Caso as condições climáticas impeçam a realização das aplicações de fungicidas no período indicado, o controle fica muito dificultado, por outro lado, se não ocorrer clima favorável à infecção, não se justifica o tratamento. A ineficiência dos tratos culturais no controle de giberela, associado à natureza quantitativa da característica de resistência, e o fato de sofrer alta influência do ambiente resultam na necessidade de identificar fontes efetivas de resistência que possam ser utilizadas em programas de melhoramento de trigo. De fato, existem culturas onde o controle das doenças mais importantes ocorre, quase que exclusivamente, por meio da resistência sendo que a identificação de genotipos que contenham resistência é um dos primeiros passos para selicioná-los. Desta forma, é fundamental o conhecimento de cultivares que apresentem níveis de resistência a giberela e métodos que possam identificar estas cultivares. Frente ao exposto o objetivo deste trabalho foi: i) separar genótipos suscetíveis e que contenham diferentes níveis de resistência utilizando avaliações morfológicas e moleculares; ii) verificar a resistência à giberela nas cultivares Frontana e Pampeano. Para isso foram utilizadas avaliações de características morfológicas citadas na literatura e associadas a resistência à giberela, assim como, o meprego de primers descritos na literatura significativamente associados à QTL de resistência à giberela. Como resultado do estudo espera-se auxiliar no desenvolvimento de ferramentas que possam ser utilizadas em programas de melhoramento de trigo visando facilitar a identificação de fontes de resistência. / The cultivation of wheat in Brazil is marked by the large number of diseases that can damage the production. Among the major diseases affecting wheat is wheat scab or Fusarium head blight (FHB), caused by the agent Gibberella zeae (Schw.) Petch (anamorph Fusarium graminearum Schwabe ) that causes negative impact not only on productivity but also the quality of the grain produced. The environmental conditions for infection are temperatures between 20-25°C and 48 consecutive hours of leaf wetness. Chemical control is performed via application of fungicides from the start extending until the end of flowering, and this type of control is characterized by its low efficiency when compared to the disease control. If the weather conditions prevent the aplication of fungicide applications within this period, there is no possibility control, on the other hand, if it does not favorable environment for infec tion treatment is not justified. The inefficiency of cultural control of FHB, associated with the quantitative trait (QTL- Quantitative traits loci)of resistance, and the fact of suffering high influence of the environment results in the need to identify effective sources of resistance that can be used in wheat breeding programs. In fact, there are cultures where control of major diseases occurs almost exclusively through resistance where the identification of genotypes containing resist ance is one of the first steps in their selection. Thus it is crucial to understand cultivar present levels of resistance to FHB and methods that can identify these cultivars.The objective of this study was: i) separate susceptible and containing different levels of resistance using molecular and morphological evaluations; ii) determine FHB resistance using cultivars Frontana and Pampeano as a source of resistance. Using morphological assessments cited in the literature and associated with resistance to FHB, as, primers as described in the literature as significantly associated with QTL for resistance to FHB. As a result of the study one expects to develop tools that can be used to facilitate the identification of sources of resistance.

Submitted by Haroudo Xavier Filho (haroudo.xavierfo@ufpe.br) on 2016-04-28T15:47:19Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_RobertoAlécio_Digital.pdf: 2746118 bytes, checksum: 727a29b8a8ea3f688e67892861afbb5b (MD5) / Made available in DSpace on 2016-04-28T15:47:19Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_RobertoAlécio_Digital.pdf: 2746118 bytes, checksum: 727a29b8a8ea3f688e67892861afbb5b (MD5) Previous issue date: 2015-11-13 / Uma previsão das propriedades mecânicas de sub-regiões da ZTA de uma junta soldada de aço API 5L X80 foi feita a partir de uma metodologia teórica e experimental. Para isso, foi utilizado um simulador termomecânico capaz de reproduzir ciclos térmicos com condições termodinâmicas de pontos discretos, semelhante à da ZTA de uma junta soldada produzida pelo processo GMAW, com um único passe, para comparação. No simulador termomecânico, três diferentes ciclos térmicos foram produzidos, a temperaturas de 900°C, 1000°C e 1100°C. Para cada temperatura foram produzidos três corpos de provas. As diferentes condições termodinâmicas impostas aos CPs reproduz a microestrutura de pontos ZTA de uma junta soldada. Em paralelo com a simulação, uma junta soldada do referido aço foi produzida para correlacionar a microestrutura de pontos discretos dessa junta. As amostras com a pseudo ZTA foram submetidos a ensaios mecânicos de tensão uniaxial e a caracterização microestrutural. Uma macrografia da ZTA da junta soldada foi realizada e esta, dividida em cinco sub-regiões e depois caracterizadas as microestruturas. As microestruturas das amostras simuladas foram comparadas com a microestrutura de ZTA das sub-regiões da junta soldada. Com base no valor das propriedades mecânicas das amostras simuladas e o tamanho do grão em ambas as microestruturas (soldada e simuladas) foram desenvolvidas um modelo numérico baseado na equação empírica de Hall-Petch, o que permitiu a determinação indireta da resistência ao escoamento de pontos discretos das sub-regiões do ZTA da junta soldada. / A prediction of the mechanical properties of the HAZ in sub-regions of a welded joint of API 5L X80 steel was made from a theoretical and experimental methodology. For this, it was used a thermomechanical simulator (TS) capable of reproducing thermal cycles with thermodynamic conditions of discrete points, similar to the HAZ of a real welded joint produced by GMAW, with a only pass, for comparations. In thermomechanical simulator, three different thermal cycles were produced at temperatures of 900°C, 1000°C and 1100°C. For each temperature three specimens were produced. The different thermodynamic conditions imposed on CPs reproduced microstructure of HAZ points of a welded joint. Parallel to the simulation, a welded joint of said steel was produced to facilitate the correlation between the microstructure of discrete points of this joint. The specimens with the so-called HAZ were subjected to mechanical tests in uniaxial tension and a microstructural characterization. A macrograph of the HAZ of the welded joint was made and divided into five sub-regions, later characterized microstructurally. The microstructures of the simulated test specimens were compared with the microstructure of HAZ sub-regions of the weld joint. Based on the value of the mechanical properties of the simulated test specimens and the grain size in both microstructures (Welded and simulated) was developed a numerical model based on the empirical equation of Hall-Petch, which enabled the indirect determination of the yield strength of discrete points of the sub-regions of the HAZ of the welded joint.

A better understanding of how the nanoscale environment affects the mechanical properties of materials, in particular metallic nanoparticles and nanocrystalline metals is vital to the development of next generation materials. Of special interest is obtaining a fundamental understanding of the inverse Hall-Petch Effect in nanocrystalline metals, and nanoindentation in individual nanoparticles. Understanding these subjects is critical to understanding how the mechanical properties of materials are fundamentally affected by nanoscale dimensions. These topics have been addressed by a combination of theoretical modeling and in-situ nanoindentation transmission electron microscopy (TEM) analysis. Specifically, the study of the inverse Hall-Petch effect in nanocrystalline metals will be investigated by a thorough review of the literature followed by a proposed novel theoretical model that better explains the experimentally observed behavior of nanocrystalline metals. On the other hand, the nanoindentation of individual nanoparticles is a very new research topic that has yet to aggregate a large body of experimental data. In this context, in-situ TEM nanoindentation experiments on silver nanoparticles will be first performed to determine the mechanisms of deformation in these nanostructures. A theoretical explanation for the observed deformation mechanisms will be then developed and its implications will be discussed. In addition to nanoparticles, this study will also provide unique and valuable insight into the deformation mechanisms of nanopillars, a growing area of research despite much controversy and speculation about their actual mechanisms of deformation. After studying the novel behavior of both nanocrystalline metals and nanoparticles, useful applications of both classes of materials will be explored. The discussion of applications will focus on utilizing the interesting behaviors explored in the dissertation. Of particular interest will be applications of nanoparticles and nanocrystalline materials to coatings, radiation resistance and super-plastic materials. / text

Microstructure controls many physical properties of a material such as strength, ductility, 1density, conductivity, which, in turn, determine the application of these materials. This thesis work focuses on studying microstructural features (such as grain size, shape, defects, orientation gradients) and mechanical properties (such as hardness and yield strength) of multilayered materials that have undergone different loading and/or operating conditions. Two materials that are studied in detail are 18 nm Cu-Nb nanolaminates and 3D printed Inconel 718. Copper-Niobium (Cu-Nb) nanolaminate is a highly stable, high strength, nuclear irradiation resistant composite, which is destabilized with application of high pressure torsion (HPT). This work focuses on understanding the deformation and failure behavior of Cu-Nb using a novel orientation mapping technique in transmission electron microscopy in (TEM) called Automated Crystal Orientation Mapping (ACOM) and Digistar (ASTARTM) or Precession Electron Diffraction (PED). A new theory is postulated to explain strengthening mechanisms at the nanoscale using a data analytics approach. In-situ TEM compression and tensile testing is performed to image dislocation movement with the application of strain. This experiment was performed by Dr. Lakshmi Narayan Ramasubramanian at Xi’an Jiaotong University in China. Another major aspect of this research focuses on the design, fabrication, and microstructural characterization of 3D printed Inconel 718 heat exchangers. Various heat exchanger designs, machine resolution, printing techniques such as build orientation, power, and velocity of the laser beam are explored. Microstructural and mechanical properties of printed parts (before and after heat treatment) are then analyzed to check consistency in grain size, shape, porosity, hardness in relation to build height, scan parameters, and design. Various tools have been utilized such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), x-ray computed microtomography (at Advanced Photon Source at Argonne National Lab), hardness and micro-pillar compression testing for this study.

In this study, mechanical properties of Al-Zn alloys were conducted, with various parameters including Zn contents, grain size, and tensile strain rate. Experimental samples were all manufactured with friction stir processing method. Samples of Al-Zn alloys with the grain size of 1.5£gm, 1£gm, or 0.5£gm and five Zn concentration were pulled in tension at strain rate of 10-3s-1,10-4s-1 and 10-5s-1 . The data set were then used to draw engineering and true tensile stress vs. strain curves , flowing stress vs. Zn contents curves, Hall-Petch equation curves, m vs. Zn contents curves and m vs. grain size curves. Quantitative analysis were conducted to discover that solid solute softening and inverse Hall-Petch relation were present in Al-Zn alloys, which were more prominent at slower tensile strain rate when grain size was less than 1£gm and the Zn contents was higher than 10wt%. Quantitative analysis of strain rate sensitivity (m) showed the trends of increasing value of m with higher Zn contents and smaller grain sizes when solid solute softening and inverse Hall-Petch relation were present. The high grain-boundary diffusion coefficient of Zn which accelerates the efficiency of dynamic recovery are considered the main reason. The effect gets more prominent with increasing Zn contents , smaller grain size , and slower tensile strain rate. For Zn concentration higher than 10wt%, dynamic recovery may drive inverse Hall-Petch relation to appear when grain size is about 1£gm large.