Multifuncionalização de nanopartículas de CoFe2O4 pela incorporação da prata na sua superfície : síntese e caracterização

Tese (doutorado)—Universidade de Brasília, Instituto de Física, 2015.

Recrystallization kinetic study of copper-bearing strip cast steels

The effects of copper on as-cast structure, recrystallization and precipitation kinetics of strip cast low carbon steel were investigated. As-cast microstructure mainly consists of polygonal ferrite and Widmanstatten ferrite. Recrystallization responses were strongly dependent on initial microstructure and Cu content. Precipitation strengthening was observed in high copper content alloys.

Effects of microstructure on the variation of the unloading behavior of DP780 steels

The nonlinear unloading behavior of three different commercial dual-phase steels (DP780 grade equivalent) was examined. These steels exhibited small variations in chemical composition (0.07 to 0.10 mass percent carbon) and martensite volume fraction (0.23 to 0.28), and they demonstrated similar hardening behavior. Uniaxial loading-unloading-loading tests were conducted at room temperature and quasi-static strain rates between engineering strains of 0.5 and 8%. Steel microstructures were examined using electron backscatter diffraction and nanoindentation techniques. The microplastic component of the unloading strain exhibited no dependence on the martensite volume fraction or the ferrite grain size within the small range encountered in this investigations. Instead, the magnitude of the microplastic component of the unloading strain increased as the strength ratio between the martensite and ferrite phases increased. Correspondingly, the apparent unloading modulus, or chord modulus, exhibited a greater reduction for equivalent increments of strain hardening as the strength ratio increased. These results suggest that springback can be reduced in structures containing two ductile phases if the strength ratio between the harder and softer phases is reduced.

Current trends in machinability research

The manufacturing index of a country relies on the quality of manufacturing research outputs. Theemergence of new materials such as composites and multi component alloy has replaced traditionalmaterials in certain design application. Materials with properties like high strength to weight ratio,fatigue strength, wear resistance, thermal stability and damping capacity are a popular choice for adesign engineer. Contrary, the manufacturing engineer is novice to the science of machining thesematerials. This paper is an attempt to focus on the current trends in machinability research and anaddition to the existing information on machining. The paper consist of information on machiningAustempered Ductile Iron (ADI), Duplex Stainless Steel and Nano-Structured Bainitic Steel. Thevarious techniques used to judge the machinability of these materials is described in this paper.Studying the chip formation process in duplex steel using a quick stop device, metallographic analysisusing heat tinting of ADI and cutting force analysis of Nano-structured bainitic steel is discussed.

Microstructure and hardness evolution during simulated coiling of a direct strip cast low carbon low niobium steel

This paper examines the impact of coiling temperature and duration on the phase transformation and precipitation behavior of a low carbon and low niobium direct strip cast steel. Coiling was performed at three carefully chosen temperatures: (1) in the ferrite (600°C), (2) during the austenite decomposition (700°C) and (3) in the austenite (850°C). The coiling conditions were found to strongly affect the final microstructure and hardness response, thus highlighting the necessity to judiciously design the coiling treatment. Optical microscopy, and scanning and transmission electron microscopy were used to characterize the microstructural constituents (polygonal ferrite, bainite and pearlite) and the NbC precipitates. Vickers macrohardness measurements are utilized to quantify the mechanical properties. The differences in hardening kinetics for the three different temperatures are shown to come from a complex combination of strengthening contributions.

Effect of retained austenite on wear resistance of nanostructured dual phase steels

Nanostructured super bainitic and quenching-partitioning (Q&P) martensitic steels with a significant amount of retained austenite obtained by low temperature bainitic transformation and Q&P respectively were studied to explore the effect of retained austenite on stirring wear resistance. The results suggest that the Q&P martensitic steel significantly enhanced the hardness of the worn surface (from 674 to 762 HV1) and increased the thickness of the deformed layer (,3.3 mm), compared to the nanostructured bainitic steel. The underlying reason is that the Q&P martensitic steel has a higher stability of retained austenite thereby providing a superior transformation induced plasticity effect to increase surface hardness and reduce wear rate during the wear process.

Investigation on decomposition behavior of austenite under continuous cooling in vanadium microalloyed steel (30MSV6)

In the present study, investigations are focused on microstructural evolution and the resulting hardness during continuous cooling transformation (CCT) in a commercial vanadium microalloyed steel (30MSV6). Furthermore, the effects of cooling rate and austenite grain size (AGS) on CCT behavior of the steel have been studied by employing high-resolution dilatometry. Quantitative metallography accompanied with scanning electron microscopy (SEM) has efficiently confirmed the dilatometric measurements of transformation kinetics and austenite decomposition products. A semi-empirical model has been proposed for prediction of microstructural development during austenite decomposition of the steel and the resultant hardness. The model consists of 8 sub-models including ferrite transformation start temperature, ferrite growth, pearlite start temperature, pearlite growth, bainite start temperature, bainite growth, martensite start temperature and hardness. The transformed fractions of ferrite, pearlite and bainite have been described using semi-empirical Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach in combination with Scheil's equation of additivity. The JMAK rate parameter for bainite has been formulated using a diffusion-controlled model. Predictions of the proposed model were found to be in close agreement with the experimental measurements.

Dependence of deformation behavior on grain size and strain rate in an ultrahigh strength-ductile Mn-based TRIP alloy

A novel ultra-high strength TRIP (transformation induced plasticity) steel, with ~1.5. GPa strength and good ductility of ~26% has been produced. The microstructure consists of ultrafine ferrite, and a large volume fraction of austenite. The flow stress was significantly increased by a reduction in the grain size, but the effect of strain rate on the flow stress was negligible. The formation of stress induced martensite was found to increase linearly with strain, and a reduction in the grain size correlated with an increase in the stress required to form the martensite.

The microstructure evolution and softening processes during high-temperature deformation of a 21Cr-10Ni-3Mo duplex stainless steel

The austenite and ferrite microstructure evolution and softening mechanisms have been investigated in a 21Cr-10Ni-3Mo duplex stainless steel, containing about 60% austenite, deformed in torsion at 1200°C using a strain rate of 0.7 s-1. The above experimental conditions led to the formation of a small volume fraction of new austenite grains through discontinuous dynamic recrystallization (DDRX), which could not account for the observed large softening on the flow curve. DDRX grains mainly formed through the strain-induced migration of the pre-existing austenite grain boundaries, known to dominate in single-phase austenite, complemented by subgrain growth in the interface regions with ferrite. A significant portion of austenite dynamic softening has been attributed to the large-scale subgrain coalescence, the extent of which increased with strain, which seems to have contributed substantially to the observed flow stress decrease. The above process thus appears to represent an alternative mode of austenite dynamic softening to the classical DDRX in the duplex austenite/ferrite microstructure, characterised by limited availability of the pre-existing austenite/austenite high-angle boundaries, deformed at a high temperature. The softening mechanism within ferrite has been classified as "continuous DRX", characterised by a gradual increase in misorientations between neighbouring subgrains with strain and resulting in the progressive conversion of subgrains into "crystallites" bounded partly by low-angle and partly by large-angle boundaries.

The role of thermomechanical routes on the distribution of grain boundary and interface plane orientations in transformed microstructures

In the current study, a series of thermomechanical routes were used to produce different microstructures (i.e., ferrite and martensite) in low-carbon low alloy steels. The five-parameter grain boundary character distribution was measured for all microstructures. The thermomechanical processing route altered the texture of the fully ferritic microstructure and significantly influenced the anisotropy of the grain boundary character distribution. Generally, the population of (111) planes increased with an increase in the γ-fiber texture for the ferritic microstructure, but it did not change the shape of the grain boundary plane distribution at specific misorientations. The most commonly observed boundaries in the fully ferritic structures produced through different routes were {112} symmetric tilt boundaries with the Σ3 = 60 deg/[111] misorientation; this boundary also had a low energy. However, the grain boundary plane distribution was significantly changed by the phase transformation path (i.e., ferrite vs martensite) for a given misorientation. In the martensitic steel, the most populous Σ3 boundary was the {110} symmetric tilt boundary. This results from the crystallographic constraints associated with the shear transformation (i.e., martensite) rather than the low-energy interface that dominates in the diffusional phase transformation (i.e., ferrite).

In situ synchrotron X-ray diffraction studies of the effect of microstructure on tensile behavior and retained austenite stability of thermo-mechanically processed transformation induced plasticity steel

Transmission electron microscopy and in situ synchrotron high-energy X-ray diffraction were used to investigate the martensitic transformation and lattice strains under uniaxial tensile loading of Fe-Mn-Si-C-Nb-Mo-Al Transformation Induced Plasticity (TRIP) steel subjected to different thermo-mechanical processing schedules. In contrast with most of the diffraction analysis of TRIP steels reported previously, the diffraction peaks from the martensite phase were separated from the peaks of the ferrite-bainite α-matrix. The volume fraction of retained γ-austenite, as well as the lattice strain, were determined from the diffraction patterns recorded during tensile deformation. Although significant austenite to martensite transformation starts around the macroscopic yield stress, some austenite grains had already experienced martensitic transformation. Hooke's Law was used to calculate the phase stress of each phase from their lattice strain. The ferrite-bainite α-matrix was observed to yield earlier than austenite and martensite. The discrepancy between integrated phase stresses and experimental macroscopic stress is about 300 MPa. A small increase in carbon concentration in retained austenite at the early stage of deformation was detected, but with further straining a continuous slight decrease in carbon content occurred, indicating that mechanical stability factors, such as grain size, morphology and orientation of the retained austenite, played an important role during the retained austenite to martensite transformation.

A modified approach to modeling of diffusive transformation kinetics from nonisothermal data and experimental verification

An inverse model is proposed to construct the mathematical relationship between continuous cooling transformation (CCT) kinetics with constant rates and the isothermal one. The kinetic parameters in JMAK equations of isothermal kinetics can be deduced from the experimental CCT kinetics. Furthermore, a generalized model with a new additive rule is developed for predicting the kinetics of nucleation and growth during diffusional phase transformation with arbitrary cooling paths based only on CCT curve. A generalized contribution coefficient is introduced into the new additivity rule to describe the influences of current temperature and cooling rate on the incubation time of nuclei. Finally, then the reliability of the proposed model is validated using dilatometry experiments of a microalloy steel with fully bainitic microstructure based on various cooling routes.

NONLINEAR EFFECTS IN MAGNETIC GARNET FILMS AND NONRECIPROCAL OPTICAL BLOCH OSCILLATIONS IN WAVEGUIDE ARRAYS

This dissertation presents detailed experimental and theoretical investigations of nonlinear and nonreciprocal effects in magnetic garnet films. The dissertation thus comprises two major sections. The first section concentrates on the study of a new class of nonlinear magneto-optic thin film materials possessing strong higher order magnetic susceptibility for nonlinear optical applications. The focus was on enlarging the nonlinear performance of ferrite garnet films by strain generation and compositional gradients in the sputter-deposition growth of these films. Under this project several bismuth-substituted yttrium iron garnet (Bi,Y) 3 (Fe,Ga)5 O12(acronym as Bi:YIG) films have been sputter-deposited over gadolinium gallium garnet (Gd 3 Ga5 O12 ) substrates and characterized for their nonlinear optical response. One of the important findings of this work is that lattice mismatch strain drives the second harmonic (SH) signal in the Bi:YIG films, in agreement with theoretical predictions; whereas micro-strain was found not to correlate significantly with SH signal at the micro-strain levels present in these films. This study also elaborates on the role of the film's constitutive elements and their concentration gradients in nonlinear response of the films. Ultrahigh sensitivity delivered by second harmonic generation provides a new exciting tool for studying magnetized surfaces and buried interfaces, making this work important from both a fundamental and application point of view. The second part of the dissertation addresses an important technological need; namely the development of an on-chip optical isolator for use in photonic integrated circuits. It is based on two related novel effects, nonreciprocal and unidirectional optical Bloch oscillations (BOs), recently proposed and developed by Professor Miguel Levy and myself. This dissertation work has established a comprehensive theoretical background for the implementation of these effects in magneto-optic waveguide arrays. The model systems we developed consist of photonic lattices in the form of one-dimensional waveguide arrays where an optical force is introduced into the array through geometrical design turning the beam sideways. Laterally displaced photons are periodically returned to a central guide by photonic crystal action. The effect leads to a novel oscillatory optical phenomenon that can be magnetically controlled and rendered unidirectional. An on-chip optical isolator was designed based on the unidirectionality of the magneto-opticBloch oscillatory motion. The proposed device delivers an isolation ratio as high as 36 dB that remains above 30 dB in a 0.7 nm wavelength bandwidth, at the telecommunication wavelength 1.55 μm. Slight modifications in isolator design allow one to achieve an even more impressive isolation ratio ~ 55 dB, but at the expense of smaller bandwidth. Moreover, the device allows multifunctionality, such as optical switching with a simultaneous isolation function, well suited for photonic integrated circuits.

Synthesis of Anisotropic Magnetic Nanoobjects Designed for Locomotion in External Magnetic Fields

The subject of the present work is the synthesis of novel nanoscale objects, designed for self-propulsion under external actuation. The synthesized objects present asymmetric hybrid particles, consisting of a magnetic core and polymer flagella and their hydrodynamic properties under the actuation by external magnetic fields are investigated. The single-domain ferromagnetic cobalt ferrite nanoparticles are prepared by thermal decomposition of a mixture of metalorganic complexes based on iron (III) cobalt (II) in non-polar solvents. Further modification of the particles includes the growth of the silver particle on the surface of the cobalt ferrite particle to form a dumbbell-shaped heterodimer. Different possible mechanisms of dumbbell formation are discussed. A polyelectrolyte tail with ability to adjust the persistence length of the polymer, and thus the stiffness of the tail, by variation of pH is attached to the particles. A polymer tail consisting of a polyacrylic acid chain is synthesized by hydrolysis of poly(tert-butyl acrylate) obtained by atom transfer radical polymerization (ATRP). A functional thiol end-group enables selective attachment of the tail to the silver part of the dumbbell, resulting in an asymmetric functionalization of the dumbbells. The calculations on the propulsion force and the sperm number for the resulting particles reveal a theoretical possibility for the propelled motion. Under the actuation of the particles with flagella by alternating magnetic field an increase in the diffusion coefficient compared to non-actuated or non-functionalized particles is observed. Further development of such systems for application as nanomotors or in drug delivery is promising.

Observatorio de salud y seguridad en el trabajo OSST - SOANDES LTDA, modelo piloto para la implementación y desarrollo

Introducción: Contar con un diagnóstico de las condiciones en seguridad y salud en el trabajo en el país permite crear estrategias para minimizar los problemas de la población trabajadora. En Colombia existe el observatorio del Instituto Nacional de Salud, sin embargo, no cuenta, en ninguno de sus tópicos, con información y análisis sobre la salud y seguridad de la población trabajadora. Objetivo: Determinar las condiciones de salud de la población atendida en la IPS SALUD OCUPACIONAL DE LOS ANDES LDTA en la ciudad de Bogotá, durante el año 2015. Materiales y métodos: Se realizó una prueba piloto del observatorio de salud y seguridad en el trabajo mediante un estudio de corte transversal, donde se tomó una base de datos de pacientes evaluados en la IPS SALUD OCUPACIONAL DE LOS ANDES LDTA, de la ciudad de Bogotá D.C. que contiene información de exámenes médicos ocupacionales realizados en el 2015 en la plataforma ISISMAWEB con una muestra representativa de 1923 registros. Se incluyeron variables sociodemográficas y laborales, los paraclínicos registrados como alterados más prevalentes, los diagnósticos y dictámenes emitidos en la población estudiada y las recomendaciones personales dadas por el sistema de gestión de la empresa. Se realizó un análisis descriptivo y para el estudio de las interacciones se empleó el Chi-cuadrado. Resultados: El 62,1% de la población fueron hombres con edad promedio de 34.8 años (DE 10,521). El 41.5% tuvieron estudios secundarios. La evaluación médica más realizada fue el examen de ingreso en el 30.5% de los casos. El cargo operadores de instalaciones y máquinas y ensambladores represento el 27.9% y en última medida los profesionales de nivel medio en operaciones financieras y administrativas con el 0.5%. El diagnostico CIE 10 emitido más frecuente fue con el 15,8% el código Z100 (Examen de salud ocupacional), seguido del Trastorno de la refracción no especificado (H527) con el 9,0%. En cuanto a las recomendaciones generales la que más se repitió fue examen periódico con un 30%. La recomendación preventiva más frecuente fue osteomuscular con el 36,5%. Las recomendaciones SVE de mayor prevalencia fueron ergonómicas con un 40,7%. Se encontraron asociaciones (p<0.05) entre las variables escolaridad, género y estrato. Conclusiones: Se deben optimizar los mecanismos de recolección del dato para ser más viable su evaluación y asociación. Hay un subregístro importante de segundos diagnósticos asociado al no registro de los paraclínicos. Este estudio plantea un modelo a seguir para poder desarrollar el observatorio nacional de salud y seguridad en el trabajo.