Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-10-25T22:17:10Z No. of bitstreams: 1 RamiroGomesDeFreitasNeto_DISSERT.pdf: 12806337 bytes, checksum: 668374ace783d763449c01ccfb9e135c (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-12-16T17:50:31Z (GMT) No. of bitstreams: 1 RamiroGomesDeFreitasNeto_DISSERT.pdf: 12806337 bytes, checksum: 668374ace783d763449c01ccfb9e135c (MD5) / Made available in DSpace on 2016-12-16T17:50:31Z (GMT). No. of bitstreams: 1 RamiroGomesDeFreitasNeto_DISSERT.pdf: 12806337 bytes, checksum: 668374ace783d763449c01ccfb9e135c (MD5) Previous issue date: 2016-04-29 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior (CAPES) / Os a?os inoxid?veis duplex s?o a?os altamente ligados caracterizados por uma estrutura bif?sica de ferrita-austenita obtida atrav?s de processos de conforma??o a quente ou tratamentos t?rmicos adequados ap?s os processos de conforma??o a frio. A presen?a de fra??es volum?tricas pr?ximas de ferrita e austenita na estrutura dos a?os duplex promove uma boa combina??o entre as propriedades de resist?ncia mec?nica e resist?ncia a corros?o. Uma das classes dos a?os duplex que vem se desenvolvendo nos ?ltimos anos ? o Lean duplex, que apresentam teores mais baixos de Cr, Ni e Mo, o que os torna mais baratos em compara??o com os austen?ticos. Para compensar a redu??o de Ni e Mo, elementos como Mn e N s?o adicionados em maior quantidade para proporcionar uma boa resist?ncia ? corros?o e conferir a estabilidade da austenita no a?o. Nos ?ltimos anos o n?mero de trabalhos relacionados ao a?o Lean duplex cresceu bastante devido ao interesse em se conhecer, desenvolver e aplicar cada vez mais essa classe de a?os. Uma das aplica??es que est? em alta ? a soldagem por difus?o no estado superpl?stico, e os a?os inox duplex possuem o comportamento superpl?stico, que exige microestruturas com gr?os pequenos e baixas taxas de deforma??o. Sendo assim, um estudo de recristaliza??o do a?o inoxid?vel Lean duplex 2101 como m?todo de refino de gr?o ? proposto para este trabalho. Diferentes lamina??es a frio, sendo elas 70, 80 e 90% de redu??o, foram empregadas no material para fornecer potencial termodin?mico para a recristaliza??o nos tratamentos t?rmicos em tr?s temperaturas, 900?C, 1000?C e 1100?C por tempos de 20 minutos, 1 e 2 horas. T?cnicas de microscopia ?ptica, eletr?nica de varredura, EDS, medidas magn?ticas, difra??o de raios-X, EBSD e microdureza foram empregadas para a caracteriza??o. Os resultados de medidas magn?ticas e microscopia ?tica indicam que h? acentuada ocorr?ncia de transforma??o induzida por deforma??o da austenita para martensita CCC, al?m de ind?cios da ocorr?ncia de outros mecanismos de deforma??o como macla??o mec?nica e deslizamento de discord?ncias, bem como ind?cios de zonas de austenita n?o transformada, aparentemente isenta de encruamento, conforme indicado pelos valores de microdureza, mesmo para redu??es de 90% na lamina??o a frio; aparentemente houve precipita??es de fases indesejadas tanto na ferrita quanto na austenita durante os tratamentos t?rmicos e aparente recristaliza??o total das amostras laminadas p?s tratamento t?rmico mesmo para os tempos mais curtos. Nas amostras analisadas por EBSD foram observados ind?cios de refino do gr?o austen?tico e uma tend?ncia de crescimento secund?rio da ferrita. Nas amostras recristalizadas identificou-se que para temperaturas e tempos de tratamento mais baixo as fra??es de ferrita se mostraram levemente maiores, enquanto que para temperaturas e tempos de tratamentos maiores as fra??es de austenita foram maiores. / Duplex Stainless Steel are an alloy based on Fe-Cr-Ni system characterized by biphasic structure of austenite-ferrite obtained through different thermomechanical cycles and they show good combination between mechanical (high tension strength and good tenacity) and corrosion resistance properties (stress corrosion cracking and pitting corrosion), besides good soldability. One of the Duplex Stainless Steel grades that is being developed in the last years is the Lean Duplex (LDSS) which have low addition of Cr, Ni, Mo and, for this, they are cheaper than austenitic and standard duplex stainless steel. To compensate the reduction of Ni and Mo, elements like Mn and N are added in larger quantities to give good corrosion resistance and ensure the stability of austenite. A possibility for application for LDSS is the manufacturing of components that are subjected corrosion conditions lesser severe for petrochemical industries, manufactured by diffusion bonding, taking advantage of superplastic behavior that these steels have since refined grains structure is present and low deformation rate are applied. Therefore, in this work was made a study of recrystallization of LDSS 2101, previously work hardened, as a method to refine grain. The material was subjected to different reduction degree by cold rolling (70, 80 and 90%), different heat treatments of recrystallization annealing in three temperatures (900, 1000 and 1100?C) and different times (20 min, 1 and 2 hours). The techniques of optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy, magnetic saturation measures, X-ray diffraction, Electron Backscattered Diffraction (EBSD) and Vickers micro hardness were employed in the microstructure characterization. The results of magnetic measures and optical microscopy show a sharp occurrence of strain induced martensite BCC (??) on austenite, besides evidences of occurrence of other deformations mechanisms such as deformation twins and dislocation glide, as well evidences of non-transformed austenite without work hardening, according to the hardness values, even for 90% reduction. Evidences of deleterious phases precipitation, as seen in SEM, were not confirmed by X-ray diffraction, which confirms the system thermodynamic simulation, which indicates phase precipitation such as ? and nitrides in temperature smaller than those used in this work. Preliminary analysis of EBSD samples indicates that recrystallization were not complete for 900?C and 20 minutes, as well a refined austenite grain structure and a trend for secondary recrystallization for ferrite. Optical microscopy also shows a trend to increase austenite over ferrite when temperature and time of recrystallization increased.

Made available in DSpace on 2014-12-17T15:42:01Z (GMT). No. of bitstreams: 1 EdissonMJ.pdf: 6565731 bytes, checksum: 5d6fdd6db6fc25a30c6100d96fff1edc (MD5) Previous issue date: 2007-08-24 / TiTanate NanoTubes (TTNT) were synthesized by hydrothermal alkali treatment of TiO2 anatase followed by repeated washings with distinct degrees of proton exchange. TTNT samples with different sodium contents were characterized, as synthesized and after heattreatment (200-800?C), by X-ray diffraction, scanning and transmission electron microscopy, electron diffraction, thermal analysis, nitrogen adsorption and spectroscopic techniques like FTIR and UV-Vis diffuse reflectance. It was demonstrated that TTNTs consist of trititanate structure with general formula NaxH2−xTi3O7?nH2O, retaining interlayer water in its multiwalled structure. The removal of sodium reduces the amount of water and contracts the interlayer space leading, combined with other factors, to increased specific surface area and mesopore volume. TTNTs are mesoporous materials with two main contributions: pores smaller than 10 nm due to the inner volume of nanotubes and larger pores within 5-60 nm attributed to the interparticles space. Chemical composition and crystal structure of TTNTs do not depend on the average crystal size of the precursor TiO2-anatase, but this parameter affects significantly the morphology and textural properties of the nanostructured product. Such dependence has been rationalized using a dissolution-recrystallization mechanism, which takes into account the dissolution rate of the starting anatase and its influence on the relative rates of growth and curving of intermediate nanosheets. The thermal stability of TTNT is defined by the sodium content and in a lower extent by the crystallinity of the starting anatase. It has been demonstrated that after losing interlayer water within the range 100-200?C, TTNT transforms, at least partially, into an intermediate hexatitanate NaxH2−xTi6O13 still retaining the nanotubular morphology. Further thermal transformation of the nanostructured tri- and hexatitanates occurs at higher or lower temperature and follows different routes depending on the sodium content in the structure. At high sodium load (water washed samples) they sinter and grow towards bigger crystals of Na2Ti3O7 and Na2Ti6O13 in the form of rods and ribbons. In contrast, protonated TTNTs evolve to nanotubes of TiO2(B), which easily convert to anatase nanorods above 400?C. Besides hydroxyls and Lewis acidity typical of titanium oxides, TTNTs show a small contribution of protonic acidity capable of coordinating with pyridine at 150?C, which is lost after calcination and conversion into anatase. The isoeletric point of TTNTs was measured within the range 2.5-4.0, indicating behavior of a weak acid. Despite displaying semiconductor characteristics exhibiting typical absorption in the UV-Vis spectrum with estimated bandgap energy slightly higher than that of its TiO2 precursor, TTNTs showed very low performance in the photocatalytic degradation of cationic and anionic dyes. It was concluded that the basic reason resides in its layered titanate structure, which in comparison with the TiO2 form would be more prone to the so undesired electron-hole pair recombination, thus inhibiting the photooxidation reactions. After calcination of the protonated TTNT into anatase nanorods, the photocatalytic activity improved but not to the same level as that exhibited by its precursor anatase / Titanatos nanoestruturados, particularmente TiTanatos NanoTubulares (TTNT), foram sintetizados por tratamento hidrot?rmico alcalino de TiO2-anat?sio seguido de repetidas lavagens com diversos graus de troca prot?nica. Amostras de TTNT com diferentes teores de s?dio foram caracterizadas na forma de p? seco e ap?s calcina??o (200-800?C) por difra??o de raios-X, microscopia eletr?nica de varredura e transmiss?o, difra??o de el?trons, an?lise t?rmica, adsor??o de nitrog?nio e t?cnicas espectrosc?picas de infravermelho e de reflet?ncia difusa no UV-Vis?vel. Demonstrou-se que tais materiais de paredes multilamelares s?o trititanatos de f?rmula geral NaxH2−xTi3O7?nH2O, retendo ?gua entre as lamelas. A remo??o de s?dio da estrutura reduz a quantidade de ?gua contraindo o espa?o interlamelar levando, combinado a outros fatores, ao aumento da ?rea e do volume de poros espec?ficos. Os TTNTs s?o materiais mesoporosos com duas contribui??es principais: poros menores que 10 nm devido ao volume interno dos nanotubos e poros entre 5 e 60 nm devido aos espa?os interpart?cula. A composi??o qu?mica e a estrutura cristalina do TTNT n?o dependem do tamanho de cristalito do TiO2-anat?sio precursor, todavia este par?metro afeta significativamente a morfologia e as caracter?sticas texturais do produto nanoestruturado. Tal depend?ncia foi racionalizada atrav?s de um mecanismo de dissolu??o-recristaliza??o que leva em conta a velocidade de dissolu??o do TiO2 de partida e sua influ?ncia sobre a taxa de crescimento de nanofolhas intermedi?rias em rela??o ? taxa de seu curvamento a nanotubos. A estabilidade t?rmica do TTNT ? definida pelo teor de s?dio e em pequena extens?o pelo tipo de anat?sio de partida. Foi demonstrado que o TTNT ap?s perder a ?gua intercalada entre 100 e 200?C se transforma pelo menos parcialmente num hexatitanato NaxH2−xTi6O13 intermedi?rio ainda nanotubular. A transforma??o t?rmica do tri- e hexatitanato nanoestruturados ocorre em maior ou menor temperatura e segue diferentes rotas dependendo do teor de s?dio. No caso de alto s?dio sinterizam e crescem at? grandes cristais de Na2Ti3O7 e Na2Ti6O13 na forma de bast?es e fitas acima de 600?C. No caso da amostra protonizada evoluem para nanotubos de TiO2(B) que facilmente se convertem em nanobast?es de anat?sio acima de 400?C. Al?m de hidroxilas e acidez de Lewis t?picos dos ?xidos de tit?nio, os TTNTs apresentam uma pequena contribui??o de acidez prot?nica capaz de se coordenar com a piridina a 150?C, e que ? perdida ap?s sua calcina??o e transforma??o ? anat?sio. O ponto isoel?trico do TTNT variou dentro da faixa 2,5- 4,0, indicando o comportamento de um ?cido fraco. Apesar de se revelar um semicondutor exibindo banda de absor??o t?pica no espectro de UV-vis?vel com energia de bandgap ligeiramente superior ao do respectivo TiO2-anat?sio precursor, os TTNTs apresentaram baixo desempenho fotocatal?tico na degrada??o de corantes cati?nico e ani?nico. Concluiu-se que a causa fundamental reside em sua estrutura de titanato lamelar que, em rela??o ? forma TiO2, apresentaria maior probabilidade de recombina??o do par el?tron-lacuna (e-/h+), inibindo as rea??es de fotoxida??o. A transforma??o do TTNT prot?nico ? nanobast?es de anat?sio melhorou a atividade fotocatal?tica, por?m ainda sem atingir o mesmo desempenho do TiO2-anat?sio precursor