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ESTUDO DA RECRISTALIZAÇÃO DINÂMICA DURANTE A DEFORMAÇÃO A QUENTE DE UM AÇO ISO 5832-9 / STUDY OF THE DYNAMIC RECRYSTALLIZATION DURING THE DEFORMATION THE HOT ONE OF A STEEL ISO 5832-9Nascimento, Luciene Araujo 25 February 2010 (has links)
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Previous issue date: 2010-02-25 / FUNDAÇÃO DE AMPARO À PESQUISA E AO DESENVOLVIMENTO CIENTIFICO E TECNOLÓGICO DO MARANHÃO / Austenitic stainless steels have low economic value compared to titanium and its alloys,
considered the most resistant to corrosion in biological environment, and for over fifty
years are widely used in the manufacture of orthopedic implants, particularly in public
health service. Currently the ISO 5832-1 steel (ASTM F 138) is the most used in the
manufacture of orthopedic prosthesis, especially the temporary ones. However, because
it is susceptible to localized corrosion when in contact with human tissue, it is being
gradually replaced by austenitic stainless steel of low carbon and high nitrogen called ISO
5832-9. The same is already used extensively in Europe and the United States, while in
Brazil it's use is more recent and smaller scale. This work investigates the behavior of
dynamic recrystallization in an ISO 5832-9 steel through hot torsion tests and optical
microscopy, for different conditions of temperature, strain and strain rate. It was found
through the plastic flow curves of the studied material crystallizes dynamically and that the
high value of its apparent activation energy for deformation can be attributed to the
presence of a large amount of precipitated particles and nitrogen dissolved in the matrix.
Micrographs confirmed that tests carried out at low temperatures reveal a strong
retardation of dynamic recrystallization. Moreover, the presence of serrated grain
boundaries and nucleation of new grains in the old deformed grain boundaries, are strong
indications that the recrystallization occurred by a necklace mechanism. The behavior of
the average recrystallized grain size, DDRX, with temperature resembles the behavior of
the austenitic grain growth in microalloyed steels. The presence of a minimum in the
behavior of DDRX with the strain rate can be attributed to a minimum strain rate required
to cause the greatest amount of dynamics precipitation. / Os aços inoxidáveis austeníticos possuem um baixo valor econômico quando
comparados ao titânio e suas ligas, considerados os mais resistentes à corrosão em meio
biológico, e há mais de cinquenta anos são amplamente utilizados na confecção de
implantes ortopédicos, em especial no serviço de saúde pública. Atualmente o aço ISO
5832-1 (ASTM F 138) é o mais utilizado na confecção de próteses ortopédicas,
principalmente as temporárias. Entretanto, devido o mesmo ser suscetível à corrosão
localizada quando em contato com tecido humano, ele está sendo gradativamente
substituído pelo aço inoxidável austenítico de baixo teor de carbono e alto teor de
nitrogênio denominado de ISO 5832-9. O mesmo já é utilizado em larga escala na
Europa e nos Estados Unidos, enquanto no Brasil a sua utilização é mais recente e em
menor escala. Este trabalho investiga o comportamento da recristalização dinâmica em
um aço ISO 5832-9, através de ensaios de torção a quente e microscopia ótica, para
diferentes condições de temperatura, deformação e taxa de deformação. Foi constatado
através das curvas de escoamento plástico que o material estudado recristaliza
dinamicamente e, que o alto valor da sua energia de ativação aparente para a
deformação pode ser atribuído à presença de uma grande quantidade de partículas de
precipitados e ao nitrogênio em solução na matriz. Micrografias confirmam que ensaios
realizados a baixas temperaturas revelam um forte retardamento da recristalização
dinâmica. Além disso, a presença de contornos de grão serrilhados e nucleação de novos
grãos nos antigos contornos de grãos deformados, são fortes indícios de que a
recristalização ocorreu por mecanismo colar. O comportamento do tamanho de grão
médio recristalizado, DDRX, com a temperatura, se assemelha ao comportamento do
crescimento de grão austeníticos de aços microligados. A presença de um mínimo no
comportamento do DDRX com a taxa de deformação pode ser atribuído a uma taxa de
deformação mínima necessária para que ocorra a maior quantidade de precipitação
dinâmica.
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Termochemie polydusíkatých heterocyklických sloučenin / Thermochemistry of high nitrogen heterocyclic compoundsBartošková, Monika January 2011 (has links)
The prediction of detonation properties of the new generation of high-nitrogen energetic materials (HNEM) is based on knowledge of their heats of formation, which are sum of values of particular nitrogen heterocyclic fragments. The diploma thesis describes theoretical calculations of heats of formation in gas phase ?f H°(298,g) for series of azines (number of N atoms 2-6) and azoles (number of N atoms 2-5) by means of quantum chemical methods. The semiempirical methods as PM3, DFT methods utilizing isodesmic approach and finally thermochemical G-recipes were used. All calculated values of heats of formation were scrutinized and for future application to HNEM materials the DFT B3LYP/cc-pVTZ method and thermochemical recipe T1 were recommended.
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Termochemické vlastnosti vysokodusíkatých energetických materiálů / Thermochemical Properties of High Nitrogen Energetic MaterialsBartošková, Monika January 2015 (has links)
The main goal of the presented thesis is a theoretical study of heat of formation for high-nitrogen energetic materials. A modification of the classical approach to the isodesmic reactions is realized with the intent that molecules on both sides of the corresponding equation have not only the same number of atoms but also approximately the same size and skeletal similarity. This approach is designated as a method "Alternative Isodesmic Reaction (AIR method)". At its base, using the DFT B3LYP / cc-pVTZ and B3PW91 / cc-pVTZ, for the high nitrogen heterocycles, which are selected from the group of triazoles, triazines, tetrazines, the enthalpy of formation values the gaseous phase f H°(298,g), were obtained whose values are close to the published f H°(298,g). Their application in the calculation of the relevant characteristics of these heterocycles detonation gave real values.
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Evaluation of the η (Eta) nitride with three laboratory meltsLind, Martin, Johansson, Cecilia January 2015 (has links)
η (eta) nitride, Cr3Ni2SiN, is a precipitate found in high temperature austenitic stainless steel and is not yet included in Thermo-Calc steel database TCFE7. The aim of this thesis is to collect thermodynamic data to enable the addition of η nitride in the databases. Three laboratory melts with varying levels of silicon, chromium and nickel have been aged at 700-1000 °C for 75 h, 300 h and 1200 h and examined by Light Optical Microscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Wavelength Dispersive Spectroscopy, Electron Backscattered Diffraction and X-ray Powder Diffraction. η nitride is in the studied alloys an equilibrium phase stabilized with nitrogen. Presence of η nitride was confirmed by Energy Dispersive Spectroscopy and X-ray Powder Diffraction. It was found to precipitate in four different ways, at primary grain boundaries, intragranularly, as a "skeleton-like" precipitate and as a border around the occurring Cr2N precipitates. The area fraction of η nitrides increases with longer aging times and is favored by silicon and nickel. The composition of η nitride is not changing regardless of material composition, aging temperature and aging time. The composition of η nitride in all three materials are 8.7-9.7 wt.% silicon, 47-54 wt.% chromium, 1.4-4.1 wt.% iron and 33-36 wt.% nickel. The nitrogen content 2 determined by Wavelength Dispersive Spectroscopy is 2.8-3.2 wt.%. No complete equilibrium was achieved and together with incomplete mixing of the alloying elements during melting, the microstructure is difficult to evaluate. Other precipitates found are Cr2N, π nitride, σ phase and two unidentified phases, M and N. Of these phases at least Cr2N is not an equilibrium phase as it dissolves during aging. Further aging to achieve complete equilibrium is necessary.
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