Made available in DSpace on 2014-10-09T12:54:28Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:42Z (GMT). No. of bitstreams: 0 / Este trabalho tem como objetivo principal investigar o comportamento frente à corrosão de aços inoxidáveis avançados em meios fisiológicos. Foram selecionados para o estudo quatro aços inoxidáveis visando avaliar o potencial destes para aplicações em implantes cirúrgicos: um aço superferrítico (DIN W. Nr. 1.4575), a Incoloy MA 956, contendo alumínio e óxido de ítrio, um aço austenítico DIN W. Nr. 1.4970 e um aço superaustenítico obtido por meio da adição de 0,87% de nitrogênio ao aço dúplex DIN W. Nr. 1.4460. Os três primeiros aços contêm baixo teor de níquel e suas películas protetoras são ricas em cromo, enquanto a Incoloy MA 956 é isenta de níquel, e rica em alumínio, o que influencia o seu filme passivo. Os materiais foram analisados usando técnicas de espectroscopia de impedância eletroquímica (EIE), polarização potenciodinâmica, técnica do eletrodo vibrante, microscopia eletroquímica de varredura, microscopia eletrônica de varredura de emissão de campo, microscopia ótica e microscopia eletrônica de varredura. Os meios escolhidos para avaliação da resistência à corrosão foram a solução de Hanks, um meio de cultura e uma solução tamponada com fosfato. Os resultados de EIE foram interpretados usando circuitos elétricos equivalentes que simularam uma camada passiva dúplex em todos os materiais analisados. Todos os materiais analisados apresentaram resistência à corrosão superior à do aço inoxidável AISI 316L, correspondente ao ASTM F-138, que é o mais utilizado na fabricação de implantes metálicos. Também foi destacada a importância do tratamento de solubilização nos aços com alto teor de nitrogênio. O DIN W. Nr. 1.4970 foi considerado citotóxico e sua potencialidade para uso como biomaterial, rejeitada. O DIN W. Nr. 1.4575 e Incoloy MA 956 podem ser usados como biomateriais, mas somente em próteses odontológicas ou de fácil remoção, devido ao seu comportamento ferromagnético. O DIN W. Nr. 1.4460 com 0,87% de nitrogênio foi o que apresentou as condições mais apropriadas para uso como biomaterial, inclusive para próteses ortopédicas. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN/CNEN-SP

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Zinc coated steels are widely used in the automotive industry, because of the improved protection against corrosion. Their use has consequences for the resistance welding process, which is the most widely used method of joining body panels. The zinc coating alloys with the copper electrode, resulting in increased electrode wear, and a reduction in electrode life. The welding current must be increased, because of the reduced contact resistance and thus heavier cables and power sources are required. A novel form of power source, the Medium Frequency Direct Current inverter, offers advantages over the traditional AC transformer. The higher operating frequency results in a lighter transformer, and a smaller welding current may be used, because the DC welding current generates heat at a constant rate, and is thus more effective than an AC power source. A potential advantage of this technology is that the increased frequency allows improved resolution in monitoring and control. Novel signal conditioning circuitry was developed, allowing significant improvement in the time resolution of the voltage and resistance signals. A series of welding trials was conducted, while monitoring the welding process. The correlation between weld quality and various process variables was assessed, and a control algorithm to compensate for electrode wear was proposed. This algorithm, based on a constant voltage principle, was implemented on a bespoke welding timer. A significant improvement in electrode life was obtained using this technique. The control algorithm was shown effective experimentally, but practical limitations do not permit testing under all possible conditions. A numerical model of the spot welding process, using Finite Difference technique, was developed. Following successful validation, the model was used to predict the performance of the control algorithm under various conditions of electrode wear. The results indicate that a constant voltage algorithm can compensate for an increase in electrode tip diameter, but that a change in contact resistance may result in unsatisfactory welds.

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The goal of this thesis is to determine the relationship between the macroscopic stress and the macroscopic strain for a variety of complex multiphase materials exhibiting rate-independent non-linear response at the micro-scale, based on experimental data obtained both at the local and macroscopic scales. A micro-macro secant mean field model (SMF model) based on the result of Eshelby and the approach of Mori and Tanaka is developed to model the behaviour of three particular systems which we have worked out by ourselves: 1) a ferrite-martensite steel produced by rolling in which we quantify the plastic anisotropy due to the morphological texture in terms of the Lankford's coefficient and pseudo yield surface; 2) a composite made of two continuous and interpenetrating phases: an aluminium matrix reinforced by a preform of sintered Inconel601 fibres. We quantify the coupled effects of temperature and phases co-continuity on the phases and overall stresses; 3) a TRIP-aided multiphase steel, in which the dispersed metastable austenite phase transforms to martensite. We derive the relationship between the overall uniaxial elastoplastic response and the progress of phase transformation, itself influenced by the thermodynamical, microstructural and mechanical properties. The stress-state dependence of the martensitic transformation is enlightened and explained. We demonstrate the existence of thermomechanical treatments leading to optima of ductility and strength-ductility balance. Finally, we show that the formability of TRIP-aided multiphase steels depends on the stability criterion.

TRIP-assisted multiphase steels exhibit an excellent balance of strength and ductility, which makes them very attractive for the automotive industry. These remarkable mechanical properties can be attributed mainly to the continuous transformation of retained austenite into martensite during straining (TRansformation Induced Plasticity). The aim of this thesis was to clarify the interaction between the hot rolling conditions, the formation of microstructure, and the resulting mechanical properties. Various rolling simulation techniques were employed to determine how the composite microstructure is formed during the various steps of multi-stage thermomechanical processing. The interaction between deformation and phase transformation is highlighted, particularly from the viewpoint of the transformation texture.

TRIP-assisted multiphase steels exhibit an excellent balance of strength and ductility, which makes them very attractive for the automotive industry. These remarkable mechanical properties can be attributed mainly to the continuous transformation of retained austenite into martensite during straining (TRansformation Induced Plasticity). The aim of this thesis was to clarify the interaction between the hot rolling conditions, the formation of microstructure, and the resulting mechanical properties. Various rolling simulation techniques were employed to determine how the composite microstructure is formed during the various steps of multi-stage thermomechanical processing. The interaction between deformation and phase transformation is highlighted, particularly from the viewpoint of the transformation texture.