Ni-rich NiTi shape memory alloys (SMAs) are capable of attaining a wide range of transformation temperatures depending on the heat treatment conditions and superior thermo-mechanical cycling stability, which are desired for repeated solid-state actuation. High Ni-content Ni-rich SMAs have very low transformation temperatures in a solutionized condition due to the high Ni-content of the matrix. Slow cooling (furnacecooling) from solutionizing temperature and additional aging heat treatments result in the formation of Ni-rich precipitates such as Ni4Ti3, Ni3Ti2 and Ni3Ti and increase transformation temperatures above ambient by depleting excess Ni from the matrix. However, the precipitates do not undergo a martensitic phase transformation and they decrease the transformation strain by reducing the volume fraction of the material capable of transforming. Meanwhile, recent preliminary work shows that Ni3Ti precipitates dominate fatigue failure. The objectives of the present study are: (1) to eliminate Ni3Ti but still have Ni4Ti3 precipitates, which are responsible for the dimensional stability and increase transformation temperatures, (2) to investigate the effect of heat treatments on the transformation strain, and (3) to select single variant Ni4Ti3 precipitates through constrained aging for the formation of oriented internal stress and eventually obtain twoway shame memory effect (TWSME) and enhanced dimensional stability. Based on these objectives, the effect of aging heat treatment on transformation temperatures, microstructural evolution, and shape memory behavior were investigated for a Ni52Ti48 shape memory alloy (SMA) by using differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and thermo-mechanical testing, including isobaric heating-cooling experiments under various stress levels. It was observed that solutionizing at 900 degree C for 24 hours eliminated Ni3Ti type precipitates, but additional aging heat treatments are needed to form Ni4Ti3 precipitates to increase transformation temperatures. Furnace-cooling and additional aging heat treatment results in the multi-stage martensitic transformation due to chemical and stress inhomogeneities in the microstructure. Aging of the controlled furnace-cooled material at 400 degree C for 48 hours resulted in the highest transformation temperatures among all processing conditions investigated due to the combination of Ni3Ti precipitates and 27 percent volume fraction of the Ni4Ti3 precipitates, which led to the depletion of Ni from the transforming matrix. However, since overaging results in losing coherency of the precipitates, dimensional stability during isobaric thermal cycling was negatively impacted.

The super duplex stainless steels are often used in applications where high mechanical strength is required, combined with corrosion resistance, especially in highly aggressive environments. However, when certain manufacturing processes are involved (hot forming or welding, for example) can occur the precipitation of undesirable phases, causing reduction of both mechanical properties as well as corrosion resistance. Particularly between 700 and 900 ÂC may occur sigma phase formation. This phase is responsible for the reduction of corrosion and mechanical resistance of super duplex steels. Through computer simulation in Thermo-CalC program, the phase diagram of ASTM A890 GRADE 1C super duplex stainless steel showed higher precipitated fraction of this phase in 830 ÂC temperature. Thus, this essay aims to evaluate the influence of isothermal treatment time in regards to this material aging, the corrosion resistance due to the formation of sigma phase. The evaluation of corrosion resistance was performed through open circuit potential monitoring, electrochemical impedance and potentiodynamic polarization tests (anodic branch) in 0.5 M H2SO4 solution, 0.5 M H2SO4 + 0.5M HCl and 0.5 M HCl at temperatures of 25, 35 and 45 ÂC. Additionally, a microstructural characterization of this material was performed by quantification phase through optical microscopy, morphological analysis by Scanning and Transmission Electron Microscopy, coupled techniques Electron backscattered diffraction and Electron Energy Dispersion Spectroscopy, and Vickers microhardness measures. The microstructural and morphological characterization confirmed the presence of sigma phase in super duplex stainless steel 1C and that the amount of this phase increases with aging heat treatment time. This phase showed a greater microhardness than the ferrite and austenite phases. The aging heat treatment influenced negatively on corrosion resistance of the steel when exposed to acidic solutions of H2SO4 and HCl at 25 ÂC, mainly when these are present in the same solution, probably due to their low pH. In this solution (H2SO4 + HCl), both increasing the aging heat treatment time as well as the temperature (35 and 45 ÂC), affects negatively the sample corrosion resistance, decreasing acoording a higher content of this phase. / Os aÃos inoxidÃveis super duplex sÃo frequentemente utilizados em aplicaÃÃes onde à necessÃria uma grande resistÃncia mecÃnica, combinada a resistÃncia à corrosÃo, especialmente em ambientes de elevada agressividade. Entretanto, ao sofrer determinados processos de fabricaÃÃo (conformaÃÃo a quente ou soldagem, por exemplo), pode ocorrer à precipitaÃÃo de fases indesejÃveis, que causam tanto reduÃÃo de propriedades mecÃnicas quanto afetam a resistÃncia à corrosÃo. Particularmente entre 700 ÂC e 900 ÂC pode ocorrer à formaÃÃo de fase sigma. Esta fase à responsÃvel pela reduÃÃo da resistÃncia à corrosÃo e mecÃnica dos aÃos inoxidÃveis super duplex. Por meio de simulaÃÃo computacional no programa Thermo-CalCÂ, o diagrama de fases do aÃo inoxidÃvel super duplex ASTM A890 GRAU 1C apresentou maior fraÃÃo precipitada desta fase na temperatura de 830 ÂC. Assim, o presente trabalho tem por objetivo avaliar a influÃncia do tempo de tratamento isotÃrmico de envelhecimento deste material, na resistÃncia à corrosÃo devido à formaÃÃo da fase sigma. A avaliaÃÃo da resistÃncia à corrosÃo foi realizada por meio do monitoramento do potencial de circuito aberto, impedÃncia eletroquÃmica e de ensaios de polarizaÃÃo potenciodinÃmica (ramo anÃdico) em soluÃÃo de H2SO4 0,5 M, H2SO4 0,5 M + HCl 0,5 M e HCl 0,5 M, nas temperaturas de 25, 35 e 45 ÂC. Adicionalmente, foi realizada a caracterizaÃÃo microestrutural desse material atravÃs de quantificaÃÃo de fases por microscopia Ãtica, anÃlise morfolÃgica por Microscopia EletrÃnica de Varredura e TransmissÃo, acoplados as tÃcnicas de DifraÃÃo de ElÃtrons Retroespalhados e Espectroscopia de DispersÃo de Energia de ElÃtrons, alÃm de medidas de microdureza vickers. A caracterizaÃÃo microestrutural e morfolÃgica confirmou a presenÃa de fase sigma no aÃo inoxidÃvel super duplex 1C e que a quantidade desta fase aumenta com o tempo de tratamento tÃrmico de envelhecimento. Esta fase apresentou microdureza maior que as fases ferrita e austenita. O tratamento tÃrmico de envelhecimento influenciou de maneira negativa na resistÃncia a corrosÃo desse aÃo quando exposto a soluÃÃes Ãcidas de H2SO4 e HCl a 25 C, principalmente quando este estÃo presentes na mesma soluÃÃo, devido provavelmente ao seu baixo pH. Nesta soluÃÃo (H2SO4 + HCl), tanto o aumento do tempo de tratamento tÃrmico de envelhecimento quanto o aumento da temperatura (35 e 45 ÂC), afetam negativamente na resistÃncia à corrosÃo do material estudado, diminuindo à medida que se tem um maior conteÃdo de fase sigma.

Orientador: Rubens Caram Junior / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-26T12:33:37Z (GMT). No. of bitstreams: 1 Salvador_CamiloAugustoFernandes_M.pdf: 6043299 bytes, checksum: d34b83077b59f31d9349e1c6f8532b3a (MD5) Previous issue date: 2015 / Resumo: Ligas biomédicas do sistema Ti-Nb-Sn são bastante promissoras em função de sua excelente biocompatibilidade, baixo módulo de elasticidade e resistência à corrosão. Estas propriedades são severamente influenciadas pela precipitação das fases ? e ? durante tratamentos térmicos de envelhecimento. Estudos prévios evidenciaram uma grande divergência entre a evolução microestrutural de amostras submetidas ao envelhecimento clássico (amostras solubilizadas, resfriadas em água e envelhecidas, RC) e de amostras submetidas à isoterma direta (ID). O objetivo deste trabalho é avaliar a evolução microestrutural de três diferentes composições - Ti-30Nb, Ti-30Nb-2Sn e Ti-30Nb-4Sn (% peso) durante os tratamentos isotérmicos RC e ID. As ligas foram fundidas em forno a arco, homogeneizadas (1000°C, 12 horas), resfriadas em água e laminadas a frio até a espessura de 2.5 mm, resultando em uma redução de espessura de 78%. Foram então obtidas amostras de 10 x 10 x 2.5 mm através de corte a frio. Um tratamento de recristalização foi empregado (800°C, 30 min), seguido dos tratamentos isotérmicos RC ou ID, a 400°C, por tempos entre 10 min e 48 h. Todas as amostras foram resfriadas em água após o envelhecimento. Em seguida, as amostras foram polidas e analisadas por MEV-FEG e DRX utilizando um porta-amostras giratório. Adicionalmente, medidas de microdureza e de módulo de foram executadas. Discos de 3 mm foram preparados para serem analisados por MET. Foram empregadas as técnicas de SAD, campo claro/campo escuro e CBED. Verificou-se que a adição de 4% de Sn suprime a formação de ? isotérmico a 400°C, dando origem a uma microestrutura de baixo módulo e baixa dureza. A adição de Sn permite a formação da fase ?" no resfriamento rápido executado após 48 h de isoterma direta. Para todas as composições, independentemente do tratamento térmico, a precipitação de ? ocorreu majoritariamente nos contornos de grão. Precipitados intragranulares em formato triangular também foram observados nas ligas com 2Sn e 4Sn / Abstract: Ti-Nb-Sn biomedical alloys have been recently developed as promising biomaterials due to their excellent biocompatibility, low elastic modulus and corrosion resistance. These desirable properties are highly influenced by the precipitation of ? and ? phases during aging heat treatments. Previous studies reported a great divergence between the microstructure evolution on specimens submitted to QA (quench and aged) and DH (direct holding from beta field, step-quenched, Route 2) aging heat treatments. The aim of this project was to evaluate the microstructure evolution of three different alloys - Ti-30Nb, Ti-30Nb-2Sn and Ti-30Nb-4Sn (wt %) during QA and DH isothermal aging heat treatments. The alloys were arc melted, homogenized (1000°C, 12 hours), water quenched and then cold rolled to 2.5 mm thickness, resulting in a total reduction rate of 78%. The ingot was cut in samples of 10 x 10 x 2.5 mm. A recrystallization heat treatment (800°C for 30 min) was employed, followed by the isothermal heat treatments (QA or DH) - from 10 minutes to 48 hours - and water quenching. The samples were finally polished and analysed by FEG-SEM and also X-ray diffraction (XRD) using a spinner device. Furthermore, microhardness and elastic modulus measurements were performed. Conventional TEM sample preparation was used in order to obtain 3 mm disks suitable for BF/DF/SAD and CBED analysis. It was verified the addition of 4% Sn supresses the ? formation during DH aging heat treatments at 400°C, resulting in a low hardness and low elastic modulus microstructure. This Sn addition also allows ?" to be formed on water quenching even after 48 h of aging. For all compositions, despite of the aging heat treatment, the majority of ? precipitation was given on grain boundaries. Intragranular, triangular-shape ? precipitates were also observed in Ti-30Nb-2Sn and Ti-30Nb-4Sn alloys / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica