The solidification process of Ti52-xNi38Cu10Nbx alloys was evaluated using the CALPHAD method and microstructural analyses of as-cast alloys with Nb additions of x = 4, 6, 8 and 10 at%. Both in Scheil calculator and structural analyses, the observed phases were TiNi, β-Nb/Ti, and Ti2Ni. TiNi is the matrix phase, and it presented similar amounts of Ti and Ni with varying Nb contents. Enrichments of Ti content and depletion of Ni are expected in the liquid with the solidification of this phase. Formation of Ti2Ni and β-Ti is related to the excess of Ti. Increasing Nb content hinders the formation of Ti2Ni, favoring β-Ti. In turn, Ni depletion and Nb accumulation led to the TiNi/β-Nb eutectic. In the intermetallic phases, coherent ratios were obtained when Cu was considered a substitute for Ni, and Nb, for Ti.

RESUMO Introdução: o tratamento térmico de fios de aço inoxidável é um procedimento clínico rotineiro adotado por muitos cirurgiões-dentistas para aliviar o estresse causado após a confecção de dobras no fio. O presente estudo avaliou a influência do tratamento térmico em fios de aço inoxidável com secção retangular de 0,016’’ x 0,022’’. Métodos: para análise da estabilidade dimensional, foram avaliadas as dimensões anteriores e posteriores de 40 arcos ortodônticos de aço inoxidável de 0,016’’ x 0,022’’ sem tratamento térmico e 30 dias após o tratamento térmico. Para análise das propriedades mecânicas, 12 segmentos de fio de aço inoxidável com a mesma secção retangular sem tratamento térmico e 30 dias após o tratamento térmico foram analisadas por testes de resistência à tração e tensão. Para verificar se houve diferenças entre as dimensões anteriores e posteriores, os resultados foram analisados pelo teste t de Student. Para comparar a resistência à tração e tensão entre os grupos, foi utilizado o teste ANOVA. O nível de significância adotado foi 95% (p< 0,05). Resultados: o tratamento térmico não interrompeu a expansão dos fios 30 dias após seu preparo, e não houve diferença estatística nos testes de resistência à tração e tensão com e sem o tratamento térmico. Conclusão: pelos achados desse estudo, conclui-se que o comportamento mecânico de fios submetidos a tratamento térmico é semelhante ao de fios de aço não submetidos ao tratamento térmico.

ABSTRACT Introduction: The heat treatment of stainless steel wires is a routine clinical procedure adopted by many dentists in order to relieve the stress caused after performing bends in the archwire. Objective: This study aimed to evaluate the influence of heat treatment of stainless steel archwires with a rectangular section of 0.016 x 0.022’-in. Methods: For analysis of the dimensional stability, the anterior and posterior dimensions of forty 0.016 x 0.022-in stainless steel orthodontic archwires without heat treatment and 30 days after heat treatment were evaluated. For analysis of the mechanical properties, 12 stainless steel wire segments with the same rectangular section without heat treatment and 30 days after heat treatment were tested through tensile strength and strain tests. To evaluate if there were differences between the anterior and posterior dimensions, the results were analyzed by the Student’s t-test. To compare the tensile strength and strain between the groups, the ANOVA test was used. The level of significance adopted was 95% (p< 0.05). Results: The heat treatment did not stop the expansion of archwires 30 days after their preparation, and there was no statistical difference in the tensile strength and strain tests with and without heat treatment. Conclusion: From the findings of this study, it can be conclude that the mechanical behavior of heat-treated stainless steel archwires is similar to that of archwires not subjected to heat treatment.

The ToF-SIMS technique was applied to analyze thin ZrO2 coatings deposited on the surface of a Ni-Ti alloy. Due to its nanostructured nature, these films are difficult to characterize by conventional techniques. ZrO2 coatings were deposited on near equiatomic Ni-Ti wires by pulse electrodeposition. Part of the samples was electrolytically polished before the coating process. The coated samples were then analyzed by ToF-SIMS to evaluate the influence of the deposition time and previous surface electropolishing on the structure of the deposited coating. The results indicate that thicker coatings were produced on the electropolished samples, in comparison with the as-received ones. The best uniformity in thickness was achieved when Ni-Ti samples were previously electropolished followed by ZrO2 electrodeposition for 1200 seconds. Moreover, it was possible to observe by this technique that the inclusions in the Ni-Ti matrix were not entirely covered by the coating.

For the biomedical application of NiTi alloys, an excellent surface finishing process is required to guarantee high corrosion resistance and biocompatibility, eliminating the allergenic and toxic effects associated with the release of nickel ions in the body. Electropolishing is a process that can reduce surface imperfections and form a thin protective layer of TiO2, even in complex-shaped devices. The main objective of our study was to find and report suitable parameters for electrolytic polishing of NiTi wires, in both the superelastic and shape memory states. The results indicate that electropolishing in a 3.5 mol•L-1 methanolic H2SO4 electrolyte at 20ºC can effectively reduce surface roughness, remove superficial nickel-rich layers and improve corrosion resistance for austenitic and martensitic NiTi alloys.

The strain hardening behavior of an AISI 304 stainless steel at different temperatures was investigated in this work. Specimens were tensile tested up to rupture at temperatures of 25, 50, 75, 100, 125 and 150 ºC by using a universal testing machine with an attached environmental test chamber. The induction of martensite by strain was assessed by X-ray diffraction and Rietveld refinement. The resultant fracture morphologies were analyzed by scanning electron microscopy. The changes in the mechanical properties as a function of temperature were evaluated through the variations in the stress-strain curve and the strain hardening behavior was described in terms of strain hardening rate, instantaneous strain hardening exponent and Crussard-Jaoul analysis. Six strain hardening stages were detected at lower temperatures, transitioning into three strain hardening stages at higher temperatures. Fracture surface was ductile at all studied temperatures, although differences in terms of dimple and void morphology were observed.

O Método Rietveld foi utilizado para investigar a estrutura de três fases intermetálicas Fe-Zn e quantificar sua proporção. As fases zeta, delta e gama foram obtidas com sucesso pelo processo de sinterização. Uma mistura simples das três fases intermetálicas foi realizada com uma porcentagem de massas conhecida e seu padrão DRX foi coletado e analisado pelo Método Rietveld. Uma diferença de 14% entre os padrões DRX simulado e experimental foi encontrada, indicando um ajuste satisfatório. Uma pequena diferença entre os valores dos parâmetros de rede calculados e aqueles contidos nos cartões ICSD foi observada. Por conta do bom ajuste, as porcentagens de fase da mistura calculadas concordaram de maneira próxima com as porcentagens experimentais. Logo, concluiu-se que o Método Rietveld é um método confiável para quantificar as fases presentes em aços galvannealed e que a mesma metodologia aplicada nesse estudo laboratorial pode ser aplicada em chapas industriais.

The Rietveld Method was used to investigate the structure of three Fe-Zn intermetallic phases and to quantify their proportion. The zeta, delta and gamma phases were successfully obtained by the sintering process. A simple mixture of the three intermetallic phases was made with a known mass percentage and the XRD pattern of this mixture was collected and analyzed by the Rietveld Method. A difference of 14% between the simulated and the experimental XRD patterns was found, indicating a satisfactory fitting. Slightly different lattice parameters from those given in the ICSD cards were observed. Because of the good fitting, the calculated phase percentages of the mixture agreed closely with the real ones. Thus, it was concluded that the Rietveld Method is a reliable method to quantify the phases in galvannealed steel sheets and the same methodology applied in this laboratorial study can be applied to industrial sheets.