The interaction between the stress-induced martensitic transformation and resistivity behavior of superelastic NiTi shape memory alloy (SMA) was studied. Strain-controlled low-cycle fatigue up to 6% was monitored by in situ electrical resistivity measurements. The experimental results show that a great motion of martensite fronts results in a significant accumulation of defects, as evidenced by transmission electron microscopy (TEM), before and after the tensile cycles. This gives rise to an overall increase of the resistivity values up to the maximum deformation. Therefore, the research suggests that shape memory alloy wire has great potential as a stress sensor inside bulk materials.
The constant search for the improvement of the performance of materials of industrial application, evaluated under aspects of weight reduction, greater resistance, greater resistance to wear and better thermal stability, among others, associated with the search for the development of ecologically viable products, that convert the context of environmental degradation in preservation and sustainability, reflects the need to conduct research that results in new materials. The objective of this work is to obtain composites of the AA6061 aluminum alloy reinforced with different contents of coke coal blast-furnace slag by powder metallurgy. The processing of these materials was done by sieving, mixing and compacting powders of reinforced aluminum alloy with 5, 10 and 15% of blast-furnace slag. The cold uniaxial compaction was realized at a pressure of 500MPa. The obtained materials were sintered at 580°C for 3h under inert atmosphere. Unreinforced aluminum alloy samples were also produced. The characterization of the materials was realized by density and hardness measurements and three-point bending tests. The analysis of its microstructure was realized by scanning electron microscopy. As results, the composites presented a homogeneous distribution of the reinforcing particles and also a progressive improvement of the hardness and the bending strength with the increase of the slag content, producing an increase of 79% in hardness and 128% in flexural strength, when compared to the material without reinforcement obtained by the same process. Such results give the coke coal blast-furnace slag a new possibility of exploitation in the metal-mechanical sector, besides contributing with the environmental issue.
The non-ferrous SMAs (shape memory alloys) have, normally, two problems that hinder the use in industrial scale: the natural aging and grain growth. The first degrades the memory effect, while the second, observed during the alloy's mechanical processing, modifies the phase transformation temperatures. Thus, the study of recrystallization kinetics is important for enabling the control of hardened state as a function of treatment time without allowing the exaggerated grain growth. The objective of this study is to determine the recrystallization kinetics in different SMAs (Cu-14Al-4Ni, Cu-12Al-0.5Be and Ni-42Ti), based on an empirical law of J-M-A (Johnson-Mehl-Avrami), as well as their activation energies for grain growth process according to the empirical Arrhenius law.Quantitative evaluations of the grain growth kinetics over a wide range of indicated DSC (differential scanning calorimetry) temperatures have been performed. The results show that the alloy less susceptible to aging in temperatures below the recrystallization peak is the Ni-42Ti, because it presented the highest activation energy, followed by the Cu-14Al-4Ni. The equations that describe the recrystallization kinetics follow the empirical law of J-M-A. The recrystallization kinetics accompanied by hardness variation was an important tool, working as an advisor for selection of treatment time as a function of temperature.
RESUMO:A atenção dos pesquisadores para os compósitos poliméricos tem crescido nos últimos anos, pois a partir de uma matriz e um reforço, pode-se obter elevada resistência mecânica e leveza. A utilização de reforço natural traz vantagens devido a afinidade com o meio ambiente e por serem biodegradáveis, recicláveis e não tóxicas. A canabrava (Gynerium sagittatum) é uma planta encontrada em toda a América do Sul, bastante aplicada em artesanato. No entanto, seu uso gera resíduos que são acumulados no meio ambiente e incineradas. Este trabalho tem como objetivo desenvolver um compósito polimérico utilizando o resíduo de canabrava como reforço. Devido a pandemia do COVID-19 não foi possível tratar o resíduo, produzir os corpos de prova do compósito polimérico e, consequentemente, avaliar as propriedades mecânicas pelos ensaios de tração uniaxial e flexão em três pontos e analisar a superfície de fratura do material por microscopia eletrônica de varredura (MEV).Palavras-chave: compósito polimérico, resíduo de canabrava, absorção de umidade Página 111Página 113
RESUMO:Os compósitos poliméricos têm chamado a atenção dos pesquisadores nos últimos anos, pois podem produzir um material com alto desempenho mecânico e baixo peso. Os reforços naturais apresentam vantagens aos compósitos devido ao seu comportamento ecologicamente correto. Este trabalho teve como objetivo produzir uma revisão da literatura sobre a utilização de fibras naturais e resíduos como reforço ou carga para o desenvolvimento de compósitos poliméricos. Foi realizada uma revisão sistemática da literatura nas principais bases de dados científicas, nomeadamente Scopus, Web of Science e Science Direct. A revisão mostrou a tendência de um número crescente de artigos científicos publicados referentes a resíduos lignocelulósicos em compósitos poliméricos. Os resultados mostraram que a maioria dos resíduos lignocelulósicos usados como reforço ou enchimento eram fibras ou materiais particulados. Nesse trabalho, foram analisados as matrizes poliméricas e fibras vegetais mais aplicados, os principais processos de fabricação e as principais propriedades dos compósitos avaliadas pelos autores. Logo, avaliou-se os parâmetros fundamentais do estudo de compósitos poliméricos, da seleção dos materiais a sua caracterização. Esses resultados serão utilizados como base para futuras pesquisas experimentais para o desenvolvimento de novo compósitos fabricados com a incorporação de resíduos lignocelulósicos. Palavras-chave: resíduos lignocelulósicos, desempenho mecânico, compósitos poliméricosPágina 124Foi realizada uma pesquisa minuciosa a fim de encontrar os artigos mais relevantes ao Página 127
DCT (deep cryogenic treatment) is commonly used in industry to improve the wear resistance characteristics of steels, especially. However, there are just a few researches about the effects on non-ferrous metals. The purpose of this work was to investigate how DCT affects the properties of Cu-14Al-4Ni alloy treated at different soak time and submitted to thermomechanical cycling. A comparative experimental analysis was performed of the thermal properties of alloys obtained on a vacuum furnace, treated by DCT and thermomechanically cyclized. The results indicates that thermomechanical cycling promoted the appearance and growth of the martensitic phase γ' 1 , less ductile than the martensitic phase β' 1 , which together with the induced hardening produced an increase in transformation temperatures and microhardness. The higher the number of cycles, the greater these effects. The DCT promoted an increase in the intensity of the diffraction peaks corresponding to the phase β' 1 and the maintenance of them during the thermomechanical cycling of the material, which indicates that the DCT stabilizes the martensitic phase β' 1 and, consequently, caused a reduction and stabilization of the martensitic transformation temperatures and the microhardness, when compared to the untreated material. The longer the soaking time of DCT, the greater these effects.
The non-ferrous shape memory alloys have, normally, two problems that hinder its use in industrial scale: the natural aging and grains growth. The first degrades the memory effect, while the second, observed during the processing of alloy, modifies the temperatures which the transformations occur. Thus, the study of kynetic of recrystallization is important for enabling the control of hardened state in function of treatment time, without causing excessive grain growth. Therefore, the objective of this study is to determine the kinetics of recrystallization of Cu-14Al-4Ni shape memory alloy, based on an empirical law of the formation of Jonhson-Mehl-Avrami, as well as their activation energies for grain growth process according to the empirical Arrhenius law. The alloy was vacuum melted in an induction furnace. After casting, the bulk samples of the alloy were homogenized for 24 hours, solubilized and hot rolled followed by water-quenching to initiate the recrystallization. Then, different samples were annealed at temperatures close to the peak, start and end of the DSC curve. Following the heat treatments, the samples were submitted to mechanical tests and the values of the properties were correlated to the fraction transformed for determination of recrystallization’s kinetic. For the characterization of the grain growth process, analyses in optical microscopy were accomplished and all annealed samples were examined by statistical metallography and the grain sizes were measured. After measurements, the ln[-ln(1-Yrec)] x ln(t) and the ln [D-Do] x 1/T diagrams were plotted to determine the parameters of Jonhson-Mehl-Avrami equation and the activation energy of the process, respectively. The results showed that the equation of the recrystallized fraction follows the empiric law of the formation of Jonhson-Mehl-Avrami for the considered property, as well as, also showed that the alloy Cu-14Al-4Ni is extremely sensitive to temperature variation in which the alloy is treated, having a dual kinetics of grain growth. In the first domain, between 670 and 710°C, the diagram provides a value for the activation energy equal to 39.32 KJ/mol, in the second domain, between 710 and 790°C, the diagram provides a value for the activation energy equal to 9.01 KJ/mol.
RESUMO:O briquete é uma excelente alternativa de fonte térmica renovável devido ao potencial energético que existe na biomassa e está em concordância com os Objetivos de Desenvolvimento Sustentável (ODS) da ONU que visa facilitar o acesso de pesquisas e o aumento da participação de energias renováveis até 2030. Este trabalho tem como objetivo avaliar as características físico-químicas e mecânicas da borra da fibra de piaçava (Attalea funifera) antes e após a briquetagem e analisar sua viabilidade para fabricação de briquetes. A biomassa foi caracterizada quanto a sua granulometria, teor de umidade, densidade a granel, análise química imediata e poder calorífico superior (PCS). Para a fabricação dos briquetes foram utilizadas granulometrias no intervalor de 0,297 -0,149 mm com um teor de umidade de 10,08%. Os briquetes da borra de piaçava padrão apresentaram uma expansão longitudinal aceitável de 22,37% e poder calorífico superior (PCS) dentro das especificações da norma com um valor de 18,73 MJ.kg¯¹. Através do ensaio de tração por compressão diametral a altura máxima de empilhamento foi 22,53 m, entretanto, o ensaio de abrasão mostrou um alto índice de friabilidade. Devido a pandemia do novo coronavírus não foi possível obter os resultados para os briquetes fabricados com aglomerante natural. A briquetagem da borra de piaçava mostrou-se capaz como uma alternativa viável para agregar valor a biomassa, podendo ser utilizada como combustível sólido renovável. Recomenda-se o estudo utilizando aglomerantes naturais para posterior avalição da sua abrasão.
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