The first stage of the solidification of metal alloys is nucleation, during this stage is defined the number of grains present in the final structure, just as the conditions in which the same will grow. The application of pressure in the solidification is mostly studied as an operational parameter, and its effects are described and commented extensively in literature. The objective of this paper is to study the effect of pressure on nucleation, still not to describe but to explain it, for this purpose the pressure is treated as a thermodynamic variable. From this analysis it was possible to determine the real influence of pressure on the activation energy, being clear that its effect is equivalent and co-dependent to that of the undercooling, and can thus cause considerable variations in the activation energy, much larger than what is proposed in previously published papers that approach the subject, causing a considerable stimulus on nucleation. It was also observed that the curve that represents the effect of pressure on the activation energy proposed in this paper, has a similar form to the curves that describe how the pressure refines structures, resulting from the process of solidification under pressure reported in literature.

RESUMO A exigência contínua de materiais leves e de alto desempenho para as indústrias aeroespaciais e automotivas, levou ao desenvolvimento de materiais compósitos com matriz metálica, destacando-se entre eles, àqueles de base alumínio. Os quasicristais são materiais metálicos com uma estrutura atômica complexa, a qual resulta uma combinação de propriedades físico-mecânicas similares às de algumas cerâmicas frágeis e compostos intermetálicos, como elevada resistência e dureza, elevado módulo de elasticidade, baixo coeficiente de atrito e boa resistência ao desgaste e à corrosão. Essas propriedades são bastante promissoras para aplicação industrial, mas a fragilidade dos quasicristais restringe sua aplicação como fase de reforço em compósitos, podendo substituir os reforços cerâmicos utilizados tradicionalmente, como SiC e Al2O3. No presente trabalho, compósitos de matriz de alumínio reforçado com 2,5%, 5% e 10% em volume de partículas quasicristalinas, foram produzidas através do método de agitação do banho de metal fundido stir casting. Etapas subsequentes de conformação por laminação foram realizadas em todos os compósitos alumínio/quasicrsital. A liga quasicristalina Al61,7Cu25,5Fe12,3Mn0,5 foi obtida via fusão convencional e posterior tratamento térmico. A moagem da liga quasicristalina, para obtenção de pós, foi realizada em moinho de alta energia. A microestrutura dos quasicristais foi investigada através da Difração de Raios-X e da Microscopia Eletrônica de Varredura (MEV). A distribuição das partículas quasicristalinas na matriz de alumínio e a influência na dureza dos compósitos foram investigadas por MEV e Microdureza Vickers, respectivamente. Os compósitos apresentaram uma boa distribuição das partículas quasicristalinas na matriz de alumínio, embora ocorrido em algumas regiões a formação de poros e aglomerados. O processo de laminação promoveu um aumento significativo na dureza em relação aos compósitos obtidos diretamente da fundição.

ABSTRACT The continuous requirement of lightweight and high performance materials for the aerospace and au-tomotive industries led to the development of composite materials with a metallic matrix, among which aluminum-based composites. The quasicrystals are metallic materials with a complex atomic structure, which results in a combination of physico-mechanical properties similar to those of some fragile ceramics and intermetallic compounds, such as high strength and hardness, high modulus of elasticity, low coefficient of friction and good resistance to wear and corrosion. These properties are very promising for industrial application, but the fragility of the quasicrystals restricts their application as a reinforcement phase in composites, and can replace the traditionally used ceramic reinforcements such as SiC and Al2O3. In the present work, reinforced aluminum matrix composites with 2,5%, 5% and 10% volume of quasicrystalline particles were produced by the stir cast method of stir casting. Subsequent steps of laminating were performed on all aluminum / quasicrystals composites. The quasicrystalline Al61,7Cu25,5Fe12,3Mn0,5 alloy was obtained by conventional melting and subsequent heat treatment. The mill of the quasicrystalline alloy, to obtain powders, was carried out in a high-energy mill. The microstructure of quasicrystals was investigated by X-ray Diffraction and Scanning Electron Microscopy (SEM). The distribution of the quasicrystalline particles in the aluminum matrix and the influence on the hardness of the composites were investigated by SEM and Vickers Microhardness, respectively. The composites presented a good distribution of the quasicrystalline particles in the aluminum matrix, although in some regions the formation of pores and agglomerates occurred. The rolling process promoted a significant increase in hardness in relation to the composites obtained directly from the casting.

RESUMO O conhecimento amplo do comportamento termomecânico das ligas com memória de forma (LMF) faz-se necessário para obtenção de ligas com elevada histerese térmica, o que torna possível eliminar a necessidade de meios criogênicos em muitas aplicações. Desta forma, os modelos matemáticos surgem como uma alternativa eficaz para descrição deste comportamento termomecânico. No presente trabalho foi desenvolvido um modelo matemático capaz de mensurar a deformação residual para uma liga Cu-Al-Be-Nb em função das variáveis: deformação, temperatura e taxa de deformação. Observa-se que os resultados obtidos por meio do modelo apresentam boa concordância quando comparado com os dados experimentais. Além disso, verificou-se que o modelo é capaz de prever as condições necessárias para se obter a máxima deformação residual.

ABSTRACT The wide knowledge of the thermomechanical behavior of shape memory alloys (SMAs) is necessary to obtain alloys with high thermal hysteresis, which makes it possible to eliminate the need for cryogenic means in many applications. In this way, the mathematical models appear as an effective alternative to describe this thermomechanical behavior. In the present work, a mathematical model was developed able to measure the residual strain for a Cu-Al-Be-Nb alloy as a function of the variables: strain, temperature and strain rate. It is observed that the results obtained through the model are presented with good agreement when compared with the experimental data. Furthermore, it was found that the model is capable of predicting the conditions necessary to obtain the maximum residual strain.

Copper-based polycrystalline shape memory alloys (SMAs) have limitations for many practical applications due to their low superelasticity and low ductility. In order to overcome this situation, in recent years, the production of copper-based SMAs by directional solidification process has attracted the interest of many researches. In this sense, the present work had as objective to evaluate, through instrumented indentation tests, the influence of the solidification direction on the elastic modulus, hardness and superelasticity of a Cu-Al-Be-Nb-Ni alloy produced by directional solidification. The results showed that the superelasticity, remnant depth, elastic modulus and hardness DHV-1 are strongly dependent on the direction of application of the load in relation to the solidification direction, that is, the alloy presented an anisotropic behavior for its mechanical properties.

The understanding of the stabilization mechanism of martensite used to obtain alloys with high hysteresis and the possibility of recovering this martensite (residual strain) by heating at temperatures higher than As and Af have been fundamental for application in bonding devices. One of the possibilities to enhance residual strain is through mechanical cycling. This work evaluated the influence of number of cycles and the effect of precipitation of Nb-rich particles as mechanisms to increase residual strain in superelastic Cu-Al-Be-Nb alloys. The results showed that the presence of Nb-rich precipitates was not a predominant factor for the enhancement of residual strain. In addition, mechanical cycling has proved to be an efficient mechanism for raising residual strain, and consequently increasing the amount of martensite retained. However, the gain of residual strain is accompanied by an increase of plastic deformation.

The effect of heat treatment on crystallization behavior of Fe73.5B15Si10Nb1.5 alloy powder prepared by mechanical alloying was studied. The powder samples were prepared by mechanical alloying (MA) and for different milling times (1, 5, 25, 70 and 100 hours). Crystalline powders of iron, boron, silicon and niobium were sealed with tungsten carbide balls in a cylindrical vial under nitrogen atmosphere. The ball-to-powder weight ratio was 20 to 1. A Fritsch Pulverizette 5 planetary ball mill was used for MA the powders at room temperature and at 250 rpm. To study the microstructural evolution, a small amount of powder was collected after different milling times and examined by X-ray diffraction, using CuKalpha radiation (lambda = 0.15418 nm). The crystallization behavior was studied by differential thermal analysis, from 25 up to 1000 °C at a heating rate of 25 °C min-1.