A castable alloy, i.e., one that flows easily to fill the entire mold cavity and also has resistance to hot tearing during solidification, must invariably contain a sufficient amount of a eutectic structure. For this reason, most traditional aluminum casting alloys contain silicon because the aluminum-silicon eutectic imparts to the alloy excellent casting characteristics. However, the solidus temperature in the Al-Si system does not exceed 577°C, and the major alloying elements (i.e., zinc, magnesium, and copper) used with silicon in these alloys further lower the solidus temperature. Also, these elements have high diffusivity in aluminum and so, while they enhance the room temperature strength of the alloy, they are not useful at elevated temperatures. Considering nickel-base super alloys, whose mechanical properties are retained up to temperatures that approach 75% of their melting point, it is conceivable that castable aluminum alloys can be developed on the same basis so that they are useful at temperatures approaching 350C. A castable aluminum alloy intended for high temperature applications must contain a eutectic structure that is stable at temperatures higher than 600°C, and must contain second phase precipitate particles that are thermodynamically stable at the service temperature. Transition metal trialuminides with the general chemical formula AlxTMy in which TM is a transition metal, are excellent candidates for both the eutectic structure and the precipitate particles. In this research, the use of transition metals in the constitution of aluminum casting alloys is investigated with emphasis on the morphology, crystallography, and mechanisms of formation of the various phases.

A large number of advanced structural materials are based on metallic materials where alloying additions play a key role in imparting the required properties. Most of the commercially important aluminium alloys are classified by the nature of the alloying additions. Among them the 2219, 2618, 5086, and 7075 are important class of lightweight alloys that plays critical role in modern engineering application. However, despite having a series of commercially useful aluminum alloys for commercial applications the increasing need of improved performance requires newer development in particular for applications that require high strength at elevated temperatures and performance at extreme environments. Precipitations of the intermetallic compounds containing copper during thermal treatments play a very important role in developing high strength aluminium alloys. Although,these precipitates are stable at fairly high temperatures, the rapid coarsening of these second phase precipitates (e.g. Al2Cu), leads to loss of strength at elevated temperature. Several approaches are explored to overcome this problem. One of them is to utilize non-equilibrium solidification route, which can increase solid solubility and hence increasing the precipitate density. Nonequilibrium processing can also alter the selection pathway of the competitive phases and evolution of the microstructure. Recently, non–equilibrium solidification by suction casting technique is becoming increasingly popular for casting of metallic materials of any shape. In this technique solidification is effected by sucking the molten alloy into water cooled copper mold using a suction force resulting from the differences between the melting chamber in Argon gas pressure and casting chamber under vacuum. The present thesis aims to develop a set of newer alloys with small amount of alloying additions primarily based on nickel that can retain reasonable strength at high temperature by utilizing the non-equilibrium solidification route. In addition to Ni (≤ 0.10at.%), the thesis present results of the effect of minor addition of Sc and Zr as ternary and quaternary additions. Following a short review in chapter 2, Chapter 3 presents the experimental techniques adopted for both preparation of alloys and their characterization. Chapter 4 deals with the results of alloying of aluminum with minor amount of nickel. The Ni in the range of 0.05-0.20at% was used to develop a high temperature template, containing a set of hardening intermetallic compounds to increase the strength of the host matrix. The microstructural investigations of the suction cast alloys reveal a characteristic feathery microstructure. At higher magnification the microstructure reveals the presence of fine dispersions of a second phase. Both x-ray and transmission electron microscopy confirms the phase in the dispersions to be primarily crystalline Al9Ni2 phase having a monoclinic crystal structure. This phase does not exist in equilibrium phase diagram. Only at higher concentration one can observe equilibrium Al3Ni (Orthorhombic) particles. The size of the particle ranges from 50-200nm. Beyond~0.5at%Ni, the microstructure changes to normal cellular type solidification morphology with interdendritic space decorated by the eutectic network of Al-Al3Ni having a rod eutectic morphology. A careful observation of alloys with small amount of Ni reveals that the feathery structure is associated with the thin cells, which have grown by continuously splitting the tip yielding a fractal like dendritic morphology. The dispersoids form at the intercellular regions. We have presented clear evidence of their origin from the interdendritic liquid, which most likely underwent Rayleigh instability. The random distribution reflects the nature of the dendritic growth. We have argued that these inter-dendritic liquid droplets, which are enriched with Ni, get undercooled. The metastable Al9Ni2 phase nucleates and grows in this liquid. In order to confirm this scenario, we have carried out a phase field simulation for dendritic growth of aluminium solid solution in the alloy melt both under the condition of constraint growth and free growth. The observed distribution of the dispersoid is well reflected in the phase field simulation. The chapter also report the response of effect of direct ageing of suction cast alloy as one expect an extension solid solubility of Ni in Al. A small increase in hardness could be observed during ageing treatment. In order to determine the thermal stability of the intermetallic particles, the samples of the suction cast alloys were exposed at 200°C for 200h and 500°C for 100h respectively. No change in the microstructure could be observed excepting a slight coarsening indicating the dispersed particles are thermally stable. After exposure at two different temperatures the maximum retained hardness was measured to be 350MPa. We have also attempted to correlate the hardness with coarsening behavior of particles. The feathery morphology of the cast structures and fine dispersion of the intermetallic phase is expected to improve the tensile strength of the alloy. The tensile yield strength of cast alloys was determined to be 150MPa ± 20 for Al- 0.09at%Ni alloy. We have tried to estimate the expected strength of the alloy from quantitative microstructural parameters using possible hardening mechanism. The estimates are in good agreement to the observed values. The chapter 5 reports attempts to develop thermally stable precipitation strengthened aluminum alloys by retaining the dispersion template developed earlier alloyed with Ni. Then, the binary alloys were added with extremely low diffusivity element Zr. The element Zr is traditionally added in the aluminium alloys as grain refiner and as a powerful agent for inhibiting recrystallization especially for high strength aluminium alloys. However, in this work we have alloyed Zr for imparting precipitation hardening. An amount of 0.15at%Zr was added to the suction cast alloys of Al-0.05, 0.09 and 0.20at%Ni. The first two alloys exhibit the formation of metastable phase Al9Ni2 during solidification stage. Increase the concentration of the alloy to Al-0.20at% Ni with 0.15at%Zr additions exhibits combination of both stable Al3Ni and Al9Ni2 metastable phases. Microstructures of these alloys show columnar cells of ~200μm with dispersions of spherical nodules of Al9Ni2 and Al3Ni with varying size ranges from 200-500nm. Particle size distribution of Zr containing aluminium alloys with 0.05at% Ni is 595nm ± 20 while the alloy having the 0.09 at% Ni has the optimum size of 290nm. Further increase of Zr composition above 0.20at % led to columnar to equiaxed transition. The as cast alloys containing Zr does not show the improvement with limited yield strength of the order of 150MPa. The equivalent hardness of the samples has been measured to be about 370-420MPa. Heat-treated alloys however show the presence of Al3Zr (L12) precipitates with ~20nm size that are coherent with the matrix. Binary suction cast Al-0.15at%Zr alloy after ageing exhibits tensile yield strength of ~200MPa. With ternary aluminium alloy with minor additions Ni and Zr, The strength increases to ~300MPa. Additionally, the alloy continue retain a maximum hardness of 870-920MPa even after long hours of aging. The Zr containing alloys were proved to be stable. When the tests were carried out on a nominally alloyed sample of Al-0.09at%Ni-0.15at%Zr peak aged and exposed to 250°C for 200h, the yield strength under compression tests was found to be 280MPa. The chapter 6 of the thesis discusses the role of Sc with the ternary Al-alloys with Ni and Zr. Addition of small quantities 0.1 and 0.2at%Sc substantially reduces the inter-particle distance of precipitates by increasing volume fraction and number of nano-sized particles. It has been observed and presented in this thesis that the Sc addition provides the highest incremental strengthening per atom percent of any alloying element. Chill-suction cast samples show equiaxed cells in the samples with dispersions of particles inside and some segregated particles at the cell boundaries. To achieve a further increase in the number density of precipitates we processed the suction cast alloys with additional heat treatment at 375 and 450°C. All the suction cast alloys with varying Ni content and keeping the Sc and Zr constant at 0.10 and 0.15at% respectively exhibit formation of Al9Ni2 phase. The alloy Al-0.20at%Ni-0.10at%Sc-0.15at%Zr also contain stable phase of Al3Ni with an eutectic morphology. The DSC experiments in the dynamic mode with heating rate of 10°C min-1 exhibit two distinct exothermic peaks due to precipitates from solution at 375 and 450°C. The TEM analysis using STEMEDX has further confirmed the existence of nano-sized particles 30-50 nm of both phases of Al3Sc and Al3 (Sc, Zr). The tensile yield strength of the as cast alloy show 200MPa while after precipitation treatment, we observe improved yield strength 350-450MPa. Thermal stability of the alloys were tested after peak aged condition and exposed to 200°C for 250h. The results show that the yield strength is unaffected implying the coarsening resistance of the precipitate particles. Overall the thesis establishes that with minimum alloying additions, it is possible to design alloys that are expected to perform for high temperature applications by the formation of set of dispersions of Al9Ni2 (monoclinic) and precipitates of ordered cubic phases of (L12) structure of Al3Zr, Al3Sc and Al3 (Sc, Zr) with required number density of particles.

Made available in DSpace on 2015-05-08T14:59:52Z (GMT). No. of bitstreams: 1 parte1.pdf: 2223950 bytes, checksum: 585473348fd57251708f0ba93240c53b (MD5) Previous issue date: 2008-10-28 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The presented work consists of a study for observation of the martensite kept in the transformation of martensite phase - austenite in a alloy of Ni - Al - V, with memory shape effect, into, a room temperature. We defined that the alloy with composition the 3,5% (in weigh) of nickel, presents better results when happens successive loading and downloading cancel though, 3,0% of deformation into a room temperature, back to austenite phase after retreat the load. Above 3,0% of deformation became evident the appearance of retrain martensite. For her identification it was necessary the development of on equipment to traction, as well as bodies of proofs dimensioned for that. The analyses were done through X rays diffraction of optical microscopy and rehearsal of traction. / O trabalho apresentado consta de um estudo para observação da martensita retida na transformação de fase de martensita austenita em uma liga de Ni Al V, com efeito de memória de forma, a temperatura ambiente. Definimos que a liga com composição 3,5% (em peso) de níquel, apresenta melhor resultado quando submetida a carregamento e descarregamento sucessivos; chegando a 3,0% de deformação a temperatura ambiente, retornando ao estado austenítico depois de retirada a carga. Acima de 3,0% de deformação, evidencia-se o surgimento de martensita retida. Para sua identificação, foi necessária a confecção de um equipamento para tracionar, como também corpos de prova dimensionados para tanto. As observações foram realizadas através de análises de raios X, microscopia ótica e ensaios de tração.

Submitted by Kilvya Braga (kilvyabraga@hotmail.com) on 2018-04-23T13:59:14Z No. of bitstreams: 1 SUELENE SILVA ARAÚJO - DISSERTAÇÃO (PPGEM) 2015.pdf: 3709690 bytes, checksum: 1d6f1a6f3675dd7c45fb2cd75c6c92bf (MD5) / Made available in DSpace on 2018-04-23T13:59:14Z (GMT). No. of bitstreams: 1 SUELENE SILVA ARAÚJO - DISSERTAÇÃO (PPGEM) 2015.pdf: 3709690 bytes, checksum: 1d6f1a6f3675dd7c45fb2cd75c6c92bf (MD5) Previous issue date: 2015-08-07 / CNPq / Neste trabalho foi avaliado o comportamento dinâmico e a fadiga termomecânica de fios de uma liga com memória de forma (LMF) CuAlNi submetidos a ensaios cíclicos utilizando um analisador dinâmico mecânico (DMA). Assim, foram realizadas análises para determinar a capacidade de amortecimento e o módulo de elasticidade dos fios e adicionalmente a fadiga estrutural sob controle de deformação em modo de flexão simples alternada, a partir da variação da amplitude de deslocamento, para três temperaturas distintas. O comportamento dinâmico revelou uma considerável capacidade de amortecimento dos fios, principalmente durante a transformação de fase martensitica, apresentando um expressivo pico de absorção de energia. Além disso, o módulo de elasticidade (rigidez) apresentado pelos fios revelou a existência de um importante aumento durante a transformação, diferenciando as duas distintas fases. Notou-se também a partir dos resultados dos ensaios de fadiga que as características funcionais dos fios dependem de forma direta da amplitude de deformação imposta, influenciando nos valores de tensão e no tempo de vida dos fios, revelando que a fadiga pode ser considerada de baixo ciclo. / This study evaluated the dynamic behavior and the thermomechanical fatigue of CuAlNi SMA wires subjected to cyclic tests using a DMA device. Therefore, analyzes were performed to determine the damping capacity and stiffness of SMA wires, in addition to structural fatigue under strain control from the amplitude variation at three different temperatures. The evolution of the wire’s behavior has shown a considerable damping capacity, mainly during the martensitic phase transformation, presenting a significant peak of energy absorption. Moreover, the storage modulus (stiffness), revealed a significant increase during transformation, differentiating the distinct phases. It has been also noted, from the results of fatigue tests, that the functional behavior of the SMA wires depend directly on the amplitude of the imposed strain, influencing on the stress values as well as on the wire’s lifetime, revealing that fatigue can be considered of low cycle type.

A castable alloy, i.e., one that flows easily to fill the entire mold cavity and also has resistance to hot tearing during solidification, must invariably contain a sufficient amount of a eutectic structure. For this reason, most traditional aluminum casting alloys contain silicon because the aluminum-silicon eutectic imparts to the alloy excellent casting characteristics. However, the solidus temperature in the Al-Si system does not exceed 577°C, and the major alloying elements (i.e., zinc, magnesium, and copper) used with silicon in these alloys further lower the solidus temperature. Also, these elements have high diffusivity in aluminum and so, while they enhance the room temperature strength of the alloy, they are not useful at elevated temperatures. Considering nickel-base super alloys, whose mechanical properties are retained up to temperatures that approach 75% of their melting point, it is conceivable that castable aluminum alloys can be developed on the same basis so that they are useful at temperatures approaching 350 °C. A castable aluminum alloy intended for high temperature applications must contain a eutectic structure that is stable at temperatures higher than 600°C, and must contain second phase precipitate particles that are thermodynamically stable at the service temperature. Transition metal trialuminides with the general chemical formula AlxTMy in which TM is a transition metal, are excellent candidates for both the eutectic structure and the precipitate particles. In this research, the use of transition metals in the constitution of aluminum casting alloys is investigated with emphasis on the morphology, crystallography, and mechanisms of formation of the various phases.

Submitted by Cleide Dantas (cleidedantas@ufpa.br) on 2014-04-14T15:04:45Z No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Dissertacao_VariaveisTermicasSolidificacao.pdf: 3783156 bytes, checksum: 09439df2a825bda18e3a37c1e6addeb0 (MD5) / Approved for entry into archive by Ana Rosa Silva (arosa@ufpa.br) on 2014-09-01T16:25:46Z (GMT) No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Dissertacao_VariaveisTermicasSolidificacao.pdf: 3783156 bytes, checksum: 09439df2a825bda18e3a37c1e6addeb0 (MD5) / Made available in DSpace on 2014-09-01T16:25:46Z (GMT). No. of bitstreams: 2 license_rdf: 23898 bytes, checksum: e363e809996cf46ada20da1accfcd9c7 (MD5) Dissertacao_VariaveisTermicasSolidificacao.pdf: 3783156 bytes, checksum: 09439df2a825bda18e3a37c1e6addeb0 (MD5) Previous issue date: 2008 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / No presente trabalho, é realizada uma seqüência de experimentos com ligas hipoeutéticas Al-Ni para analisar a solidificação unidirecional vertical ascendente em condições transitórias de fluxo de calor. Abordagens experimentais são desenvolvidas para a determinação quantitativa de variáveis térmicas de solidificação, tais como: tempos locais de solidificação; velocidades de deslocamento das isotermas liquidus; taxas de resfriamento à frente da isoterma liquidus e gradientes térmicos à frente da interface sólido / líquido. O trabalho analisa também a dependência dos espaçamentos dendríticos secundários em relação às variáveis térmicas de solidificação e ao teor de soluto das ligas, além de relacionar estes parâmetros com a resistência à corrosão destas ligas. Esses parâmetros dendríticos experimentais, referentes à solidificação das ligas Al - 1, 1,4 e 1,8 % Ni, não puderam ser comparados com os principais modelos teóricos de crescimento dendrítico da literatura, pois, para as ligas em estudo há uma escassez de propriedades termofísicas, impossibilitando assim esta comparação. O comportamento corrosivo é analisado pela técnica de polarização potenciodinâmica e técnica de polarização potenciodinâmica cíclica conduzidas em solução de 3,5% NaCl em temperatura ambiente, estruturas dendríticas mais grosseiras tendem a aumentar as taxas de corrosão das ligas hipoeutéticas do sistema Al-Ni. Os resultados experimentais obtidos através de ensaios de corrosão são correlacionados com a microestrutura dendrítica. Dessa forma, são determinadas tendências experimentais de crescimento dendrítico e resistência à corrosão para a solidificação unidirecional vertical ascendente. / In this research is carried out a sequence of experiments with Al-Ni hypoeutectic alloys to analyze the upward vertical directional solidification on transient conditions of heat flow. Experimental approaches were developed to quantitative the thermal variables of solidification, as follow: local time solidification; dislocation rate of liquidus isotherms, tip cooling rates and thermal gradients. This study analyzes the dendrite secondary arms spacing in relation to solidification thermal variables and to alloy composition, beyond link these parameters with corrosion resistance of these alloys. These experimental dendrite parameters, concerning to alloys solidification as Al - 1, 1,4 and 1,8%Ni, couldn’t be compared with the main theoretical model of dendrite growth by others researches, because, to alloy studied there shortage of thermophysic properties, impossible this comparison. The corrosive behavior is analyzed by potentiodynamic polarization technique and cyclic polarization technique lead into 3,5%NaCl solution on room temperature, coarse structure dendritics tend to increase the corrosion rates of Al-Ni hypoeutectic alloys. The experimental results obtained by the corrosion tests were correlated with dendrite microstructure. This way, experimental tendencies of growth dendrite were determined to upward vertical directional solidification.

Les poudres d'alliages aluminium-nickel produites par atomisation peuvent être traitées pour préparer du nickel de Raney, un catalyseur utilisé dans de nombreux procédés industriels. L'activité du catalyseur dépend fortement du déroulement des multiples réactions de solidification pendant l'atomisation. Un modèle de microségrégation pour la solidification d'alliages métalliques est alors développé. En considérant des flux de diffusion finis, des cinétiques de réactions dendritique, péritectique et eutectique et des surfusions de germination, une alternative plus évoluée est proposée aux modèles de Gulliver-Scheil ou de la loi des leviers. Le couplage avec des calculs d'équilibre thermodynamique est effectué pour évaluer les compositions des interfaces et les termes d'enthalpie dans le bilan d'énergie. Le modèle est appliqué à un alliage binaire, avec des densités de phases constantes, pour simuler le procédé d'atomisation de gouttes d'alliage Al-Ni. Un modèle dédié est choisi pour les conditions aux limites d'échange de chaleur. Les résultats sont comparés à des mesures expérimentales obtenues par diffraction de neutron. Des interprétations sont alors établies sur les comportements non triviaux des alliages Al-Ni solidifiés rapidement. Le modèle proposé permet ainsi d'appréhender les effets concurrents des différentes cinétiques (diffusion chimique, bilan d'énergie, vitesse croissance des microstructures, etc.) lors de la solidification hors équilibre. Les principaux développements envisageables autour de ce travail incluent : l'extension aux alliages multicomposés, l'inclusion de densités variables, le couplage avec des calculs macroscopiques.

No description available.

Submitted by Cássio da Cruz Nogueira (cassionogueirakk@gmail.com) on 2017-02-03T11:45:45Z No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_InfluenciaAdicaoNanotubos.pdf: 6893729 bytes, checksum: eae2f8951b81e53f035b40fce6837e00 (MD5) / Approved for entry into archive by Edisangela Bastos (edisangela@ufpa.br) on 2017-02-07T16:33:51Z (GMT) No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_InfluenciaAdicaoNanotubos.pdf: 6893729 bytes, checksum: eae2f8951b81e53f035b40fce6837e00 (MD5) / Made available in DSpace on 2017-02-07T16:33:51Z (GMT). No. of bitstreams: 2 license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Tese_InfluenciaAdicaoNanotubos.pdf: 6893729 bytes, checksum: eae2f8951b81e53f035b40fce6837e00 (MD5) Previous issue date: 2015-11-27 / Neste trabalho foi fabricada uma matriz hipoeutética de Al-2%Ni e adicionado nela concentrações variadas de Nanotubos de Carbono, com o objetivo de verificar a evolução da resistência mecânica do novo material processado. O processo de fabricação do material estudado se deu por fundição e resfriamento brusco unidirecional ascendente, com extração transiente de fluxo de calor e determinação de variáveis térmicas de solidificação. Imagens de Microscopia Eletrônica de Varredura mostram o arranjo dos nanotubos na matriz e com a Espectroscopia Raman foi possível determinar as bandas “D” e “G” e analisar a ação do processo sobre os mesmos. Por meio de ensaios metalográficos e mecânicos foi possível caracterizar o comportamento macro, microestrutural e as propriedades mecânicas. O acréscimo de pequenas concentrações de nanotubos aumentou significativamente a resistência mecânica da matriz. / In this work, we produced a hypoeutectic Al-2wt%Ni matrix, to which we added various concentrations of carbon nanotubes, in order to evaluate the evolution of the mechanical strength of the new material. The manufacturing process occurred by melting and ascending unidirectional abrupt cooling, with extraction of the heat transient and determination of the thermal variables involved in the solidification. Scanning Electron Microscopy images show how the nanotubes are settled in the matrix, and by means of Raman Spectroscopy, we were able to determine the “D” and “G” bands and analyze the action of the process over them. By means of metallographic and mechanical tests, we were able to characterize the macro and micro-structural behaviors and the mechanical properties. The addition of small quantities of nanotubes led to a significant raise in the mechanical strength of the matrix.

Amorphous metals have been discovered in 1960 and, because of their structures, exhibit very unique mechanical, magnetic and chemical properties that can have various applications. These properties qualify them as the potential material of the future. This work focuses on a new laser ablation technique to synthesize nanoparticles of amorphous metals from aqueous feedstock. One of the critical factors in the production of amorphous metal is the cooling rate used to synthesize them. The laser ablation of microparticle aerosol (LAMA) process used in this work, with a cooling rate estimated of 10¹² K/s, has all the characteristics required for the production of such metallic glasses. A Collison nebulizer is used to generate microdroplets of a nitrate solution containing the corresponding ratio of metals for the production of a Zr-Al-Ni based alloy. Once dried and conditioned, these microdroplets leave solid microparticles which are ablated by an excimer laser producing nanoparticles that are then filtered by virtual impaction. In order to characterize the nanoparticles obtained with this process nanoparticulate films produced by LAMA have been analyzed by optical profilometry, scanning electron microscopy (SEM) equipped with energy-dispersive x-ray spectroscopy (EDS) and transmission electron microscopy (TEM) equipped with EDS. The results agree with a hypothesis that the films contain oxidized, amorphous metal on the surface of the films. When the films are thin, they are fully oxidized, and simultaneous segregation of Zr occurs to the surface. The hypothesis and the results are discussed. / text