No description available.

The purpose of this investigation is to make more detailed studies of transformations. Fourteen compounds have been examined by high temperature X-ray diffraction for this purpose. The investigations have been carried out in such a way as to reveal: 1. the existence of transformations, 2. the influence of polarizability on thermal expansion, 3. the anisotropy of expansion, and 4. the discontinuity of thermal expansion.

Ligas de Urânio são candidatas ao uso como combustível nuclear em reatores avançados, dentre essas ligas se destacam as ligas de Urânio com Nióbio e com Zircônio. Este trabalho investigou como os teores de Nb e Zr, assim como a processamento termomecânico afetam as microestruturas e as propriedades mecânicas de 3 ligas U-XNb-YZr onde X+Y=12. Duas amostras contendo 50g cada, foram fabricadas através de fusão à plasma nos teores U-3Nb-9Zr (liga 39), U- 6Nb-6Zr (liga 66) e U-9Nb-3Zr (liga 93). Uma das amostras de cada liga foi tratada termicamente por 5h a 1000°C para a realização de homogeneização química. A amostra homogeneizada e a amostra bruta de fusão de cada liga foram conjuntamente encapsuladas em aço para a realização de laminação a quente seguida de um recozimento final a 1000°C por 2h. À rota adotada pela amostra bruta de fusão chamou-se \" Rota C\" e a rota adotada pela amostra homogeneizada chamou-se \"Rota H\". A caracterização microestrutural foi feita por microscopia óptica e eletrônica de varredura. Todas as amostras, independente do processamento, apresentaram precipitados ricos em Nióbio e Zircônio em adição a uma matriz rica em Urânio. A caracterização das amostras brutas de fusão mostra que os teores de elementos de liga influenciam diretamente a morfologia das dendritas evidentes na microestrutura assim como das demais fases presentes em cada amostra. A liga 39 apresentou predominantemente fase ?\', a liga 66 a fase ?\" com traços de fase y e a liga 93 a fase y com traços de fase ?\". Após o tratamento térmico de homogeneização, a liga 39 apresentou fase ?\" na forma celular enquanto a liga 66 apresentou as fases y0 e y e a liga 93 apresentou apenas fase y. As propriedades mecânicas das ligas foram avaliadas por ensaios de dureza e de dobramento simples. As amostras da Rota C apresentaram redução de dureza em relação à condição inicial. Todas as microestruturas das amostras laminadas a quente exibiram duas fases ricas em U. A liga 39 apresentou as fases ?\" na forma celular e ?\' após a laminação a quente. Após o recozimento final na rota C, a liga 39 apresentou fase ?\" na forma acicular enquanto as ligas 66 e 93 apresentaram as fases ?\" e y após a laminação e fase y. A fração de área da amostra pobre em U elevou-se nas ligas 39 e 66 e reduziu-se na liga 93. As amostras da Rota H apresentaram redução de dureza em relação à condição bruta de fusão. A liga 39 apresentou fase ?\" na forma celular com orientação e traços da fase ?, a liga 66 exibiu as fases y0 e y e a liga 93 as fases y e y0. Após o recozimento final, a liga 39 mostrou-se novamente na forma ?\" na forma celular, mas sem orientação. A liga 66 apresentou fase y e a liga 93 fase y0. Os testes de dobramento simples mostraram que as ligas da Rota C exibem plasticidade, retendo parte da deformação plástica após a ruptura das amostras testadas. Já as amostras da Rota H mostraram comportamento super elástico, possibilitando maiores deformações mas sem reter deformação plástica após a ruptura das amostras. A melhor relação entre deformação total e residual foi observada na liga 93 fabricada pela Rota C. As análises nos perfis de fratura das amostras da Rota C mostram fraturas transgranulares em todas as amostras. O perfil de fratura de na amostra recozida liga 39 mostra que a fase ?\" na forma acicular tende a deforma-se por deslizamento. As análises dos perfis de fratura nas amostras da Rota H confirmaram a ausência de deformação plástica mesmo em escala microscópica. Para essa condição, a fase ?\" na forma celular com orientação (liga 39) aparenta deformar-se por maclação. As análises de superfície de fratura indicam que a fase pobre em U tem participação durante o processo de crescimento e propagação da fratura na Rota H, atuando como caminho para bifurcação de trincas acelerando o processo de ruptura, enquanto na Rota C, a fase pobre em U deforma-se conjuntamente com a matriz de U. Em uma segunda etapa do trabalho, a estabilidade das microestruturas resultantes na amostras processadas foi investigada por Ensaios de Calorimetria Diferencial Exploratória (DSC) e por calorimetria de queda livre (esta apenas para a amostra 93 da rota H). O teor de Nb e Zr também afeta a estabilidade das fases presentes em cada amostra. foram realizados com as amostras da condição homogeneizada e laminada. A quantidade de transformações assim como o estado final de cada liga diferiu de acordo com a razão Nb/Zr. Após o ciclo de aquecimento e resfriamento da análise térmica, a liga 39 apresentou fase ?\', a liga 66 fase ?\" e a liga 93 fase y. No ensaio de calorimetria por queda livre foi possível observar as diferentes etapas de reação de envelhecimento da matriz g, correspondendo a à transformação y -> y0 (entre 525 e 530 K), a transformação y -> ?\". (entre 623 e 651 K) e à transformação y\' -> y3+? (entre 825 e 925 K). / Uranium alloys are candidates to be used as nuclear fuel in research reactors, among the U alloys, the Nb and Zr containing alloys are promising. This work evaluated how the Nb and Zr content and the thermomechanical processing affects the microstructure and mechanical properties of 3 alloys U-XNb-YZr were X+Y=12. Two 50g slugs of each sample were fabricated using plasma arc melting according to U-3Nb-9Zr (alloy 39), U-6Nb-6Zr (alloy 66) and U-9Nb-3Zr (alloy 93). One slug of each alloy was heat treated for 5h at 1000°C to perform the chemical homogenization. The homogenized sample and the as-cast one were encapsulated in the same steel frame in order to perform hot rolling. After the rolling process, the samples were annealed by 2h at 1000°C. The route that uses only as-cast samples was nominated \"Route C\" and the route that uses the homogenized sample was nominated \"Route H\". The microstructural characterization was performed by optical and scanning electron microscopy. All samples, regardless the processing route, presented Nb and Zr rich precipitates in addition to U rich matrix. The characterization of as-cast samples shows that the content of the alloying element has a direct influence on dendrite morphology as in the phases presented for each alloy. The alloy 39 presented predominantly ?\' phase, the alloy 66 the ?\" phase with a small quantity of ? phase and the alloy 93 presented the ? phase with small quantity of ?\" phase. Afte the homogenization, the alloy 39 presented cellular ?\" phase, the alloy 66 presented ?0 and ?, the alloy 93 presented only ? phase. The mechanical properties were evaluated by hardness measurements and free bending tests. The Route C samples presented hardness reduction in comparison to the initial condition. All microstructures of hot-rolled samples of this route exhibit two U rich phases. The alloy 39 exhibited cellular ?\" and ?\', after the final annealing the alloy 39 presented acicular ?\". The alloys 66 and 93 exhibited ?+?\" after hot rolling and ? phase after the final annealing. The area fraction of poor U phase increased in the alloys 39 and 66, but reduced in alloy 93. The Route H samples presented hardness reduction in comparison to as-cast samples. The alloy 39 presented cellular oriented ?\" phase and a small quantity of ? phase. The alloy 66 exhibited ?0 and ?, the alloy 93 ? and ?0. After the final annealing, the alloy 39 presented the ?\" again, but without orientation. The alloy 66 presented ? phase and the alloy 93 presented ?0. The free bending tests show that Route C samples have real plasticity, retaining part of deformation after rupturing as plastic strain. The Route H samples exhibited superelastic behavior, allowing higher deformations but retaining no plastic strain after sample breaking. The better balance between total and residual strain was observed in alloy 93 fabricated by Route C. The cracking profile analysis of Route C samples shows transgranular fractures in all samples. The Cracking profile of final 39 sample shows that acicular ?\" tends to deform by slipping. The cracking profile analysis of Route H samples confirmed the absence of plasticity even on the microscopic scale. This condition, the oriented cellular ?\" phase (alloy 39) apparently deforms by twinning. The crack surface analysis indicates that the U poor phase has a direct participation in crack growing and propagation, acting as forking points to the fracture and accelerating the fracture process. In the Route C samples, the poor U phase deforms alongside the U matrix. The stability of resulting microstructures of homogenized and hot rolled samples was investigated by Differential Scanning Calorimetry (DSC) and Drop Differential Scanning Calorimetry (only for the homogenized hot rolled 93 sample). The Nb and Zr also affect the stability of present phases in each sample. The number of transformations and the final structure is directly influenced by the Nb/Zr ratio. After the thermal cycle imposed by the DSC analysis, the alloy 39 exhibited ?\' phase, the alloy 66 exhibited ?\" phase and the alloy 93 exhibited ? phase. The Drop-DSC allowed observing the different stages of reaction in ? matrix, corresponding to ? -> ?0 (between 525 and 530 K), ? -> ?\" transformation (between 623 and 651 K) and ?\' -> ?3+? transformation (between 825 and 925 K).

Ligas de Urânio são candidatas ao uso como combustível nuclear em reatores avançados, dentre essas ligas se destacam as ligas de Urânio com Nióbio e com Zircônio. Este trabalho investigou como os teores de Nb e Zr, assim como a processamento termomecânico afetam as microestruturas e as propriedades mecânicas de 3 ligas U-XNb-YZr onde X+Y=12. Duas amostras contendo 50g cada, foram fabricadas através de fusão à plasma nos teores U-3Nb-9Zr (liga 39), U- 6Nb-6Zr (liga 66) e U-9Nb-3Zr (liga 93). Uma das amostras de cada liga foi tratada termicamente por 5h a 1000°C para a realização de homogeneização química. A amostra homogeneizada e a amostra bruta de fusão de cada liga foram conjuntamente encapsuladas em aço para a realização de laminação a quente seguida de um recozimento final a 1000°C por 2h. À rota adotada pela amostra bruta de fusão chamou-se \" Rota C\" e a rota adotada pela amostra homogeneizada chamou-se \"Rota H\". A caracterização microestrutural foi feita por microscopia óptica e eletrônica de varredura. Todas as amostras, independente do processamento, apresentaram precipitados ricos em Nióbio e Zircônio em adição a uma matriz rica em Urânio. A caracterização das amostras brutas de fusão mostra que os teores de elementos de liga influenciam diretamente a morfologia das dendritas evidentes na microestrutura assim como das demais fases presentes em cada amostra. A liga 39 apresentou predominantemente fase ?\', a liga 66 a fase ?\" com traços de fase y e a liga 93 a fase y com traços de fase ?\". Após o tratamento térmico de homogeneização, a liga 39 apresentou fase ?\" na forma celular enquanto a liga 66 apresentou as fases y0 e y e a liga 93 apresentou apenas fase y. As propriedades mecânicas das ligas foram avaliadas por ensaios de dureza e de dobramento simples. As amostras da Rota C apresentaram redução de dureza em relação à condição inicial. Todas as microestruturas das amostras laminadas a quente exibiram duas fases ricas em U. A liga 39 apresentou as fases ?\" na forma celular e ?\' após a laminação a quente. Após o recozimento final na rota C, a liga 39 apresentou fase ?\" na forma acicular enquanto as ligas 66 e 93 apresentaram as fases ?\" e y após a laminação e fase y. A fração de área da amostra pobre em U elevou-se nas ligas 39 e 66 e reduziu-se na liga 93. As amostras da Rota H apresentaram redução de dureza em relação à condição bruta de fusão. A liga 39 apresentou fase ?\" na forma celular com orientação e traços da fase ?, a liga 66 exibiu as fases y0 e y e a liga 93 as fases y e y0. Após o recozimento final, a liga 39 mostrou-se novamente na forma ?\" na forma celular, mas sem orientação. A liga 66 apresentou fase y e a liga 93 fase y0. Os testes de dobramento simples mostraram que as ligas da Rota C exibem plasticidade, retendo parte da deformação plástica após a ruptura das amostras testadas. Já as amostras da Rota H mostraram comportamento super elástico, possibilitando maiores deformações mas sem reter deformação plástica após a ruptura das amostras. A melhor relação entre deformação total e residual foi observada na liga 93 fabricada pela Rota C. As análises nos perfis de fratura das amostras da Rota C mostram fraturas transgranulares em todas as amostras. O perfil de fratura de na amostra recozida liga 39 mostra que a fase ?\" na forma acicular tende a deforma-se por deslizamento. As análises dos perfis de fratura nas amostras da Rota H confirmaram a ausência de deformação plástica mesmo em escala microscópica. Para essa condição, a fase ?\" na forma celular com orientação (liga 39) aparenta deformar-se por maclação. As análises de superfície de fratura indicam que a fase pobre em U tem participação durante o processo de crescimento e propagação da fratura na Rota H, atuando como caminho para bifurcação de trincas acelerando o processo de ruptura, enquanto na Rota C, a fase pobre em U deforma-se conjuntamente com a matriz de U. Em uma segunda etapa do trabalho, a estabilidade das microestruturas resultantes na amostras processadas foi investigada por Ensaios de Calorimetria Diferencial Exploratória (DSC) e por calorimetria de queda livre (esta apenas para a amostra 93 da rota H). O teor de Nb e Zr também afeta a estabilidade das fases presentes em cada amostra. foram realizados com as amostras da condição homogeneizada e laminada. A quantidade de transformações assim como o estado final de cada liga diferiu de acordo com a razão Nb/Zr. Após o ciclo de aquecimento e resfriamento da análise térmica, a liga 39 apresentou fase ?\', a liga 66 fase ?\" e a liga 93 fase y. No ensaio de calorimetria por queda livre foi possível observar as diferentes etapas de reação de envelhecimento da matriz g, correspondendo a à transformação y -> y0 (entre 525 e 530 K), a transformação y -> ?\". (entre 623 e 651 K) e à transformação y\' -> y3+? (entre 825 e 925 K). / Uranium alloys are candidates to be used as nuclear fuel in research reactors, among the U alloys, the Nb and Zr containing alloys are promising. This work evaluated how the Nb and Zr content and the thermomechanical processing affects the microstructure and mechanical properties of 3 alloys U-XNb-YZr were X+Y=12. Two 50g slugs of each sample were fabricated using plasma arc melting according to U-3Nb-9Zr (alloy 39), U-6Nb-6Zr (alloy 66) and U-9Nb-3Zr (alloy 93). One slug of each alloy was heat treated for 5h at 1000°C to perform the chemical homogenization. The homogenized sample and the as-cast one were encapsulated in the same steel frame in order to perform hot rolling. After the rolling process, the samples were annealed by 2h at 1000°C. The route that uses only as-cast samples was nominated \"Route C\" and the route that uses the homogenized sample was nominated \"Route H\". The microstructural characterization was performed by optical and scanning electron microscopy. All samples, regardless the processing route, presented Nb and Zr rich precipitates in addition to U rich matrix. The characterization of as-cast samples shows that the content of the alloying element has a direct influence on dendrite morphology as in the phases presented for each alloy. The alloy 39 presented predominantly ?\' phase, the alloy 66 the ?\" phase with a small quantity of ? phase and the alloy 93 presented the ? phase with small quantity of ?\" phase. Afte the homogenization, the alloy 39 presented cellular ?\" phase, the alloy 66 presented ?0 and ?, the alloy 93 presented only ? phase. The mechanical properties were evaluated by hardness measurements and free bending tests. The Route C samples presented hardness reduction in comparison to the initial condition. All microstructures of hot-rolled samples of this route exhibit two U rich phases. The alloy 39 exhibited cellular ?\" and ?\', after the final annealing the alloy 39 presented acicular ?\". The alloys 66 and 93 exhibited ?+?\" after hot rolling and ? phase after the final annealing. The area fraction of poor U phase increased in the alloys 39 and 66, but reduced in alloy 93. The Route H samples presented hardness reduction in comparison to as-cast samples. The alloy 39 presented cellular oriented ?\" phase and a small quantity of ? phase. The alloy 66 exhibited ?0 and ?, the alloy 93 ? and ?0. After the final annealing, the alloy 39 presented the ?\" again, but without orientation. The alloy 66 presented ? phase and the alloy 93 presented ?0. The free bending tests show that Route C samples have real plasticity, retaining part of deformation after rupturing as plastic strain. The Route H samples exhibited superelastic behavior, allowing higher deformations but retaining no plastic strain after sample breaking. The better balance between total and residual strain was observed in alloy 93 fabricated by Route C. The cracking profile analysis of Route C samples shows transgranular fractures in all samples. The Cracking profile of final 39 sample shows that acicular ?\" tends to deform by slipping. The cracking profile analysis of Route H samples confirmed the absence of plasticity even on the microscopic scale. This condition, the oriented cellular ?\" phase (alloy 39) apparently deforms by twinning. The crack surface analysis indicates that the U poor phase has a direct participation in crack growing and propagation, acting as forking points to the fracture and accelerating the fracture process. In the Route C samples, the poor U phase deforms alongside the U matrix. The stability of resulting microstructures of homogenized and hot rolled samples was investigated by Differential Scanning Calorimetry (DSC) and Drop Differential Scanning Calorimetry (only for the homogenized hot rolled 93 sample). The Nb and Zr also affect the stability of present phases in each sample. The number of transformations and the final structure is directly influenced by the Nb/Zr ratio. After the thermal cycle imposed by the DSC analysis, the alloy 39 exhibited ?\' phase, the alloy 66 exhibited ?\" phase and the alloy 93 exhibited ? phase. The Drop-DSC allowed observing the different stages of reaction in ? matrix, corresponding to ? -> ?0 (between 525 and 530 K), ? -> ?\" transformation (between 623 and 651 K) and ?\' -> ?3+? transformation (between 825 and 925 K).

Thesis (MSc)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: Buying and selling in financial markets are driven by demand. The demand can be quantified by the imbalance in the number of shares QB and QS transacted by buyers and sellers respectively over a given time interval t. The demand in an interval t is given by (t) = QB − QS. The local noise intensity is given by = h|aiqi − haiqii|i where i = 1, . . . ,N labels the transactions in t, qi is the number of shares traded in transaction i, ai = ±1 denotes buyer- initiated and seller- initiated trades respectively and h· · · i is the local expectation value computed from all the transactions during the interval t. In a paper [1] based on data from the New York Stock Exchange Trade and Quote database during the period 1995-1996, Plerou, Gopikrishnan and Stanley [1] reported that the analysis of the probability distribution P( | ) of demand conditioned on the local noise intensity revealed the surprising existence of a critical threshold c. For < c, the most probable value of demand is roughly zero; they interpreted this as an equilibrium phase in which neither buying nor selling predominates. For > c two most probable values emerge that are symmetrical around zero demand, corresponding to excess demand and excess supply; they interpreted this as an out-of-equilibrium phase in which the market behaviour is buying for half of the time, and selling for the other half. It was suggested [1] that the two-phase behaviour indicates a link between the dynamics of a financial market with many interacting participants and the phenomenon of phase transitions that occurs in physical systems with many interacting units. This thesis reproduces the two-phase behaviour by means of experiments using sequences of random variables. We reproduce the two-phase behaviour based on correlated and uncorrelatd data. We use a Markov modulated Bernoulli process to model the transactions and investigate a simple interpretation of the two-phase behaviour. We sample data from heavy-tailed distributions and reproduce the two-phase behaviour. Our experiments show that the results presented in [1] do not provide evidence for the presence of complex phenomena in a trading market; the results are a consequence of the sampling method employed. / AFRIKAANSE OPSOMMING: Aankope en verkope in finansi¨ele markte word deur aanvraag gedryf. Aanvraag kan gekwantifiseer word in terme van die ongebalanseerdheid in die getal aandele QB en QB soos onderskeidelik verhandel deur kopers en verkopers in ’n gegewe tyd-interval t. Die aanvraag in ’n interval t word gegee deur (t) = QB −QS. Die lokale geraasintensiteit word gegee deur = h|aiqi − haiqii|i waar i = 1, . . . ,N die transaksies in t benoem, qi die getal aandele verhandel in transaksies verwys, en h· · · i op die lokale verwagte waarde dui, bereken van al die tansaksies tydens die interval t. In ’n referaat [1] wat op data van die New York Effektebeurs se Trade and Quote databasis in die periode tussen 1995 en 1996 geskoei was, het Plerou, Gopikrishnan en Stanley [1] gerapporteer dat ’n analise van die waarskynlikheidsverspreiding P( | ) van aanvraag gekondisioneer op die lokale geraasintensiteit , die verrassende bestaan van ’n kritieke drempelwaarde c na vore bring. Vir < c is die mees waarskynlike aanvraagwaarde nagenoeg nul; hulle het dit ge¨ınterpreteer as ’n ekwilibriumfase waartydens n`og aankope n`og verkope die oormag het. Vir > c is die twee mees waarskynlike aanvraagwaardes wat te voorskyn kom simmetries rondom nul aanvraag, wat oorenstem met ’n oormaat aanvraag en ’n oormaat aanbod; hulle het dit geinterpreteer as ’n buite-ewewigfase waartydens die markgedrag die helfte van die tyd koop en die anderhelfte verkoop. Daar is voorgestel [1] dat die tweefase gedrag op ’n verband tussen die dinamiek van ’n finansiele mark met baie deelnemende partye, en die verskynsel van fase-oorgange wat in fisieke sisteme met baie wisselwerkende eenhede voorkom, dui. Hierdie tesis reproduseer die tweefase gedrag deur middel van eksperimente wat gebruik maak van reekse van lukrake veranderlikes. Ons reproduseer die tweefase gedrag gebaseer op gekorreleerde en ongekorreleerde data. Ons gebruik ’n Markov-gemoduleerde Bernoulli proses om die transaksies te moduleer en ondersoek ’n eenvoudige interpretasie van die tweefase gedrag. Ons seem steekproefdata van “heavy-tailed” verspreidings en reproduseer die tweefase gedrag. Ons ekperimente wys dat die resultate in [1] voorgested is nie bewys lewer vir die teenwoordigheid van komplekse verskynsel in’n handelsmark nie; die resultate is as gevolg van die metode wat gebruik is vir die generering van die steekproefdata.

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This thesis describes experimental studies of colloidal particles dispersed in solvents which themselves have phase transitions. One common definition of soft matter is: a material characterized by a mesoscopic length scale. This length scale is, for example, the colloid size or the ordered domain size. Here we combine a complex host with one characteristic length scale with dispersed particles that have a different size. It might be anticipated that new behaviour will occur. Two limits of particle characteristics are probed: the case of dilute sterically-stabilized particles and the case of a weak gel of attractive particles. The two systems are polymer particles dispersed in a phaseseparating microemulsion and silica nanoparticles dispersed in a low molecular weight liquid crystal. In each system a temperature driven phase transition plays a crucial role. In the microemulsion case we observe how transitional and pre-transitional phenomena create effective interactions between particles and how new behaviour emerges in the host solvent in the late stage of the phase separation. We show that the pre-transitional clustering of the PMMA particles is due to an adsorbed layer of dodecane. Subsequently heterogeneous nucleation of the gas phase is seen. After phase separation has occurred in off-critical samples the particles remain in either the continuous or dispersed phase depending on the original microemulsion composition. In the late stage of the phaseseparation the coalescence and coarsening behaviour changes significantly, after more material exchange between the phases has taken place. This behaviour is reminiscent of viscoelastic phase separation in polymer based samples. In the liquid crystal case we discover the anisotropy of the liquid crystal persists over large length scales and modifies the local dynamics of the gel. Using electron microscopy and scattering techniques we demonstrate that the silica embedded in the liquid crystal forms agglomerates with a fractal structure. Rheological characterization demonstrates that the resulting composite is a gel. Investigation of the composite’s local dynamics using x-ray photon correlation spectroscopy shows anisotropy and intermittency in the dynamics on significant length scales. In both systems we have studied new behaviour seen due to the influence of one component on the dynamic characteristics of the other The pre- and post- phase transition phenomena are only crucial in the microemulsion case where the particles have purely repulsive interactions. Our results illustrate the subtle balances that occur in soft composite systems.

A cinética da transformação de fase UAl3 + Al &rarr; UAl4 foi estudada em duas ligas Al-U, com 31,4 % e 33,4 % U em peso respectivamente, através de metalografia quantitativa. Os resultados mostraram que esta transformação é um processo de nucleação e crescimento termicamente ativado, com a nucleação ocorrendo heterogeneamente nas interfaces UAl3/AI(&infin;) e o crescimento sendo controlado por difusão em volume. A energia de ativação empírica do processo foi determinada, cujo valor médio é da ordem de 54,8 kcal/mol. Foi verificado que a cinética de crescimento da fase UAl4 obedece uma lei parabólica. As interfaces UAl4/ UAl3 e UAl4/Al(&infin;) migram em direções opostas, sendo que a velocidade da interface UAl4/ UAl3 é aproximadamente cinco vezes maior que aquela da interface UAl4/Al(&infin;). O coeficiente de difusão química do Al e do U na fase UAl4 foi avaliado ser da ordem de 10-9 cm 2/s a 600°C. / The kinetics of phase transformation UAl3 + Al &rarr; UAl4 of two Al-U alloys, with 31.4 and 33.4 wt % U respectively, was studied by quantitative microscopy. The results have shown that this transformation is a nucleation and thermally activated growth process. The nucleation occurs heterogeneously at the UAl3/AI(&infin;) interfaces and growth is controlled by volume diffusion. The empirical activation energy of the process was determined, whose mean value is of about 54.8 kcal/mol. It was verified that a parabolic growth law is obeyed. The UAl4/ UAl3 and UAl4/Al(&infin;) interfaces migrates in opposite directions, with the UAl4/ UAl3 interface velocity was approximately 5 times greater than that of UAl4/Al(&infin;) interface. The chemical diffusion coefficient of Al and U in the UAl4 phase were evaluated to be of the order of 10-9 cm 2/s at 600°C.