Made available in DSpace on 2017-07-21T20:43:44Z (GMT). No. of bitstreams: 1 Sheila Cristina Jacumasso.pdf: 3640428 bytes, checksum: 9f48682e159a5b7129a15abc7bb73945 (MD5) Previous issue date: 2014-06-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The AA7050 class from the 7XXX series alloys based on Al-Zn-Mg-Cu are widely used in aerospace structures and fuselage components, when the same are subjected to high stress loading due to its high ratio mechanical strength to density, in addition, to its corrosion resistance. The increased resistance of these alloys is obtained by heat treatment that involves solution treatment and ageing followed precipitation of a fine and homogeneous phase from the own chemical composition. In this, the present study aimed to perform the of AA7050 aluminum alloy microstructural characterization with different ageing heat treatment from the T7451, T6 and T6I4-65 conditions. Thus, different characterization techniques from the Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Extraction of second phase particles, X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) were used. It was evaluated which condition results is higher precipitation hardening phases that are responsible for the increased resistance of the alloy. The most significant results were obtained by TEM where it was possible to determine the morphology of the GPII zones in needles form with approximate size of 30nm and 10nm for both T6 and T6I4-65 conditions, respectively. Moreover, the metastable η' phase was identified in platelets form in the T7451, T6 and T6I4-65 conditions with approximate size of 50nm, 20nm and 10nm, respectively. The surface roughness analyses obtained by AFM have revealed that the T6I4-65 condition has higher surface roughness (Ra=14,87nm) when compared to the T7451 (Ra=7,65nm) and T6 (Ra=8,35nm) conditions. Indicating in this case, a higher density of small particles homogeneously distributed in the T6I4-65 aluminum alloy matrix. / As ligas da série 7XXX, da classe AA7050, à base de Al-Zn-Mg-Cu, são muito utilizadas na indústria aeroespacial, em estruturas de fuselagem e componentes sob alta tensão de carregamento, devido à sua alta relação entre resistência mecânica e densidade, além de sua resistência à corrosão. O aumento da resistência destas ligas é obtido por tratamento térmico de solubilização e envelhecimento, através da precipitação de uma fase fina e homogênea, proveniente da própria composição química. Neste contexto, o presente trabalho teve como objetivo principal realizar a caracterização microestrutural de uma liga de alumínio AA7050, submetida a diferentes tratamentos térmicos de envelhecimento nas condições T7451, T6 e T6I4-65. Desse modo, fez-se o uso de diferentes técnicas de caracterização entre elas Microscopia Ótica (MO), Microscopia Eletrônica de Varredura (MEV), Microscopia de Força Atômica (AFM), Extração de Partículas de segunda fase, Difração de Raios X (DRX) e Microscopia Eletrônica de Transmissão. Consequentemente, avaliou-se qual condição apresenta maior precipitação de fases endurecedoras que são responsáveis pelo aumento de resistência da liga. Os resultados mais significativos foram obtidos por MET onde foi possível determinar a morfologia das zonas GPII presentes na forma acicular (needles) com tamanho aproximado de 30nm na condição T6 e 10nm na condição T6I4-65. A fase metaestável η’ foi identificada na forma de plaquetas finas (platelets) nas condições T7451, T6 e T6I4-65 com tamanho aproximado de 50nm, 20nm e 10nm respectivamente. A rugosidade superficial obtida por AFM revelou que a condição T6I4-65 apresenta maior rugosidade superficial (Ra=14,87nm) quando comparado às condições T7451(Ra=7,65nm) e T6 (Ra=8,35nm), indicando maior densidade de partículas de pequena dimensão distribuída homogeneamente na matriz da liga de alumínio.

This thesis systematically investigates the effects of a recently developed modified ageing procedure of aluminium alloys, termed the T6I6 temper, on the microstructural development and mechanical properties of the Al ??? Mg ??? Si - Cu alloy 6061. For the T6I6 temper, a conventional single stage T6 temper is interrupted by an ageing period at a reduced temperature (65??C) to facilitate secondary precipitation, before resuming the final ageing at the temperature of the initial T6 treatment. The T6I6 temper was found to cause simultaneous increases in tensile properties, hardness, and toughness as compared with 6061 T6. Al ??? Mg ??? Si ??? Cu alloys are medium strength alloys widely used in the automotive industry and their further improvement is underpinned by stringent demands for weight reduction placed on the transportation industry in recent years. The potential for further improvement of the mechanical properties was found in the control of secondary precipitation that may take place even in some fully aged alloys when exposed to reduced temperatures. The overall improvement in the mechanical properties of 6061 T6I6 was attributed to the formation of finer and more densely dispersed precipitates in the final microstructure. The refinement of precipitates was facilitated by control of the precipitation processes and gradual evolution of the microstructure throughout each stage of the T6I6 treatment. The results indicated that the concentration and the chemical environment of the vacancies controlled the precipitation processes in this alloy. Findings also show that the proportion of the different precipitate phases present in the final microstructure, as well as the amount of the solute in these precipitates, can be controlled and modified utilizing secondary precipitation. A number of analytical techniques were used in this study. The evolution of the microstructure was studied using Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Three Dimensional Atom Probe (3DAP). Vacancy-solute interactions were studied using Positron Annihilation Lifetime Spectroscopy (PALS) and 3DAP. The distribution of the solute was studied using 3DAP and Nuclear Magnetic Resonance (NMR). Differential Scanning Calorimetry (DSC) was used to identify precipitation reactions and to determine the stability of vacancy-associated aggregates.

This thesis systematically investigates the effects of a recently developed modified ageing procedure of aluminium alloys, termed the T6I6 temper, on the microstructural development and mechanical properties of the Al ??? Mg ??? Si - Cu alloy 6061. For the T6I6 temper, a conventional single stage T6 temper is interrupted by an ageing period at a reduced temperature (65??C) to facilitate secondary precipitation, before resuming the final ageing at the temperature of the initial T6 treatment. The T6I6 temper was found to cause simultaneous increases in tensile properties, hardness, and toughness as compared with 6061 T6. Al ??? Mg ??? Si ??? Cu alloys are medium strength alloys widely used in the automotive industry and their further improvement is underpinned by stringent demands for weight reduction placed on the transportation industry in recent years. The potential for further improvement of the mechanical properties was found in the control of secondary precipitation that may take place even in some fully aged alloys when exposed to reduced temperatures. The overall improvement in the mechanical properties of 6061 T6I6 was attributed to the formation of finer and more densely dispersed precipitates in the final microstructure. The refinement of precipitates was facilitated by control of the precipitation processes and gradual evolution of the microstructure throughout each stage of the T6I6 treatment. The results indicated that the concentration and the chemical environment of the vacancies controlled the precipitation processes in this alloy. Findings also show that the proportion of the different precipitate phases present in the final microstructure, as well as the amount of the solute in these precipitates, can be controlled and modified utilizing secondary precipitation. A number of analytical techniques were used in this study. The evolution of the microstructure was studied using Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Three Dimensional Atom Probe (3DAP). Vacancy-solute interactions were studied using Positron Annihilation Lifetime Spectroscopy (PALS) and 3DAP. The distribution of the solute was studied using 3DAP and Nuclear Magnetic Resonance (NMR). Differential Scanning Calorimetry (DSC) was used to identify precipitation reactions and to determine the stability of vacancy-associated aggregates.

L’objectif de ce travail est d’améliorer la durée de vie en fluage d’un alliage résistant à haute température. L’alliage étudié, nommé « C », appartient à la classe des aciers austénitiques de type HP utilisés pour la fabrication des tubes de reformage. L’évolution microstructurale de l’alliage « C » a été étudiée dans une vaste gamme de températures, s’étendant de 700 à 1040°C pour des temps de vieillissement allant jusqu’à 1000 h. La caractérisation de ces états vieillis a été réalisée au moyen de la microscopie électronique (MEB-FEG, MET) et de la diffraction des rayons X. L’accent a été mis sur une caractérisation fine de la précipitation secondaire présente Ces résultats ont ensuite été utilisés afin d’identifier les conditions thermiques optimales pour l’affinement de la précipitation en vue d’amélioration du comportement macroscopique de l’alliage. La cinétique de précipitation a été modélisée à l’aide du logiciel PRISMA ThermoCalc. Un bon accord entre la simulation et les mesures expérimentales a pu être obtenu.Dans la gamme de températures étudiée, la précipitation secondaire est majoritairement constituée de deux carbures : M23C6 (M=Cr, Fe) et NbC. En condition de service (980°C), la croissance du M23C6 est rapide. La coalescence des précipités survient dès 200 h de vieillissement. Nous avons prouvé qu’un vieillissement à des températures plus basses (700-750°C) permet d’affiner cette précipitation. De plus, notre étude a montré l’efficacité d’un prétraitement à des températures basses, effectué avant la mise en service du matériau à 980°C. Une nette amélioration de la résistance en fluage dans des essais accélérés a été obtenue pour l’alliage « C » ayant subi le prétraitement cité ci-dessus. Outre l’affinement et le retardement de la coalescence du M23C6, la présence d’une précipitation nanométrique du NbC sur des lignes de dislocations est probablement à l’origine de cet effet. / The purpose of this work is to optimise the microstructure of a creep-resistant alloy of the type HP, called “C” (industrial denomination). These austenitic steels are used for the manufacture of reformer tubes. The microstructural evolution of the alloy "C" has been studied in a wide range of temperatures, ranging from 700 to 1040 °C for aging times up to 1000 h. The characterization of these aged states was performed using electron microscopy (FEG-SEM, TEM) and X-ray diffraction, with emphasis on a detailed characterization of this secondary precipitation. This knowledge was then used to identify the optimal thermal conditions for the refinement of precipitation to improve the macroscopic behaviour of the alloy. The precipitation kinetics was modelled using the PRISMA ThermoCalc. A good agreement between simulation and experimental measurements has been obtained.In the studied range of temperature, the secondary precipitation consists mainly of two carbides M23C6 (M = Cr, Fe) and NbC. In the service conditions (980°C), the growth of M23C6 is fast. The coalescence of the precipitates starts after only 200h of aging. Aging at lower temperatures (700-750°C) refines this precipitation. Our study showed the efficacy of pre-treatment of the alloy at low temperatures, before the service of the material at 980°C. In the alloy "C", treated in such conditions, a significant increase in creep resistance was obtained in accelerated testing. In addition to refinement of the secondary precipitation and delaying the effects of coalescence of M23C6, the presence of a nanoscale precipitation of NbC on dislocation lines is probably the origin of this effect.