Os vidros metálicos baseados em Cu-Zr representam uma classe bastante promissora para a categoria de materiais estruturais, tendo em vista suas interessantes propriedades resultantes da natureza amorfa. Sabe-se que o oxigênio tem grande influência na formação da estrutura amorfa e consequentemente nas propriedades dessa classe de materiais. No presente trabalho, ligas amorfas baseadas em Cu-Zr foram analisadas termicamente a fim de se observar o comportamento das mesmas frente à contaminação com oxigênio. As análises térmicas foram realizadas em um equipamento de calorimetria exploratória diferencial (DSC), e as temperaturas características como de transição vítrea, cristalização, fusão e líquidus foram determinadas. Concluiu-se que conforme a literatura apresenta, o processo de cristalização é favorecido pela presença de oxigênio, causando uma queda na energia de ativação dos processos de cristalização das ligas trabalhadas. Através de ensaios que simularam tratamentos térmicos, cristalizou-se controladamente as amostras amorfas baseadas em Cu-Zr a fim de formar compósitos entre cristais e vidros metálicos buscando diminuir a fragilidade das ligas. / Cu-Zr-based bulk metallic glasses represent a very promising class of structural materials with interesting properties resulting from the amorphous nature. It is known that oxygen has a great influence on the formation of the amorphous structure and consequently on the properties of these materials. In the present work, Cu-Zr-based amorphous alloys were thermally analyzed in order to observe their behavior against oxygen contamination. Thermal analyzis were performed on a differential scanning calorimetry (DSC) equipment, and characteristic temperatures as glass transition, crystallization, melting and liquidus were determined. It was concluded that, according to the literature, the crystallization process is favored by the presence of oxygen, causing a decrease in the activation energy of the crystallization processes of the worked alloys. Through tests that simulated heat treatments, the amorphous samples were crystallized in order to form composites between crystals and metallic glasses in order to reduce the brittleness of the alloys.

Os vidros metálicos baseados em Cu-Zr representam uma classe bastante promissora para a categoria de materiais estruturais, tendo em vista suas interessantes propriedades resultantes da natureza amorfa. Sabe-se que o oxigênio tem grande influência na formação da estrutura amorfa e consequentemente nas propriedades dessa classe de materiais. No presente trabalho, ligas amorfas baseadas em Cu-Zr foram analisadas termicamente a fim de se observar o comportamento das mesmas frente à contaminação com oxigênio. As análises térmicas foram realizadas em um equipamento de calorimetria exploratória diferencial (DSC), e as temperaturas características como de transição vítrea, cristalização, fusão e líquidus foram determinadas. Concluiu-se que conforme a literatura apresenta, o processo de cristalização é favorecido pela presença de oxigênio, causando uma queda na energia de ativação dos processos de cristalização das ligas trabalhadas. Através de ensaios que simularam tratamentos térmicos, cristalizou-se controladamente as amostras amorfas baseadas em Cu-Zr a fim de formar compósitos entre cristais e vidros metálicos buscando diminuir a fragilidade das ligas. / Cu-Zr-based bulk metallic glasses represent a very promising class of structural materials with interesting properties resulting from the amorphous nature. It is known that oxygen has a great influence on the formation of the amorphous structure and consequently on the properties of these materials. In the present work, Cu-Zr-based amorphous alloys were thermally analyzed in order to observe their behavior against oxygen contamination. Thermal analyzis were performed on a differential scanning calorimetry (DSC) equipment, and characteristic temperatures as glass transition, crystallization, melting and liquidus were determined. It was concluded that, according to the literature, the crystallization process is favored by the presence of oxygen, causing a decrease in the activation energy of the crystallization processes of the worked alloys. Through tests that simulated heat treatments, the amorphous samples were crystallized in order to form composites between crystals and metallic glasses in order to reduce the brittleness of the alloys.

Die Motivation zur Untersuchung ternärer und quaternären CuZr-Legierungen bestand in der Annahme, dass die Zugabe von Kobalt den Stabilitätsbereich von B2 CuZr bis zur Raumtemperatur erweitert und Aluminium einen signifikanten Effekt auf die Glasbildungsfähigkeit des CuZr-Systems hat. Die vorliegende Dissertation befasst sich mit der Herstellung und Charakterisierung von Cu50-xCoxZr50 (0 ≤ x ≤ 20) und Cu50-xCoxZr45Al5- (x = 2, 5, 10 und 20) Legierungen. Hierbei wurden die Phasenbildung, die thermische Stabilität, die Mikrostruktur, die Martensitbildung und die mechanischen Eigenschaften der Legierungen untersucht. Die Abhängigkeit der Phasenbildung von der Erstarrungsrate und der thermodynamischen Stabilität von Cu-Co-Zr-Legierungen zeigte, dass die Zugabe von Kobalt die Glasbildungsfähigkeit von Cu-Co-Zr-Legierungen absenkt und die stabilen kristallinen Produkte des Systems von Cu10Zr7 + CuZr2 zu (Cu,Co)Zr Phase mit einer B2 Struktur verändert. Die Ergebnisse weisen darauf hin, dass bei den schmelzgesponnene Bänder mit wenigstens 5 Atom-% Co das Glas direkt in B2 (Cu,Co)Zr kristallisiert, während Massivproben mit Co-Gehalten zwischen 0 ≤ x < 5 die monokline (Cu,Co)Zr Phase und Cu10Zr7 sowie CuZr2 beinhalten, wobei für x ≥ 10 die B2 (Cu,Co)Zr Phase bei Raumtemperatur im Gleichgewicht ist. Des Weiteren werden mit steigendem Co-Gehalt die Martensitumwandlungstemperaturen zu niedrigeren Werten verschoben. Die Phasenbildung im ternären System wird im pseudo-binären (Cu,Co)Zr-Phasendiagramm zusammengefasst, welches die Entwicklung neuer Formgedächtnislegierungen sowie metallischer Glas-Komposite bei Zugabe des Glasbildungselementes Aluminium vereinfacht. In den Vierstofflegierungen erhöht Al die Glasübergangs- und Kristallisationstemperaturen und verbessert dadurch die Glasbildungsfähigkeit des Systems. Die röntgenographische Analyse zeigte, dass die Kristallisationsprodukte der schmelzgesponnenen Bänder variieren: von Cu10Zr7 + CuZr2 + AlCu2Zr zu (Cu,Co)Zr + AlCu2Zr, wenn Co ≤ 5 und Co ≥ 10. Die Herstellung von Massivproben mit unterschiedlichen Durchmessern führte zu einem vollständig amorphen Gefüge, einem metallischen Glas-Komposit oder einem vollständig kristallinen Gefüge. Für Co ≤ 5 tritt neben (Cu,Co)Zr und AlCu2Zr ebenfalls Cu10Zr7 auf. Mittels Rasterelektronen (REM)- und Transmissionselektronenmikroskopie (TEM) erfolgte die Analyse des Einflusses von Al- und Co-Zugaben auf die Mikrostruktur von CuZr-Legierungen. Für die Cu-Co-Zr-Al-Legierungen sowie Cu30Co20Zr45Al5 (ø = 4 mm) und Cu45Co5Zr45Al5 (ø = 2 mm) wurden mikrostrukturelle Untersuchungen mittels TEM durchgeführt. Nachfolgend wurde die Heterogenität der Mikrostruktur in der Cu40Co10Zr45Al5 (ø = 2 mm) untersucht. Der Einfluss von Co auf die mechanischen Eigenschaften von rascherstarrten Cu50-xCoxZr50 (x = 2, 5, 10 und 20 Atom-%) Legierungen zeigt, dass das Verformungsverhalten der Stäbe unter Druckbeanspruchung stark von der Mikrostruktur der (Cu,Co)Zr Phase und somit von der Legierungszusammensetzung abhängt. Kobalt beeinflusst die Bruchfestigkeit der Gussproben. Weiterhin zeigen Proben mit martensitischem Gefüge eine Kaltverfestigung. Neben der Kaltverfestigung zeigen die Legierungen mit hohem Co-Gehalt eine verformungsinduzierte Martensitumwandlung und weisen zwei Streckgrenzen auf. Für die Vierstofflegierungen wurde der Einfluss der Kühlrate und der chemischen Zusammensetzung auf die mechanischen Eigenschaften untersucht. Für Cu48Co2Zr45Al5 (ø = 1.5, 2, 3 und 4 mm) und Cu45Co5Zr45Al5 (ø = 3 mm) wurde der Einfluss der Kühlrate und der Heterogenität diskutiert. Die Ergebnisse zeigen, dass die mechanischen Eigenschaften der Cu50-xCoxZr45Al5-Legierungen stark von der Makrostruktur und dem Volumenanteil der amorphen und kristallinen Phase abhängen. Die verformungsinduzierte Martensitumwandlung in Cu40Co10Zr50- und Cu40Co10Zr45Al5-Gussstäben wurde mittels hochenergetischer Röntgenstrahlung durchgeführt. Die In-situ- Druckversuche erfolgten weg- und kraftkontrolliert. Das makroskopische und mikroskopische Spannung-Dehnungs-Verhalten sowie die Phasenumwandlungskinetik wurden dabei betrachtet. Die relativen Veränderungen der vollständig integrierten Intensität der ausgewählten B2- und Martensitreflexe, die auf die Veränderungen der Volumenanteile der entsprechenden Phasen unter Verformung hinweisen, wurden als Phasenumwandlungsvolumen M/M+B2 beschrieben. / The fact that the presence of Co extends the stability range of B2 CuZr to room temperature, together with the significant effect of Al on improving the glass forming ability of the CuZr system was the motivation to investigate the ternary and quaternary CuZr alloys with the aim of synthesizing BMG composites containing B2 (Cu,Co)Zr crystals. This PhD thesis deals with preparation and characterization of Cu50-xCoxZr50 (0 ≤ x ≤ 20) and Cu50-xCoxZr45Al5 (x = 2, 5, 10 and 20) alloys. The phase formation, thermal stability, microstructure, martensitic transformation and mechanical properties of these alloys were investigated. The dependence of phase formation on solidification rate and the thermodynamically stability of Cu-Co-Zr alloys reveals that the addition of Co decreases the glass forming ability (GFA) of the Cu-Co-Zr alloys and changes the stable crystalline products of the system from Cu10Zr7 + CuZr2 to (Cu,Co)Zr phase with a B2 structure. The results indicate that for the melt-spun ribbons with at least 5 % Co, the glass crystallizes directly into B2 (Cu,Co)Zr, while in the case of bulk specimens, compositions with 0 ≤ x < 5 of Co contain the monoclinic (Cu,Co)Zr phase and Cu10Zr7 and CuZr2, whereas, for x ≥ 10, the B2 (Cu,Co)Zr phase is the equilibrium phase at room temperature. Furthermore, increasing the cobalt content decreases the martensitic transformation temperatures to lower temperatures. The phase formation in the ternary system is summarized in a pseudo-binary (Cu,Co)Zr phase diagram, that helps for designing new shape memory alloys, as well as bulk metallic glass composites with the addition of glass former elements. In the quaternary alloys, Al increases the glass transition and crystallization temperatures and hence improves the GFA of the system. The X-ray analysis illustrates that for the melt-spun ribbons, the crystallization products vary from Cu10Zr7 + CuZr2 + AlCu2Zr to (Cu,Co)Zr + AlCu2Zr when Co ≤ 5 and Co ≥ 10, respectively. Depending on the cooling rates, the bulk samples represent a fully amorphous structure or BMG composites or a fully crystalline structure. For Co ≤ 5, beside (Cu,Co)Zr and AlCu2Zr, Cu10Zr7 exists as well. Scanning (SEM) and transmission (TEM) electron microscopy investigations were done to investigate the effect of Al and Co addition to the microstructure of CuZr alloys. In the case of Cu-Co-Zr-Al alloys, Cu30Co20Zr45Al5 (ɸ = 4 mm) and Cu45Co5Zr45Al5 (ɸ = 2 mm) compositions were selected for the microstructure verification using TEM. Later, the heterogeneity of the microstructure in Cu40Co10Zr45Al5 (ɸ = 2 mm) alloy was considered. The effect of Co on the mechanical properties of rapidly solidified Cu50-xCoxZr50 (x = 2, 5, 10 and 20 at.%) alloys depict that the deformation behavior of the rods under compressive loading strongly depends on the microstructure, and as a results, on the alloy composition. Cobalt affects the fracture strength of the as-cast samples; and deformation is accompanied with two yield stresses for high Co-content alloys, which undergo deformation-induced martensitic transformation. Instead samples with a martensitic structure show a work-hardening behavior. For quaternary alloys, the effects of cooling rate and chemical composition on mechanical properties of the alloys were investigated. Cu48Co2Zr45Al5 (ɸ= 1.5, 2, 3 and 4 mm) and Cu45Co5Zr45Al5 (ɸ = 3 mm) compositions were selected to discuss the effect of cooling rate and heterogeneity, respectively. The results depict that the mechanical properties of Cu50-xCoxZr45Al5 alloys strongly depend on the microstructure and the volume fraction of the amorphous and crystalline phases. The deformation-induced martensitic transformation of Cu40Co10Zr50 and Cu40Co10Zr45Al5 as-cast rods, was studied by means of high-energy X-rays. The in situ compression measurements were performed in track control and load control modes. The macroscopic and microscopic stress-strain behavior, as well as the phase transformation kinetics were considered. The relative changes in the fully integrated intensity of the selected B2 and martensite peaks, which indicate the changes in volume fraction of the corresponding phases under deformation, was described as phase transformation volume, M/M+B2.

Die Motivation zur Untersuchung ternärer und quaternären CuZr-Legierungen bestand in der Annahme, dass die Zugabe von Kobalt den Stabilitätsbereich von B2 CuZr bis zur Raumtemperatur erweitert und Aluminium einen signifikanten Effekt auf die Glasbildungsfähigkeit des CuZr-Systems hat. Die vorliegende Dissertation befasst sich mit der Herstellung und Charakterisierung von Cu50-xCoxZr50 (0 ≤ x ≤ 20) und Cu50-xCoxZr45Al5- (x = 2, 5, 10 und 20) Legierungen. Hierbei wurden die Phasenbildung, die thermische Stabilität, die Mikrostruktur, die Martensitbildung und die mechanischen Eigenschaften der Legierungen untersucht. Die Abhängigkeit der Phasenbildung von der Erstarrungsrate und der thermodynamischen Stabilität von Cu-Co-Zr-Legierungen zeigte, dass die Zugabe von Kobalt die Glasbildungsfähigkeit von Cu-Co-Zr-Legierungen absenkt und die stabilen kristallinen Produkte des Systems von Cu10Zr7 + CuZr2 zu (Cu,Co)Zr Phase mit einer B2 Struktur verändert. Die Ergebnisse weisen darauf hin, dass bei den schmelzgesponnene Bänder mit wenigstens 5 Atom-% Co das Glas direkt in B2 (Cu,Co)Zr kristallisiert, während Massivproben mit Co-Gehalten zwischen 0 ≤ x < 5 die monokline (Cu,Co)Zr Phase und Cu10Zr7 sowie CuZr2 beinhalten, wobei für x ≥ 10 die B2 (Cu,Co)Zr Phase bei Raumtemperatur im Gleichgewicht ist. Des Weiteren werden mit steigendem Co-Gehalt die Martensitumwandlungstemperaturen zu niedrigeren Werten verschoben. Die Phasenbildung im ternären System wird im pseudo-binären (Cu,Co)Zr-Phasendiagramm zusammengefasst, welches die Entwicklung neuer Formgedächtnislegierungen sowie metallischer Glas-Komposite bei Zugabe des Glasbildungselementes Aluminium vereinfacht. In den Vierstofflegierungen erhöht Al die Glasübergangs- und Kristallisationstemperaturen und verbessert dadurch die Glasbildungsfähigkeit des Systems. Die röntgenographische Analyse zeigte, dass die Kristallisationsprodukte der schmelzgesponnenen Bänder variieren: von Cu10Zr7 + CuZr2 + AlCu2Zr zu (Cu,Co)Zr + AlCu2Zr, wenn Co ≤ 5 und Co ≥ 10. Die Herstellung von Massivproben mit unterschiedlichen Durchmessern führte zu einem vollständig amorphen Gefüge, einem metallischen Glas-Komposit oder einem vollständig kristallinen Gefüge. Für Co ≤ 5 tritt neben (Cu,Co)Zr und AlCu2Zr ebenfalls Cu10Zr7 auf. Mittels Rasterelektronen (REM)- und Transmissionselektronenmikroskopie (TEM) erfolgte die Analyse des Einflusses von Al- und Co-Zugaben auf die Mikrostruktur von CuZr-Legierungen. Für die Cu-Co-Zr-Al-Legierungen sowie Cu30Co20Zr45Al5 (ø = 4 mm) und Cu45Co5Zr45Al5 (ø = 2 mm) wurden mikrostrukturelle Untersuchungen mittels TEM durchgeführt. Nachfolgend wurde die Heterogenität der Mikrostruktur in der Cu40Co10Zr45Al5 (ø = 2 mm) untersucht. Der Einfluss von Co auf die mechanischen Eigenschaften von rascherstarrten Cu50-xCoxZr50 (x = 2, 5, 10 und 20 Atom-%) Legierungen zeigt, dass das Verformungsverhalten der Stäbe unter Druckbeanspruchung stark von der Mikrostruktur der (Cu,Co)Zr Phase und somit von der Legierungszusammensetzung abhängt. Kobalt beeinflusst die Bruchfestigkeit der Gussproben. Weiterhin zeigen Proben mit martensitischem Gefüge eine Kaltverfestigung. Neben der Kaltverfestigung zeigen die Legierungen mit hohem Co-Gehalt eine verformungsinduzierte Martensitumwandlung und weisen zwei Streckgrenzen auf. Für die Vierstofflegierungen wurde der Einfluss der Kühlrate und der chemischen Zusammensetzung auf die mechanischen Eigenschaften untersucht. Für Cu48Co2Zr45Al5 (ø = 1.5, 2, 3 und 4 mm) und Cu45Co5Zr45Al5 (ø = 3 mm) wurde der Einfluss der Kühlrate und der Heterogenität diskutiert. Die Ergebnisse zeigen, dass die mechanischen Eigenschaften der Cu50-xCoxZr45Al5-Legierungen stark von der Makrostruktur und dem Volumenanteil der amorphen und kristallinen Phase abhängen. Die verformungsinduzierte Martensitumwandlung in Cu40Co10Zr50- und Cu40Co10Zr45Al5-Gussstäben wurde mittels hochenergetischer Röntgenstrahlung durchgeführt. Die In-situ- Druckversuche erfolgten weg- und kraftkontrolliert. Das makroskopische und mikroskopische Spannung-Dehnungs-Verhalten sowie die Phasenumwandlungskinetik wurden dabei betrachtet. Die relativen Veränderungen der vollständig integrierten Intensität der ausgewählten B2- und Martensitreflexe, die auf die Veränderungen der Volumenanteile der entsprechenden Phasen unter Verformung hinweisen, wurden als Phasenumwandlungsvolumen M/M+B2 beschrieben. / The fact that the presence of Co extends the stability range of B2 CuZr to room temperature, together with the significant effect of Al on improving the glass forming ability of the CuZr system was the motivation to investigate the ternary and quaternary CuZr alloys with the aim of synthesizing BMG composites containing B2 (Cu,Co)Zr crystals. This PhD thesis deals with preparation and characterization of Cu50-xCoxZr50 (0 ≤ x ≤ 20) and Cu50-xCoxZr45Al5 (x = 2, 5, 10 and 20) alloys. The phase formation, thermal stability, microstructure, martensitic transformation and mechanical properties of these alloys were investigated. The dependence of phase formation on solidification rate and the thermodynamically stability of Cu-Co-Zr alloys reveals that the addition of Co decreases the glass forming ability (GFA) of the Cu-Co-Zr alloys and changes the stable crystalline products of the system from Cu10Zr7 + CuZr2 to (Cu,Co)Zr phase with a B2 structure. The results indicate that for the melt-spun ribbons with at least 5 % Co, the glass crystallizes directly into B2 (Cu,Co)Zr, while in the case of bulk specimens, compositions with 0 ≤ x < 5 of Co contain the monoclinic (Cu,Co)Zr phase and Cu10Zr7 and CuZr2, whereas, for x ≥ 10, the B2 (Cu,Co)Zr phase is the equilibrium phase at room temperature. Furthermore, increasing the cobalt content decreases the martensitic transformation temperatures to lower temperatures. The phase formation in the ternary system is summarized in a pseudo-binary (Cu,Co)Zr phase diagram, that helps for designing new shape memory alloys, as well as bulk metallic glass composites with the addition of glass former elements. In the quaternary alloys, Al increases the glass transition and crystallization temperatures and hence improves the GFA of the system. The X-ray analysis illustrates that for the melt-spun ribbons, the crystallization products vary from Cu10Zr7 + CuZr2 + AlCu2Zr to (Cu,Co)Zr + AlCu2Zr when Co ≤ 5 and Co ≥ 10, respectively. Depending on the cooling rates, the bulk samples represent a fully amorphous structure or BMG composites or a fully crystalline structure. For Co ≤ 5, beside (Cu,Co)Zr and AlCu2Zr, Cu10Zr7 exists as well. Scanning (SEM) and transmission (TEM) electron microscopy investigations were done to investigate the effect of Al and Co addition to the microstructure of CuZr alloys. In the case of Cu-Co-Zr-Al alloys, Cu30Co20Zr45Al5 (ɸ = 4 mm) and Cu45Co5Zr45Al5 (ɸ = 2 mm) compositions were selected for the microstructure verification using TEM. Later, the heterogeneity of the microstructure in Cu40Co10Zr45Al5 (ɸ = 2 mm) alloy was considered. The effect of Co on the mechanical properties of rapidly solidified Cu50-xCoxZr50 (x = 2, 5, 10 and 20 at.%) alloys depict that the deformation behavior of the rods under compressive loading strongly depends on the microstructure, and as a results, on the alloy composition. Cobalt affects the fracture strength of the as-cast samples; and deformation is accompanied with two yield stresses for high Co-content alloys, which undergo deformation-induced martensitic transformation. Instead samples with a martensitic structure show a work-hardening behavior. For quaternary alloys, the effects of cooling rate and chemical composition on mechanical properties of the alloys were investigated. Cu48Co2Zr45Al5 (ɸ= 1.5, 2, 3 and 4 mm) and Cu45Co5Zr45Al5 (ɸ = 3 mm) compositions were selected to discuss the effect of cooling rate and heterogeneity, respectively. The results depict that the mechanical properties of Cu50-xCoxZr45Al5 alloys strongly depend on the microstructure and the volume fraction of the amorphous and crystalline phases. The deformation-induced martensitic transformation of Cu40Co10Zr50 and Cu40Co10Zr45Al5 as-cast rods, was studied by means of high-energy X-rays. The in situ compression measurements were performed in track control and load control modes. The macroscopic and microscopic stress-strain behavior, as well as the phase transformation kinetics were considered. The relative changes in the fully integrated intensity of the selected B2 and martensite peaks, which indicate the changes in volume fraction of the corresponding phases under deformation, was described as phase transformation volume, M/M+B2.

In this study, the mechanical properties of Cu-Zr binary bulk metallic glasses (BMG) were investigated at the nano-scale. The stable amorphous structures and corresponding energies of BMG structures are performed by density functional theory (DFT) calculation as reference data. This study will combine the Force-Matching (FM) method and Basin-Hopping (BH) method to develop a new method for fitting the Cu-Zr Tight-binding (TB) potential parameters. Moreover, the Bulk modulus, Shear modulus, Young's modulus and Poisson ratio of Cu46Zr54, Cu50Zr50 and Cu64Zr36 structures are calculated with the fitting TB parameters. In addition, the compression process of BMG materials is simulated by the Molecular Statics. The stress and strain are obtained to investigate the deformation mechanism of CuZr/Zr/CuZr nanolaminates at 0 and 45 inclined degree. Finally, we investigate the angle in the deformation process under different strain in the shear band, shear transformation zones (STZs) and force caused by the slip of the atomic distribution of TFMGs layer.

O processo de soldagem por resistência de chapas galvanizadas automotivas é parte indispensável na concepção de uma carroceria de automóvel. Utilizado na união das chapas metálicas que formam um veículo, a grande importância deste processo deve-se basicamente ao fato de não agregar peso à carroceria, ter um desempenho satisfatório em relação à qualidade e produtividade nas linhas de montagem e ainda não onerar o custo final do produto, visto que seu custo de operação pode ser considerado baixo de modo geral. As variáveis específicas deste processo demandam atenção permanente quanto aos parâmetros dos equipamentos, a limpeza e a manutenção, a refrigeração dos eletrodos, a dressagem e a eliminação da camada de latão formada na superfície de contato entre o eletrodo e a chapa. A durabilidade e o desgaste do eletrodo estão ligados basicamente a estes cuidados, podendo ainda gerar ganhos em produtividade e qualidade do ponto além de reduzir o nível de problemas durante e após a soldagem da chapa. Durante a realização deste trabalho, procurou-se avaliar o comportamento do eletrodo fabricado através da liga Cu-Zr, o desgaste do eletrodo em relação ao número de pontos soldados, sua relação com a dureza e variação de temperatura durante o processo, os cuidados que podem retardar o desgaste e a microestrutura do material ao longo do número de pontos e do desgaste deste eletrodo. / The resistance welding process of automotive galvanized steel is an indispensable part in the design of a body car. Used in the union of the metal plates that form a vehicle, the great importance of this process is basically due to the fact not add weight to the body, have a satisfactory performance in relation to quality and productivity in assembly lines and still does not burden the final cost the product, the operating cost can be considered low in general. The specific variables this process require constant attention regarding the parameters of the equipment, cleaning and maintenance, cooling the electrodes, the dressing and the elimination of the brass layer formed on the contact surface between the electrode and the plate. The durability and the electrode wear are basically related to such care, and may generate gains in productivity and quality of point and reduce the level of problems during and after welding the plate. During this work, we tried to evaluate the electrode manufactured by Cu-Zr alloy, the electrode wear in relation to the number of welding points, their relationship with the hardness and temperature variation during the process, the care can slow the wear and the microstructure of the material during the number of welding points and the wear of this electrode.

O processo de soldagem por resistência de chapas galvanizadas automotivas é parte indispensável na concepção de uma carroceria de automóvel. Utilizado na união das chapas metálicas que formam um veículo, a grande importância deste processo deve-se basicamente ao fato de não agregar peso à carroceria, ter um desempenho satisfatório em relação à qualidade e produtividade nas linhas de montagem e ainda não onerar o custo final do produto, visto que seu custo de operação pode ser considerado baixo de modo geral. As variáveis específicas deste processo demandam atenção permanente quanto aos parâmetros dos equipamentos, a limpeza e a manutenção, a refrigeração dos eletrodos, a dressagem e a eliminação da camada de latão formada na superfície de contato entre o eletrodo e a chapa. A durabilidade e o desgaste do eletrodo estão ligados basicamente a estes cuidados, podendo ainda gerar ganhos em produtividade e qualidade do ponto além de reduzir o nível de problemas durante e após a soldagem da chapa. Durante a realização deste trabalho, procurou-se avaliar o comportamento do eletrodo fabricado através da liga Cu-Zr, o desgaste do eletrodo em relação ao número de pontos soldados, sua relação com a dureza e variação de temperatura durante o processo, os cuidados que podem retardar o desgaste e a microestrutura do material ao longo do número de pontos e do desgaste deste eletrodo. / The resistance welding process of automotive galvanized steel is an indispensable part in the design of a body car. Used in the union of the metal plates that form a vehicle, the great importance of this process is basically due to the fact not add weight to the body, have a satisfactory performance in relation to quality and productivity in assembly lines and still does not burden the final cost the product, the operating cost can be considered low in general. The specific variables this process require constant attention regarding the parameters of the equipment, cleaning and maintenance, cooling the electrodes, the dressing and the elimination of the brass layer formed on the contact surface between the electrode and the plate. The durability and the electrode wear are basically related to such care, and may generate gains in productivity and quality of point and reduce the level of problems during and after welding the plate. During this work, we tried to evaluate the electrode manufactured by Cu-Zr alloy, the electrode wear in relation to the number of welding points, their relationship with the hardness and temperature variation during the process, the care can slow the wear and the microstructure of the material during the number of welding points and the wear of this electrode.

No description available.

This thesis is an attempt to understand ways to improve thermo-mechanical and structural properties of nano-structured materials. A detailed study on computational design and analysis of metallic nanowires is carried out. Molecular dynamic simulation method is applied. In particular, FCC metallic nanowires, NiAl, and CuZr nanowires are studied. Various bottom-up approaches are suggested with improved structural and thermo¬mechanical properties. In the first part of the thesis, Cu nanowires are considered. Existence of a novel and stable pentagonal multi-shell nanobridge structure of Cu under high strain rate tensile loading is reported. Such a structure shows enhanced mechanical properties. A three-fold pseudo-elastic-plastic shape recovery mechanism in such nanowires is established. This study also shows that the length of the pentagonal nanobridge structures can be characterized by its inelastic strain. It is also reported that an initial FCC structure is transformed into a new HCP structure. The evidence of HCP structure is confirmed with the help of experimental data published in the literature. Subsequent to the above study, a novel mechanism involving coupled temperature-stress dependent reorientation in FCC nanowires is investigated. A detailed map is generated for size dependent stress-temperature induced solid-solid reorientation in Cu nanowires. In the second part of the thesis, deformation mechanisms in NiAl based intermetallic nanowires are studied. A novel mechanism of temperature and cross-section dependent pseudo-elastic/pseudo-plastic shape and strain recovery by an initial B2 phase of NiAl nanowire is reported. Such a recoverable strain, which is as high as ~ 30%, can potentially be utilized to realize various types of shape memory and strain sensing phenomena in nano-scale devices. An asymmetry in tensile and compressive yield strength behavior is also observed, which is due to the softening and hardening of the nanowires under tensile and compressive loadings, respectively. Two different deformation mechanisms dominated by twinning under tension and slip under compression are found. Most interestingly, a superplastic behavior with a failure strain of up to 700% in the intermetallic NiAl nanowires is found to exist at a temperature of 0.36Tm. Such superplastic behavior is attributed to the transformation of the nanowire from a crystalline phase to an amorphous phase after yielding of the nanowire. In the last part the work, another type of nanowires having Cu-Zr system is considered. A novel stress induced martensitic phase transformation from an initial B2 phase to BCT phase in a CuZr nanowire under tensile loading is reported. It is further shown that such a stress induced martenistic phase transformation can be achieved under both tensile as well as compressive loadings. Tensile-compressive asymmetry in the stress-strain behavior is observed due to two different phase transformation mechanisms having maximum transformation strains of ~ 5% under compressive loading and ~ 20% under tensile loading. A size and temperature dependent tensile phase transformation in the nanowire is also observed. Small nanowires show a single step tensile phase transformation whereas the nanowires with larger size show a two step deformation mechanism via an intermediate R-phase hardening followed by R-phase yielding. A study of energetic behavior of these nanowires reveals uniform distribution of stress over the nanowire cross-section and such stress distribution can lead to a significant improvement in its thermo-mechanical properties. Similar improvement is demonstrated by designing the nanowires via manipulating the surface configuration of B2-CuZr system. It is found that the CuZr nanowires with Zr atoms at the surface sites are energetically more stable and also give a uniform distribution of stresses across the cross-section. This leads to the improvement in yield strength as well as failure strain. An approach to design energetically stable nano-structured materials via manipulating the surface configurations with improved thermo-mechanical properties is demonstrated which can help in fundamental understanding and development of similar structures with more stability and enhanced structural properties. Further ab-initio and experimental studies on the confirmation of the stability of the nanowires via manipulating the surface site is an open area of research and related future scopes are highlighted in the closure.

Al-Zn-Mg-Cu-Zr based AA7010 aluminum alloy belongs to the class of heat treatable alloys and the semi-finished products are generally produced by hot rolling, forging or extrusion processes. It is well known that the thermo-mechanical processing parameters strongly influence both the evolution of texture as well as microstructure in the material. As a result, the semi-finished products exhibit anisotropy in mechanical properties causing legitimate concerns on the applicability of the alloys. In the present thesis, a systematic study on the evolution of texture and microstructure and its implications on the mechanical properties anisotropy of AA7010 alloy has been attempted. A brief introduction on the development of texture and its influence on the anisotropy of the mechanical properties of 7xxx series aluminum alloys is presented first with a view to explore the scopes for further investigation. An overview of the relevant literature is described subsequently. The development of texture and microstructure in an Al-Zn-Mg-Cu-Zr based 7010 aluminum alloy during uneven, hot cross-rolling is presented. Materials processing involves three different types of uneven cross-rolling. The variations in relative intensity of the β-fibre components as a function of cross rolling modes have been investigated. It has been shown that the main attributes to the texture evolution in the present study are (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component following each successive pass, (b) recrystallization resistance of Bs oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids. The stability of the unique single, rotated Brass-{110}(556) component developed in the present alloy, during long term thermal annealing and cold rolling deformation has been systematically investigated further. Subsequently, the influence of development of microstructure and texture on the in-plane anisotropy (AIP) of yield strength, work hardening behavior and yield locus anisotropy has been presented. The AIP and work hardening behavior are evaluated by tensile testing at 0o, 45o and 90o to the rolling direction, whilst yield loci have been generated by Knoop hardness method. It has been observed that in spite of having strong rotated Brass texture, the specimens show low AIP especially in peak aged temper. The in-plane anisotropy (AIP) of yield strength, and work hardening behavior of a heat treated 7010 aluminum alloy sheet having strong, rotated Brass-{110}556 component with different texture intensity and volume fraction of recrystallization has been further evaluated. It is observed that the AIP increases with increase in texture intensity and volume fraction of recrystallization. In the subsequent chapter, the tensile flow and work hardening behavior are described using constitutive equations. Room temperature tensile properties have been evaluated as a function of tensile axis orientations in as-hot rolled as well as peak aged conditions. It has been found that both the Ludwigson and a generalized Voce-Bergström relation adequately describe the tensile flow behavior in all conditions compared to the Hollomon relation. The Voce-Bergström parameters define the slope of - plots in the stage-III regime when the specimens show a classical linear decrease in hardening. Further analysis of work hardening behavior throws light on the effect of texture on the dislocation storage and dynamic recovery. An overall summary of the experimental results and the scopes for future studies have been presented at the end.