Titanium and its alloys are widely used in applications ranging from aeroengines and offshore equipment to biomedical implants and sporting goods, owing to their high ratio of strength to density, excellent corrosion resistance, and biomedical compatibility. Among the titanium alloys used in aerospace, Ti-6Al-4V (an α+β alloy) is the most widely used, in applications in which the temperature may reach 350°C, at which point it retains good fatigue and fracture properties as well as moderate tensile strength and ductility. These alloy properties are dependent on variables such as crystalline structure, alloy chemistry, manufacturing techniques and environmental conditions during service. These variables influence the microstructure and mechanical properties of titanium alloys. With regard to the alloy chemistry and operating environment, the focus of the present work is to understand the influence of boron and hydrogen on the microstructure and selected mechanical properties of cast Ti-6Al-4V. The addition of boron to cast Ti-6Al-4V (0.06 and 0.11 wt% in this work) refines the coarse “as cast” microstructure, which is evaluated quantitatively using FoveaPro image analysis software. Compression testing was performed using a Gleeble 1500 instrument, by applying a 10% strain at different strain rates (0.001, 0.1 and 1 s-1) for temperatures in the range 25-1100°C. The tests were performed to evaluate the effect of boron on the mechanical properties of the alloy. It was observed that there is an increase in the compressive strength, predominantly at room temperature, of cast Ti-6Al-4V after the addition of boron. Metallographic evaluation showed that this increase in strength is a likely result of reductions in both the prior β grain and α colony dimensions, which is caused by boron addition. Studies in a hydrogen environment at 150 bar showed that cast Ti-6Al-4V exhibited lower yield strength and lower ultimate tensile strength in comparison with those properties measured in an air environment. No significant change in the ductility was observed. It was also noted that in a high strain range (≈2%) the low cycle fatigue (LCF) life was significantly reduced in hydrogen compared with air. Microstructural and fractographic characterization techniques were used to establish the role of hydrogen on the deformation mechanism by analysing the crack propagation path through the microstructure. It is seen that cracks tend to propagate along the interface between prior β grain boundaries and/or along the α colony boundaries

The work presented aims to find microstructure and mechanical properties after remelting of aluman-16 alloy by using high pressure die casting (HPDC). Alloy has been casted with specific composition. The alumna-16 alloy has been casted in 2 different composition of different Si content of 0.22% and 0.062%. This work describes a method for creating samples for tensile tests through experimental techniques on standard samples. Microstructural analysis utilizing scheil simulation and fracture analysis have also been undertaken. The results have been compared by using received data to available data. The difference between the two alloys was noted in the microstructure study, where the two Fe-rich intermetallic Al8Fe2Si and Al9Fe2Si2 were not visible in the ThermoCalc software. Elongation percentage was found to decrease with remelting and to increase when Si weight percentage increased. / Not applicable

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The goal of this research is to aid the development of large-scale additive manufacturing of jointless underbody hulls for the Army Ground Vehicle Systems by 1) generating an improved mechanical and metallurgical database and 2) understanding the Additive Friction Stir Deposition (AFSD) process. AFSD is a solid-state additive manufacturing process that is a high strain rate and a hot working process that deforms material onto a substrate and builds a component layer by layer. This unique, solid-state additive manufacturing process has the potential for scalability into ground vehicle applications on the extra large-scale due to its solid-state nature. Two different aluminum alloys were investigated: Al-Mg-Si (6061) and Al-Zn-Mg-Cu (7075). AFSD builds were evaluated in the transverse or through layer direction (Z) and the 6061 material was also evaluated in the longitudinal direction (X). Uniaxial tensile testing was performed to generate mechanical property data while fractography, and metallography were used to better understand the metallurgical implications of this process. This research determined that the refinement of the grain size caused by the AFSD process had little or no strengthening effect on the mechanical properties of either alloy. Instead, the as-deposited condition in both alloys were soft with good ductility due to the dissolution of the strengthening particles. After heat treatment, the elongation and fracture mode of the 6061 alloy was dependent on the layer direction. Failure often initiated at interfaces and affected the materials' elastic-plastic behavior. For the 7075 alloy, the strength and failure mechanism of the material were affected by the presence of the graphite lubricant used during processing. The use of graphite increased the variability of the mechanical properties results and caused premature failure in numerous samples. In both alloys, the heat treatment caused grain coarsening to varying degrees which can affect the mechanical behavior. From these results, it was found that a precipitation strengthening heat treatment is required for material deposited with AFSD to achieve the minimum mechanical property standards for a forging. Recommendations and future work include 1) investigating the effect of residual stresses on AFSD components, 2) determining the fatigue properties of AFSD materials, 3) continuing to increase the database of mechanical properties for AFSD materials, and 4) developing additional lubricants for the AFSD process. / Master of Science / The results of this research will be used to help generate design requirements for large-scale additively manufactured parts such as underbody tank hulls. This research generated and expanded on the mechanical and metallurgical understanding of solid-state additively manufactured aluminum. The solid-state additive process used was Additive Friction Stir Deposition. Like its name, this process uses a rotating tool head to apply friction to a solid bar of aluminum that then generates heat which makes the metal soft enough to stir and deposit into a layer. Another layer is then deposited on top and repeated layer by layer until the final part is completed. Other metal additive manufacturing processes that involve melting and then rapidly cooling the material degrade the quality of the metal material. The first part of this research investigated the mechanical properties in different layer directions either pulling along the build direction or against the layers. The results showed that a heat treatment was required to improve the strength of the aluminum to meet current standards of quality. However, the ability of the aluminum to elongate depended on the orientation of the layers. The second part of this research investigated the effect that a graphite lubricant used on the aluminum feedstock to help prevent the material from sticking in the tool head affected the mechanical properties. The results show that the graphite lubricant did not dissolve or disappear into the metal and caused a reduction in the elongation of the aluminum. Recommendations for extra large-scale metal additive manufacturing are to design parts to apply the highest stress along the layer direction and to eliminate the use of the graphite lubricant.

[pt] A utilização de materiais de construção civil a base de cimento reforçado com fibras vem aumentando rapidamente nos últimos anos. No Brasil um vasto programa experimental para avaliar o comportamento mecânico desses materiais através de ensaios de flexão monotônicos e de compressão, vem sendo desenvolvidos na PUC/RIO desde 1979. Este trabalho procura dar continuidade a essa linha de pesquisa, sendo o primeiro a estudar o comportamento em fadiga de compósitos cimentícios reforçados com polpa de bambu, através de curvas de vida-fadiga S-N e da cinética de crescimento de trincas. As curvas S-N foram levantadas para compósitos entalhados e não entalhados, contendo 6% em massa de polpa em relação a massa de cimento. Essas curvas foram modeladas, com base nas propriedades mecânicas básicas levantadas nos ensaios de compressão e flexão. Com o objetivo de verificar a aplicabilidade da lei de Paris à cinética de crescimento de trincas de fadiga nesses compósitos, foi levantada a relação entre o comprimento da trinca a e o número de ciclos N durante a propagação estável da mesma, adotando-se teores de reforço de 6 e 14% em relação a massa de cimento. Finalmente as superfícies de fratura foram avaliadas por meio de microscópio eletrônico de varredura. / [en] The use of fiber reinforced cementious composites as construction materials in civil engineering has rapidly grown in the last few years. In Brasil, a large experimental program for evaluating the mechanical behavior of these materials has been developed in PUC-RIO since 1979. The present study has the purpose of evaluating the fatigue behavior of cementitious composites by means of determining the S-N curves for notched and unnotched specimens. The fatigue curves were modeled using basic mechanical properties determined by means of compression and slow bend tests. With the purpose of verifying the applicability of Paris law to the fatigue crack growth kinetics, the crack length was determined as a function of the number of cycles N during stable crack propagation, for composites containing 6% and 14% weight percentage of bamboo pulp relative to the weight of cement. Finally, the fracture surface was analyzed by means of scanning electron microscopy.

Compósitos de matriz metálica (CMM) reforçados com dois tipos de particulado cerâmico foram produzidos por meio do processo ARB (Accumulative Roll Bonding) a fim de estudar os efeitos destes no que diz respeito às propriedades mecânicas e microestruturais. ARB é um processo de deformação plástica severa aplicada originalmente a uma pilha de lâminas metálicas, a qual é laminada, seccionada em duas metades, as quais são empilhadas e novamente laminadas, e assim por diante, desenvolvido com o propósito de reduzir o tamanho de grão e aumentar a resistência mecânica do produto final. O processo é econômico e capaz de produzir de folhas ultrafinas a placas espessas, sem que haja restrição de quantidade. Confeccionou-se CMM de alumínio reforçados com partículas de carbeto de silício (Al+SiCp) e alumina (e Al+Al2O3p) com granulometria média de 40µm, as quais foram caracterizadas microestruturalmente e ensaiadas em tração até a falha, cuja análise foi conduzida via microscopia eletrônica de varredura. Ambas as amostras obtiveram ganho em sua resistência mecânica, comparadas ao alumínio monolítico (sem adição de partículas de reforço) e alumínio recozido. Foram ensaiados em tração corpos de prova com e sem presença de entalhe, sendo que as peças entalhadas apresentaram comportamento esperado de aumento de resistência mecânica e baixo alongamento e fratura de aspecto frágil. De acordo com análise feita por fratografia houve boa ancoragem e dispersão das partículas de reforço na matriz. / Metal matrix composite (CMM) reinforced with two types of ceramic particles have been produced through the process ARB (Accumulative Roll Bonding) in order to study their effect as regards the mechanical and microstructural properties. ARB is a severe plastic deformation process originally applied to a stack of metal sheets, which is laminated, sectioned into two halves, which are stacked and rolled again, and so on, developed with the purpose of reducing the grain size and increase the mechanical strength of the final product. The process is economical and capable of producing ultrafine sheets to thicker plates without much restriction. Were fabricated CMM of the aluminum reinforced with particles of silicon carbide (Al + SiCp) and alumina (and Al + Al2O3p) with an average particle size of 40μm, which are characterized microstructurally and tested in tension until failure, whose analysis was conducted via scanning electron microscopy. Both samples were successful in its mechanical strength compared to the monolithic aluminum (without addition of reinforcing particles) and annealed aluminum. They were tested for tensile specimens with and without the presence of notch, and the carved pieces showed strength-enhancing behavior and low elongation and frail fracture. According to analysis by fractography was good anchoring and reinforcement particles dispersed in the matrix.

Turbine engine blades are subjected to extreme conditions characterized by significant and simultaneous excursions in both stress and temperature. These conditions promote thermo-mechanical fatigue (TMF) crack growth which can significantly reduce component design life beyond that which would be predicted from isothermal/constant load amplitude results. A thorough understanding of the thermo-mechanical fatigue crack behavior in single crystal superalloys is crucial to accurately evaluate component life to ensure reliable operations without blade fracture through the use of "retirement for cause" (RFC). This research was conducted on PWA1484, a single crystal superalloy used by Pratt & Whitney for turbine blades. Initially, an isothermal constant amplitude fatigue crack growth rate database was developed, filling a void that currently exists in published literature. Through additional experimental testing, fractography, and modeling, the effects of temperature interactions, load interactions, oxidation and secondary crystallographic orientation on the fatigue crack growth rate and the underlying mechanisms responsible were determined. As is typical in published literature, an R Ratio of 0.7 displays faster crack growth when compared to R = 0.1. The effect of temperature on crack growth rate becomes more pronounced as the crack driving force increases. In addition secondary orientation and R Ratio effects on crack growth rate were shown to increase with increasing temperature. Temperature interaction testing between 649°C and 982°C showed that for both R = 0.1 and 0.7, retardation is present at larger alternating cycle blocks and acceleration is present at smaller alternating cycle blocks. This transition from acceleration to retardation occurs between 10 and 20 alternating cycles for R = 0.1 and around 20 alternating cycles for R = 0.7. Load interaction testing showed that when the crack driving force is near KIC the overload size greatly influences whether acceleration or retardation will occur at 982°C. Semi-realistic spectrum testing demonstrated the extreme sensitivity that relative loading levels play on fatigue crack growth life while also calling into question the importance of dwell times. A crack trajectory modeling approach using blade primary and secondary orientations was used to determine whether crack propagation will occur on crystallographic planes or normal to the applied load. Crack plane determination using a scanning electron microscope enabled verification of the crack trajectory modeling approach. The isothermal constant amplitude fatigue crack growth results fills a much needed void in currently available data. While the temperature and load interaction fatigue crack growth results reveal the acceleration and retardation that is present in cracks growing in single crystal turbine blade materials under TMF conditions. This research also provides a deeper understanding of the failure and deformation mechanisms responsible for crack growth during thermo-mechanical fatigue. The crack path trajectory modeling will help enable "Retirement for Cause" to be used for critical turbine engine components, a drastic improvement over the standard "safe-life" calculations while also reducing the risk of catastrophic failure due to "chunk liberation" as a function of time. Leveraging off this work there exists the possibility of developing a "local approach" to define a crack growth forcing function in single crystal superalloys.

Compósitos de matriz metálica (CMM) reforçados com dois tipos de particulado cerâmico foram produzidos por meio do processo ARB (Accumulative Roll Bonding) a fim de estudar os efeitos destes no que diz respeito às propriedades mecânicas e microestruturais. ARB é um processo de deformação plástica severa aplicada originalmente a uma pilha de lâminas metálicas, a qual é laminada, seccionada em duas metades, as quais são empilhadas e novamente laminadas, e assim por diante, desenvolvido com o propósito de reduzir o tamanho de grão e aumentar a resistência mecânica do produto final. O processo é econômico e capaz de produzir de folhas ultrafinas a placas espessas, sem que haja restrição de quantidade. Confeccionou-se CMM de alumínio reforçados com partículas de carbeto de silício (Al+SiCp) e alumina (e Al+Al2O3p) com granulometria média de 40µm, as quais foram caracterizadas microestruturalmente e ensaiadas em tração até a falha, cuja análise foi conduzida via microscopia eletrônica de varredura. Ambas as amostras obtiveram ganho em sua resistência mecânica, comparadas ao alumínio monolítico (sem adição de partículas de reforço) e alumínio recozido. Foram ensaiados em tração corpos de prova com e sem presença de entalhe, sendo que as peças entalhadas apresentaram comportamento esperado de aumento de resistência mecânica e baixo alongamento e fratura de aspecto frágil. De acordo com análise feita por fratografia houve boa ancoragem e dispersão das partículas de reforço na matriz. / Metal matrix composite (CMM) reinforced with two types of ceramic particles have been produced through the process ARB (Accumulative Roll Bonding) in order to study their effect as regards the mechanical and microstructural properties. ARB is a severe plastic deformation process originally applied to a stack of metal sheets, which is laminated, sectioned into two halves, which are stacked and rolled again, and so on, developed with the purpose of reducing the grain size and increase the mechanical strength of the final product. The process is economical and capable of producing ultrafine sheets to thicker plates without much restriction. Were fabricated CMM of the aluminum reinforced with particles of silicon carbide (Al + SiCp) and alumina (and Al + Al2O3p) with an average particle size of 40μm, which are characterized microstructurally and tested in tension until failure, whose analysis was conducted via scanning electron microscopy. Both samples were successful in its mechanical strength compared to the monolithic aluminum (without addition of reinforcing particles) and annealed aluminum. They were tested for tensile specimens with and without the presence of notch, and the carved pieces showed strength-enhancing behavior and low elongation and frail fracture. According to analysis by fractography was good anchoring and reinforcement particles dispersed in the matrix.