A liga Ti-6Al-4V apresenta excelentes propriedades mecânicas, baixa densidade, alta dureza e resistência à corrosão. Entretanto, tratamentos superficiais como a nitretação por plasma são amplamente utilizadas para melhorar a resistência a fadiga desta liga. O objetivo deste trabalho foi tratar termoquimicamente a liga Ti-6Al-4V por meio da nitretação por plasma a fim de verificar a sua influência nas propriedades de fadiga e comparar os resultados obtidos com as não nitretadas. Este tratamento superficial foi realizado com o objetivo de retardar a etapa de nucleação da trinca. A nitretação por plasma foi realizada em temperaturas em torno de 720°C por 4 horas, numa atmosfera de N2, Ar e H2 (5:5:1). Foi realizado um tratamento térmico nas amostras para se obter a microestrutura de Widmanstätten. A temperatura usada foi de 1050°C por 30 minutos. A caracterização microestrutural das amostras nitretadas e não nitretadas foi realizada usando difração de raios X (DRX), microscopia óptica (MO), e microscopia eletrônica de varredura (MEV). Os resultados da difração de raios X das amostras nitretadas mostram a presença de fases: α, β, ε-Ti2N, e δ-TiN e nas amostras não nitretadas as fases α e β. Essas análises revelaram a presença de uma superfície rica em nitretos seguida de uma zona de difusão com nitrogênio na matriz. A rugosidade superficial foi estudada, e foi observada um aumento na rugosidade de 61% após a nitretação. Os testes de fadiga foram realizados em uma máquina de ensaios flexo rotativa, com uma razão de carregamento R=-1 . Os resultados obtidos mostraram que as propriedades de fadiga foram melhoradas após a nitretação por plasma, retardando a etapa de nucleação de trinca. / The Ti-6Al-4V alloy presents excellent mechanical properties, low density, high toughness and corrosion resistance. However, surface treatments like plasma nitriding are widely used to improve the fatigue resistance of this alloy. Therefore, the aim of this work was to treat thermochemically Ti-6Al-4V alloy by means of plasma nitriding to verify its influence in fatigue properties and compare the results with the untreated ones. This surface treatment was performed with the purpose of retards the crack initiation. The plasma nitriding was performed in temperatures around 720°C for 4 hours, with a gas atmosphere N2, Ar and H2 (5:5:1). A heat treatment was made in the samples to obtain a Widmanstätten microstructure. The temperature used was 1050°C for 30 minutes. Microstructural characterizations of nitrided and non-nitrided samples were carried out using X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). The XRD results of the nitrided sample showed the presence of the α, β, ε-Ti2N, and δ-TiN phases. These analyses revealed a surface layer rich in nitrides on the surface, followed by a diffusion layer of nitrogen in the matrix. The surface roughness was compared between the nitrided and non-nitrided layer, and it was observed that the roughness increased 61% after the plasma nitriding. Fatigue tests were performed in a rotating bending testing machine with a stress ratio (R=-1). The results obtained in fatigue tests showed that the fatigue properties were improved after plasma nitriding, retarding the crack initiation.

Esta dissertação, investiga o comportamento em fadiga da liga Ti-6Al-4V após o tratamento superficial denominado oxidação térmica, para aplicação em pinos fusíveis mecânicos. A oxidação térmica controlada do titânio e suas ligas apresenta propriedades interessantes na resistência à corrosão-desgaste, devido a sua alta estabilidade e dureza, contudo, há lacunas na literatura se esse benefício é recíproco nas propriedades da vida em fadiga. Corpos-de-prova foram tratados termicamente em atmosfera de ar ambiente e após foram submetidos a ensaios de rugosidade, metalografia, dureza, tração, fadiga e análise fractográfica. Os resultados obtidos em fadiga foram comparados com a mesma liga não oxidada, mostrando que as propriedades em fadiga não são recíprocas às propriedades de corrosão-desgaste da liga Ti- 6Al-4V, após o tratamento superficial de oxidação térmica. Enquanto para uma determinada condição de tratamento superficial, houve melhora de até 30x na resistência à corrosãodesgaste (redução de 97% na taxa de desgaste), nas propriedades mecânicas houve redução de 18% no limite de resistência à fadiga. / This dissertation investigates the fatigue behavior of thermally oxidized Ti-6Al-4V alloy, applied to break bolts. The controlled thermal oxidation of titanium and its alloys presents interesting properties in fretting-corrosion resistance, especially to the high stability and hardness. However, there are gaps in the literature regarding the effect of this oxide layer on fatigue properties. Thermally Oxidized specimens were tested for roughness, metallography, hardness, tensile strength, fatigue and fractographic analysis. The results were compared with the fatigue results of the same alloy when untreated, showing that the fatigue properties and the fretting-corrosion properties of thermally oxidized Ti-6Al-4V are not reciprocal. While with a given condition for the thermal oxidation, an improvement up to 30x occurred on fretting-corrosion resistance (97% reduction in the rate of wear), the endurance decreased 18%.

Esta dissertação, investiga o comportamento em fadiga da liga Ti-6Al-4V após o tratamento superficial denominado oxidação térmica, para aplicação em pinos fusíveis mecânicos. A oxidação térmica controlada do titânio e suas ligas apresenta propriedades interessantes na resistência à corrosão-desgaste, devido a sua alta estabilidade e dureza, contudo, há lacunas na literatura se esse benefício é recíproco nas propriedades da vida em fadiga. Corpos-de-prova foram tratados termicamente em atmosfera de ar ambiente e após foram submetidos a ensaios de rugosidade, metalografia, dureza, tração, fadiga e análise fractográfica. Os resultados obtidos em fadiga foram comparados com a mesma liga não oxidada, mostrando que as propriedades em fadiga não são recíprocas às propriedades de corrosão-desgaste da liga Ti- 6Al-4V, após o tratamento superficial de oxidação térmica. Enquanto para uma determinada condição de tratamento superficial, houve melhora de até 30x na resistência à corrosãodesgaste (redução de 97% na taxa de desgaste), nas propriedades mecânicas houve redução de 18% no limite de resistência à fadiga. / This dissertation investigates the fatigue behavior of thermally oxidized Ti-6Al-4V alloy, applied to break bolts. The controlled thermal oxidation of titanium and its alloys presents interesting properties in fretting-corrosion resistance, especially to the high stability and hardness. However, there are gaps in the literature regarding the effect of this oxide layer on fatigue properties. Thermally Oxidized specimens were tested for roughness, metallography, hardness, tensile strength, fatigue and fractographic analysis. The results were compared with the fatigue results of the same alloy when untreated, showing that the fatigue properties and the fretting-corrosion properties of thermally oxidized Ti-6Al-4V are not reciprocal. While with a given condition for the thermal oxidation, an improvement up to 30x occurred on fretting-corrosion resistance (97% reduction in the rate of wear), the endurance decreased 18%.

Esta dissertação, investiga o comportamento em fadiga da liga Ti-6Al-4V após o tratamento superficial denominado oxidação térmica, para aplicação em pinos fusíveis mecânicos. A oxidação térmica controlada do titânio e suas ligas apresenta propriedades interessantes na resistência à corrosão-desgaste, devido a sua alta estabilidade e dureza, contudo, há lacunas na literatura se esse benefício é recíproco nas propriedades da vida em fadiga. Corpos-de-prova foram tratados termicamente em atmosfera de ar ambiente e após foram submetidos a ensaios de rugosidade, metalografia, dureza, tração, fadiga e análise fractográfica. Os resultados obtidos em fadiga foram comparados com a mesma liga não oxidada, mostrando que as propriedades em fadiga não são recíprocas às propriedades de corrosão-desgaste da liga Ti- 6Al-4V, após o tratamento superficial de oxidação térmica. Enquanto para uma determinada condição de tratamento superficial, houve melhora de até 30x na resistência à corrosãodesgaste (redução de 97% na taxa de desgaste), nas propriedades mecânicas houve redução de 18% no limite de resistência à fadiga. / This dissertation investigates the fatigue behavior of thermally oxidized Ti-6Al-4V alloy, applied to break bolts. The controlled thermal oxidation of titanium and its alloys presents interesting properties in fretting-corrosion resistance, especially to the high stability and hardness. However, there are gaps in the literature regarding the effect of this oxide layer on fatigue properties. Thermally Oxidized specimens were tested for roughness, metallography, hardness, tensile strength, fatigue and fractographic analysis. The results were compared with the fatigue results of the same alloy when untreated, showing that the fatigue properties and the fretting-corrosion properties of thermally oxidized Ti-6Al-4V are not reciprocal. While with a given condition for the thermal oxidation, an improvement up to 30x occurred on fretting-corrosion resistance (97% reduction in the rate of wear), the endurance decreased 18%.

Burnishing is a chipless finishing process used to improve surface integrity by severe plastic deformation (SPD) of surface asperities. As surface integrity in large measure defines the functional performance and fatigue life of aerospace alloys, burnishing is thus a means of increasing the fatigue life of critical components, such as turbine and compressor blades in gas turbine engines. Therefore, the primary objective of this dissertation is to characterize the burnishing-induced surface integrity of Ti-6Al-4V alloy in terms of the implemented processing parameters. As the impact of cooling mechanisms on surface integrity from SPD processing is largely unexplored, a particular emphasis was placed upon evaluating the influence of cryogenic cooling with liquid nitrogen in comparison to more conventional methodologies. Analysis of numerical and experimental results reveals that burnishing facilitates grain refinement via continuous dynamic recrystallization. Application of LN2 during SPD processing of Ti-6Al-4V alloy suppresses the growth of new grains, leading to the formation of near-surface nanostructures which exhibit increased microhardness and compressive residual stress fields. This is particularly true in cryogenic multipass burnishing, where successive tool passes utilizing lower working pressures generate thermally stable work hardened surface layers, uniform nano-level surface finishes, and significantly deeper layers of compressive residual stresses.

Problems with musculoskeletal system, such as of developmental disorders, fractures or damage of the bone by age, inflammatory or tumor diseases, are still increasing in orthopaedics. Sometimes the bone tissue is not capable to completely regenerate to exert its physiological function in the organism. For this reason, using the bone replacements is necessary and common nowadays. Despite of an intensive research and testing of a wide range of the potential biomaterials and their combinations, the usage of metal materials for construction of the bone implants, still remains to be the gold standard. Ti-6Al-4V alloy is one of the commercialy used metal materials, which is known for the high mechanical and chemical resistance and a good biocompatibility. For a good biological response of the patient's organism for the bone implant, is an ability of osteointegration into the surrounding bone tissue, the key. This ability can be influenced in the case of the metals, by their surface structure. As it is known from earlier studies, the surface topography of the material is very important for the adhesion and proliferation of the bone cells, which are able to discriminate, very sensitively, between various stages of the material surface roughness. For this reason we have focused on studying of an influence...

Titanium and its alloys constitute an important class of materials for aerospace, biomedical, and chemical industries, primarily due to their high specific strength and fracture toughness with good corrosion resistance. Owing to their hexagonal crystal structure at room temperature, both microstructure and texture play a crucial role in the processing and hence the properties of titanium alloys. The basis for tailoring the microstructure and texture in titanium alloys centers around the transformation of high temperature β (body-centered cubic) to the low temperature α (hexagonal close packed) phase. One of the most widely used titanium alloy is Ti-6Al-4V, which exists as two phase (α+β) alloy at room temperature. The cast structure of the alloy Ti-6Al-4V is generally coarse and has strong solidification texture that leads to inferior properties. Recently, trace boron addition has been reported to produce substantial refinement in cast microstructure for Ti-6Al-4V. Significant improvements in some of the mechanical properties have been reported for the Ti-6Al-4V-0.1B alloy in the as-cast condition. The reasons for microstructural refinement in the boron modified alloy and associated improvements in properties, however, needs to be investigated since the property attributes strongly depend on finer microstructural details including crystallographic texture. In addition, the titanium alloys are processed through thermo-mechanical treatments that involve deformation and annealing response of the alloy. The effect of boron modification on the processing response during thermo-mechanical treatments (TMP) has also not been studied. All these aspects shape the framework of the thesis, wherein microstructure and texture evolution is probed from starting cast condition through different stages of TMP. Micro-mechanisms are identified at every stage from the interrelation of these two intrinsic factors. In the first part of the study, the spatial variation of microstructure and texture in the cast ingot has been studied using SEM-EBSD technique. It has been found that trace boron addition (0.1 wt%) to Ti-6Al-4V alloy ensures excellent microstructural homogeneity throughout the cast ingot. A subdued thermal gradient due to constitutional undercooling persists during solidification and maintains equivalent β grain growth kinetics at different locations in the ingot. For Ti-6Al-4V-0.1B alloy, both high temperature β and room temperature α phase textures weaken. The microstructural attributes of boron addition manifests as the absence of grain boundary α-phase and the presence of TiB particles. Both these features strongly affect the mechanism of β→α phase transformation and consequently weaken the α phase texture. The evolution of microstructure and texture during β-processing of Ti-6Al-4V-0.1B alloy is examined in the second part of the work. Boron modified alloy shows the typical features of β worked microstructure with fine prior β grains, however without the formation of shear bands, which is generally observed in the microstructure of β worked Ti-6Al-4V alloy. The transformed α texture is almost same for the two alloys indicating similarity in the transformation behaviour of boron modified and boron free Ti-6Al-4V alloy due to complete dynamic recrystallization during β processing. The microstructural features as well as the crystallographic texture indicates dominant grain boundary sliding for the boron added alloy which leads to homogeneous deformation response without instability (shear band) formation. In the third part of the study, the deformation response in the (α+β) regime has been studied by carrying out hot compression tests at different temperature under constant true strain rate to simulate experimental processing conditions for the cast Ti-6Al-4V-0.1B alloy. The critical combinations of temperature and strain rate suitable for processing are identified based on flow curves and kinetic analyses. Microstructural features display dynamic recovery of the α-phase at low temperatures and softening due to globularization and/or dynamic recrystallization at high temperatures irrespective of boron addition. The transition temperature for the two mechanisms is comparatively lower for boron added alloy. Unlike Ti-6Al-4V, no sign of instability formation has been observed in Ti-6Al-4V-0.1B. The absence of macroscopic instabilities and early initiation of softening mechanisms has been attributed to microstructural features and texture of boron modified alloy in the initial as-cast condition. In the fourth part, the large strain deformation response has been studied for the Ti-6Al-4V-0.1B alloy by rolling in the (α+β) regime. Microstructure in near α rolling regime is characterized by a few kinked and bent α colonies while others are elongated along the rolling direction. Dynamic softening at higher temperatures is more dominant for the boron added alloy. Microstructural features are strongly orientation sensitive while relative differences are inherited from the starting cast alloys. Texture evolution, however, does not markedly vary for the two alloys and indicates little difference in the slip based deformation processes under plane strain condition. The influence of transformation texture appears early for the boron added alloy and affects the final texture in much stronger way at higher temperature. Subsequent to the (α+β) rolling, static annealing of warm rolled alloys has been carried out. A faster annealing kinetics for boron added alloy has been observed, which is related to deformation prior to annealing leading to additional diffusion pathways due to microstructural factors. Texture of the annealed material is similar to the deformed state for shorter annealing times but substantially modifies by epitaxial growth of primary α phase during long time annealing. The final part of the work deals with the deformation response of boron added alloy under superplastic conditions. Out of the two alloys with similar microstructure and texture, higher elongation for boron modified alloy is justified by the absence of slip based deformation and improved grain boundary sliding. Increase in α/β interfaces due to globularization during warm rolling and static annealing contribute to the grain boundary sliding. The outcomes of the thesis have been presented as a summary at the end and suggestions have been made indicating the scope for future investigations pertaining to this area.

This diploma thesis is focused on a productive machining of titanium alloys. At the beginning it deals about titanium and its alloys. It describes chip generation mechanism, tool blunting and surface quality. Further it contains modern strategies of efficient titanium alloys machining. Then it analyzes contemporary manufacturing technology of hinge made of titanium alloy Ti-6Al-4V in Frentech Aerospace s.r.o. company, and at the end finds possibility of savings by inovation of roughing process.

The alloys of Ti are extensively used in a number of industries with the α+β alloy Ti-6Al-4V (referred to as Ti64 hereafter) being the most popular. Recently, it was demonstrated that the addition of a small amount of B – as small as 0.04 wt.% – results in an order-of-magnitude reduction in the as-cast grain size. Consequently, there is considerable current interest in understanding the mechanical behaviour of B-modified alloys, with particular emphasis on correlating the microstructural changes with the property variations and the deleterious effects – if any – of TiB particles especially in the context of fatigue. Prior studies have indicated that the addition of 0.1 wt.% B to Ti64 yields the most optimum combination of room temperature properties. The research reported in the current thesis builds further on it, with the objective of exploring the utility of Ti64-B alloys in the engineering applications context. Towards this end, mechanical behaviour of cast and wrought Ti64-B alloys at cryogenic and high temperatures, the possible effect of hydrogenation on the tensile properties, and strain-controlled low cycle fatigue was experimentally evaluated as detailed below. While extensive work is reported on as-cast alloys, the mechanical properties of wrought alloys have not been examined hitherto. Keeping this in view, room temperature tensile and fatigue properties of wrought Ti64-B alloys were investigated. Microstructures of wrought alloys show kinking of the  lamellae and alignment of TiB particles along the flow direction. Marginal enhancement in tensile and fatigue properties upon forging is noted. Decrease in fatigue strength of wrought Ti64-0.04B is observed due to increase in volume fraction of the grain boundary α phase with B addition, which acts as a crack nucleation site. No significant effect of TiB particles on tensile and fatigue properties is observed. Next, strain-controlled fatigue behaviour was investigated. Results show significant softening when the strain amplitudes, ΔεT/2, are ≥0.75%. B addition was found to improve the fatigue life for ΔεT/2 ≤ 0.75% as it corresponds to the elastic regime and hence strength dominated. At ΔεT/2 = 1%, in contrast, the base alloy exhibits higher life as TiB particle cracking due to strain incompatibility renders easy crack nucleation in the B-modified alloys. To examined whether the addition of B to Ti64 is beneficial in enhancing its high temperature mechanical behavior, tensile and creep tests are carried out in the temperature range of 475-550 °C. Experimental results show that the B addition enhances both elevated temperature strength and creep properties of Ti64, especially at the lower end of the temperatures investigated. The steady state creep rate in the B-modified alloys were lower than that in the base alloy, and both the strain at failure as well as the time for rupture increases with the B content. These marked improvements in the creep resistance due to B addition to Ti64 were attributed primarily to the increased number of inter-phase interfaces – a direct consequence of the microstructural refinement that occurs with the B addition – that provide resistance to dislocation motion. Titanium alloys are widely used in various ambient and high temperature applications. However, in some instances these alloys are exposed to hydrogen and low operating temperature environments. Ti64 alloy shows poor ductility in hydrogen and cryogenic environments. Whether the microstructural refinement that occurs with the B addition also improves its relative mechanical performance in such environments is examined. For this purpose, alloys were H charged at 500 and 700 °C for up to 4 h. Microstructures and room temperature tensile properties of the resulting alloys have been evaluated. Experimental results show that charging at 700 °C for 2 h leads to the formation of titanium hydride in the microstructure, which in turn causes severe embrittlement. For shorter durations of charging, a marginal increase in strength was noted, which is attributed to the solid solution strengthening by hydrogen. The mechanical performance of the B modified alloys was found to be relatively better, implying that B addition is beneficial in applications that involve H environment. Finally, the utility of B-modified Ti64 for cryogenic applications is examined through notched and unnotched tensile tests at 77 and 20 K. While the addition of B up to 0.3 wt.% increases the strength at both 77 and 20 K. However, the ductility of the alloys decreases drastically with decrease in temperature. The tensile stress-strain responses of Ti64-B alloys exhibit serrations beyond yielding at 20 K. The extent of serrations were found to be maximum in coarse grained B-free Ti64 alloy, while only one serration could be identified in B-containing alloys. Activation of deformation twinning at 20 K results in the formation of serrations. Three twinning modes were identified in coarse grained B-free Ti64 alloy- {10 ̅2}, {11 ̅1} and {5 ̅1 ̅} while only{10 ̅2}twinning mode was activated in B-containing alloys. Extensive deformation through twinning results in higher ductility of B-free Ti64 alloy at 20 K in comparison to B containing alloys.