@phdthesis{Lemos2020, author = {Lemos, Georges}, title = {Development of Ni-based superalloy metal matrix composites, featuring high creep resistance}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-53985}, school = {BTU Cottbus - Senftenberg}, year = {2020}, abstract = {The increasing demand for competitive, whilst also environment-friendly airplane travel, compels the design of highly efficient engines in the aeronautical field. A potential for improvement of traditional polycrystalline Ni-based superalloys, aiming higher creep resistance, was investigated. The approach adopted the concept of metal matrix composites (MMCs) to incorporate a rigid discontinuous phase, in the form of particles, to a γ'-strengthened Ni-based superalloy. In order to make the concept feasible, different microstructures resulting from diverse manufacturing techniques were investigated. By using distinct mixing and sintering methods, powders of Inconel X-750 and TiC were combined to form composites containing 15 vol.\% of reinforcing particles. Powders were prepared with low and high energy milling processes, and formed by uniaxial pressure sintering and spark plasma sintering methods. Non-reinforced variants and composites had microstructures thoroughly examined at their initial state and after long isothermal aging treatments. Selected variants were further submitted to tensile and compression creep tests at temperatures between 700 and 800 °C, in the stress range of 200 to 500 MPa. A comprehensive analysis was conducted using techniques such as EBSD, XRD Rietveld refinement, EDS and TEM to evaluate the development of γ', η and TiC phases, determining the achievable microstructures with each fabrication method and establishing their evolution after aging treatments over times up to 1000 h. Likewise, creep properties were analyzed by obtaining parameters such as creep exponents, threshold stresses and activation energies. A creep life estimation was conducted with the use of a Monkman-Grant relationship and a Larson-Miller parametrization. Lastly, the potential for a reduction in creep strain rates in a working turbine blade, considering the density of investigated materials as a parameter, was evaluated. All produced composites presented power law creep, with dislocations surpassing γ' particles by climb. The variant produced by high energy ball milling and spark plasma sintering exhibited the highest creep rates, resulting from intense diffusion through grain boundaries. It also presented η phase after long isothermal aging, which affects negatively the creep resistance. Contrastingly, in the variant produced by low mixing combined with pressure sintering the lowest creep rates were observed. It was proposed that reinforcing TiC particles effectively acted as a load bearing phase, counterbalancing the adverse effects of the intergranular diffusion in the refined microstructure. Furthermore, a higher microstructural stability was observed in this variant, resulting from limited interaction between TiC particles and the matrix during fabrication.}, subject = {Superalloys; MMC; Creep resistance; TiC; Isothermal aging; Superlegierungen; MMC; Kriechbest{\"a}ndigkeit; TiC; Isotherme Alterung; Metallmatrix-Verbundwerkstoff; Titancarbid; Superlegierung; Kriechfestigkeit}, language = {en} } @phdthesis{Xu2020, author = {Xu, Peng}, title = {Enhancement of tensile fracture resistance of metal-injection-molded β titanium alloys biomaterials via diverse sintering pathways}, doi = {10.26127/BTUOpen-5512}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-55126}, school = {BTU Cottbus - Senftenberg}, year = {2020}, abstract = {The powder metallurgically produced beta titanium alloys (traditional PM beta Ti-alloys) have long been plagued by high impurities contamination. For binder-based powder technologies, they originate from the sintering atmosphere, the debinding processes and the starting powders. In general, a normal carbon residual of binder-based powder technologies is capable of incurring the formation of aligned TiCx particles along beta grain boundaries (GB-TiCx) in most classes of beta Ti-alloys. Whereas, oxygen atoms are likely to deteriorate the ductility of PM Ti alloys by promoting the formation of diverse brittle phases and/or altering the deformation modes. Such materials exhibiting rather low toughness to strain ratios are not an option for critical structural applications, where catastrophic damage is completely unacceptable. In this study, biotolerant metastable beta Ti-20Nb-10Zr alloys, containing a certain amount of carbon, oxygen residuals originated from materials processing and consequently 0.5 vol.\% in situ synthesized TiCx particles, were fabricated via metal-injection-molding (MIM). With varying yttrium (Y) addition, the effects of Y-induced oxygen scavenging, beta-grain refinement and porosity increment on tensile properties were systematically investigated. To scavenge oxygen from the beta Ti-matrix, the Y elemental powder with a maximum particle size of 15 µm (e.g. <12 µm or 1200 mesh) is more appropriate than the commonly used <45 µm (i.e. 325 mesh) sized powder or larger ones and without significant detrimental effect on the as-sintered density of beta Ti-alloys. A novel toughening strategy was proposed by regulating TiCx precipitation evolution and resultantly adjusting particles distribution pattern. Synchrotron radiation identified that two separate TiCx precipitation-type reactions occurred at the beta phase region and the alpha/beta region. In a narrow temperature range between these two precipitation reactions, dissolution of carbides was observed just below alpha/beta transus. Y addition can postpone TiCx precipitation. On the basis of those mechanisms, adjusting TiCx particle distribution was proposed for the first time, specifically a combination of yttrium addition (Y) and carbide spheroidization reprecipitation annealing (CSRA). As a result, aligned GB-TiCx particles were adjusted to dispersed intragranular TiCx particles. An apparent toughening effect (≈ 113\% increment reaching elongation = 8.3\%) was achieved after TiCx redistribution, while non-optimally aligned TiCx pattern seriously limited tensile toughness of materials. Here, the mechanisms of TiCx redistribution behavior and its toughening are elucidated systematically.}, subject = {Metal-Injection-Molding; Titanium; Carbide; Fracture; Oxygen; Metallspritzguss; Titan; Karbid; Fraktur; Sauerstoff; Biomaterial; Metallspritzguss; Titanlegierung; Carbide; Sauerstoff; Bruchverhalten}, language = {en} } @phdthesis{Zhang2020, author = {Zhang, Haoyin}, title = {Novel type of biomedical titanium-manganese-niobium alloy fabricated by arc melting and metal injection moulding}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-51280}, school = {BTU Cottbus - Senftenberg}, year = {2020}, abstract = {Titanium and its alloys have been widely used as implant biomaterials due to their suitable combination of mechanical properties and biological compatibilities. At present, about 70-80\% of implants are made of metallic biomaterials. Compared with magnesium alloys, stainless steel and cobalt alloys, titanium alloys have a higher specific strength, high corrosion resistance, and excellent biocompatibility. With research, Ti-Nb biomedical titanium alloys have been constantly developed. In the Ti-Nb alloy, the amount of Nb is usually from 16\% to 42\% (wt.\%) which is about 10\% to 30\% (at.\%). As reported, Mn as a trace element to the human body has the potential to be used in bio-materials. Therefore, this study aims at the partial replacement of Nb by Mn to reduce the costs, without deteriorating the mechanical properties. Moreover, it must be ensured good biocompatibility and corrosion resistance. This is the first investigated on Ti-Mn-Nb ternary alloys. In this work, According to β single-phase field, Ti-xMn-yNb (x=4, 10, 16; y=2, 8, 14, at.\%) alloys (arc-melted) have been fabricated. The Ti-Mn-Nb alloys are investigated by optical microscopy (OM), X-ray diffraction (XRD), hardness test, transmission electron microscopy (TEM) and mechanical testing. By screening study on alloy, Ti-10Mn-14Nb (at.\%) (Ti-10Mn-23.7Nb (wt.\%)) is the optimal alloy with tensile strength (760 MPa) and elongation (10.5\%). After that, the Metal Injection Moulding (MIM) is used to prepare Ti-Mn-Nb alloys. The MIM method can greatly reduce the processing cost. MIM Ti-xMn-yNb (x=3, 4, 6; y=1, 2, 4, at.\%), Ti-4Mn-14Nb and Ti-10Mn-14Nb alloys are fabricated. Among them, a very good combination of mechanical properties is achieved for MIM processed Ti-4Mn-2Nb (at.\%) (Ti-4.5Mn-3.8Nb (wt.\%)), namely a YS of 642 MPa, UTS of 725 MPa and high ductility of 16\% elongation to fracture. With further investigations, when the yttrium content is 0.1\% (at.\%), the tensile strength of Ti-4Mn-2Nb-0.1Y (at.\%) (Ti-4.5Mn-3.8Nb-0.18Y (wt.\%)) is increased to 785 MPa while elongation of 12.9\%. These mechanical properties already exceed Ti-6Al-4V (ASTM F2885 Grade 5 undensified). In the in vitro evaluation, in comparison with MIM pure titanium, human osteoblasts MG63 adhered as well and proliferated on the surface of MIM Ti-Mn-Nb specimens. In the supernatant after cell culture, the Ti-Mn-Nb alloy shows similar osmolality and pH value results as MIM pure titanium. By LDH assay and DNA isolation, the MIM Ti-Mn-Nb alloys are not found to be toxic to MG63 cells. In the study of corrosion resistance in Hanks' balanced salt solution (HBSS) at 37 °C, the corrosion current densities as well as the impedance of MIM Ti-Mn-Nb alloys are all better than those of MIM pure titanium and even better than those of MIM Ti-6Al-4V alloy.}, subject = {Titanium; Metal injection moulding; Niobium; Mangane; Biomedical; Titan; Mangan; Niob; Metallspritzguss; Biomedizin; Biomedizin; Lichtbogen; Metallspritzguss; Nioblegierung; Schmelzen; Manganlegierung; Titanlegierung}, language = {en} } @phdthesis{Wang2014, author = {Wang, Li}, title = {Mechanisms of carbide precipitation and carbon solubility in high Nb containing TiAl Alloys}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-30392}, school = {BTU Cottbus - Senftenberg}, year = {2014}, abstract = {TiAl alloys with a high addition of Nb have recently been applied in the aerospace and automotive fields due to their excellent high-temperature properties. The C additions to TiAl alloys have been reported to improve their strength and creep resistance through solid-solution hardening and / or precipitation hardening. In this work powder metallurgy (PM) based Ti-45Al-5Nb-xC (x=0, 0.5 0.75 and 1.0 at. \%) alloys were systematically investigated after different heat treatments. The research is mainly concerned with the temperature range from 800 to 1000ºC which is interesting for processing as well as applications. The results show that the addition of C influences the phase transformations in Ti-45Al-5Nb and that the addition of Nb may influence the carbon solubility in TiAl alloys at high temperatures. The carbon solubility in Ti-45Al-5Nb is > 1.0 at. \% at 1400°C, but between 0.5 and 0.75 at. \% at 1000°C, and less than 0.5 at. \% at 800°C. The carbide precipitation and development of carbide morphology are discussed in great detail. The thermal stability of P-Ti3AlC carbides in Ti-45Al-5Nb-xC alloys is increased, which might be attributed to the addition of high amounts of Nb, or the high amount of carbon, or a combination of both. In heat-treated Ti-45Al-5Nb-0.5C and Ti-45Al-5Nb-0.75C, H-type carbides are not detected to form during annealing. The addition of Nb may increase the formation temperature of the H-Ti2AlC precipitate phase.}, subject = {High Nb containing TiAl alloys; Transmission electron microscope; High-energy X-ray diffraction; Carbide; Thermal stability; Hochniobhaltige TiAl-Legierungen; Transmissionselektronenmikroskopie; Hochenergie-R{\"o}ntgenbeugung; Karbide; Thermische Stabilit{\"a}t; Titanaluminide; Carbide; Temperaturbest{\"a}ndigkeit}, language = {en} }