Department of Metallurgical and Materials Engineering

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    ELECTROCHEMICAL NITRIDATION OF YTTRIA STABILIZED ZIRCONIA TO PRODUCE NITRIDE CONDUCTING SOLID ELECTROLYTES
    ( 2023-01-20) ÖZTÜRK, Onur ; DOĞU, Doruk
    The gradual increase in the world population has many effects, both positive and negative, globally. Due to the increasing population, the amount of carbon emissions per capita is also increasing. If the carbon emissions continue to increase in this way, the world average temperature will increase by 4 ℃ by 2050. With the Paris Climate Agreement, which our country has also signed, this temperature is tried to be kept at 2 ℃. Ammonia is the second most produced chemical in the world, which is used in many fields from the food sector to the energy sector. Ammonia is currently produced by a process called Haber-Bosch. This process takes place at high temperatures and pressure. As a result, it is a production method with a high energy requirement and a high amount of carbon emissions. For these reasons, scientists are looking for alternative production methods. The production of ammonia using electrochemical methods is one of the most promising alternatives. These systems enable ammonia to be produced at low-pressure values. At the same time, the fact that these systems are easily scalable is another advantage. They ensure that the ammonia production can be done onsite and on demand where it is needed, and thus offers a great energy saving. The production method using high-temperature solid oxide electrolytes is one of the most popular methods. Currently, oxide conductive electrolytes and proton conductive electrolytes are used in studies using solid oxide electrolytes. These systems aim to ionize water vapor or hydrogen, which is the source of hydrogen in both systems, and react with nitrogen. But here, due to the strong triple bond between nitrogen atoms, the ionized hydrogens recombine instead of reacting with nitrogen and turn into H2. This reduces ammonia production rate and selectivity. On the other hand, if nitride conductive electrolytes are used, nitrogen can be fed to the reaction site in ionic form reducing hydrogen recombination and increasing ammonia selectivity. Within the scope of this thesis, nitrite conductive electrolytes, which can also solve the problem in the production methods mentioned above, have been developed for the first time. First of all, the reaction system was installed and designed, then the reactor was designed with the help of Autodesk Inventor 2019. 310 stainless steel material was used in the production of the reactor. Glass seal was used as sealing material in the system and crofer mesh was used. The experiments were carried out at 550 °C. Nitrogen ion conduction experiments were carried out using 8%YSZ electrolytes (127- 140-270 µm) of different thicknesses and using an LSM-YSZ (symmetric cell) catalyst. During these experiments, a current scan was made between 0.1-300 mA and it was observed that the cell cracked at high currents. In all experiments, 15 sccm of N2 gas was fed from the cathode side and 20 sccm of He gas was fed to the anode side. The anode gas outlet is connected to the mass spectrometer gas analyzer where the signals are monitored. Nitrogen ion transfer is observed starting around 200 mA. With the data taken at different currents with 40 minutes intervals, faradaic efficiency calculations were made and the maximum efficiency was found around 40% at 300 mA current value. Electrochemical nitridation was also performed using N-Mayenite and Ce-doped LaFeO3 anode catalysts. In addition to the electrochemical nitridation study, powder nitridation studies were also carried out. Nitridation of 8% YSZ and ZrO2 powders under different temperatures, time, and flow rates in N2 rich atmosphere has been tried. In addition to these, ZrO2 powder was tried to be reduced by the carbothermic reduction method.
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    EFFECT OF AUSTEMPERING ON THE MICROSTRUCTURAL AND BALLISTIC PROPERTIES OF MIL-A-12560 AND AISI 4340 STEELS
    ( 2012-10-25) DEMİRCİ, Ali Tunç ; TEKİN, Erdoğan
    This thesis is prepared in order to show the possibilites of the development of bainitic steel armor plates from U.S. Military Standard, MIL-A-12560 and alternatively, AISI 4340 steel to protect the civillian and military vehicles against external threats.
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    INVESTIGATION OF THE HIGH TEMPERATURE OXIDATION BEHAVIOR OF CVD ALUMINIZED INCONEL 718 AND INCONEL 738LC SUPERALLOYS
    ( 2022-03-09) Telbakiroğlu, Yusuf Burak ; Konca, Erkan
    This study was undertaken to investigate and compare the high temperature oxidation behavior of aluminized Inconel 718 and Inconel 738LC nickel based superalloys. Bare and high activity chemical vapor deposition (CVD) aluminized Inconel 718 and Inconel 738LC samples were oxidized in air at 925, 1000 and 1050°C for 200 hours. Detailed cross-sectional examinations, elemental analyses, weight change measurements, and x-ray diffraction studies were performed in order to investigate and evaluate the oxidation mechanisms of the samples. It was observed that the oxidation resistances of both 718 and 738LC alloys were significantly improved by the protective Al2O3 layer formed on the NiAl phase that was created on the surfaces of the samples during aluminizing. The beneficial effect of aluminizing was found to be more evident in the case of 738LC alloy samples which showed lower oxidation rates at all test temperatures. It is suggested that the higher aluminum content of the 738LC alloy substrate slows down the diffusion flux of Al away from the NiAl phase and hence this is the main reason for its superior oxidation resistance.
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    MECHANICAL AND BIOLOGICAL PROPERTIES OF ALUMINA AND TITANIA CO-DOPED ZIRCONIA CERAMICS
    ( 2017-01-07) Ağaç, Özlem ; PARK, Jongee
    Zirconia-based ceramics have generated considerable interests in the dental community as restorative dental materials due to their superb mechanical and chemical properties, biocompatibilty, and aesthetic characteristics. This study was to produce 3Y-TZP ceramics by adding titania and alumina as a dopant. Various amounts (ranging from 0 to 2 wt%) of TiO2 and Al2O3 were mono and co-doped to tetragonal zirconia ceramic containing 3 mol% yttria (3Y-TZP) by mechanical ball milling. The shaped samples were pressureless sintered at 1350, 1450 and 1550 °C for 2 hrs. Density, hardness, fracture toughness, and cell attachment of the co-doped 3Y-TZP ceramics were measured with respect to dopant addition. The mechanical property was determined using the method of Vickers indentation after sintering. XRD was operated to examine the crystalline phases in the titania and/or alumina-added zirconia ceramics. SEM was used for analysis of grain size and surface morphology. The results show that density decreased gradually as the amount of dopant was increased. The mechanical properties showed the maximum value when 0.5 wt% TiO2 and 1.0 wt% Al2O3 were co-doped to 3Y-TZP. Crystalline phase formation and microstructural morphology were investigated by XRD and SEM analyses to explain the variations in the properties. Co-doping of TiO2 and Al2O3 to 3Y-TZP did not have an influence on the phases present, but decreased the grain size. The co-doping also affected the cell attachment and the growth on the surface of the zirconia ceramics.
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    NONDESTRUCTIVE CHARACTERIZATION OF BEARING STEELS BY MAGNETIC BARKHAUSEN NOISE TECHNIQUE
    ( 2017-06-02) Arslan, Ebru ; Davut, Kemal ; Şimşir, Caner
    One of the most popular type of bearing steels is 100Cr6. The strength, toughness, hardness, fatigue life, wear resistance of it can be improved by heat treatment applications which also changes the microstructure. For controlling the microstructure, traditional metallographic and XRD- based methods, that involve taking representative specimens from lots, are used. Magnetic Barkhausen noise (MBN) technique may provide an alternative nondestructive, fast and practical measurement method to those traditional techniques. This study aims at investigating the possibility of nondestructive characterization of microstructure of heat treated 100Cr6 bearing steels by using the MBN technique. For that purpose, 100Cr6 steel specimens were heat treated in a quenching dilatometer under different austenitization conditions and then quenched to room temperature and to -130°C, in order to generate variations in their microstructures. After heat treatment applications, microstructural properties of the specimens including the fraction and distribution of carbides, amount of retained austenite and also hardness were determined by metallographic and XRD analysis. Moreover, MBN measurements were performed and then the results were correlated to the microstructural parameters. MBN signals correlate with the microstructure variations in the 100Cr6 steel samples via simple linear relations; however, the prediction bands were quite wide and the MBN technique was not sufficiently sensitive, for direct characterization of hardness, retained austenite and carbide fraction of the specimens that were quenched to room temperature. In order to characterize these interdependent microstructural parameters via MBN technique, non-linear relations based on carbide dissolution kinetics are needed. On the other hand, the MBN measurement results of the sub-zero treated specimens showed that; elimination of retained austenite significantly improved the goodness of fit of on those linear relations. Although coefficient of determinations of both carbide fractions and hardness were acceptable, it can be improved by developing a newer non-linear model. Nevertheless, all of the results were promising for the future applications of MBN technique on nondestructive characterization of microstructure variations in 100Cr6 steels.