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    Effects of heat treatment temperatures on phase transformation, thermodynamical parameters, crystal microstructure, and electrical resistivity of NiTiV shape memory alloy
    (Springer, 2020) Kok, Mediha; Al-Jaf, Ahmed Omar Ali; Cirak, Z. Deniz; Qader, Ibrahim Nazem; Ozen, Ecem
    Changing physical property of NiTi shape memory alloy by adding new element and heat treatment technique has become interesting in the last decades. In this study, equiatomic NiTi alloy was doped by 1 and 5 atomic percentages of vanadium element. For each case, heat treatment was accomplished for 873, 973, 1073, and 1173 K, and its effect on phase transformation temperatures was analyzed by differential scanning calorimetry. Crystal structure and chemical compound found in XRD pattern showed the different situation for each case, e.g., TiO2 appeared in heat-treated samples, whereby the intensity of peaks varied by changing the temperature of heat treatment. In NiTi-5V alloy, each enthalpy, entropy, Gibbs free energy, and elastic energy were decreased by increasing heat treatment temperature, while grain size of the alloy extended with respect to non-heat-treated sample. Electrical resistivity of the alloys was measured as a function of temperature, which was increased by increasing vanadium composition and heat treatment temperature.
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    The effects of substituting Sn for Ni on the thermal and some other characteristics of NiTiSn shape memory alloys
    (Springer, 2021) Qader, Ibrahim Nazem; Kok, Mediha; Cirak, Zehra Deniz
    In this study, the thermal stability and phase transformation of Ni50-xTi50Snx (x = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 at%) SMAs have been investigated. Some thermal, mechanical, and microstructural characteristics were measured using DSC, Vickers microhardness, XRD, and SEM-EDX devices. Calculations, such as grain size and entropy change, were also performed based on the DSC results. It is found that by increasing the amount of Sn in NiTiSn alloy, fragmentation has occurred in the alloy, which may be due to the high concentration of Sn and Ti in the grooves. Furthermore, the microhardness and grain size are in harmony, i.e., the microhardness of the alloys was influenced by the grain size. Additionally, martensite and austenite peaks were shifted to the lower temperature by increasing the cycling process, while the rate of shifting increased generally by adding more Sn element into the alloy.
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    Examination of phase changes in the CuAl high-temperature shape memory alloy with the addition of a third element
    (Springer, 2018) Kok, Mediha; Ata, Sahin; Yakinci, Zehra Deniz; Aydogdu, Yildirim
    In the present study, a ternary CuAl-based alloy was produced by adding 2% chromium, niobium, titanium and hafnium instead of 2% copper from the Cu88Al12 (% in mass) shape memory alloy, and the phase changes in the alloy were examined. As a result of the X-ray analyses performed at room temperature, the alpha phase, which is rich in copper, was detected in the main sample, i.e., the Cu88Al12 alloy, and the beta (1)(1) and gamma (A +/-)(1) phases were detected in the four of the Cu86Al12Cr2, Cu86Al12Nb2, Cu86Al12Ti2 and Cu86Al12Hf2 alloys. All of phases were clearly seen in SEM images. As a result of the mapping performed during chemical analysis, it was observed clearly that there appeared a precipitation phase in the Cu86Al12Cr2, Cu86Al12Nb2, Cu86Al12Ti2 alloys due to the additions. It was also observed that the additions were effective in forming a martensite phase in the Cu88Al12 alloy. In differential scanning calorimetry (DSC) measurements, which were taken to support these measurements, no martensitic phase transformations were detected in dual primary alloy (Cu88Al12); however, a clear martensite phase transformation was detected in ternary alloys (Cu86Al12Cr2, Cu86Al12Nb2, Cu86Al12Ti2 and Cu86Al12Hf2) in the first DSC measurement. Then, when the DSC cycle was applied to the ternary alloy, both the austenite transformation and martensite transformation temperatures were clearly seen, and it was claimed that all the alloys showed high-temperature shape memory alloy properties.
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    IN VITRO STUDY, CORROSION, AND THERMAL OXIDATION BEHAVIOR OF WIRE-SHAPED NITISN SHAPE MEMORY ALLOY
    (World Scientific Publ Co Pte Ltd, 2021) Qader, Ibrahim Nazem; Kok, Mediha; Cirak, Zehra Deniz; Pekdemir, Mustafa Ersin; Saydam, Sinan; Aydogdu, Yildirim
    The development of NiTi-based shape memory alloys (SMAs) for some medical applications has been limited by releasing Ni element into the organic tissue. In this study, Ni47Ti50Sn3-wire (at.%) SMA was produced using the arc-melting technique, and the alloy was subjected to non-isothermal processes. Some characteristics of the alloys, such as oxidation behavior, corrosivity, and in vitro study in simulated body fluid and physiological solution at body temperature, were investigated. The corrosion rate of the as-casted alloy was lower than the equiatomic-NiTi alloy found in the literature. The estimated ICP-MS results showed that the release of alloying constituents into the solutions is in accordance with the standards.
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    Investigation of the thermal and microstructural changes of CuAlNiNb quaternary shape memory alloys by different niobium amount
    (Springer Heidelberg, 2018) Cirak, Z. Deniz; Kok, Mediha
    Among the CuAl-based alloys, the most useful one is the CuAlNi ternary shape memory alloy. There are studies on the development of these alloys. In this study, beta-Nb element was added to the CuAlNi alloy as a fourth element at various rates. The effect of the doped Nb element on the martensitic transformation temperature of the CuAlNi-shape memory alloy was determined by differential scanning calorimetry (DSC). When the value of transformation temperature in quaternary CuAlNi alloys increased, as a result, austenite start and martensite start temperatures are close to each other. Crystalline structure analysis with X-ray diffractometer revealed that beta 1' and gamma 1' phases were found in Cu83.5Ni3.5Al13 alloy, which is the main alloy, while crystal structure of beta-Nb phase was also found with Nb addition. Likewise, the same phases were determined by SEM-EDX analysis. In addition, the result of Vickers hardness measurements indicated that the hardness value of CuAlNi alloys increased by extent of contribution of Nb.
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    Thermal Properties, Microstructure and Microhardness of Cu-Al-Co Shape Memory Alloy System
    (Springer India, 2014) Aydogdu, Yildirim; Kurum, Ferdi; Kok, Mediha; Yakinci, Zehra Deniz; Aydogdu, Ayse
    In this study, the effects of Co content on the crystal structure, transformation temperatures and microstructure of Cu-Al-Co shape memory alloy system were investigated. It was found that Cu-Al-Co alloy system has also 18R type martensitic structure, which is commonly observed in copper-based systems. The transformation temperatures were found to be higher than 250 A degrees C and they do not show a linear increase or decrease with Co content. The microstructural examination revealed the presence of martensite phase and precipitates. It was realized that the size of precipitate increases with increasing Co content. It can be stated that the hardness increased with increasing Co content. All these results point out that increasing Co content increases the strength of Cu-Al-Co shape memory alloy system.
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    Valorization of bio-calcium carbonate based Chamelea gallina shell waste fillers in shape memory polymer composites
    (Wiley, 2024) Pekdemir, Sibel Selcuk; Onay, Hatice; Oner, Ecem Ozen; Pekdemir, Mustafa Ersin; Kok, Mediha; Ates, Burhan; Aydogdu, Yildirim
    In recent years, the focus has been on the use of calcium carbonate-based seashell wastes in the production of new thermoplastic and thermoset polymer materials, paving the way for their use as biofillers in polymeric composites. In this study, it is aimed to obtain a new polymeric composite material by doping Chamelea gallina shells, on polylactic acid (PLA)/polyethylene glycol (PEG) blend. Structural characterization of the obtained PLA/PEG blend/C. gallina composite films was performed with attenuated total reflection infrared spectroscopy (ATR-IR). When the thermal properties of composite materials were examined by thermogravimetric analysis (TGA), it was determined that the thermal stability of polymeric composites increased with the addition of C. gallina. SEM images showed that the polymer blend films, which appeared to have a porous structure, filled the pores with increasing C. gallina ratio. It was observed that the biodegradability of PLA/PEG blend composite films decreased with increasing C. gallina shells addition. However, C. gallina had a positive effect on the swelling and water absorption capacities of polymeric composites. The increase in tensile strength and elongation at break values of PLA/PEG blend/C. gallina composite films with increasing C. gallina means that the mechanical properties of the polymer are improved.