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Öğe Design of Novel Non-equiatomic Cu-Ni-Al-Ti Composite Medium-Entropy Alloys(Springer, 2020) Polat, Gokhan; Erdal, Ziya Anil; Kalay, Yunus ErenThere has been great attention on high-entropy alloys (HEAs) over the past decade. Unlike conventional alloy systems, HEAs commonly include at least five principal elements with equiatomic or near-equiatomic ratio. HEAs with their superior mechanical, magnetic, and thermal properties are promising materials for critical engineering applications. Medium-entropy alloys (MEAs), which consist of less than five principal elements, have very similar structural features with HEAs such as robust thermodynamic stability and exceptional mechanical performance. The insights of MEAs have not been fully revealed yet. In the present study, novel MEAs (Cu20Ni20Al30Ti30, Cu25Ni25Al25Ti25, Cu34Ni22Al22Ti22, and Cu35Ni25Al20Ti20) have been designed using thermo-physical calculations and Thermo-Calc software. These MEAs were then produced using copper heart arc melting and suction cast into cylindrical rods with 3 mm diameters. X-ray diffraction (XRD), optical microscope (OM), transmission electron microscope (TEM), scanning electron microscope (SEM), and energy-dispersive spectroscopy (EDS) were used for structural characterization. The corresponding results reveal that the Cu20Ni20Al30Ti30, MEA, consists of a body-centered cubic (BCC-B2) phase with intermetallic compounds (ICs), whereas Cu25Ni25Al25Ti25 has single BCC-B2 phase. When the amounts Cu and Ni are increased, system drives itself toward a face-centered cubic (FCC) structure. A dual BCC and FCC composite Cu35Ni25Al20Ti20 has been detected as the most promising MEA among the others with 820 and 1338 MPa measured yield and compressive strength, respectively.Öğe An economical electrocoagulation process of a hazardous anionic azo dye wastewater with the combination of recycled electrodes and solar energy(Springer Heidelberg, 2023) Akkaya, Gulizar Kurtoglu; Polat, Gokhan; Nalcaci, Gamze; Eker, Yasin RamazanThe energy and electrode costs are the restrictions of applying electrocoagulation (EC) in wastewater treatment and many attempts have been made to decrease these costs. In this study, an economical EC was investigated to treat a hazardous anionic azo dye wastewater (DW) that threatens the environment and human health. Firstly, an electrode for EC process was produced from recycled aluminum cans (RACs) by remelting in an induction melting furnace. The performance of the RAC electrodes in the EC was evaluated for COD, color removal, and the EC operating parameters such as initial pH, current density (CD), and electrolysis time. Response surface methodology which is based on central composite design (RSM-CCD) was used for the optimization of the process parameters which were found to be pH 3.96, CD 15 mA/cm(2), and electrolysis time 45 min. The maximum COD and color removal values were determined as 98.87% and 99.07%, respectively. The characterization of electrodes and the EC sludge was conducted by XRD, SEM, and EDS analyses for the optimum variables. In addition, the corrosion test was conducted to determine the theoretical lifetime of the electrodes. The results showed that the RAC electrodes show an extended lifetime as compared to their counterparts. Secondly, the energy cost required to treat DW in the EC was aimed to decrease by using solar panels (PV), and the optimum number of PV for the EC was determined by the MATLAB/Simulink. Consequently, the EC with low treatment cost was proposed for the treatment of DW. An economical and efficient EC process for waste management and energy policies was investigated in the present study which will be instrumental in the emergence of new understandings.Öğe Effect of Composition, Mechanical Alloying Temperature and Cooling Rate on Martensitic Transformation and Its Reversion in Mechanically Alloyed Stainless Steels(Korean Inst Metals Materials, 2021) Polat, Gokhan; Kotan, HasanStainless steels with Fe/Cr/Ni ratios of 74/18/8, 71/17/12, and 55/20/25 were produced from elemental powders by high energy mechanical alloying at both room and cryogenic temperatures. The effect of mechanical alloying temperature on martensitic transformation, the reversion of deformation-induced martensite-to-austenite upon annealing, and the influence of cooling rate on the thermal stability of reversed austenite upon cooling to room temperature were investigated in detail by in-situ and ex-situ X-ray diffraction (XRD) experiments, transmission electron microscopy (TEM) and Thermo-Calc simulations. A relative comparison of stainless steels after room temperature mechanical alloying indicated that the low nickel-containing steel underwent an almost complete martensitic transformation. However, martensitic transformation by deformation through mechanical alloying at room temperature would not be possible with increasing nickel contents but was created partially at cryogenic temperature, the degree of which depended on the steel composition. The in-situ XRD studies exhibited that the deformation-induced martensite completely transformed to austenite at elevated temperatures. The complete reverse transformation temperature simulated by Thermo-Calc software was found to be lower than that of the experimentally determined ones. Additionally, the different cooling rates from the reversed austenite demonstrated that the slower cooling increased the thermal stability of reversed austenite at room temperature. GraphicÖğe Effect of Hf additions on phase transformation, microstructural stability, and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying(Elsevier, 2021) Kotan, Hasan; Polat, Gokhan; Yildiz, A. Busra304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 degrees C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase trans -formation completed after 105 min annealing at 900 degrees C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was-10 nm after MA and remained-167 and-293 nm after annealing at 900 and 1100 degrees C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 degrees C. The hardness of 304L increased from-200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 degrees C with Hf addition as opposed to 195 HV hardness of 304L. (c) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.Öğe Effect of milling time, MWCNT content, and annealing temperature on microstructure and hardness of Fe/MWCNT nanocomposites synthesized by high-energy ball milling(Elsevier, 2021) Polat, Gokhan; Canbolat, I. Emre; Uzunoglu, Aytekin; Kotan, HasanNanocrystalline pure Fe and Fe/MWCNT nanocomposites powders with 0.25, 0.5, 1, and 10 wt% MWCNT contents were synthesized by high-energy ball milling (HEBM). The as-milled powders were coldcompacted and annealed at 400 degrees C and 600 degrees C for 1 h in Ar atmosphere. The effect of ball milling on pristine MWCNT and Fe/MWCNT composite powders was also investigated as a function of milling time up to 20 h. The physical properties of MWCNT were imaged by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after HEBM. The structural damage of MWCNT as a function of milling time and MWCNT content was studied using Raman spectroscopy. The structural characterization of MWCNT and Fe/MWCNT composites was conducted by X-ray diffraction (XRD) as a function of milling time, MWCNT content, and annealing temperature. The chemical properties of the synthesized composite powders were investigated using X-ray photoelectron spectroscopy (XPS). The microhardness test was performed to assess the effect of milling time, annealing temperature, and MWCNT content on the mechanical properties. The results indicated that after the ball milling process, the structure of MWCNT was destroyed, and the formation of the amorphous carbon phase was observed, which was confirmed by XRD and TEM analyses. In addition, decreased defect and carbon intensity ratios (ID/IG) were calculated from the Raman results with longer ball milling processes, which is attributed to the destruction of carbon bonds. The XPS results confirmed the presence of FeAC bonds as a result of the formation of carbide phases. A fine dispersion of precipitated carbides determined by TEM is found to promote the grain size stability below 100 nm in the nanocrystalline Fe matrix. The results from the micro-hardness tests showed that Orowan particle strengthening resulting from the carbide formation, as well as grain size hardening, is an important contributor to strengthening in Fe/MWCNT composites. (C) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.Öğe Effect of Y addition on the structural transformation and thermal stability of Ti-22Al-25Nb alloy produced by mechanical alloying(Walter De Gruyter Gmbh, 2021) Cetin, Mehmei Emin; Polat, Gokhan; Tekin, Mustafa; Batibay, Ahmet Burcin; Kotan, MasanIn this study, a Ti-22Al-25Nb alloy with nanocrystalline structure was produced by high energy mechanical alloying (HEMA) and 1 at.-% yttrium was added as a thermal stabilizer. The as-milled samples were annealed at various temperatures up to 900 degrees C in a protective gas atmosphere, and the samples were allowed to cool to room temperature in the furnace. The phase transformations and microstructural changes as a function of the annealing temperatures and alloy compositions were studied using room- and high-temperature X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning electron microscopy (SEM). The mechanical properties of the samples were interpreted based on the hardness results and their correlation with the microstructures. The results showed that the as-milled nanocrystalline structure of Ti-22Al-25Nb alloy increased from 3.4 nm to 350 nm after annealing at 800 degrees C due to the high driving force induced by the large grain boundary area. Consequently, the as-milled hardness of the Ti-22Al-25Nb alloy dropped from 7.63 +/- 0.18 GPa to 5.37 +/- 0.28 GPa. The grain size stability of the Ti-22Al-25Nb alloy after annealing at elevated temperature was ensured through the addition of yttrium. Thus, the grain size remained at the level of 125 nm, and the hardness value was maintained at around 6.98 +/- 0.43 GPa after annealing at 800 degrees C.Öğe Effect of Y addition on the structural transformation and thermal stability of Ti-22Al-25Nb alloy produced by mechanical alloying(Walter De Gruyter Gmbh, 2021) Cetin, Mehmei Emin; Polat, Gokhan; Tekin, Mustafa; Batibay, Ahmet Burcin; Kotan, MasanIn this study, a Ti-22Al-25Nb alloy with nanocrystalline structure was produced by high energy mechanical alloying (HEMA) and 1 at.-% yttrium was added as a thermal stabilizer. The as-milled samples were annealed at various temperatures up to 900 degrees C in a protective gas atmosphere, and the samples were allowed to cool to room temperature in the furnace. The phase transformations and microstructural changes as a function of the annealing temperatures and alloy compositions were studied using room- and high-temperature X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning electron microscopy (SEM). The mechanical properties of the samples were interpreted based on the hardness results and their correlation with the microstructures. The results showed that the as-milled nanocrystalline structure of Ti-22Al-25Nb alloy increased from 3.4 nm to 350 nm after annealing at 800 degrees C due to the high driving force induced by the large grain boundary area. Consequently, the as-milled hardness of the Ti-22Al-25Nb alloy dropped from 7.63 +/- 0.18 GPa to 5.37 +/- 0.28 GPa. The grain size stability of the Ti-22Al-25Nb alloy after annealing at elevated temperature was ensured through the addition of yttrium. Thus, the grain size remained at the level of 125 nm, and the hardness value was maintained at around 6.98 +/- 0.43 GPa after annealing at 800 degrees C.Öğe Grain size stabilization of oxide dispersion strengthened CoCrFeNi-Y2O3 high entropy alloys synthesized by mechanical alloying(Elsevier Science Sa, 2021) Tekin, Mustafa; Polat, Gokhan; Kalay, Y. Eren; Kotan, HasanNanocrystalline CoCrFeNi high entropy alloys (HEAs) with 1 and 4 wt% nanosized Y2O3 were synthesized by high energy mechanical alloying and subjected to annealing treatments at different temperatures up to 1100 degrees C. X-ray diffraction (XRD), focused ion beam microscopy (FIB), and transmission electron microscopy (TEM) were used to investigate the microstructures of as-milled and annealed HEAs as a function of annealing temperature and Y2O3 content. The results have shown that the as-milled HEAs were solid solutions with face-centered cubic (fcc) crystal structure, which remained stable even after annealing at 1100 degrees C. The as-milled nanocrystalline CoCrFeNi HEA revealed grain growth upon annealing, reaching 293 nm and 1.45 mu m after annealing at 900 and 1100 degrees C, respectively. This suggests that the nanocrystalline microstructure of CoCrFeNi is not thermally stable at high temperatures. The grain size stability was found to reach around 72 nm with nanosized Y2O3 particles after annealing at 1100 degrees C. Accordingly, 477 +/- 20 HV asmilled hardness of CoCrFeNi was dramatically reduced to 220 +/- 14 HV after annealing at 1100 degrees C due to severe grain coarsening but retained around 450 +/- 23 HV with 4 wt% Y2O3 addition. The correlation between microstructure and hardness was utilized to evaluate the mechanical properties. (c) 2021 Elsevier B.V. All rights reserved.Öğe Highly stable silver-platinum core-shell nanowires for H2O2 detection(Royal Soc Chemistry, 2021) Koylan, Serkan; Tunca, Sensu; Polat, Gokhan; Durukan, Mete Batuhan; Kim, Dongkwan; Kalay, Yunus Eren; Ko, Seung HwanSilver nanowire (Ag NW) networks have great potential to replace commercial transparent conducting oxides due to their superior properties in conjunction with their competitive cost, availability and mechanical flexibility. However, there are still challenges to overcome for the large scale utilization of Ag NWs in devices due to oxidation/sulfidation of NWs, which leads to performance loss. Here, we develop a solution-based strategy to deposit a thin platinum (Pt) shell layer (15 nm) onto Ag NWs to improve their chemical, environmental and electrochemical stabilities. Environmental and thermal stabilities of the core-shell NW networks were monitored under different relative humidity conditions (RH of 43, 75 and 85%) and temperature settings (75 degrees C for 120 hours and 150 degrees C for 40 hours) and compared to those of bare Ag NWs. Afterwards, stability of core-shell NW networks in hydrogen peroxide was investigated and compared to that of bare Ag NW networks. The potential window for electrochemical stability of the Ag NW networks was broadened to 0-1 V (vs. Ag/AgCl) upon Pt deposition, while bare Ag NWs were stable only in the 0-0.6 V range. Moreover, Ag-Pt core-shell NWs were used for the detection of hydrogen peroxide, where a high sensitivity of 0.04 mu A mu M-1 over a wide linear range of concentrations (16.6-990.1 mu M) with a low detection limit (10.95 mu M) was obtained for the fabricated sensors. All in all, this highly effective and simple strategy to improve the stability of Ag NWs will certainly open new avenues for their large-scale utilization in various electrochemical and sensing devices.Öğe An investigation of abnormal grain growth in Zr doped CoCrFeNi HEAs through in-situ formed oxide phases(Elsevier Sci Ltd, 2022) Tekin, Mustafa; Polat, Gokhan; Kotan, HasanAbnormal grain growth (AGG) in nanocrystalline (CoCrFeNi)(100-x)Zr-x (x = 1 and 4 at. %) HEAs, prepared through high energy mechanical alloying, was comprehensively investigated upon annealing. Transmission electron microscopy (TEM), including high angle annular dark field imaging (HAADF) and energy dispersive spectroscopy (EDS) mapping, focused ion beam microscopy (FIB), and X-ray diffraction experiments (XRD) were utilized to investigate the microstructures as a function of added Zr content and temperature exposures. The results showed that nanocrystalline grains of the as-milled HEAs did not increase significantly upon annealing up to 700 degrees C as the nanocrystalline grain sizes were retained. However, grain growth was observed in (CoCrFeNi)(99)Zr-1 after annealing at 900 degrees C, which turned into AGG after annealing at a higher temperature of 1100 degrees C, disrupting the equiaxed grain structures observed at 900 degrees C. Although the increased amount of Zr doping reduced the average grain size in (CoCrFeNi)(96)Zr-4, bimodal grain structure existed in the microstructure composed of a matrix with 255 nm grain size and abnormally grown grains up to 3 mu m. The observed AGG was attributed to the pinning effect of in-situ formed secondary oxide phases. The microstructural evolution as a function of Zr doping and annealing temperatures was also correlated with the microhardness test results. The AGG and bimodal grain structure reported for the Zr doped CoCrFeNi HEA may open a new avenue to produce HEAs with the enhanced strength-ductility combination due to the incorporation of larger grains and in-situ formed oxide phases in a fine-grained matrix.Öğe Role of yttrium addition and annealing temperature on thermal stability and hardness of nanocrystalline CoCrFeNi high entropy alloy(Elsevier Sci Ltd, 2022) Polat, Gokhan; Tekin, Mustafa; Kotan, HasanCoCrFeNi high entropy alloys (HEAs) with yttrium (Y) additions (1 and 4 at. %) were nanostructured by mechanical alloying process and annealed at various temperatures between 500 degrees C and 1100 degrees C. The structure, grain growth, and hardness were studied as a function of solute addition and annealing temperature using X-ray diffraction (XRD), focused ion beam (FIB), and scanning transmission electron microscope (S/TEM) techniques, and hardness test. The thermo-physical calculations were utilized to discuss the phase evolution after mechanical alloying and annealing with respect to added solutes. The results showed that Y additions did not affect the main crystal structure of the base CoCrFeNi HEA as the solid solution with a single face-centered cubic (fcc) crystal structure was maintained even after 1 h annealing at 1100 degrees C. The as-milled nanocrystalline grain size of CoCrFeNi HEA yielded extensive grain growth with the temperature exposures reaching 291 nm and 1.4 mu m after annealing at 900 degrees C and 1100 degrees C, respectively. However, Y additions retarded the grain growth and decreased the average grain size upon annealing as compared to the base HEA. That is, 1 and 4 at. % Y additions stabilized the grain size around 88 nm and 95 nm (both determined by TEM) after annealing at 900 degrees C and 1100 degrees C, respectively. Accordingly, the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to 220 HV after annealing at 1100 degrees C, while the reduction in hardness was relatively gradual with Y additions and retained around 435 HV with 4 at. % Y addition even after annealing at 1100 degrees C. Such thermal stability may facilitate the use of HEAs at high temperatures and enable the consolidation routes of powders into dense nanocrystalline compact HEAs.Öğe Understanding microstructural evolution and hardness of nanostructured Fe89.5Ni8Zr2.5 alloy produced by mechanical alloying and pressureless sintering(Elsevier - Division Reed Elsevier India Pvt Ltd, 2020) Polat, Gokhan; Batibay, Ahmet B.; Kotan, HasanFe89.5Ni8Zr2.5 alloy was synthesized by mechanical alloying followed by pressureless sintering at various temperatures up to 900 degrees C. Microstructural evolution as a function of processing temperature was characterized using focused ion beam microscopy, transmission electron microscopy and X-ray diffraction techniques. The dependence of hardness on the microstructure was utilized to study the mechanical changes. The experimental results showed that microstructural stability can be enhanced by segregation of solutes to grain boundaries at low temperatures and by precipitation of second phases at elevated temperatures. Eventually, at higher processing temperatures the stability was lost due to the coarsening of the precipitated second phases leaving behind ultra-fined grained microstructure. Despite the coarsening of the grain size with increasing processing temperatures, the in-situ formed second phases were found to induce an Orowan strengthening effect leading to approximately 5.5 GPa hardness after 1 h sintering at 900 degrees C. (c) 2020 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).