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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 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 Microstructural Characterization and Hardness Study of Nanostructured CoCrFeNi High Entropy Alloys with Dual Effect of Y and Nano-Sized Y2O3 Additions(Springer India, 2022) Tekin, Mustafa; Kotan, HasanThe motivation of this work is to increase the grain size stability of nanocrystalline CoCrFeNi high entropy alloys (HEA) with Y and Y2O3 additions to strike a balance between thermodynamic and kinetic stabilizations. The nanocrystalline HEAs prepared by mechanical alloying were annealed at different temperatures and characterized by X-ray diffraction, focused ion beam microscopy and micro-hardness test. The results revealed that as-milled nanostructured grain size yielded grain growth upon annealing reaching to 350 nm and 1.3 mu m after annealing at 900 degrees C and 1100 degrees C, respectively, while the addition of Y and Y2O3 appeared to stabilize the grain size in the nano-range after annealing at the same temperatures. Consequently, while the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to around 200 HV after annealing at 1100 degrees C, the elevated hardness of 430 HV was retained with Y and Y2O3 additions after annealing at the same temperature.Öğe Nano-kristal yapılı yüksek entropi alaşımlarının termal kararlılığının ve mekanik özelliklerinin iyileştirilmesi(Necmettin Erbakan Üniversitesi Fen Bilimleri Enstitüsü, 2022) Tekin, Mustafa; Kotan, HasanYapılan çalışmalarda, eş atomik ya da eş atomik orana yakın kompozisyonlarda en az 5 elementin bir araya gelerek oluşturduğu yeni bir alaşım türü olan Yüksek entropili alaşımlar (YEA) keşfedilmiştir. YEA’lar en az 5 elementin bir araya gelerek oluşturduğu, basit katı çözelti oluşturma yatkınlığı ile geliştirilmiş mikroyapısal ve mekanik özelliklere sahip malzemeler olarak tanımlanmışlardır. YEA’lar geleneksel malzemelere kıyasla özellikle yüksek sertlik/dayanım, üstün aşınma dayanımı, yüksek sıcaklık dayanımı, iyi korozyon direnci gibi geliştirilmiş mekanik özellikler sayesinde son dönemlerde önemli oranda araştırılmakta ve çalışılmaktadır. Bu alaşımların mekanik alaşımlama yöntemi ile üretilmesi sayesinde, mikroyapısal ve mekanik özellikleri daha da geliştirilebilmektedir. Mekanik alaşımlama ile elde edilen nanokristal yapılı malzemelerin sıcaklığa karşı tane büyümesi eğiliminin üstesinden gelebilmek amacıyla, YEA’lar da dahil olmak üzere, alaşım sistemlerine termal kararlılığı artırıcı element ilaveleri yapılmaktadır. Bu çalışma kapsamında, nanokristal yapılı eşatomik CoCrFeNi alaşımı mekanik alaşımlama yöntemi ile üretilerek, 500 ile 1100 °C arasında tavlanmıştır. Termal kararlılık sağlayıcı ilaveler olarak, İtriyum (Y) ve Zirkonyum (Zr) tozları ana alaşıma atomca %1 ve %4 olacak şekilde ve İtriyum Oksit (Y2O3) tozu da ağırlıkça %1 ve %4 olacak şekilde ilave edilmiştir. Tavlama sıcaklığı ve termal kararlılık sağlayıcıların oranının bir fonksiyonu olarak, mekanik alaşımlanmış ve tavlanmış YEA'ların mikro yapı araştırmaları için X-ışını kırınımı (XRD), odaklanmış iyon ışını mikroskobu (FIB) ve geçirimli elektron mikroskobu (TEM) kullanılmıştır. Sonuçlar, mekanik alaşımlanmış CoCrFeNi alaşımının, 1100 °C'de tavlamadan sonra bile yüzey merkezli kübik (ymk) kristal yapıya sahip olduğunu göstermiştir. Artan tavlama sıcaklıkları ile birlikte CoCrFeNi alaşımının tane büyümesine maruz kaldığı, dolayısı ile termal olarak kararlı bir yapıda olmadığı görülmüştür. Değişen oranlarda Y, Zr ve Y2O3 ilaveleri ile ana alaşımın termal kararlılığının farklı düzeylerde artırıldığı çalışma kapsamında belirlenmiştir. Ayrıca artan tavlama sıcaklığı ile ciddi oranda düşüş gösteren CoCrFeNi alaşımının sertliği, termal kararlılık sağlayıcı ilaveler ile yüksek sıcaklıklarda dahi tane büyümesi engellenerek belirli seviyelerde tutulmuştur. Sonuç olarak, bu çalışma kapsamında CoCrFeNi alaşımına Y, Zr ve Y2O3 ilaveleri ile elde edilen tane boyutu kararlılığının, nanokristal yapılı CoCrFeNi YEA tozlarının konsolidasyon sürecini basitleştirmesi ve özelliklerinde önemli bir azalma olmaksızın potansiyel yüksek sıcaklıklarda uygulamalarında kullanımına olanak sağlaması beklenmektedir.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 Wear behavior of in-situ oxide dispersion strengthened Fe-8Ni alloy with Zr additions(Chulalongkorn University, 2023) Tekin, Mustafa; Muhaffel, Faiz; Kotan, Hasan; Baydoğan, MuratIn this study, in-situ oxide dispersion strengthened (ODS) Fe91Ni8Zr1 and Fe88Ni8Zr4 alloys were produced by combination of high energy mechanical alloying (HEMA) and high temperature equal channel angular extrusion (HT-ECAE). The wear behaviors of the consolidated samples were investigated under different loads from 1 N to 4 N by reciprocating wear tests at room temperature. The Scanning electron microscopy (SEM) was used to examine the wear tracks to analyze the wear characteristics as a function of applied loads. The relative comparison of the wear results showed that under the lower loads of 1 N and 2 N, Fe88Ni8Zr4 alloy has lower wear rate than Fe91Ni8Zr1 alloy whereas under the higher loads of 3 N and 4 N, it is vice versa. Additionally, the friction coefficient of Fe91Ni8Zr1 alloy was found to be lower than that of Fe88Ni8Zr4 alloy under all the applied loads. The results were comparatively discussed with respect to microstructural features of 1 at% Zr and 4 at% Zr containing ODS alloys produced by HEMA followed by ECAE. The obtained results of ODS alloys with different grain size, precipitate size, and number density of the precipitates, may disclose a new sight for using such alloys in wear applications just as cutting tools, turbine blades, and discs.
