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Öğe An Overview of the Microstructural, Physical, Mechanical, and Tribological Performance of Beta-Type Titanium Alloys for Total Hip Replacement(Springer International Publishing Ag, 2025) Dahmani, Marwa; Hezil, Naouel; Fellah, Mamoun; Benoudia, Mohamed-Cherif; Kotan, Hasan; Larios, Alejandro Perez; El-Hiti, Gamal A.The design of total hip replacements is subject to ongoing research and development in the structures and materials used. This study provides an overview of the main properties (structural, physical, mechanical, and tribological) of new beta-type titanium alloys designed for total hip prosthesis. The literature review demonstrated that alloying elements addition and the production process significantly impact microstructural changes in Ti alloys, as well as the microstructure adjustments in beta-type Ti alloys could achieve a low elastic modulus, high strength, improved micro-hardness and overall physical characteristics. Furthermore, the tribological behavior of beta-type Ti-alloys is critical for biomedical applications, as these materials must exhibit low wear rates and high wear resistance to prevent wear debris and metallic ion release. Previous studies demonstrated superior wear resistance in the studied alloys, and the strong relationship between the wear resistance and microstructure characteristics. Collectively, these findings suggest that beta-type titanium alloys possess significant potential to address mechanical biocompatibility and wear-related challenges in biomedical applications.Öğe Effect of in-situ formed oxide and carbide phases on microstructure and corrosion behavior of Zr/Y doped CoCrFeNi high entropy alloys prepared by mechanical alloying and spark plasma sintering(Elsevier Sci Ltd, 2023) Kotan, Hasan; Tekin, Mustafa; Bayatli, Aleyna; Bayrak, Kuebra Gurcan; Kocabas, Mustafa; Ayas, ErhanThe present work has examined the microstructural evolution, thermal stability, hardness, and corrosion behavior of Zr/Y doped CoCrFeNi HEAs prepared through high-energy mechanical alloying followed by spark plasma sintering (SPS) at 1100 degrees C. The achieved microstructures were investigated by XRD and TEM techniques. The results showed that investigated HEAs consist of an fcc solid solution of CoCrFeNi matrix with in-situ formed Cr-C carbides and Cr/Zr/Y based oxide phases. The SPS processing of CoCrFeNi yielded grain growth to 370 & PLUSMN; 60 nm, while 240 & PLUSMN; 160 nm grain size with bimodal grain size distribution and 165 & PLUSMN; 38 nm grain size were achieved with Zr and Y additions, respectively. The effects of microstructural changes on the hardness and corrosion behaviors of HEAs were also investigated. Compared with 372 & PLUSMN; 15 HV hardness of CoCrFeNi HEA, 445 & PLUSMN; 26 HV and 563 & PLUSMN; 58 HV hardness values were determined with Zr and Y doped HEAs, respectively. The increase in hardness is mainly ascribed to the precipitation strengthening of carbide and oxide phases as well as smaller grain sizes. The corrosion analysis showed that, although the achieved smaller grain sizes and the presence of different oxide types when dopped with Y and Zr impaired the corrosion resistance, the investigated HEAs have reasonable resistance to corrosion when compared to SS304 stainless steel.Öğe Geleneksel sinterleme ve spark plazma sinterleme yöntemlerinin nanokristal yapılı CoCrFeNi yüksek entropili alaşımın mikroyapısal özellikleri ve sertliği üzerine etkilerinin araştırılması(2024) Baloğlu, Ali Rıza; Tekin, Mustafa; Kotan, HasanCoCrFeNi YEA’ları mekanik alaşımlama yöntemiyle üretilerek geleneksel sinterleme ve spark plazma sinterleme yöntemleriyle konsolide edilmiştir. Sinterleme yöntemi ve sıcaklıklarının bir fonksiyonu olarak mikroyapıların incelenmesi için x-ışınları difraksiyonu (XRD), odaklanmış iyon demeti mikroskobu (FIB) ve geçirimli elektron mikroskobu (TEM) yöntemleri kullanılmıştır. Elde edilen sonuçlar, alaşımlanmış yapıların tek fazlı yüzey merkezli kübik (ymk) kristal yapıya sahip olduğunu göstermiştir. Bununla birlikte, aynı sıcaklıklarda uygulanan spark plazma sinterleme sonrasında alaşımların mikroyapılarında ymk kristal yapıya sahip matris fazına ilave olarak Cr7C3 fazının da oluştuğu tespit edilmiştir. Mekanik alaşımlanmış yapıların tane boyutu 10 nm civarında iken, 1000 ve 1100 °C’deki geleneksel sinterleme sonrasında tane boyutu sırasıyla 450 nm ve 1,5 µm değerlerine ulaşmış, bu da nanokristal yapılı CoCrFeNi alaşımının geleneksel sinterleme ile termal kararlığını koruyamadığını göstermektedir. Mekanik alaşımlanmış tozların spark plazma sinterleme ile 1100 °C’de konsolidasyonu sonrası YEA’nın tane boyutu yaklaşık 355 nm olarak elde edilmiş olup bu değer aynı sıcaklıkta geleneksel sinterleme ile elde edilmiş alaşımın tane boyutundan daha küçüktür. Buna göre, CoCrFeNi YEA’sının mekanik alaşımlama sonrası 4,6 GPa olarak tespit edilen sertliği, 1100 °C’deki geleneksel sinterleme sonrasında görülen tane büyümesi nedeniyle 2,1 GPa’ya düşmüş, ancak 1100 °C’de spark plazma sinterleme ile konsolidasyon sonucunda sertlik değeri 3,6 GPa olarak korunmuştur.Öğe Hybrid effect of Sm and B4C on grain size stability and hardness of nanocrystalline CoCrFeNi high entropy alloy after isothermal annealing(Elsevier Science Sa, 2025) Tasdemir, Esma; Koc, Recep C.; Batibay, Ahmet B.; Kotan, HasanIn this study, a hybrid effect of Sm and B4C additions is explored to enhance the thermal stability of mechanically alloyed nanocrystalline CoCrFeNi HEA after exposures to long-term annealing at 700 degrees C and 900 degrees C. The structural and microstructural investigations were carried out using X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (EDS). The results revealed that 7 nm grain size of as-milled CoCrFeNi HEA coarsened to 247.38 f 69 nm after 1 h of annealing at 900 degrees C. However, the hybrid addition of Sm and B4C significantly retarded grain growth, reducing the average grain size to 98.74 f 47 nm after 24 h of annealing at the same temperature. The resistance to grain growth is attributed to Sm and B4C additions and the formation of Sm-based oxide phases during annealing. The influence of microstructures on hardness was utilized to examine the mechanical changes as a function of annealing temperature and annealing time. Accordingly, the hardness of CoCrFeNi, measured as 517.8 f 25 HV after mechanical alloying, decreased significantly to 321 f 10 HV and 277 f 13 HV after 1 and 24 h of annealing at 900 degrees C, respectively. In contrast, the hybrid HEA maintained a hardness of 439.5 f 10 HV even after 24 h of annealing at 900 degrees C. Compared to the individual additions of Sm and B4C, the hybrid addition also proved more effective in preserving hardness at elevated temperatures over time. Overall, exploring the hybrid effect of alloying and second-phase addition provides a promising strategy for improving the grain size stability and the mechanical properties of nanocrystalline CoCrFeNi HEAs.Öğe Indentation creep behavior of Fe-8Ni-xZr oxide dispersion strengthened alloys(Walter De Gruyter Gmbh, 2023) Tekin, Mustafa; Muhaffel, Faiz; Kotan, Hasan; Baydogan, MuratThis study was conducted to understand the creep behavior of two oxide dispersion strengthened alloys containing Zr as the alloying addition by performing indentation creep tests at room temperature. The oxide dispersion strengthened alloys were Fe-8Ni-xZr (x = 1 and 4 at.%, i.e., Zr-1 and Zr-4 alloys, respectively), which had been previously fabricated by mechanical alloying; followed by consolidation via equal channel angular extrusion at 1000?. The indentation tests were conducted under a maximum load of 100 mN with the loading rates at 300 and 400 mN min(-1). The hardness was calculated by the Oliver-Pharr method, and the creep properties, such as the creep displacement, creep strain rate, creep stress, and stress exponent n, were determined. The results showed that the Zr-4 alloy was harder than the Zr-1 alloy. However, the creep resistance of the Zr-1 alloy was better than that of the Zr-4 alloy. It was further demonstrated that both the hardness and creep resistance depended on the loading rate. Moreover, a possible creep mechanism was proposed. Although the tests were performed at room temperature, they can provide insight into the effect of an oxide dispersion strengthened alloys microstructure on creep at higher temperatures.Öğe Influence of Isochronal and Isothermal Annealing on Microstructure and Hardness of Nanocrystalline CoCrFeNi-B4C High Entropy Alloy Composites(Springer, 2025) Koc, Recep C.; Polat, Gokhan; Batibay, Ahmet B.; Kotan, HasanAdding nano-sized reinforcements to nanocrystalline alloys is an effective strategy for achieving stable microstructures with enhanced mechanical properties. This study investigates the effect of nano-sized B4C particles on the microstructural stability and mechanical properties of nanocrystalline CoCrFeNi high-entropy alloys (HEAs) synthesized by high-energy mechanical alloying. Comprehensive characterizations, including x-ray diffraction (XRD), focused ion beam (FIB) microscopy, and transmission electron microscopy (TEM) revealed that B4C addition improved microstructural stability resulting in an average grain size of 225 nm after annealing at 900 degrees C for 1 h. The in-situ formation of secondary phases, such as Cr7C3 and Cr2O3, during annealing process further contributed to the retardation of grain growth in CoCrFeNi HEA, improving the overall thermal stability. The mechanical properties, particularly hardness, were enhanced by the addition of B4C following mechanical alloying and annealing at 900 degrees C for up to 6 h. Beyond this duration, the hardness values plateaued, exhibiting minimal variations with B4C additions. Specifically, the as-milled hardness increased from 518 HV to 573 HV with incorporating 4 wt.% B4C. After annealing for longer than 6 h at 900 degrees C, the B4C containing HEA showed a hardness of 309 HV, compared to 278 HV for the HEA without B4C. Despite some reduction after prolonged annealing at elevated temperatures, this enhancement shows the alloy's potential to maintain mechanical performance under such conditions, while further optimization is needed to ensure durability over extended temperature exposures.Öğe Inkjet-printed flexible electrochemical sensors based on palladium and silver-decorated, N-doped holey graphene and nano graphene(Elsevier, 2025) Ceylan, Ebru; Gurbuz, Havva Nur; Kotan, Hasan; Uzunoglu, AytekinInkjet printing is a low-cost method to deposit conductive layers with high precision for electrochemical applications. This work reports the development of flexible electrochemical sensors by inkjet printing of palladium and silver (PdAg)-decorated nitrogen-doped holey graphene and nanographene. Our novel ink formulations were printed on flexible polyethylene tetraflate (PET) substrates to fabricate robust and high-performance flexible electrochemical sensors. The physicochemical properties of the inks, including stability, surface tension, viscosity, and printability, were determined. In addition, the printed electrodes' homogeneity, resistance, and bending resilience were assessed. The electrochemical performance of the electrodes was evaluated against hydrogen peroxide (H2O2) and glucose (Glc). The defective graphene structures had a paramount influence on the ink stability and the ultimate electrochemical performance of the flexible sensors. The sensors showed a low detection limit (LOD) of 3.5 and 0.41 mu M against H2O2 and glucose, respectively, with linear ranges of 0.1-5.6 mM H2O2 and 1-19 mM glucose. Furthermore, the real sample analysis results indicated the applicability of the sensors in real samples.Öğe Investigation of the effects of conventional sintering and spark plasma sintering methods on the microstructural properties and hardness of nanostructured CoCrFeNi high entropy alloy(Gazi Univ, Fac Engineering Architecture, 2024) Baloglu, Ali Riza; Tekin, Mustafa; Kotan, HasanGraphical/Tabular CoCrFeNi HEAs were synthesized by mechanical alloying and consolidated via conventional sintering and spark plasma sintering, and microstructural properties and hardness were investigated as a function of sintering type and temperature. The result are shown in Figure A. The findings showed that the as -milled single-phase face centered cubic (fcc) crystal structure retained after conventional sintering at 1000 and 1100 degrees C whereas spark plasma sintering yielded additional Cr-rich carbide (Cr 7 C 3 ) phases at the same temperatures. Figure A. XRD and hardness results of the HEAs correlated with the microstructures Purpose: To investigate the effect of sintering type and temperature on the microstructural properties and hardness by using X-ray diffraction (XRD), focused ion beam microscopy (FIB), transmission electron microscopy (TEM), and microhardness test. Theory and Methods: High energy mechanical alloying was used to synthesize the equiatomic CoCrFeNi HEAs in nanocrystalline structure by SPEX 8000D shaker mill. The as -milled powders were consolidated by conventional sintering and spark plasma sintering methods. Results: The as -milled grain size of 10 nm increased to 450 nm and 1.5 mu m after conventional sintering at 1000 and 1100 degrees C, respectively, which shows that nanocrystalline CoCrFeNi alloy does not remain thermally stable after long temperature exposures at elevated temperatures. After consolidation of as -milled powders by SPS at 1100 degrees C, the grain size of the HEA was retained around 353 nm. This striking thermal stability of HEA is correlated with high heating rates and lower sintering duration by SPS, and the Zener pinning of the boundaries by nano -sized Cr-rich carbide phases. Accordingly, the as -milled hardness of the CoCrFeNi HEA reduced from 4.6 GPa to 2.1 GPa after conventional sintering at 1100 degrees C due to the significant grain growth, while the enhance hardness of 3.6 GPa was maintained after consolidation with SPS at 1100 degrees C. Conclusion: The findings suggest that as -milled CoCrFeNi HEA is not thermally stable particularly at high homologous processing temperatures. Consolidation with spark plasma sintering technique at 1100 degrees C provided higher density and higher thermal stability with a retarded grain growth of around 353 nm.Öğe Microstructural Evolution and Mechanical Properties of Y Added CoCrFeNi High-entropy Alloys Produced by Arc- melting(2024) Polat, Gökhan; Kotan, HasanThe CoCrFeNi high entropy alloy (HEA) with face-centered cubic (FCC) crystal struc- ture exhibits excellent ductility values even at cryogenic temperatures. However, since this HEA is relatively weak in strength, it may not meet the requirements of industrial applications in terms of strength-ductility trade-off. Therefore, the systematic addition of yttrium (Y) into CoCrFeNi HEA was investigated in the present study to increase the strength by solid solution and second phase strengthening. The HEAs were produced by vacuum arc melting, suction casting, and subsequent homogenization at 1150 °C for 24 h. The structural development of the HEAs was investigated by using the X-ray diffraction (XRD) technique which revealed the formation of a solid solution phase and CaCu5-type hexagonal structure (HS) second phase. The corresponding microstructure of the HEAs was examined under a scanning electron microscope (SEM) revealing the transformation of the microstructure from elongated grains to nearly equiaxed grains with the increase of Y content from 2 at. % to 4 at. %. The mechanical properties of the HEAs were investigated by using hardness and compression tests. The results exhibited a dramatic increase in the hardness from 143 (±2) HV to 335 (±7) HV and in the yield strength from 130 MPa to 1025 MPa with 4 at. % Y addition. Our study has revealed that the addition of rare earth Y element results in further development in the strength of the CoCrFeNi for potential engineering applications.Öğe Microstructural evolution, nanoindentation creep response, and wear properties of Y2O3-modified CoCrFeNi high entropy alloys(Elsevier Science Inc, 2026) Tekin, Mustafa; Kotan, Hasan; Balci, Erdem; Kaba, Mertcan; Baydogan, Murat; Bayrak, Kubra Gurcan; Ayas, ErhanThe combined effects of wear and creep largely determine the long-term reliability of alloys in demanding thermal and mechanical environments, but conventional structural materials show limited resistance to these degradation mechanisms. High-entropy alloys (HEAs), though inherently robust, have gained attention as potential candidates for such environments, particularly when reinforced with stable oxide dispersions. In this study, oxide-dispersion-strengthened Co-Cr-Fe-Ni HEAs containing 1 and 4 wt% Y2O3 were synthesized through mechanical alloying and spark plasma sintering to evaluate this approach. Microstructural characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the retention of the fcc crystal lattice. Pronounced grain refinement was achieved, decreasing from 360 +/- 70 nm in the unreinforced HEA to 95 +/- 15 nm in the 4 wt% ODS composition, accompanied by a substantial increase in hardness to 685 +/- 30 HV. Wear experiments revealed a fourfold reduction in specific wear rate. This improvement was accompanied by a transition in wear mode from extensive surface damage in the unreinforced HEA to predominantly oxidative and fatigue-assisted mechanisms in the ODS HEAs, facilitated by the formation of protective tribo-oxide layers. Nanoindentation creep analysis revealed a decrease in stress exponent from 16.05 to 5.72 with increasing Y2O3 content. This change signifies a transition toward dislocation-controlled creep and tunable creep resistance. Collectively, these findings establish that rare-earth oxide dispersion is an effective strategy for simultaneously enhancing surface durability and controlling time-dependent deformation in HEAs, thereby extending their potential for demanding structural and tribological applications.Öğe Remarkable thermal stability of nanocrystalline CoCrFeNi high entropy alloy achieved through the incorporation of rare-earth element samarium(Elsevier Sci Ltd, 2025) Kotan, Hasan; Koc, Recep C.; Batibay, Ahmet B.High entropy alloys (HEAs) with nanocrystalline grain sizes have received significant interest in recent years; however, their microstructural integrity is compromised by a tendency for grain growth due to their high-volume fraction of grain boundaries. Here, nanocrystalline CoCrFeNi with Sm addition was synthesized through mechanical alloying, followed by annealing at temperatures up to 1100 degrees C and for durations of up to 24 h. The results have revealed that the 16 +/- 6 nm as-milled grain size of CoCrFeNi experienced grain coarsening during the annealing process, reaching similar to 1.35 +/- 0.5 mu m and similar to 4.5 +/- 1.1 mu m after 1 and 24 h annealing at 1100 degrees C, respectively. This indicates that the nanocrystalline microstructure of CoCrFeNi lacks thermal stability at elevated temperatures. The average grain size was maintained at 110 nm after 1 h annealing at 1100 degrees C (T/T-m = 0.74) with Sm addition. Furthermore, while large grains (similar to 1.5 mu m) appeared after 24 h of annealing at 1100 degrees C, pockets of nano-sized grains were still present in the microstructure. The resistance to grain growth is ascribed to the presence of rare earth element, Sm, and the formation of Sm-based additional mixed oxide phases (Sm/Cr-O). Consequently, 517.8 +/- 25 HV as-milled hardness of CoCrFeNi decreased dramatically to 221.5 +/- 11 HV due to extensive grain growth but remained elevated at 442.5 +/- 15 HV (84 % of as-milled hardness) with Sm addition after annealing at 1100 degrees C. These findings highlight the potential for optimizing the thermal and mechanical performance of CoCrFeNi HEAs in various applications.Öğe Unveiling the creep mechanisms of rare earth element yttrium added and SPS consolidated CoCrFeNi high entropy alloys(Walter De Gruyter Gmbh, 2025) Tekin, Mustafa; Kotan, Hasan; Baydogan, Murat; Kaba, Mertcan; Balci, Erdem; Bayrak, Kubra Gurcan; Ayas, ErhanAs high entropy alloys (HEAs) continue to be increasingly studied for next-generation structural materials, gaining a comprehensive understanding of their mechanical properties, including their creep behaviors, remains essential. In this work, rare earth element yttrium (Y) added CoCrFeNi HEAs are produced by mechanical alloying, followed by consolidation via spark plasma sintering (SPS) with ultrafine grain sizes. The microstructures after SPS consolidation are examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The creep properties, including creep displacement, creep strain rate, creep stress, and stress exponent, are evaluated using a nanoindentation test with a Berkovich tip indenter. The results reveal that the average grain size of CoCrFeNi HEA is determined to be 385 +/- 65 nm after SPS consolidation, which reduces to 190 +/- 30 nm and 155 +/- 55 nm with 1 and 4 at.% Y additions, respectively. Accordingly, HEA with the addition of 4 at.% Y exhibits increased hardness, attributed to the presence of additional Y-based oxides and the reduced grain size in its microstructure. Furthermore, the creep mechanisms for the investigated CoCrFeNi HEAs are primarily dominated by dislocation-precipitation interaction based on the calculated stress exponent values.
