Yazar "Tekin, Mustafa" seçeneğine göre listele
Listeleniyor 1 - 6 / 6
Sayfa Başına Sonuç
Sıralama seçenekleri
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 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 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, 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 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.
