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    Cr Modifiyeli Cu-Ni-Si Alaşımlarının Mekanik ve Elektriksel Özellikleri Üzerine Aşırı Plastik Deformasyon Etkisi
    (Osman SAĞDIÇ, 2021) Elibol, Çağatay
    Bakır alaşımları, sahip oldukları yüksek mukavemet, korozyon direnci ve iletkenlikleri nedeniyle otomotiv, elektronik, petrokimya ve nükleer uygulamalarında yaygın olarak kullanılmaktadır. Bu endüstrilerde üstün özelliklerinden dolayı çoğunlukla berilyum içeren bakır alaşımları kullanılmakla birlikte, berilyumun toksik etkileri nedeniyle alternatif bakır alaşımlarına ihtiyaç duyulmaktadır. Bunlar arasında Corson alaşımları olarak bilinen Cu-Ni-Si alaşımları yaygın olarak tercih edilmektedir. Hem alaşımlama hem de termomekanik işlemler, bu alaşım sisteminde berilyum içeren Cu alaşımlarına kıyasla benzer ve hatta daha iyi özellikler elde etme noktasında önemli rol oynamaktadır. Bu araştırmada, aşırı plastik deformasyon ve akabinde gerçekleştirilen ısıl işlemlerin (çökelme sertleşmesi) Cr ile modifiye edilmiş Cu-Ni-Si alaşımlarının mukavemeti, sertliği ve elektrik iletkenliği üzerine etkisini karakterize etmek için kombine termomekanik işlem uygulanmıştır. Farklı kimyasal bileşime (Cr içeriğine) sahip bakır alaşımları, çözeltiye alma ve su verme işleminden sonra eş kanallı açısal pres (EKAP) yöntemi kullanılarak şekillendirilmiştir. Cr modifiyeli alaşımların davranışı ayrıntılı olarak karakterize edilmiş ve kombine termomekanik işlem ile alaşımdaki Cr içeriğinin nihai mekanik ve fiziksel özellikler üzerine etkisi irdelenmiştir. Elde edilen sonuçlar, ağırlıkça %0,4, 0,6 ve 0,8 Cr içeren çözeltiye alınmış alaşımların yaşlandırılması sırasında hem sertlikte hem de elektriksel iletkenlikte önemli bir artış gözlendiğini ve artan Cr içeriğinin Cu-Ni-Si alaşımının sertliğini ve iletkenliğini artırdığını göstermektedir. İki paso EKAP ile işlemi neticesinde, incelenen tüm alaşımların sertliği çözeltiye alınmış duruma kıyasla önemli ölçüde artarken, elektriksel iletkenliği aşırı plastik deformasyondan negatif etkilenmiştir. EKAPlanmış alaşımlarının mekanik özellikleri, EKAP sonrası yaşlandırma ile daha da geliştirilebilmektedir. Bu nedenle, yaygın olarak kullanılan bakır-berilyum alaşımlarına alternatif Cu-Ni-Si alaşımlarının aşırı plastik deformasyon ile geliştirilmesi mümkündür. İlk sonuçlar, son zamanların yüksek öncelikli alanları arasında yer alan elektronik endüstrisi, yüksek hız raylı sistemler, uzay/havacılık uygulamaları ile nükleer santrallerde kullanılabilecek yapı malzemelerinin geliştirilmesine ışık tutmaktadır.
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    Effect of equal-channel angular pressing on ordering kinetics and twinning in an 18-carat AuCuAg alloy
    (2023) Elibol, Çağatay; Illgen, Christian; Colas, Damien; Bohne, Benjamin; Fritsch, Sebastian; Dietrich, Dagmar
    The ability to modify microstructural features and the resulting properties of red gold provides an attractive potential for applications in electronics, dental devices or jewelry. In this study, the effect of severe plastic deformation by equal-channel angular pressing (ECAP) on microstructural evolution in an 18-carat AuCuAg alloy, and on the ordering kinetics during subsequent aging, is investigated. Differential scanning calorimetry measurements show that the ordering process is significantly influenced by a plastically deformed microstructure and lattice defects introduced by one or two passes of ECAP. Electron backscatter diffraction and transmission electron microscopy demonstrate that the microstructural evolution during ordering, accompanied by the formation of an L10 superstructure, substantially depends on the thermo-(mechanical) history of the material: While the undeformed material clearly shows a hierarchical structure of twins on different length scales, only a few nanotwins are observed within the strongly deformed microstructures after ECAP. Moreover, after ECAP, static recrystallization is considerably suppressed during long-term annealing at elevated temperatures close to the phase transition temperature. These results clearly highlight the potential of ECAP in combination with heat treatments for microstructural optimization of AuCuAg alloys and their applications.
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    Effect of Severe Plastic Deformation on the Mechanical and Electrical Properties of Cr-modified Cu-Ni-Si Alloys
    (2021) Elibol, Çağatay
    Copper alloys are widely used in automotive, electronics, petrochemical and nuclear applications due to their good mechanical/ corrosion resistance and conductivity. Although mostly beryllium containing copper alloys are utilized in these industries due to their superior properties. However, due to the toxic effects of beryllium, there is a need for alternative copper alloys. Among the alternative alloys, Cu-Ni-Si alloys known as Corson alloys are commonly preferred. Both alloying and thermomechanical treatments play important roles to attain similar or better properties in this alloy system, compared to the beryllium containing Cu alloys. In this research, a combined thermomechanical treatment is performed in order to characterize the effect of severe plastic deformation and subsequent heat treatments (i.e., precipitation hardening) on strength, hardness and electrical conductivity of Corson alloys modified by Chromium. The copper alloys having different chemical compositions (Cr contents) are processed using equal-channel angular pressing (ECAP) after solution annealing and quenching. The behavior of Cr-modified alloys is characterized in detail and the effect of the combined thermomechanical treatment and of the Cr content on the final mechanical and physical behavior is discussed. The results show that during aging of solution treated alloys containing 0.4, 0.6 and 0.8 wt-% Cr, a significant increase in both hardness and electrical conductivity is observed, and that increasing Cr content also leads to an increase of both properties. After processing by two-pass ECAP, the hardness of all investigated alloys is significantly increased compared with the solution treated state, whereas the electrical conductivity is adversely affected by severe plastic deformation. The mechanical behavior of the ECAPed alloys can be enhanced even further by performing post-ECAP aging. Therefore, it allows us to develop Cu-Ni-Si alloys by severe plastic deformation process, as an alternative to commonly used beryllium containing copper alloys. The preliminary results provide new insights for the development of construction materials that can be used in the electronic industry, high-speed rail systems, aerospace applications and nuclear power plants, which are recently high priority topics.
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    Effect of severe plastic deformation on the precipitation kinetics and the properties of CuCoNiBe alloys
    (Elsevier, 2022) Elibol, Çağatay
    The effect of the precipitation hardening by means of the conventional heat treatment and the combined thermomechanical treatment (i.e., equal channel angular pressing (ECAP) with subsequent aging) on the electrical conductivity, mechanical properties and precipitation kinetics of precipitation hardening, quadruple CuCo1Ni1Be alloy was investigated. No experimental studies on the effect of ECAP on the electrical conductivity and precipitation kinetics of CuCoNiBe alloys have been reported. Systematic microstructural and mechanical investigations show that both ECAP processing and post-ECAP aging result in a significant increase in the hardness and strength of the alloy due to the grain boundary strengthening by means of the grain refinement (and the formation of subgrains) and due to the precipitation hardening by means of the formed precipitates (primary and secondary beryllides) in the fine-grained structure, respectively. Moreover, the electrical conductivity of both solution treated and ECAPed alloys increases with increasing aging time due to the precipitation of the solute atoms in alpha-Cu matrix leading to decreasing electrical resistivity. However, the CuCo1Ni1Be alloys subjected to ECAP exhibit a lower conductivity than the solution treated ones, which can be attributed to an increase in resistivity caused by increased probability of electrons scattering by severe plastic deformation. The linear relationship between electrical conductivity and volume fraction of the precipitates according to the Martition's law and the empirical Avrami equation are used to investigate the precipitation kinetics of the CuCo1Ni1Be alloy. The S-shaped volume fraction of precipitates vs. aging time curves clearly indicate that the precipitation kinetics of the CuCoNiBe alloys can be significantly promoted by ECAP processing. The results of the present study provide new insights into the interaction of the severe plastic deformation, the kinetics of precipitation, and the microstructural, mechanical and physical properties of the CuCo1Ni1Be alloy.
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    Effect of water absorption and hydroxyapatite addition on mechanical and microstructural properties of dental luting cements
    (Walter de Gruyter, 2022) Elibol, Çağatay; Atalay, Ender Gökşen; Sağır, Kadir; Yoruç, Afife Binnaz Hazar
    In recent years, dental cements have gained more and more importance due to the increasing clinical expectations. They are used as restorative materials and as materials providing critical functions such as chemical and physical adhesion in restorations and protective lining in the cavity. In this work, the effect of water absorption on the mechanical and microstructural properties of different commercial dental cements is systematically investigated. Furthermore, the effect of addition of biocompatible hydroxyapatite on the mechanical and microstructural properties of the cements is characterized. The results show that Voco Meron containing glass ionomer provides the highest values in water absorption and volume change tests. Ketac Cem Radiopaque (209 HV) containing glass ionomer exhibits the highest hardness increase after immersion for 28 days in water, whereas the highest water uptake value is observed in Adhesor Carbofine cement containing zinc polycarboxylate. The hardness of Adhesor ZincPhospate increases with increasing content of hydroxyapatite, whereas in Ketac Cem Radiopaque, the hardness decreases with increasing content of hydroxyapatite significantly. Test results are verified by microstructural analysis of different types of dental cements using scanning electron microscopy and the effect of particle size/distribution on the properties that are of crucial clinical importance is characterized in detail.
  • Küçük Resim
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    Eş kanallı açısal pres (EKAP) yöntemi ile şekillendirilmiş Ti-6Al-4V alaşımının mekanik özellikleri ve makroskobik deformasyon davranışı arasındaki ilişki
    (2020) Elibol, Çağatay
    Bu çalışmada, Ti-6Al-4V alaşımının kaba taneli ve eşit kanallı açısal presleme (EKAP) ile üretilen ultra ince taneli durumda, quasi-statik tek eksenli çekme yüklemesi altında (yaklaşık 10-3 s-1 deformasyon hızı ile) sergilediği mekanik davranış ve makroskobik deformasyon modu (homojen vs. lokalize) karakterize edilmiştir. EKAP işleminin ve farklı kanal iç açılarının çekme dayanımı, kopma uzaması (süneklik), üniform uzama ve sertlik gibi malzemenin mekanik özelliklerine etkisi kapsamlı bir şekilde incelenmiştir. Dijital görüntü korelasyon (DIC) tekniği ile tüm çekme testleri esnasında yüzey deformasyon alanları insitu olarak ölçülmüştür. Çift paso EKAP işlemi sonrası malzemenin çekme dayanımı, akma dayanımı ve sertliğinin sırasıyla 795,8 MPa, 660 MPa ve 255 HV’den, 120° kanal iç açılı kalıpla EKAPlanmış numunede 918,3 MPa, 850 MPa, 303 HV ve 90° kanal iç açılı kalıpla EKAPlanmış numunede ise 990,5 MPa, 890 MPa, 343 HV değerlerine ulaştığı görülmüştür. Ancak, tanelerin EKAP prosesi neticesinde incelmesi, malzemenin müteakip kırılmasını tetikleyebilecek olan erken zamanlı deformasyon lokalizasyonuna bağlı olarak üniform uzama ve kopma uzamasında önemli bir gerilemeyi beraberinde getirmektedir. DIC verileri, başlangıç numunesinde (EKAP öncesi) deformasyonun homojen olarak ilerlediğini; çift paso EKAP işlemi sonrası çekme yüklemesine tabi tutulan numunede ise deformasyonun daha ziyade inhomojen/lokalize bir mod eğilimi sergilediğini açıkça ortaya koymaktadır. Bu çalışma kapsamında elde edilen sonuçlar, EKAP sonrası plastik deformasyona tabi tutulan malzemenin makroskobik deformasyon modu ile sünekliği arasındaki ilişkiye dair yeni bakış açıları kazandırmaktadır.
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    Investigation of Cu whisker growth by molecular beam epitaxy
    (Walter de Gruyter, 2021) Elibol, Çağatay; Strunk, Horst Paul
    There is a trend towards smaller and smaller structures (nanostructures/miniaturization) which is well known in microelectronic, energy and semiconductor applications. Nanoengineering is expected to lead to significant improvements in the intrinsic properties of structures, e. g., in energy storage for supercapacitors. In this context, a deeper understanding of the growth mechanisms of the thinnest crystal layers is of crucial importance for the controlled growing of nanowhiskers with outstanding properties. In the present study, we consider a simple whisker growth model based on the surface energy (i. e., wettability) of the components and investigate the effect of the carbon interlayer deposited on a Si (111) wafer using the magnetron sputtering technique on the whisker formation during the subsequent molecular beam epitaxy process in the Si-C-Cu system. In the present study, the topographic holes in the carbon layer which are the preferred nucleation areas of whiskers were identified by a series of scanning tunneling microscopy analyses, and the natural hole density was statistically determined. Using atomic force microscopy, the surface roughness of the carbon layer was characterized. The results of our investigations indicate that there is a correlation between the hole density in the carbon layer and the density of Cu nanowhiskers. This may validate the supposition that the holes in the carbon layer are the preferred nucleation sites for whiskers - an effect that could be relevant for future works on the growth of nanowhiskers at predefined positions.
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    Numerical and experimental formability analysis of aluminum 3105 sandwich panels produced by continuous hot-press forming
    (Walter de Gruyter, 2022) Elibol, Çağatay; Wapande, Sadam Hamis
    Aluminum-plastic composites as building materials are widely used for different construction purposes, for instance, for exterior wall cladding, ventilated facades, and interior decoration of buildings, in billboards, trains, and automotive industry. The main goal of this study is to investigate the formability of AA3105/LDPE/AA3105 sandwich composite in detail, which would exhibit a higher formability compared to AA3105 due to its higher strain rate sensitivity and strain hardening exponent. This is of decisive importance for the manufacturing process. The Nakajima tests are performed to experimentally determine the forming limit curves (FLCs) using in situ optical technique digital image correlation. Furthermore, numerical simulations of Nakajima tests are conducted using a modified Gurson-Tvergaard-Needleman damage model to compare the numerically and experimentally determined FLCs. The results show that the sandwich composite underwent inhomogeneous deformation during the Nakajima test, and that the FLC has no typical patterns exhibited by metal sheets. The FLCs predicted by the numerical model used in this study exhibit a very good correlation with the FLCs determined experimentally. The results of the present study provide new insights into the analysis and understanding of the deformation behavior of the sandwich composite sheet that may undergo complex stress and strain states.
  • Küçük Resim
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    Superelastic anisotropy and meso-scale interface regions in textured NiTi sheets
    (Gazı University Faculty Engineering Architecture, 2021) Elibol, Çağatay
    Typical manufacturing processes of NiTi shape memory alloys (SMAs) involve drawing and rolling operations leading to crystallographic texture and superelastic anisotropic behavior. Therefore, an in-depth understanding of directional material properties and underlying mechanisms may help to guide the production process to create suitable textures for applications requiring different mechanical properties. During stress-induced martensitic transformation (SIMT), macroscopic deformation occurs only in the mesoscale interface regions between fully martensitic and fully austenitic regions. In case of localized deformation, the detailed characterization of these transition regions is therefore of crucial practical importance. In this study, the effects of crystallographic texture and specimen geometry on mechanical/localized deformation behavior of polycrystalline NiTi superelastic sheets were systematically analyzed. Local surface strain fields are recorded during deformation by digital image correlation (DIC) in order to characterize interface regions in detail. The results show that the mechanical behavior has a clear directional dependency, and that the orientation/texture effect is reduced with decreasing width/thickness ratio of specimen. For different sample geometries and orientations, martensite/austenite interface has different angles which changes during SIMT. The results provide new insights into the interaction of crystallographic texture, SIMT characteristics, superelastic anisotropy and specimen geometry which is essential for engineering applications of NiTi.
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    Tension-compression asymmetry of quadruple CuCoNiBe alloys processed by high-temperature multi-pass equal channel angular pressing (ECAP)
    (Walter de Gruyter, 2022) Elibol, Çağatay
    Among copper alloys, the precipitation (age) hardening quadruple CuCoNiBe alloys due to their superior mechanical properties in terms of the highest strength and elasticity achieved by peak aging have been used over the years in a wide range of industrial applications including aircrafts, coil systems, blast-proof materials, and molding dies. Combined thermomechanical treatment (i.e., equal channel angular pressing [ECAP] processing followed by post-ECAP aging) can result in a further enhancement of the strength of CuCoNiBe alloys due to the grain refinement and the formation of fine precipitates from alpha-Cu supersaturated solid solution. In this study, for the first time, the effect of severe plastic deformation by ECAP processing on the mechanical behavior of CuCoNiBe alloys is discussed thoroughly based on the material responses to tensile and compressive loading conditions. The results show that, besides a considerable strength enhancement, ECAP processing leads to a strong tension-compression asymmetry and significantly accelerated precipitation kinetics in CuCoNiBe alloys.
  • Küçük Resim
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    Virtual extensometer analysis of martensite band nucleation, growth, and strain softening in pseudoelastic NiTi subjected to different load cases
    (Mdpi, 2018) Elibol, Çağatay; Wagner, Martin F. -X.
    Pseudoelastic NiTi shape memory alloys exhibit different stress-strain curves and modes of deformation in tension vs. compression. We have recently shown that under a combination of compression and shear, heterogeneous deformation can occur. In the present study, we use digital image correlation to systematically analyze how characteristic features of the nominally uniaxial engineering stress-strain curves (particularly the martensite nucleation peak and the plateau length) are affected by extensometer parameters in tension, compression, and the novel load case of shear-compression. By post-experimental analysis of full surface strain field data, the effect of the placement of various virtual extensometers at different locations (with respect to the nucleation site of martensite bands or inhomogeneously deforming regions) and with different gauge lengths is documented. By positioning an extensometer directly on the region corresponding to the nucleating martensite band, we, for the first time, directly record the strain-softening nature of the material-a specific softening behavior that is, for instance, important for the modeling community. Our results show that the stress-strain curves, which are often used as a basis for constitutive modeling, are affected considerably by the choice of extensometer, particularly under tensile loading, that leads to a distinct mode of localized deformation/ transformation. Under compression-shear loading, inhomogeneous deformation (without lateral growth of martensite bands) is observed. The effects of extensometer gauge length are thus less pronounced than in tension, yet systematic-they are rationalized by considering the relative impact of differently deforming regions.