Mühendislik Fakültesi

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Designing thermochemical processes for ti-alloys produced by additive manufacturing techniques
(2019) Esen, Ziya; Doğu, Merve Nur; Bilgin, Güney Mert; Tan, Evren; Berkay Gümüş, Berkay; Dericioğlu, Arcan F.; 52373; Ortak Dersler Bölümü
Ti-6Al-4V alloys are extensively used in biomedical, aerospace and petro-chemical applications mainly due to their high specific strength, excellent corrosion resistance and high fatigue strength. In contrast to conventional techniques, additive manufacturing techniques make possible production of parts with complex geometries in one step by minimizing the usage of raw materials. However, post heat treatment processes need to be applied to reduce residual stresses created and to get uniform controllable microstructure so as to obtain desired mechanical properties. Conventional annealing heat treatments are effective in eliminating the residual stress and increasing the ductility. But, they usually degrade the strength and result in coarsening the microstructure. In this study, thermochemical process, called thermo-hydrogen process (THP), was applied to Ti-6Al-4V alloy parts produced by selective laser melting (SLM) and electron beam melting (EBM) with the aim of increasing ductility without degrading the alloy strength. Additively manufactured parts initially hydrogenated at 650oC for 1 h under H2+Ar gas mixture and then, dehyrogenated at 700oC for 18 h under high vacuum. As a result of alloying with hydrogen and dealloying turned martensitic microstructure in SLM parts into fine + lamellar phase mixture. On the other hand, microstructural refinement in EBM parts were limited since as-manufactured parts were already composed of fine lamellar microstructure. THP prevented grain growth and grain boundary α-phase formation in SLM parts. Ductility of the alloys increased by 110 %, while the strength decreased slightly only by 10 %. However, the effect of the THP on mechanical properties of EBM samples couldn’t be observed due to their high surface roughness which induced notch effect.
Effect of SiC addition on the thermal diffusivity of SiAlON ceramics
(2017) Kuşhan Akın, Şeniz Reyhan; Turan, Servet; Gençoğlu, Pervin; Mandal, Hasan; 224219
Despite the fact that thermal conductivity is a crucial parameter for SiAlON ceramics with respect to their suitability in various applications, including high-temperature structural components, wear parts, and cutting tools, studies on SiAlON ceramics reported thus far mainly focus on the improvement of their mechanical properties. In view of the lack of sufficient studies on the thermal conductivity of SiAlON ceramics, this study investigates the improvement in the thermal diffusivity behaviour of SiAlON ceramics by the addition of highly conductive SiC particles. As solid-solution SiAlON ceramics exhibit complex crystal structures typically composed of defects, the phonon scattering increases, subsequently decreasing diffusivity. In particular, the improvement in the thermal diffusivity of both α- and β-SiAlONs was investigated by the addition of 0.25 wt% SiC. In addition, the effect of the SiC particle size on the thermal diffusivity of β-SiAlON was examined. Using inverse diffusivity data, intrinsic and extrinsic scattering parameters were determined, and compared to intrinsic scattering, extrinsic scattering was a dominant factor. Furthermore, transmission electron microscopy (TEM) images of SiCp-reinforced α and -β-SiAlON ceramics were recorded to examine the SiC particle distribution.
Effects of Process Parameters on Fe2O3/α-Al2O3 Luster Pigments Produced By Precipitation Method
(2020) Günhan, Büşra; Kaya, Güray; Kök, Mesut; Weinekotter, Çiğdem; Poyraz, H. Bogaç; Ceylantekin, Rasim; Kuşhan Akın, Şeniz Reyhan; 224219
Common known examples of substrate-based pigments are mica/TiO2 and mica/Fe2O3 luster pigments produced using wet chemical methods and gas-phase reaction techniques. Luster coatings, in particular those prepared using iron(III) oxide, come into prominence thanks to their characteristics of good hiding power and weather resistance, to name a few. Mica/Fluorphlogopite/Silica-based luster pigments have certain disadvantages due to following reasons: natural mica contains iron (Fe2O3) impurity which imparts a yellow color to the material, synthetic fluorphlogopite is quite expensive compared to natural mica and silica has a high production cost. In addition to these, fragility of mica bases due to their low mechanical strength adversely affects the luster effect. Reaction kinetics control is easier in wet chemical methods than in gas-phase reaction techniques since the coarse substrate particles can be kept suspended more easily by stirring. In this study, α-Al2O3/Fe2O3 luster pigments were produced using the wet chemical method/precipitation method by preferring plate-like alumina substrates in order to eliminate the afore-said problems. The pigments produced were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) methods and color analysis was performed. As a result, it was determined that the increase in titration flow rate and stirring rate in the production process of Fe2O3/α-Al2O3 luster pigment using the wet chemical method had no effect on the crystal structure of the resulting coating. However, while the stirring rate does not have a significant effect on the quality of coating, the increase in titration flow rate adversely affects the quality of coating.
Epitaxial Graphene and Carbon Nanotubes on Silicon Carbide
(CRC Press-Taylor & Francis Group, 2014) Buke, Göknur Cambaz
Kinetics and mechanisms of isothermal devitrification inamorphous cu50zr50
(Springer, 2015) Kalay, İlkay; Kalay, İlkay; Kalay, Y. Eren; Kramer, Matt; Napolitano, Ralph; 101579; Mekatronik Mühendisliği
The crystallization kinetics and microstructural dynamics associated with devitrifying a melt-spun Cu50Zr50 metallic glass were investigated using isothermal treatments, in situ high-energy synchrotron X-ray diffraction, conventional and high-resolution transmission electron microscopy, and differential scanning calorimetry. The analysis of isothermal transformations allows us to more clearly unravel the complex interplay between nucleation and growth of competing stable and metastable phases. The isothermal devitrification response was found to involve the Cu10Zr7, CuZr2, and CuZr phases, consistent with previously reported constant heating rate experiments, but here we have resolved the phase evolution and structural characteristics of the transformation, including the very early stages of crystallization. At 671 K (398 A degrees C), the isothermal transformation starts with the formation of the Cu10Zr7 phase, which grows in a generally equiaxed morphology. At a size of approximately 100 nm, the growth of the Cu10Zr7 particles is interrupted by the precipitation of a thin layer of the CuZr2 phase, upon which the metastable CuZr (B2) grows epitaxially. Crystallization kinetics are quantified here though in situ measurements (HEXRD, DSC) and ex situ microstructural analysis (TEM, HRTEM). Finally, the influences of chemical partitioning, diffusion, and crystallographic orientation on this sequence are examined.
Microstructural Refinement of Ti-6Al-4V Alloy Fabricated by Additive Manufacturing Using Thermochemical Processes
(2016) Esen, Ziya; Esen, Ziya; Kuşhan Akın, Şeniz Reyhan; Dericioğlu, Arcan Fehmi; 52373; 224219; Ortak Dersler Bölümü
Temperature Dependence of the Brillouin Frequency Shift and the Linewidth of the LA Mode in the Ferroelectric Phase of PZT-x (PbZr1-xTixO3)
(IEEE, 2015) Yurtseven, F.; Korkmaz, C. Şişman; 42475
The temperature dependence of the Brillouin frequency and the linewidth of the LA mode is studied for the concentration of x=0.45 in the ferroelectric phase of PbZr1-xTixO3 (PZT-x) within the temperature range of 443 to 656 K (T-C=657 K). Using the experimental data for the Brillouin frequency of the LA mode as an order parameter below T-C the temperature dependence of the linewidth (FWHM) is calculated by the pseudospin-phonon coupled model and the energy fluctuation model for the lead titanate zirconate (x=0.45). Additionally, the activation energies are compared from the damping constant (linewidth) using both models for the temperature range of 443 to 656 K in the ferroelectric phase of PbZr1-xTixO3. Our calculated values for the damping constant are in agreement with the observed data for the ferroelectric phase of PbZr1-xTixO3 single crystals. The activation energies calculated from both models are much higher in the ferroelectric phase than the value of k(B)T(C)=0.056 eV at the transition temperature for PbZr1-xTixO3 (x=0.45).

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