Malzeme Bilimi ve Mühendisliği Bölümü Yayın Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/396

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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
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.
Citation - WoS: 32
Citation - Scopus: 36
Optimization of the Mechanical Properties of Ti-6al Alloy Fabricated by Selective Laser Melting Using Thermohydrogen Processes
(Elsevier Science Sa, 2017) Esen, Ziya; Akin, Seniz Kushan; Dericioglu, Arcan F.; Bilgin, Guney Mert
2-step Thermo Hydrogen Process (THP) including hydrogenation and dehydrogenation steps was applied to Ti-6Al-4V alloy fabricated by selective laser melting (SLM) process to refine the microstructure and to increase the ductility of the alloy. It was observed that as-fabricated alloy's surface was composed of oxides of titanium and aluminum, which may alter the hydrogenation kinetics. The hydrogen treatment for 1 hat 650 degrees C, the maximum hydrogen solubility temperature of the alloy, transformed starting non-equilibrium alpha'-martensitic phase to 13 and 8 (TiH2) phases. On the other hand, very fine discontinuous beta-phase was formed along with alpha-phase as a result of dehydrogenation at 700 degrees C for 18 h, which decreased the hydrogen level well below the starting value and caused transformation of delta-phase to fine alpha-phase. In contrast to commonly used THP treatments consisting of betatizing at high temperature, 2-step treatment prevented grain boundary alpha-phase formation as well as excessive grain growth. About 110% and 240% increments were detected in % Elongation (EL) and % Reduction in Area (RA) values, respectively, as a result of 2-step THP, while the strength decrease was limited to 10%. The change in ductility of the alloy was also verified by transformation of flat and shiny fracture surfaces seen in SLM fabricated alloy to a fracture surface containing equiaxed dimples after THP treatment. The hardness drop in the alloy after hydrogenation and dehydrogenation treatments was attributed to relief of residual stress and formation of equilibrium alpha and beta-phases.
Citation - WoS: 8
Citation - Scopus: 9
Effect of Sm on Thermal and Mechanical Properties of Cu-Zr Bulk Metallic Glasses
(Elsevier Science Sa, 2019) Sikan, F.; Atabay, S. E.; Motallebzadeh, A.; Ozerinc, S.; Kalay, I.; Kalay, Y. E.
The effect of rare-earth (Sm) microalloying on the thermal stability and phase selection along with the effect of nanocrystallization on the mechanical properties of amorphous melt-spun ribbons of Zr50Cu40Al10, Zr49Cu39.2Al9.8Sm2 and Zr48Cu38.4Al9.6Sm4 alloys were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Vickers and nanoindentation hardness tests and micropillar compression analysis. XRD and TEM analyses showed that all samples were fully amorphous in as-spun state; however, crystallization sequences for the Sm-free and the Sm micro-alloyed samples were different during devitrification. Combined study of XRD, DSC and TEM on melt-spun ribbons show that Zr48Cu38.4Al9.6Sm4 have nanocrystallization of Cu2Sm phase with an average diameter of 10 nm, which was absent in Zr50Cu40Al10, prior to crystallization of Cu10Zr7 phase. The nanoindentation and micropillar compression tests revealed Cu2Sm nanocrystals embedded in Zr48Cu38.4Al9.6Sm4 alloy improves strength and hardness. On the other hand, presence of these nanocrystals deteriorate shear band stability and thus result in a catastrophic brittle fracture through a single shear band burst.
Citation - WoS: 31
Citation - Scopus: 30
Ionization Energies, Coulomb Explosion, Fragmentation, Geometric, and Electronic Structures of Multicharged Boron Clusters Bn (n=2-13)
(Amer Physical Soc, 2011) Tas, Murat; Ozdogan, Cem; Boustani, Ihsan; Akman, Nurten
Based on the ab initio quantum chemical methods, we have determined fragmentation channels, ionization energies, and the Coulomb explosion of multicharged boron clusters B-n (n = 2-13), where n is the cluster size. The electron-deficient boron clusters sustain more stability and hardly fragment when they are negatively charged. Stability of boron clusters decreases with increasing ionization. Only by the first ionization the odd-size clusters are more stable than the even-size clusters. Further ionizations cause the repulsive Coulomb force between the constituent atoms to get stronger, and lead first to metastable states, then to the Coulomb explosion of clusters. None of the cationic boron clusters studied remain stable after six times ionization. The critical charge for metastability is estimated as Q(m) <= n/2 for even-size clusters, and Q(m) <= 1/2 (n - 1) for odd-size clusters. In addition, the critical charge for the Coulomb explosion is found to be Q(c) = n/2 + 1 for even-size clusters, and Q(c) = 1/2(n + 1) for odd-size clusters. Relative stability of clusters with respect to their nearest neighbors is determined from the analysis of their second energy difference data. Several dissociation channels of B-n(+) and B-13(Q) isomers with the lowest fragmentation energies are presented. All of the vibrational frequencies are found positive indicating that no transition state is possible for the clusters studied. Reliability of our data is verified with a good agreement with experimental results.
Citation - WoS: 34
Citation - Scopus: 43
Clinoptilolite/Pcl-peg-pcl Composite Scaffolds for Bone Tissue Engineering Applications
(Sage Publications Ltd, 2017) Pazarceviren, Engin; Erdemli, Ozge; Keskin, Dilek; Tezcaner, Aysen
The aim of this study was to prepare and characterize highly porous clinoptilolite/poly(epsilon-caprolactone)-poly(ethylene glycol)-poly(epsilon-caprolactone) composite scaffolds. Scaffolds with different clinoptilolite contents (10% and 20%) were fabricated with reproducible solvent-free powder compression/particulate leaching technique. The scaffolds had interconnective porosity in the range of 55-76%. Clinoptilolite/poly(epsilon-caprolactone)-poly(ethylene glycol)poly(epsilon-caprolactone) scaffolds showed negligible degradation within eight weeks and displayed less water uptake and higher bioactivity than poly(epsilon-caprolactone)-poly(ethylene glycol)-poly(epsilon-caprolactone) scaffolds. The presence of clinoptilolite improved the mechanical properties. Highest compressive strength (5.6 MPa) and modulus (114.84 MPa) were reached with scaffold group containing 20% clinoptilolite. In vitro protein adsorption capacity of the scaffolds was also higher for clinoptilolite/poly(epsilon-caprolactone)-poly(ethylene glycol)-poly(epsilon-caprolactone) scaffolds. These scaffolds had 0.95 mg protein/g scaffold adsorption capacity and also higher osteoinductivity in terms of enhanced ALP, OSP activities and intracellular calcium deposition. Stoichiometric apatite deposition (Ca/P = 1.686) was observed during cellular proliferation analysis with human fetal osteoblasts cells. Thus, it can be suggested that clinoptilolite/poly(epsilon-caprolactone)poly(ethylene glycol)-poly(epsilon-caprolactone) composite scaffolds could be promising carriers for enhancement of bone regeneration in bone tissue engineering applications.
Citation - WoS: 25
Citation - Scopus: 27
Crystallization Kinetics and Phase Transformation Mechanisms in Cu56zr44 Glassy Alloy
(Springer, 2015) Kramer, Matthew J.; Napolitano, Ralph E.; Kalay, Ilkay
The kinetics and phase selection mechanisms involved in the crystallization of an amorphous Cu-Zr alloy of eutectic composition (Cu56Zr44) were investigated using in situ high-energy X-ray diffraction (HEXRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) under isothermal and constant heating rate conditions. In situ HEXRD results for 10 K/min (10 A degrees C/min) heating indicate that the amorphous alloy devitrifies into CuZr2 and mainly Cu10Zr7 at the crystallization temperature of 725 K (452 A degrees C). The sequence continues with the precipitation of CuZr (B2) at 1004 K (731 A degrees C), where these three phases coexist until the decomposition of CuZr2 is observed at 1030 K (757 A degrees C). The two equilibrium phases Cu10Zr7 and CuZr (B2) remain present on further heating until melting at the eutectic temperature for the Cu56Zr44 alloy. TEM investigation of the isothermal [705 K (432 A degrees C)] crystallization sequence reveals primary nucleation and growth of the Cu10Zr7 phase, where growth of the Cu10Zr7 crystals is initially planar with a transition to a cellular morphology, associated with partitioning of Zr at the growth front. Related cellular structures and composition profiles are quantified. (C) The Minerals, Metals & Materials Society and ASM International 2015
Citation - WoS: 23
Citation - Scopus: 25
Microstructural and Texture Evolution During Thermo-Hydrogen Processing of Ti6al4v Alloys Produced by Electron Beam Melting
(Elsevier Science inc, 2020) Esen, Ziya; Davut, Kemal; Tan, Evren; Gumus, Berkay; Dericioglu, Arcan F.; Dogu, Merve Nur
The present study was conducted to reveal the effects of building angles and post heat-treatments (2-step Thermo-Hydrogen Processing (THP) and conventional annealing treatment) on the density, microstructure and texture of Ti6Al4V alloy parts produced by Electron Beam Melting (EBM). The results showed that regardless of the building angle; the density, microstructure and crystallographic texture (defined with respect to building angle) of the as-produced samples were identical; having Widmanstatten a structure and columnar beta-grains which are parallel to building direction. The main texture component for the alpha phase was (10 (1) over bar0)//building direction, and for beta phase (001)//building or heat flow direction. The first step of THP, namely, the hydrogenation step, produced a needle-like microstructure and increased the local misorientations due to lattice distortion. On the other hand, after application of the second step of THP, dehydrogenation step, microstructure was refined, particularly alpha-grains that were larger than 10 mu m and located at grain boundaries. Moreover, THP randomized the crystallographic texture since it involves beta to alpha phase transformation, at which one beta-grain can produce 12 distinct alpha-variants. The grain boundary misorientation distributions also changed in accordance with the microstructural changes during the 2-step THP. On the other hand, annealing coarsened the grain boundary and Widmanstatten alpha phases; moreover, it changed the texture so that the basal planes (0001) rotated 30 degrees around the building direction.
Citation - WoS: 11
Citation - Scopus: 11
Effect of Sn Alloying on the Diffusion Bonding Behavior of Al-Mg Alloys
(Springer, 2017) Esen, Ziya; Dericioglu, Arcan F.; Atabay, Sila Ece
Effect of Sn as an alloying element on the diffusion-bonding behavior of Al-Mg-Si alloy has been studied by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and mechanical testing of the diffusion-bonded joint. XRD results revealed the formation of Mg2Sn and (Sn) phases during solidification following induction casting. DSC results showed local liquid (Sn) formation during the bonding process for Sn-containing alloys, where its amount was found to be increasing with the increasing Sn content. Results revealed that Sn addition leads to an increase in the bond shear strength of the diffusion-bonded joints and elimination of the irregularities formed on the bonded interface. Fractured surfaces showed that formation of (Sn) layer at the bonded interface causes the fracture to transform from the ductile to the mixed fracture mode.
Citation - WoS: 14
Citation - Scopus: 21
Fabrication, Morphology Analysis, and Mechanical Properties of Ti Foams Manufactured Using the Space Holder Method for Bone Substitute Materials
(Mdpi, 2019) Esen, Ziya; Yildirim, Bora; Cetinel, Oktay
Porous titanium (Ti) offers several key attributes as a biomedical material. Among the known characteristics of Ti relevant to biomedical applications, the mechanical performance and effects of a pore structure on the deformation characteristics under compressive loading were examined. The space holder method was employed to generate Ti foams with target porosities of 60%, 70%, and 80%. A micro-computed to mography analysis and light and scanning electron microscopy were performed to examine the pore morphology and microstructure. The mechanical properties along with the elastic modulus and compressive strength were evaluated via uniaxial compression testing. Ti foam samples with three porosity levels displayed average elastic moduli and compressive strengths comparable with those of human cancellous and cortical bone. All the Ti foam samples had elastic moduli similar to those of cancellous bone with their open porous structures. Although the foam samples with similar to 60% porosity had compressive strength comparable to that of cortical bone, the samples with similar to 80% porosity displayed compressive strength similar to that of cancellous bone. The results indicate that Ti foam scaffolds produced using the space holder method have great potential for applications in hard tissue engineering, as their mechanical properties and pore structures are similar to those of bone.
Citation - WoS: 11
Citation - Scopus: 14
Nanocrystallization in Cu-Zr Bulk Metallic Glasses
(Springer, 2018) Yasar, Bengisu; Kalay, Ilkay; Sikan, Fatih
The effect of rare-earth element (Sm) microalloying on the thermal stability and crystallization kinetics of melt-spun ribbons and suction-cast rods of Zr48Cu38.4Al9.6Sm4 alloy were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). The XRD results of constant heating rate annealing indicated that amorphous Zr48Cu38.4Al9.6Sm4 melt-spun ribbons devitrifies into Cu2Sm at 673 K (400 A degrees C). The sequence continues with the precipitation of Cu10Zr7 and then these two phases coexist. XRD and TEM studies on 1 mm diameter as suction-cast rods indicated the precipitation of 30-nm-mean size Cu2Sm crystals during solidification. TEM investigation of the isothermal crystallization sequence of melt-spun ribbons and 1-mm-diameter suction-cast rods revealed the precipitation of Cu2Sm nanocrystals at the onset of crystallization and the restriction of the growth of these nanocrystals up to 10 nm diameter with further annealing. APT analysis of 1-mm-diameter suction-cast rods showed that the limited growth of Cu2Sm nanocrystals is due to sluggish diffusion of Sm and Al-Zr pile up at the interface.
Citation - WoS: 102
Citation - Scopus: 119
Characterization of Ti-6al Alloy Foams Synthesized by Space Holder Technique
(Elsevier Science Sa, 2011) Bor, Sakir; Esen, Ziya
Ti-6Al-4V foams, biomedical candidate materials, were synthesized by powder metallurgical space holder technique as a result of evaporation of magnesium to achieve desired porosity content. Final products contained porosities in the range similar to 43-64% with an average macropore size between 485 and 572 mu m and a lamellar type Widmanstatten microstructure composed of alpha-platelets and beta-laths. Unlike the case of bulk Ti-6Al-4V alloy tested under compression loading, compression stress-strain curves of manufactured Ti-6Al-4V foams were similar to those of elastic-plastic foams, which contain a linear elastic region; a plateau stage: and a densification stage. In the plateau region deformation bands perpendicular to the compression axis were developed and cell collapsing took place together with the buckling and fracture of some of the cell walls and edges in a ductile manner. Calculated elastic modulus and yield strength were in the range 1.42-14.7 GPa and 28.2-150 MPa, respectively, and the foam mechanical properties were found to be dependent on micro porous cell wall properties, which in turn depends on neck size between powder particles. Around 330 MPa yield strength value was calculated for porous cell walls by the use of Ti-6Al-4V alloy powder samples sintered in loose and compacted conditions, which were utilized to simulate the cell wall structure of foams. In addition, overall mechanical properties of foam s were investigated considering macro porosity fraction, p(macro), and the yield strength of foams exhibited a power law dependence, similar to commonly used minimum solid area models, in the form of A*(1 - p(marco))(n), where the proportionality constant "A" was found to be the yield strength of micro porous cell walls. (C) 2011 Elsevier B.V. All rights reserved.
Kinetics and mechanisms of isothermal devitrification inamorphous cu50zr50
(Springer, 2015) Cullinan, Tim; Kalay, İlkay; Kalay, Y. Eren; Kramer, Matt; Napolitano, Ralph
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.
Citation - WoS: 50
Citation - Scopus: 51
Local Chemical and Topological Order in Al-Tb and Its Role in Controlling Nanocrystal Formation
(Pergamon-elsevier Science Ltd, 2012) Kalay, Y. E.; Kalay, I.; Hwang, Jinwoo; Voyles, P. M.; Kramer, M. J.
How the chemical and topological short- to medium-range order develops in Al-Tb glass and its ultimate effect on the control of the high number density of face-centered-cubic-Al (fcc-Al) nuclei during devitrification are described. A combined study using high-energy X-ray diffraction (HEXRD), atom probe tomography (APT), transmission electron microscopy and fluctuation electron microscopy (FEM) was conducted in order to resolve the local structure in amorphous Al90Tb10. Reverse Monte Carlo simulations and Voronoi tessellation analysis based on HEXRD experiments revealed a high coordination of Al around Tb atoms in both liquid and amorphous states. APT results show Al-rich and Al-depleted regions within the as-quenched alloy. A network structure of Tb-rich clusters divides the matrix into nanoscale regions where Al-rich clusters are isolated. It is this finely divided network which allows the amorphous structure to form. Al-rich regions are the locus for fcc-Al crystallization, which occurs before the intermetallic crystallization. FEM reveals medium-range ordered regions similar to 2 nm in diameter, consistent with fcc-Al and trigonal-like Al3Tb crystal structures. We propose that the high coordination of Al around Tb limits diffusion in the intermetallic network, allowing for the isolated Al-rich regions to form at high density. These regions are responsible for the extremely high density of Al nanocrystal nuclei. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Citation - WoS: 7
Citation - Scopus: 7
A Novel Approach for Synthesis of Monticellite Based Bioactive Ceramic Powders From Boron Derivative Waste
(Elsevier Science Bv, 2017) Butev, Ezgi; Esen, Ziya; Ayas, Erhan; Koroglu, Levent
Monticellite bioactive ceramics have a high usage potential as bone graft substitutes due to their superior mechanical properties compared to hydroxyapatite and excellent bioactivity. Turkey has almost 72% of the global boron reserves and the fairly high amount of emerged boron derivative waste causes storage problems and environmental pollutions. The aim of this study was the solid state synthesis of monticellite based ceramic powders from boron derivative waste and the evaluation of bioactivity characteristic of produced powders. The monticellite based ceramic powders were synthesized at low temperature (800 degrees C). The powders were incubated in Simulated Body Fluid and their surfaces were examined using a scanning electron microscope. The surfaces of synthesized powders were bioactive and allowed formation of bone-like apatite layer within 15 days. Results of the study confirmed that cost-effective and ecofriendly monticellite based bioactive ceramic powders can be synthesized from boron derivative waste. (C) 2017 Elsevier B.V. All rights reserved.
Citation - WoS: 36
Corrosion of Metallic Biomaterials
(Springer-verlag Berlin, 2015) Esen, Ziya; Duygulu, Ozgur; Gungor, Serap; Dikici, Burak
Metallic materials have been used as biomedical implants for various parts of the human body for many decades. The physiological environment (body fluid) is considered to be extremely corrosive to metallic surfaces; and corrosion is one of the major problems to the widespread use of the metals in the human body since the corrosion products can cause infections, local pain, swelling, and loosening of the implants. Recently, the most common corrosion-resistant metallic biomaterials are made of stainless steels and titanium and its alloys along with cobalt chromium molybdenum alloys. It is well known that protective surface films of the alloys play a key role in corrosion of the metallic implants. Key documents on the corrosion behavior of the metallic biomaterials in human body have been compiled under this chapter as a review.
Citation - WoS: 9
Citation - Scopus: 11
Boriding Kinetics and Mechanical Properties of Borided Commercial-Purity Nickel
(Natl inst Science Communication-niscair, 2017) Uçar, Nuri; Calik, A.; Ucar, N.; Delikanli, K.; Carkci, M.; Karakas, S.; Bankacılık ve Finans
Kinetics of boride layer growth and tensile behaviour in bonded commercial-purity nickel was investigated. Bonding was carried out in a solid medium consisting of Ekabor-II powders at 1173, 1223 and 1273 K for periods of 3, 5 and 8 h. Scanning electron microscopy (SEM) and optical microscopy showed column morphology in the boride layer. X-ray diffraction (XRD) analyses indicated that the boride layer formed on the surface consisted mainly of Ni2B, with precipitates of Ni6Si2B. A parabolic relationship between layer thickness and processing temperature was observed. The obtained results showed that although the boride layer thickness increased with increasing boriding temperature and time, boriding parameters had no significant effect on the hardness of the boride layer or the matrix. Tensile properties were negatively influenced by the bonding treatment; both yield and tensile strength values decreased due to the presence of the hard yet brittle surface coating. In addition, the growth kinetics of boride layers was also analysed. The results showed a nearly parabolic relationship between the layer thickness and the process temperature, with activation energy of 47.3 kJ mol(-1).
Citation - WoS: 39
Citation - Scopus: 42
Effect of Post Fabrication Aging Treatment on the Microstructure, Crystallographic Texture and Elevated Temperature Mechanical Properties of In718 Alloy Fabricated by Selective Laser Melting
(Elsevier Science Sa, 2022) Bilgin, Guney Mert; Davut, Kemal; Esen, Ziya; Dericioglu, Arcan F.; Ozer, Seren
The effect of building direction and post fabrication aging treatment on the microstructure, crystallographic texture and high temperature mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) method was investigated. After aging, arc-shaped structures seen in as-fabricated samples dis-appeared and converted into a mixture of columnar and equiaxed grains. Nano-sized gamma '' and/or gamma' precipitates were formed upon aging; however, MC type carbides and Laves phase encountered in as-fabricated samples were not dissolved completely after aging. Moreover, aging did not alter the texture ((001)//building direction (BD)) of as-fabricated samples. Mechanical properties of the alloys under tension were influenced by the build direction, aging time and test temperature. As-fabricated samples produced in vertical direction exhibited higher room temperature strengths with lower ductility due to orientation of overlapped prior melt pools. Room temperature tensile test results revealed that peak aging caused a significant improvement in ultimate tensile strength (UTS), from 1066.5 MPa and 998.4 MPa to 1408.5 MPa and 1330.4 MPa whereas elongation values decreased from 27.5% and 32.2% to 19.6% and 23.7% in vertically and horizontally built samples, respectively. Peak-aged samples (aged at 700 degrees C for 8 h) tested at 600 degrees C displayed serrated regions in their stress-strain curves due to dynamic strain aging (DSA). Although strength values of the samples displayed an expected decrease by temperature, ductility of the samples reduced to minimum at temperatures around 700-800 degrees C, which was attributed to intermediate temperature embrittlement.
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.
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.
Citation - WoS: 23
Citation - Scopus: 26
In Vitro Bioactivity Investigation of Alkali Treated Ti6al7nb Alloy Foams
(Elsevier, 2015) Esen, Ziya; Bor, Sakir; Butev, Ezgi
Biocompatible Ti6Al7Nb alloy foams with 70% porosity manufactured by space holder method were activated via alkali treatment using 5 M NaOH solution at 60 degrees C. The interconnected pore structures enabled formation of homogenous sodium rich coating on the foam surfaces by allowing penetration of alkali solution throughout the pores which had average size of 200 mu m. The resulted coating layer having 500 nm thickness exhibited porous network morphology with 100 nm pore size. On the other hand, heat treatment conducted subsequent to alkali treatment at 600 degrees C in air transformed sodium rich coating into crystalline bioactive sodium titanate phases. Although the coatings obtained by additional heat treatment were mechanically stable and preserved their morphology, oxidation of the samples deteriorated the compressive strength significantly without affecting the elastic modulus. However, heat treated samples revealed better hydroxyapatite formation when soaked in simulated body fluid (SBF) compared to alkali treated foams. On the other hand, untreated surfaces containing bioactive TiO2 layer were observed to comprise of Ca and P rich precipitates only rather than hydroxyapatite within 15 days. The apatite formed on the treated porous surfaces was observed to have flower-like structure with Ca/P ratio around 1.5 close to that of natural bone. (C) 2014 Elsevier B.V. All rights reserved.
Reaksiyon Bağlı Silisyum Nitrür Seramiklerinde Sinterleme İlavesinin Mikroyapı ve Antibakteriyel Davranışa Etkisinin İncelenmesi
(2021) Kuşhan Akın, Şeniz Reyhan
Silisyum Nitrür (Si3N4) genellikle yüksek sıcaklık ve yapısal uygulamalarda tercih edilen, üstün özellikli bir oksit dışı seramiktir. Çok sert bir malzeme olmasının yanı sıra, çubuksu beta () tanelerin gelişimi ile seramik malzemeler içinde yüksek kabul edilebilecek tokluk değerleri elde etmek mümkün olabilmektedir. Bu özelliklerine ilave olarak biyo uyumluluğunun da anlaşılması ile birlikte özellikle, bu malzemenin biyoseramik olarak kullanım potansiyelini arttırmak amacıyla son on yılda yapılan çalışmaların sayısı da artmıştır. Bu çalışmada da reaksiyon bağlama tekniği ile, çevre biyolojik dokularla entegrasyon açısından avantajlı olan gözenekli Si3N4 seramiklerinin üretilmesi amaçlanmıştır. Sinterleme ilavesi olarak sisteme yapılan CaO, Y2O3 ve CeO2 katkılarının, elde edilen Si3N4 seramiklerin fiziksel ve mikroyapısal özelliklerine ve bunların sonucu olarak antibakteriyel davranışlarına etkileri incelenmiştir. Elde edilen sonuçlar CeO2’in hem E. coli hem de S. aureus açısından dezavantajlı bir sinterleme ilavesi olduğunu, Y2O3’in ise en avantajlı ilave olduğunu görülmüştür. CaO ilavesi yapılan sistem ise E. coli için düşük, S. aureus için yüksek oranda bakteri üremesi ile sonuçlanmıştır.

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