Browsing by Author "Aydogmus, Tarik"

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Enhanced Sintering of Tini Shape Memory Foams Under Mg Vapor Atmosphere
(Springer, 2012) Aydogmus, Tarik; Bor, Sakir
TiNi alloy foams are promising candidates for biomaterials to be used as artificial orthopedic implant materials for bone replacement applications in biomedical sector. However, certain problems exist in their processing routes, such as formation of unwanted secondary intermetallic phases leading to brittleness and deterioration of shape memory and superelasticity characteristics; and the contamination during processing resulting in oxides and carbonitrides which affect mechanical properties negatively. Moreover, the eutectic reaction present in Ti-Ni binary system at 1391 K (1118 A degrees C) prevents employment of higher sintering temperatures (and higher mechanical properties) even when equiatomic prealloyed powders are used because of Ni enrichment of TiNi matrix as a result of oxidation. It is essential to prevent oxidation of TiNi powders during processing for high-temperature (> 1391 K i.e., 1118 A degrees C) sintering practices. In the current study, magnesium powders were used as space holder material to produce TiNi foams with the porosities in the range of 40 to 65 pct. It has been found that magnesium prevents secondary phase formation and contamination. It also prevents liquid phase formation while enabling employment of higher sintering temperatures by two-step sintering processing: holding the sample at 1373 K (1100 A degrees C) for 30 minutes, and subsequently sintering at temperatures higher than the eutectic temperature, 1391 K (1118 A degrees C). By this procedure, magnesium may allow sintering up to temperatures close to the melting point of TiNi. TiNi foams produced with porosities in the range of 40 to 55 pct were found to be acceptable as implant materials in the light of their favorable mechanical properties.
Microstructure and Mechanical Behavior of Pumice Particle Reinforced Magnesium Matrix Composites
(Springer Heidelberg, 2024) Aydogmus, Tarik; Kosedag, Ertan
In the present study, novel, lightweight and low-cost Mg/pumice composite materials have been developed for the first time. Mg matrix composites reinforced with 3-12 vol.% pumice powders were produced by hot pressing at 600 degrees C for 1 h under 50 MPa pressure. Microstructural analysis revealed that in-situ Mg2Si and MgO phases formed as a result of chemical reaction between pure Mg powders and SiO2 present in the pumice powders during processing. The room and elevated temperature (150 degrees C and 200 degrees C) yield and compressive strength of the composites increased with increasing reinforcement content. The increments up to 47.5% in yield strength and 53% in compressive strength at room temperature have been observed. Similarly, the utmost increments in yield and compressive strength at elevated temperatures were 72% and 50.5%, respectively. Ductility (especially at the room temperature) of the composites on the other hand was almost independent of the reinforcement content.
Microstructure and Mechanical Properties of a Novel Tini Particulate Reinforced Az91 Metal Matrix Composite
(Elsevier Science Bv, 2018) Kelen, Fevzi; Gavgali, Mehmet; Aydogmus, Tarik
A new AZ91/TiNi composite has been developed in the present study. The composite containing 20% (vol.) TiNi reinforcement micro-particles was produced by hot pressing of initial prealloyed AZ91D and Ti49.2Ni50.8 powders at 420 degrees C for 1 h. Microstructural analysis revealed that matrix and reinforcement phases did not react each other during processing and a good bonding which is free from porosity, oxide and carbides was attained. Ti49.2Ni50.8 reinforcement increased room temperature yield strength, compressive strength and ductility of AZ91D alloy 5%, 50% and 30%, respectively. Most importantly, yield strength of the composite at 100 degrees C was found to be 267 MPa, 50% higher than that of AZ91D alloy (176 MPa) due to stress-induced martensitic transformation observed in the reinforcing alloy. (C) 2018 Elsevier B.V. All rights reserved.
Microstructured Prealloyed Titanium-Nickel Powder as a Novel Nonenzymatic Hydrogen Peroxide Sensor
(Academic Press inc Elsevier Science, 2018) Kazici, Hilal Celik; Caglar, Aykut; Aydogmus, Tarik; Aktas, Nahit; Kivrak, Hilal
At present, commercial pure Titanium (Ti) and microstructured pre-alloyed Titanium-Nickel (TiNi) powders are employed as a sensitive electrochemical hydrogen peroxide (H2O2) sensor. Surface characterization of these materials are performed by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical characterization is achieved via cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) on Ti and TiNi modified glassy carbon electrode (GCE). The electrochemical behavior of H2O2 at the pure Ti/GCE and microstructure pre-alloyed TiNi/GCE are studied by CV in 0.1 M phosphate buffer solution (PBS) containing as the supporting electrolyte. In addition, CA is employed for the determination of H2O2 at the applied potential of 0 V vs. Ag/AgCl. The sensor has a linear response range of 0.5-17.5 mM with a sensitivity of 280 mu A mM(-1) cm(-2). Moreover, the limit of detection (LOD) and limit of quantification (LOQ) are 0.5 mu M and 1.7 mu M, respectively. The electrochemical sensor exhibits fast and selective responses to H2O2 concentration. The applicability of the sensor is checked using a hair coloring as a real sample with satisfactory results. (C) 2018 Elsevier Inc. All rights reserved.
Phase Transformation Behavior of Porous Tini Alloys Produced by Powder Metallurgy Using Magnesium as a Space Holder
(Springer, 2011) Aydogmus, Tarik; Bor, Elif Tarhan; Bor, Sakir
Porous TiNi alloys with porosities in the range of 51 to 73 pct were prepared successfully applying a new powder metallurgy fabrication route in which magnesium was used as a space holder, resulting in either single austenite phase or a mixture of austenite and martensite phases dictated by the composition of the starting powders, but entirely free from secondary brittle intermetallics, oxides, nitrides, and carbonitrides. Since transformation temperatures are very sensitive to composition, deformation, and oxidation, for the first time, transformation temperatures of porous TiNi alloys were investigated using chemically homogeneous specimens in as-sintered and aged conditions eliminating secondary phase, contamination, and deformation effects. It was found that the porosity content of the foams has no influence on the phase transformation temperatures both in as-sintered and aged conditions, while deformation, oxidation, and aging treatment are severely influential.
Processing of Interpenetrating Mg-Tini Composites by Spark Plasma Sintering
(Elsevier Science Sa, 2015) Aydogmus, Tarik
Mg/Ti(49)ANi(50.6) interpenetrating composites have been produced via spark plasma sintering technique. Interpenetrating matrix and the reinforcement networks were formed simply by adjustment of the particle sizes of the starting pure Mg and TiNi alloy powders. Microstructural characterization revealed a good interfacial bonding, i.e. free from porosity, oxide and intermetallic compounds, between Mg matrix and the reinforcement. It has been found that, the microstructure of TiNi shape memory alloy reinforcement material is highly sensitive to processing conditions especially to sintering time. Different sintering times resulted in different phase formations and accordingly different phase transformation temperatures due to the overlap of sintering temperature with aging temperature. Room temperature yield and compressive strength of the composite samples were in the range of 53-113 MPa and 226-315 MPa respectively, depending on the reinforcement content (10-30 vol%) and the texture formation during processing. Ductility of the composites on the other hand was in the range of 9.6-20%. 30% TiNi addition resulted in minimum ductility, whereas composite samples with 20% TiNi reinforcement exhibited the maximum ductility values. Mg/TiNi composites displayed extraordinary mechanical properties at high temperatures. Yield strength and elastic modulus of the composites increased with increasing temperature up to 150 degrees C due to the increase of transformation stress and the elastic modulus of TiNi shape memory reinforcements with increasing temperature. Beyond 150 degrees C both properties decreased with further temperature increase. TiNi alloys seem to be promising candidates for reinforcement of pure Mg and Mg alloys to be used in automotive powertrain applications. Composites produced with 20% and 30% TiNi content meet the high strength and elastic modulus requirements at the service conditions of 150-200 degrees C and the stress levels of 50-70 MPa. (C) 2014 Elsevier B.V. All rights reserved.
Processing of Porous Β-Type Ti74nb26 Alloys for Biomedical Applications
(Elsevier Science Sa, 2021) Aydogmus, Tarik; Palani, Dana Kareem Hameed; Kelen, Fevzi
Ti74Nb26 alloys with porosities in the range of 45-68% were produced employing Mg space holder technique combined with hot pressing using elemental Ti, Nb and Mg powders. Powder mixtures were hot pressed at 600 degrees C (below the melting point of Mg, 650 degrees C) for 1 h under a constant pressure of 50 MPa and flowing argon gas. After hot pressing samples were sintered in a vertical furnace at 1200 degrees C (above the boiling temperature of Mg, 1090 degrees C) for 4 h under flowing inert argon gas atmosphere. Evaporation and removal of Mg particles from the samples were carried out during sintering simultaneously. X-ray diffraction and scanning electron microscopy analysis revealed that beta phase was the main phase in the microstructure of all the samples. A small amount of a and undissolved pure Nb were also observed. Room temperature Young's moduli and compressive strength of the specimens increased with decreasing porosity and were in the range of 1.6-14 GPa and 14-136 MPa, respectively. The mechanical properties of the 45%, 54% and 59% porous alloys were sufficient for both cortical and cancellous bone replacement applications. The alloy with a porosity content of 68% demonstrated mechanical properties suitable only for the cancellous bone implants. (C) 2021 Elsevier B.V. All rights reserved.
Superelasticity and Compression Behavior of Porous Tini Alloys Produced Using Mg Spacers
(Elsevier, 2012) Aydogmus, Tarik; Bor, Sakir
In the scope of the present study, Ni-rich TiNi (Ti-50.6 at %Ni) foams with porosities in the range 38-59% were produced by space holder technique using spherical magnesium powders as space formers. Single phase porous TiNi alloys produced with spherical pores were subjected to loading-unloading cycles in compression up to 250 MPa stress levels at different temperatures in as-processed and aged conditions. It has been observed that strength, elastic modulus and critical stress for inducing martensite decrease with increasing porosity. Partial superelasticity was observed for all porosity levels at different test temperatures and conditions employed. Irrecoverable strain was found to decrease with pre-straining and with increasing test temperature. Unlike in bulk TiNi alloys a constant stress plateau has not been observed during the compression testing of porous TiNi alloys. Instead linear superelasticity with a quite steep slope allowing 5% applied strain to be recovered after pre-straining or aging was observed. Even at test temperatures higher than austenite finish temperature in as-sintered and aged condition, strain applied could not be recovered fully. due to martensite stabilization resulting from heavy deformation of macro-pore walls and sintering necks. TiNi foams produced with porosities in the range of 38-51% meet the main requirements of biomaterials in terms of mechanical properties for use as bone implant. (c) 2012 Elsevier Ltd. All rights reserved.
Tini Particle-Reinforced Magnesium Matrix Composites: Production, Microstructure, Phase Transformations, and Mechanical Properties
(Wiley-v C H verlag Gmbh, 2025) Kelen, Fevzi; Gavgali, Mehmet; Aydogmus, Tarik
Magnesium matrix composites reinforced with 5-25 vol% Ti49.2Ni50.8 microparticles are produced by uniaxial hot pressing technique. Powder mixtures are hot pressed in cylindrical graphite dies under 50 MPa pressure for 1 h at 600 degrees C. Density measurements show that specimens reach almost full density and do not contain any pores. Microstructural analyses demonstrate that undesired secondary phases or compounds do not form at matrix/reinforcement interface. As a result of aging heat treatment performed simultaneously with the manufacturing process, martensite phase (B19 ') and Ti3Ni4 precipitates are also observed in the microstructure of reinforcing materials in addition to the main austenite (B2) phase. The room-temperature yield strength and compressive strength of the composite materials range between 60 and 86 MPa and 232 and 296 MPa, respectively. Although the ductility values decrease with increasing reinforcement content, the decrease is less than those of ceramic and other metallic reinforced composites. The ductility is determined to be 17.6% for the composite with the highest reinforcement content. In contrast to the reference and conventional materials, the yield strengths of the composite samples produced increase significantly with the increasing temperature up to 100 degrees C. This unusual behavior is attributed to the stress-induced martensitic transformations observed in TiNi alloys.