Hakan Yilmazer

Mechanical properties of a medical β-type titanium alloy with specific microstructural evolution through high-pressure torsion

The effect of high-pressure torsion (HPT) processing on the microstructure and mechanical biocompatibility includes Youngs modulus, tensile strength, ductility, fatigue life, fretting fatigue, wear properties and other functionalities such as super elasticity and shape memory effect, etc. at levels suitable for structural biomaterials used in implants that replace hard tissue in the broad sense (Sumitomo et al., 2008 [4]). In particular, in this study, the mechanical biocompatibility implies a combination of great hardness and high strength with an adequate ductility while keeping low Youngs modulus of a novel Ti-29Nb-13Ta-4.6Zr (TNTZ) for biomedical applications at rotation numbers (N) ranging from 1 to 60 under a pressure of 1.25 GPa at room temperature was systematically investigated in order to increase its mechanical strength with maintaining low Youngs modulus and an adequate ductility. TNTZ subjected to HPT processing (TNTZHPT) at low N exhibits a heterogeneous microstructure in micro-scale and nano-scale consisting of a matrix and a non-etched band, which has nanosized equiaxed and elongated single β grains, along its cross section. The grains exhibit high dislocation densities, consequently non-equilibrium grain boundaries, and non-uniform subgrains distorted by severe deformation. At high N which is N>20, TNTZHPT has a more homogeneous microstructure in nano-scale with increasing equivalent strain, εeq. Therefore, TNTZHPT at high N exhibits a more homogenous hardness distribution. The tensile strength and 0.2% proof stress of TNTZHPT increase significantly with N over the range of 0≤N≤5, and then become saturated at around 1100 MPa and 800 MPa at N≥10. However, the ductility of TNTZHPT shows a reverse trend and a low-level elongation, at around 7%. And, Youngs modulus of TNTZHPT decreases slightly to 60 GPa with increasing N and then becomes saturated at N≥10. These obtained results confirm that the mechanical strength of TNTZ can be improved while maintaining a low Youngs modulus in single β grain structures through severe plastic deformation.

Developing biomedical nano-grained β-type titanium alloys using high pressure torsion for improved cell adherence

Proper surface characteristics for a titanium implant are crucial for the formation of different cellular protrusions known as filopodia and lamellipodia, both of which have a significant impact on cell attachment, spreading, migration, and proliferation. Microstructural features such as grain boundaries and defects of implant surface can modulate the cellular components and structure at the leading edge of cells. Here, a nano-grained Ti–29Nb–13Ta–4.6Zr (NG TNTZ) substrate was produced by high-pressure torsion (HPT) for improved biofunctionality. Cellular response of human osteoblast cells on nano-grained TNTZ substrates is evaluated and compared with the cellular response of those on coarse-grained TNTZ. High wettability, which depends on high internal energy due to the nano-sized grains that are full of boundaries, interfaces, and high dislocation density, influenced the hOBs cells on NG TNTZ to form highly developed cellular protrusions. Large number of filopodia protrusions resulted in excellent cell attachment as consistent with high level of vinculin and superior cell proliferation. This study demonstrates the advantages of nanocrystalline surface modification using HPT for processing metallic biomaterials that are proper for orthopedic implants.

Microstructural evolution and mechanical properties of biomedical Co-Cr-Mo alloy subjected to high-pressure torsion

The effects of severe plastic deformation through high-pressure torsion (HPT) on the microstructure and tensile properties of a biomedical Co-Cr-Mo (CCM) alloy were investigated. The microstructure was examined as a function of torsional rotation number, N and equivalent strain, εeq in the HPT processing. Electron backscatter diffraction analysis (EBSD) shows that a strain-induced martensitic transformation occurs by the HPT processing. Grain diameter decreases with increasing εeq, and the HPT-processed alloy (CCMHPT) for εeq=45 exhibits an average grain diameter of 47nm, compared to 70μm for the CCM alloy before HPT processing. Blurred and wavy grain boundaries with low-angle of misorientation in the CCMHPT sample for εeq<45 become better-defined grain boundaries with high-angle of misorientation after HPT processing for εeq=45. Kernel average misorientation (KAM) maps from EBSD indicate that KAM inside grains increases with εeq for εeq<45, and then decreases for εeq=45. The volume fraction of the ε (hcp) phase in the CCMHPT samples slightly increases at εeq=9, and decreases at εeq=45. In addition, the strength of the CCMHPT samples increases at εeq=9, and then decrease at εeq=45. The decrease in the strength is attributed to the decrease in the volume fraction of ε phase, annihilation of dislocations, and decrease in strain in the CCMHPT sample processed at εeq=45 by HPT.

Microstructure and Mechanical Properties of a Biomedical β-Type Titanium Alloy Subjected to Severe Plastic Deformation after Aging Treatment

Strengthening by Grain Refinement and Increasing Dislocation Density through High-Pressure Torsion (HPT), which Is an Attractive Technique to Fabricate Ultrafine Grained and Nanostructured Metallic Materials, Is Expected to Provide β-Type Ti-29Nb-13Ta-4.6Zr (TNTZ) Higher Mechanical Strength while Maintaining Low Young’s Modulus because they Keep the Original β Phase. However, the Ductility Shows Reverse Trend. Greater Strength with Enhanced Ductility Can Be Achieved by Controlling Precipitated Phases through HPT Processing after Aging Treatment. Aged TNTZ Subjected to HPT Processing at High N Exhibits a Homogeneous Microstructure with Ultrafine Elongated Grains Having a High Dislocation Density and Consequently Non-Equilibrium Boundaries and Distorted Subgrains with Non-Uniform Shapes and Nanostructured Intergranular Precipitates of αphases. Therefore, the Effect of HPT Processing on the Microstructure and Mechanical Hardness of TNTZ after Aging Treatment Was Systematically Investigated in this Study. TNTZ, which Was Subjected to Aging Treatment at 723 K for 259.2 Ks in Vacuum Followed by Water Quenching, Subjected to HPT Processing at Rotation Numbers (N) of 1 to 20 under a Pressure of around 1.25 GPa at Room Temperature. The Microstructure of TNTZAT Consisted of Precipitated Needle-Like α Phases in β Grains. However, TNTZAHPT at N ≥ 10 Comprises Very Fine α and Small Amount ω Phases in Ultrafine β Grains. Furthermore, the Hardness of Every TNTZAHPT Was Totally much Greater than that of TNTZAT. The Hardness Increased from the Center to Peripheral Region of TNTZAHPT. In Addition, the Tensile Strength of Every TNTZAHPT Was Greater than that of TNTZAT. The Tensile Strength of TNTZAHPT Increased, but the Elongation Decreased with Increasing N and then both of them Saturated at N ≥ 10.

Synthesis and Characterization of Hydroxyapatite/TiO2 Coatings on the β-Type Titanium Alloys with Different Sintering Parameters using Sol-Gel Method

In this study, hydroxyapatite (HA) based composite films were successfully syntheses on the β-type Ti29Nb13Ta4.6Zr (TNTZ). The solutionized TNTZ substrates coated with HA and HA/Titania (TiO2) bioactive composite coatings by sol-gel method under various sintering parameters related to sintering temperatures and heating ramp rates. Microstructural observations of the coatings revealed that apatite was formed on the substrates. The hardness values of the coatings increase with increasing both the sintering temperature and the TiO2 concentration in the coatings layer. However, it was found that the heating ramp rate of the sintering was not affecting the hardness values so much. Also, the hardness values of the HA/TiO2 composite coatings at all sintering temperatures were higher than only HA coated TNTZ samples due to the existence TiO2 phases in the HA matrix. Results indicating that the doping of HA with TiO2, improve the physical consistency between the coating layer and the substrates and provide a better inter-particle bonding due to the existence TiO2 phases in the HA.

ZİRKONYA TAKVİYELİ HİDROKSİAPATİT (HA) BAZLI BİYOAKTİF HİBRİD KAPLAMALARIN KOROZYON DUYARLILIKLARI

Titanyum (Ti) ve alasimlari sahip olduklari dusuk elastik modul, yuksek dayanim, iyi biyouyumluluk ve korozyona karsi gelismis direncleri nedeniyle implant uygulamalarinda en sik kullanilan metalik biyomalzemelerdir. Bu calismada; ticari safliktaki titanyum (CP Ti) altliklar uzerine sol ‐ jel teknigi kullanilarak hidroksiapatit (HA: Ca 5 (PO 4 ) 3 (OH)) bazli zirkonya (ZrO 2 ) katkili biyoaktif hibrid kaplamalar ile kaplanmasi ve bu kaplamalarin in-vitro ortamlardaki elektrokimyasal korozyon duyarliliklari arastirilmistir. Kaplamalarin korozyon duyarliliklari potansiyodinamik polarizasyon (PDS) testleri ile belirlenmistir. Karakterizasyon calismalarinda XRD, SEM ve EDS cihazlari kullanilmistir. Elde edilen bulgular HA icerisine katilan ZrO 2 partikullerinin implantin yuk tasima kapasitelerini artirmakla birlikte kaplanmamis numunelere oranla yuzey pasivizasyon ozelliklerini iyilestirdigi gorulmustur.

Electrochemical Behaviors of Biomedical Nanograined β-Type Titanium Alloys

The microstructural evolution and its effect on biocompatibility of TNTZ through HPT processing were investigated systematically in this study. TNTZAHPT shows an enhanced mechanical biocompatibility, which is characterized by a higher tensile strength (1375 MPa) and hardness (450 HV) than those of TNTZST, TNTZAT, and Ti64 ELI while maintaining a relatively low Young’s modulus. In this study, such microstructural refinement of TNTZ and its effect on electrochemical biocompatibility through HPT processing are investigated systematically in this study. The microstructure of TNTZAT consists of randomly distributed needle-like α precipitates in the equiaxed β grains with a diameter of approximately 40 m. The microstructure of TNTZAHPT consists of nanograined (NG) elongated β grains that have subgrains of non-uniform morphologies resulting from distortion by severe torsional deformation. Furthermore, the β grains and subgrains are surrounded by non-equilibrium grain boundaries. The needle-like α precipitates are completely refined to a nanograined. TNTZAHPT exhibits an enhanced combination of excellent corrosion performance and improved cellular response compared to TNTZST, TNTZAT, and Ti64 ELI.

Microstructural Analysis of Biomedical Co-Cr-Mo Alloy Subjected to High-Pressure Torsion Processing

The effect of high-pressure torsion (HPT) processing on the microstructure and Vickers hardness of Co-Cr-Mo (CCM) alloys were investigated in this study. The microstructure of initial CCM alloy contains equiaxed grains with a grain diameter of approximately 50 μm and twins. The clear grain boundaries of equiaxed grains and twins disappear after HPT processing at a rotation number, N, of 10. The phase maps of initial CCM alloy and CCM alloy subjected to HPT processing at N = 5 measured by electron backscatter diffraction exhibit that the ratio of γ phase decreases from 93.5% to 34.1% and the ratio of ε phase increases from 6.5% to 65.9% by applying HPT processing. These results indicate that the ε phase is formed by high-strain, which is induced by the HPT processing. The Vickers hardness values on the surfaces of the CCM alloys subjected to HPT processing at N = 1, 5, and 10 increase with increasing the equivalent strain, εeq. These results suggest that an increase of Vickers hardness is correlated to an increase of the ratio of ε phase and the dislocation density, and grain refinement, which are caused by the high-strain induced by HPT processing.

Nanostructure and Fatigue Behavior of β-Type Titanium Alloy Subjected to High-Pressure Torsion after Aging Treatment

A novel β-type, Ti-29Nb-13Ta-4.6Zr, referred to as TNTZ has been developed for biomedical applications. Its fatigue strength is one of the most important mechanical biocompatibilities of TNTZ because, in surgical applications, it will be used under cyclic loading conditions. The effect of the microstructural refinement by high-pressure torsion (HPT) on the fatigue behaviour of TNTZ is systematically investigated in this study. TNTZ subjected to HPT processing where the rotation number (N) is 20 (TNTZAHPT) after aging treatment (AT) shows a unique microstructure having ultrafine elongated grains (285 nm in length and 36 nm in width) with high-density dislocations, a large fraction of blurred and wavy boundaries consisting of non-uniform subgrains with high misorientation and nanostructured precipitated α phase. Remarkably, a good combination of high mechanical strength (1375 MPa) and low Young’s modulus (87 GPa), compared to that of Ti-6Al-4V (Ti64) ELI, is achieved for TNTZAHPT at N = 20. TNTZAHPT a great fatigue strength, which is comparable to those of (Ti64) ELI.

Effect of high–pressure torsion processing on microstructure and mechanical properties of a novel biomedical β–type Ti–29Nb–13Ta–4.6Zr after cold rolling

High mechanical biocompatibility, which implies excellent mechanical properties such as great strength and hardness with keeping low Youngs modulus in a new biomedical β–type titanium alloy, Ti–29Nb–13Ta–4.6Zr (TNTZ), can be achieved by microstructural control. Strengthening of TNTZ by grain refinement and increasing dislocation density is expected to provide high mechanical strength with keeping low Youngs modulus because they maintain the original β phase. In this case, high–pressure torsion (HPT) processing is one of the effective ways to obtain these properties simultaneously in this alloy. This study systematically investigated the effect of HPT processing on the microstructure and the mechanical properties of TNTZ. The microstructure of TNTZ, which was subjected to HPT processing after cold rolling, exhibits a single β phase composed of grains with diameter of less than a few hundred nanometres and high–angle boundaries. The grains have non–uniform subgrains with high angle misorientation and high dislocation density due to severe plastic deformation. The tensile strength of the specimen after HPT processing increases significantly compared with the specimen processed by cold rolling.