Papers by Keyword: Metallic Biomaterial

Abstract: Recently, biomaterial α + β type Ti alloys with relatively low Young’s modulus and high specific strength have been widely used all over the world. Martensite (M) phase in α + β type Ti alloy has been reported to improve the toughness and ductility, therefore, there is high possibility of improvement in the mechanical properties easily by controlling the volume fraction of M phase. In this study, the change in mechanical properties of α + β type Ti-6Al-7Nb (Ti67) with various volume fractions of M phase were systematically investigated through the various heat treatments and thermo-mechanical treatments. Microstructures of Ti67 subjected to ST at 1173 K to 1273 K below the temperature of β transus were composed of martensite and primary α phases. The volume fraction of M phase increased with an increase in ST temperature. Tensile strength increased simply with an increase in the volume fraction of M phase, while the elongation, reduction of area and Young’s modulus showed a reverse trend. Fatigue limit of Ti67 subjected to ST at 1243K showed the highest value of 880 MPa.

Abstract: Ti-29Nb-13Ta-4.6Zr (TNTZ), which is one of metastable beta-type Ti alloys, has developed as one of representative biomedical and dental Ti alloys in Japan. TNTZ subjected to solution treatment shows Young’s modulus of 60 GPa, which is close to that of cortical bone. In addition, TNTZ has very low cytotoxicity and good bone biocompatibility as well. Heat treatment like solution treatment and aging (STA) is mainly used for improving the mechanical properties of metastable beta-type Ti alloys because of alpha precipitates, while Young’s modulus also rises drastically. This study was investigated the effects of mechanical surface modifications such as fine particle bombarding (FPB) with steel and hydroxyapatite particles or friction stir processing (FSP) on the mechanical strength of TNTZ in order to maintain low Young’s modulus. The relative bone contact ratios between the cancellous bones of Japanese white rabbits and column-shaped TNTZ subjected to FPB of steel particles were also evaluated. Vickers hardness (HV) of TNTZ subjected to FPB with fine particles of steel and hydroxyapatite particles increased by HV30 to 200 at the edge of the specimen surface to around 100 to 300 mm in depth as compared with that of TNTZ subjected to solution treatment. The hydroxyapatite layer was formed on the specimen surface by FPB with fine particles of hydroxyapatite particles, although the trend was not significant by FPB with steel particles. Furthermore, the fatigue strength in high cycle fatigue region of TNTZ subjected to FPB with steel particles was improved and the fatigue limit showed around 400 MPa, although that of TNTZ subjected to FPB with fine particles of hydroxyapatite particles were around 60 MPa higher than that to TNTZ subjected to solution treatment (230 MPa). TNTZ with a rough surface texture (Ra: 0.65 μm) showed a relative bone contact ratio of more than 80% after undergoing FPB with fine particles of steel particles; this value was significantly higher than that of TNTZ with a surface texture (Ra: 0.07 μm). Lastly, the microstructure of TNTZ subjected to FSP showed the recrystallization area by the frictional heating with very fine equiaxed beta phase with an average grain diameter of 3.0 μm. The change in Vickers hardness of TNTZ subjected to FSP was almost identical to that of Young’s modulus and showed the almost same trend of FPB.

Abstract: Stainless steel, titanium alloys and cobalt chromium molybdenum alloys are classified under the metallic biomaterials whereby various surgical implants, prosthesis and medical devices are manufactured to replace missing body parts which may be lost through accident, trauma, disease, or congenital conditions. Among these materials, cobalt chromium molybdenum alloys are the common cobalt base alloy used for orthopedic implants due their excellence properties which include high corrosion resistance, high strength, high hardness, high creep resistance, biocompatibility and greater wear resistance. This paper summarises the various aspects and characteristic of metallic biomaterials such as stainless steel, titanium and cobalt chromium alloys for medical applications especially for orthopedic implant. These include material properties, biocompatibility, advantages and limitations for medical implants applications.

Abstract: 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, ε_eq. 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.

Abstract: Beta-type Ti-29Nb-13Ta-4.6Zr (TNTZ) was recently developed as a representative biomedical Ti alloy. As-solutionized TNTZ has a low Young’s modulus less than 60 GPa close to that of cortical bone along with very low cytotoxicity and good bone biocompatibility. Solution treatment and aging (STA) is a typical heat treatment for improving the mechanical properties of beta-type titanium alloys. However, STA also drastically increases the Young’s modulus. Therefore, this study investigated the effects of surface modification, micro-shot peening, on the mechanical properties of TNTZ subjected to severe thermomechanical treatment in order to maintain a relatively low Young’s modulus. The bone contact characteristics of TNTZ samples subjected to surface modification and cancellous bone were also compared. The Vickers hardness of cold-swaged TNTZ (TNTZ_SW) subjected to micro-shot peening was significantly increased within 20 mm from the very edge of the specimen surface. The fatigue strength of TNTZ_SW subjected to micro-shot peening increased especially in the high cycle fatigue life region. The fatigue limit was around 400 MPa. The bone formations on TNTZ_SW subjected to micro-shot peening and TNTZ_SW with the mirror surface as comparison material were almost identical to each other. However, the relative bone contact ratio of TNTZ_SW subjected to micro-shot peening was better than that of TNTZ_SW with the mirror surface.

Abstract: The wear mechanisms of a conventional titanium alloy, Ti–6Al–4V extra-low interstitial (Ti64), and a new titanium alloy, Ti–29Nb–13Ta–4.6Zr alloy (TNTZ) were studied to investigate the wear properties of a Ti64/TNTZ combination for spinal fixation devices. Balls and discs made of Ti64 and TNTZ were prepared to be used as wear-test specimens. Frictional wear tests of Ti64 and TNTZ discs were carried out against Ti64 and TNTZ balls in air using a ball-on-disc frictional wear testing system. The wear mechanisms were investigated by analysis of worn surfaces and wear debris using scanning electron microscopy. Volume losses of the TNTZ discs were found to be larger than those of the Ti64 discs, regardless of mating ball. Furthermore, the morphologies of wear tracks and debris were found to be different between TNTZ and Ti64 discs. It is considered that the wear mechanism for a Ti64 disc is oxidative wear, whereas that for a TNTZ disc is delamination wear, regardless of mating ball material.

Abstract: The present paper put in evidence the influence of applying different heat treatments on some metallic materials such as cobalt and titanium alloys, usualy used for medical applications. The alloys were cobalt base alloy type CoCrMo (22%Cr, 6%Mo, rest Co), in nontreated state, quenching at 1100°C /1h/aer, quenching at 1100°C /1h/air+ sensiblizing at 550°/4h/ air, 600°/4h/air and/or 650°C/4h/air and titanium base alloys - alloy TiMo0.3Ni0,7 (0.23%Mo, 0.72%Ni, Ti rest), alloy TiAl5Fe2V2Mo1,5 (1.52%Mo, 5.15%Al, 2.56%Fe, 2.35%V, Ti rest), each of them in non treated state, annealing at 1050°C/1h/air and annealing at 850°C/1h/air. Corrosion tests were made at potentiostat-galvanostat AUTOLAB, in Ringer solution (for both alloys) and NaCl infusion solution (only for cobalt base alloy) by drawing the polarization curves. Our conclusion is that by applying correct and proper heat treatments to both at cobalt alloys and in titanium alloys there is an improving of the corrosion resistance.

Abstract: The present paper put in evidence the mechanical behaviour of some commercial dental cobalt alloys, after own classification. The experimental cobalt alloys were classified in according either with molybdenum content, or with Cr+Mo content. A hierarcy of the alloys was finally made. The fractographic surfaces were investigate both by stereomacroscopy and scanning electron microscopy, putting in evidence the main structural features.

Abstract: 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 TNTZ_AT Consisted of Precipitated Needle-Like α Phases in β Grains. However, TNTZ_AHPT at N ≥ 10 Comprises Very Fine α and Small Amount ω Phases in Ultrafine β Grains. Furthermore, the Hardness of Every TNTZ_AHPT Was Totally much Greater than that of TNTZ_AT. The Hardness Increased from the Center to Peripheral Region of TNTZ_AHPT. In Addition, the Tensile Strength of Every TNTZ_AHPT Was Greater than that of TNTZ_AT. The Tensile Strength of TNTZ_AHPT Increased, but the Elongation Decreased with Increasing N and then both of them Saturated at N ≥ 10.

Abstract: The main drawback of conventional stenting procedure is the high risk of restenosis. The idea of a stent that "disappears" after having fulfilled its mission is very intriguing and fascinating. The stent mass should diminishing in time to allow the gradual transmission of the mechanical load to the surrounding tissues. Magnesium and its alloys seem to be among the most appealing materials to design biodegradable stents. The objective of this work is to develop, in a finite element (FE) framework, a model of magnesium degradation able to predict the corrosion rate and thus providing a valuable tool to design biodegradable stents. Continuum damage approach is suitable for modelling different damage mechanisms, including several types of corrosion. Corrosion is modelled by a scalar damage field which accounts for the material strength loss due to geometrical discontinuities. As damage progresses, the material stiffness decreases. Corrosion damage results as the superposition of stress corrosion process and uniform corrosion. The former describes the stress-mediated localization of the corrosion attack through a stress-dependent evolution law similar to the one used in analytical models, while the latter affects the free surface of the material exposed to an aggressive environment. The effects of both phenomena described are modelled through a linear composition of the two specific damage evolution laws. The model, developed in a FE framework, manages the mesh dependency, typical of strain-softening behaviour, including the FE characteristic length in the damage evolution law definition. The developed model is able to reproduce the behaviour of different magnesium alloys subjected to static and slow-strain-rate corrosion tests. Moreover, 3D stenting procedures accounting for the interaction with the arterial vessel are simulated.