Β-type Titanium Alloy Research Articles

Microstructure, elastic deformation behavior and mechanical properties of biomedical β-type titanium alloy thin-tube used for stents

Cold-deformability and mechanical compatibility of the biomedical β-type titanium alloy are the foremost considerations for their application in stents, because the lower ductility restricts the cold-forming of thin-tube and unsatisfactory mechanical performance causes a failed tissue repair. In this paper, β-type titanium alloy (Ti–25Nb–3Zr–3Mo–2Sn, wt%) thin-tube fabricated by routine cold rolling is reported for the first time, and its elastic behavior and mechanical properties are discussed for the various microstructures. The as cold-rolled tube exhibits nonlinear elastic behavior with large recoverable strain of 2.3%. After annealing and aging, a nonlinear elasticity, considered as the intermediate stage between “double yielding” and normal linear elasticity, is attributable to a moderate precipitation of α phase. Quantitive relationships are established between volume fraction of α phase (Vα) and elastic modulus, strength as well as maximal recoverable strain (εmax-R), where the εmax-R of above 2.0% corresponds to the Vα range of 3–10%. It is considered that the “mechanical” stabilization of the (α+β) microstructure is a possible elastic mechanism for explaining the nonlinear elastic behavior.

Fatigue behavior of bulk β-type titanium alloy Ti–15Mo–5Zr–3Al annealed in high temperature nitrogen gas

β-type titanium alloy, Ti–15Mo–5Zr–3Al, was annealed in nitrogen gas at the temperature of 1200°C for 0.5 or 1h. Subsequently, four-point bending fatigue tests were carried out using the annealed specimens. Based on the results obtained, the effect of the solid solution of nitrogen on fatigue properties was discussed. The hardness of specimen surface was increased markedly from 311HV to about 480HV by the annealing in nitrogen gas. The increase of hardness was due to the solid solution of nitrogen, formation of titanium nitride (TiN) and precipitation of α-phase resulting in Widmannstaetten (needle-like α- and β-phases) layer near the specimen surface. In spite of the improvement of hardness at the specimen surface, the fatigue strengths of the annealed specimens were lower than that of the untreated specimen. EDX analyses revealed that the brittle TiN was formed along grain boundaries near specimen surface. Though the re-solution treatment at 850°C for 1h dissolved TiN, the fatigue strength of re-solution treated specimen was not recovered compared to the untreated specimen. It was considered that the decrease in fatigue strength was mainly attributed to the grain coarsening during the annealing.

A mean-field interatomic potential for a multi-component β-type titanium alloy

By employing the mean-field concept and the embedded-atom method, an interatomic potential was constructed for a β-type titanium alloy, Ti–24Nb–4Zr–7.9Sn (Ti2448). The alloy was assumed to consist of one single special “element”, and its properties were obtained by averaging the interaction between all elements in the alloy. The lattice parameter, shear modulus, bulk modulus, anisotropy ratio and vacancy formation energy from experiments and the cohesive energy from ab initio calculations were used for fitting the potentials. Verification of the present potential was performed by the calculation of the cohesive energies under various lattice structures, the equation of state and point defect energies. Molecular dynamics simulations showed that the present potential is capable of reproducing various manners of plastic deformation, e.g., dislocation nucleation and propagation, deformation twinning, phase transformation, etc. The results show that for such disordered alloys, while retaining the reliability, the mean-field scheme is much simplified compared with traditional methods of potential construction, hence making it possible to study multi-component engineering alloys, such as Ti2448.

Microstructural evolution of precipitation-hardened β-type titanium alloy through high-pressure torsion

In order to elucidate the microstructural refinement mechanism and the effect of secondary phase on the microstructural evolution of β-type titanium alloy, severe plastic deformation was conducted on samples of a precipitation-hardened Ti–29Nb–13Ta–4.6Zr (TNTZ). Specifically, TNTZ that was precipitation-hardened through an aging treatment (TNTZAT) was subjected to high-pressure torsion (HPT) processing (TNTZAHPT). The microstructure of TNTZAHPT, which has been evaluated as a function of the torsional rotation number, N, exhibits ultrafine elongated β grains. The needle-like α precipitates in TNTZAT, which exhibit a diameter of approximately 12nm, are homogeneously distributed within the β grains. The dislocation density and subgrain diameter, estimated by X-ray line profile analysis, saturate at approximately 4.2×1016m−2 and 12.2nm, respectively, at N⩾10. The β grains contain nanostructured subgrains having non-uniform morphologies surrounded by blurred and wavy boundaries. A saturated hardness distribution at approximately 450HV indicates that microstructural homogeneity has been achieved at N⩾10. The α precipitates enhance the β grain refinement and microstructural homogeneity is achieved in TNTZAHPT at N⩾10, whereas this occurs at later stages (N>20) in TNTZ which is solution-treated and therefore does not contain any α precipitates.

Effects of surface nanocrystallization on corrosion resistance of β-type titanium alloy

Surface mechanical attrition treatment (SMAT) was performed on biomedical β-type TiNbZrFe alloy for 60 min at room temperature to study the effect of surface nanocrystallization on the corrosion resistance of TiNbZrFe alloy in physiological environment. The surface nanostructure was characterized by TEM, and the electrochemical behaviors of the samples with nanocrystalline layer and coarse grain were comparatively investigated in 0.9% NaCl and 0.2% NaF solutions, respectively. The results indicate that nanocrystallines with the size of 10–30 nm are formed within the surface layer of 30 μm in depth. The nanocrystallized surface behaves higher impedance, more positive corrosion potential and lower corrosion current density in 0.9% NaCl and 0.2% NaF solutions as compared with the coarse grain surface. The improvement of the corrosion resistance is attributed to the rapid formation of stable and dense passive film on the nanocrystallized surface of TiNbZrFe alloy.

Electrochemical and biological characterization of coatings formed on Ti–15Mo alloy by plasma electrolytic oxidation

β-Type titanium alloys are considered the future materials for bone implants. To improve the bioactivity of Ti–15Mo, the surface was modified using the plasma electrolytic oxidation (PEO) process. Tricalcium phosphate (TCP, Ca3PO4), wollastonite (CaSiO3) and silica (SiO2) were selected as additives in the anodizing bath to enhance the bioactivity of the coatings formed during the PEO process. Electrochemical analysis of the samples was performed in Ringer's solution at 37°C. The open-circuit potential (EOCP) as a function of time, corrosion potential (ECORR), corrosion current density (jCORR) and polarization resistance (Rp) of the samples were determined. Surface modification improved the corrosion resistance of Ti–15Mo in Ringer's solution. In vitro studies with MG-63 osteoblast-like cells were performed for 1, 3 and 7days. After 24h, the cells were well adhered on the entire surfaces, and their number increased with increasing culture time. The coatings formed in basic solution with wollastonite exhibited better biological performance compared with the as-ground sample.

Enhancement of mechanical properties of biocompatible Ti–45Nb alloy by hydrostatic extrusion

β-Type titanium alloys are promising materials for orthopaedic implants due to their relatively low Young’s modulus and excellent biocompatibility. However, their strength is lower than those of α- or α + β-type titanium alloys. Grain refinement by severe plastic deformation (SPD) techniques provides a unique opportunity to enhance mechanical properties to prolong the lifetime of orthopaedic implants without changing their chemical composition. In this study, β-type Ti–45Nb (wt%) biomedical alloy in the form of 30 mm rod was subjected to hydrostatic extrusion (HE) to refine the microstructure and improve its mechanical properties. HE processing was carried out at room temperature without intermediate annealing in a multi-step process, up to an accumulative true strain of 3.5. Significant microstructure refinement from a coarse-grained region to an ultrafine-grained one was observed by optical and transmission electron microscopy. Vickers hardness measurements (HV0.2) demonstrated that the strength of the alloy increased from about 150 to 210 HV0.2. Nevertheless, the measurements of Young’s modulus by nanoindentation showed no significant changes. This finding is substantiated by X-ray diffraction analyses which did not exhibit any phase transformation out of the bcc phase being present still before processing by HE. These results thus indicate that HE is a promising SPD method to obtain significant grain refinement and enhance strength of β-type Ti–45Nb alloy without changing its low Young’s modulus, being one prerequisite for biomedical application.

Nanostructure Of β-type Titanium Alloys Through Severe Plastic Deformation

A novel β-type titanium alloy Ti–29Nb–13Ta–4.6Zr (TNTZ) has been developed and extensively researched to achieve highly desirable mechanical properties such as a high strength while maintaining a low Young’s modulus that is close to that of bone, as an alternative candidate for conventional titanium metallic biomaterials such as Ti-6Al-4V ELI. Therefore, strengthening by grain refinement and increasing dislocation density is expected to provide TNTZ high mechanical strength while keeping a low Young’s modulus because they keep the original β phase. In this case, severe plastic deformation, such as high-pressure torsion (HPT) processing, is a potential treatment for obtaining these properties. Furthermore, HPT processing is effective for producing ultrafine-grained TNTZ having high dislocation density in single β structure. The obtained promising results, which are a tensile strength of around 1100 MPa and a Young's modulus of around 60 GPa, motivated that the above mentioned mechanical properties can be achieved by microstructural refinement through HPT processing However, the mechanism of microstructural refinement is unclear for TNTZ during HPT processing. Therefore, the aim of this study is to investigate microstructural changes of TNTZ through HPT processing by X-ray diffraction analysis and transmission electron microscopy. The microstructures of TNTZ subjected to cold rolling (TNTZCR) and HPT processing (TNTZHPT) comprised single β grains; however, the intense β {110} peak reveals that the preferred orientation is β for TNTZHPT. While the microstructure of TNTZCR shows a comparatively high dislocation density (2.3 x 10

Low modulus Ti–Nb–Hf alloy for biomedical applications

β-Type titanium alloys with a low elastic modulus are a potential strategy to reduce stress shielding effect and to enhance bone remodeling in implants used to substitute failed hard tissue. For biomaterial application, investigation on the mechanical behavior, the corrosion resistance and the cell response is required. The new Ti25Nb16Hf alloy was studied before and after 95% cold rolling (95% C.R.). The mechanical properties were determined by tensile testing and its corrosion behavior was analyzed by potentiostatic equipment in Hank's solution at 37°C. The cell response was studied by means of cytotoxicity evaluation, cell adhesion and proliferation measurements. The stress–strain curves showed the lowest elastic modulus (42GPa) in the cold worked alloy and high tensile strength, similar to that of Ti6Al4V. The new alloy exhibited better corrosion resistance in terms of open circuit potential (EOCP), but was similar in terms of corrosion current density (iCORR) compared to Ti grade II. Cytotoxicity studies revealed that the chemical composition of the alloy does not induce cytotoxic activity. Cell studies in the new alloy showed a lower adhesion and a higher proliferation compared to Ti grade II presenting, therefore, mechanical features similar to those of human cortical bone and, simultaneously, a good cell response.

Relationship between Heterogeneous Microstructure and Fatigue Strength of Ti-Nb-Ta-Zr Alloy for Biomedical Materials Subjected to Aging Treatments

β-type titanium alloys such as a Ti–29Nb–13Ta–4.6Zr alloy (TNTZ) are potential candidates for next-generation metallic biomaterials. However, the mechanical strength of β-type titanium alloys with a single β phase is not enough to be approved as materials for fabricating medical implant devices that are subjected to heavy loads, such as a spinal fixation device. Therefore, β-type titanium alloys are often subjected to aging treatments in order to improve their mechanical strength through precipitation hardening. However, β-type titanium alloys exhibit a heterogeneous microstructure because of microscale elemental segregation. In this study, the heterogeneous microstructure caused by the microsegregation of secondary phases was characterized by field emission scanning electron microscopy (FESEM) in TNTZ subjected to aging treatments. Furthermore, the influence of the heterogeneous distribution of secondary phases in TNTZ on mechanical properties was revealed by comparing its properties to the homogeneously structured TNTZ subjected to long-term homogenization.

Biofunctional Surface Layer and its Bonding Strength in Low Modulus β-Type Titanium Alloy for Biomedical Applications

Recently, low-modulus β-type titanium alloys have been the focus of considerable attention because of their high biocompatibility and their low moduli that make them effective for inhibiting bone atrophy and for enhancing bone remodeling. However, the biofunctinalities of titanium alloys, such as bone conductivity, blood compatibility, and soft tissue compatibility, are poor. Therefore, surface modification techniques such as bioactive ceramic surface modification and blood-and soft-tissue-compatible polymer surface modification are applied to titanium alloys. Hydroxyapatite (HAp) surface modification via metal organic chemical vapor deposition (MOCVD) and segmented polyurethane (SPU) surface modification via silane coupling treatment are effective techniques to add biofunctionalites to titanium alloys. HAp surface modification via MOCVD and SPU surface modification using three kinds of silane coupling agents on a low modulus beta-type titanium alloy, namely, TNTZ, are discussed. Moreover, the bonding strengths of HAp and SPU on the surface of TNTZ, which are important parameters, are also discussed.

Biomedical Polymer Surface Modification of Beta-Type Titanium Alloy for Implants through Anodic Oxide Nanostructures

Anodic oxide nanostructures (nanopores and nanotubes) were formed on a biomedical β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), in order to improve adhesive strength by the anchor effect of a segmented polyurethane (SPU) with soft tissue compatibility. The nanotube structure was formed beneath the nanoporous structure. The adhesive strength between the SPU coating and the nanoporous structure formed on TNTZ by anodization is more than 1.5 times that of an SPU coating on as-polished TNTZ with a mirror finish. After removal of the nanoporous structure by etching with HF solution, the adhesive strength of the SPU coating on the exposed nanotube structure is decreased.

Changeable Young’s modulus with large elongation-to-failure in β-type titanium alloys for spinal fixation applications

Materials used for implant rods in spinal fixation systems must show changeable Young’s moduli, high strength and large elongation-to-failure. A d-electron design method was used to determine the chemical composition of Ti–10Cr, which showed all these properties and significant work-hardening characteristics owing to multiple plastic deformation mechanisms, such as deformation-induced ω-phase transformation, {3 3 2} mechanical twinning and dislocation gliding. Therefore, Ti–10Cr exhibits great potential for use in spinal fixation applications.

Electrochemical characterization of nanostructured Ti–24Nb–4Zr–8Sn alloy in 3.5% NaCl solution

A multifunctional β-type titanium alloy Ti–24Nb–4Zr–8Sn (Ti2448) with high strength and low elastic modulus has advantage of rapid grain refinement by conventional cold processes due to its special highly localized plastic deformation behavior. In the paper, electrochemical behavior of the cold-rolled alloy with nanostructured (NS) grains was studied in 3.5% NaCl solution by potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS) measurement, Lgi-Lgt curve and Mott–Schottky analyses, as well as X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results showed that the NS alloy has better corrosion resistance than the coarse-grained (CG) alloy because the former has more stable and thicker protective passive layer. Such dioxide layer consists of an inner barrier layer and an outer porous layer for both the CG and NS alloy whereas the NS alloy is preponderant in thickness. The passive film is n-type semiconductor and its donor density can be decreased by grain refinement. Therefore, the above positive effect can be explained by the homogeneous and dense passive film formed on the NS Ti2448 alloy due to its high density of grain boundary.

A review of surface modification of a novel low modulus β-type titanium alloy for biomedical applications

Ti-29Nb-13Ta-4.6Zr (TNTZ) is a novel β-type titanium alloy that has great potential use in biomedical implants because of its relatively low Young’s modulus (around 60 GPa), which minimise bone atrophy due to stress shielding. However, the biocompatibility and bio-functionality of TNTZ is poor. Therefore, a bioactive ceramic surface modification or soft-tissue-compatible polymer surface modification is advantageous for enhancing the biological properties of TNTZ. The bio-functionality of TNTZ itself, and that modified with a coating hydroxyapatite (HAp) using alkali treatment, electrochemical treatment, and dip-coating treatment with calcium phosphate glass ceramic or metal organic chemical vapour deposition (MOCVD) is assessed. In addition, modification of TNTZ with soft-tissue-compatible segmented polyurethane (SPU) using different silane coupling agents is described. The bonding strength of the bioactive ceramic and soft-tissue compatible polymer with the TNTZ is assessed with respect to variations in surface roughness.

A New Low-Cost β-Type High-Strength Titanium Alloy with Lower Alloying Percentage for Spring Applications

Spring applications require materials that possess a low Young’s modulus and high strength, in addition to having a lower alloying percentage for reduced costs. In this study, a new low-cost β-type titanium alloy with a lower alloying percentage than conventional β-type alloys was developed for spring applications using [Al]eq and [Mo]eq. The alloy is located in the near-β region and its β transus temperature is about 1023 K (750°C). This alloy could be cold rolled to a reduction ratio of 86.7% without any intermediate heat treatment. The solutionized alloy exhibits an ultimate strength of 925 MPa, a 0.2% proof stress of 910 MPa, an elongation of 18%, and a Young’s modulus of 78 GPa; the comprehensive characteristics are better than those of conventionally used alloys (Ti-15-3 and βC) for spring applications.

Surface Age-Hardening Layer Formation by Two-Step Solution Treatment of β-Type Titanium Alloys

Surface Age-Hardening Layer Formation by Two-Step Solution Treatment of β-Type Titanium Alloys

Adhesive strength of medical polymer on anodic oxide nanostructures fabricated on biomedical β-type titanium alloy

Anodic oxide nanostructures (nanopores and nanotubes) were fabricated on a biomedical β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), by anodization in order to improve the adhesive strength of a medical polymer, segmented polyurethane (SPU), to TNTZ. TNTZ was anodized in 1.0M H3PO4 solution with 0.5 mass% NaF using a direct-current power supply at a voltage of 20V. A nanoporous structure is formed on TNTZ in the first stage of anodization, and the formation of a nanotube structure occurs subsequently beneath the nanoporous structure. The nanostructures formed on TNTZ by anodization for less than 3600s exhibit higher adhesive strengths than those formed at longer anodization times. The adhesive strength of the SPU coating on the nanoporous structure formed on top of TNTZ by anodization for 1200s improves by 144% compared to that of the SPU coating on as-polished TNTZ with a mirror surface. The adhesive strength of the SPU coating on the nanotube structure formed on TNTZ by anodization for 3600s increases by 50%. These improvements in the adhesive strength of SPU are the result of an anchor effect introduced by the nanostructures formed by anodization. Fracture occurs at the interface of the nanoporous structure and the SPU coating layer. In contrast, in the case that SPU coating has been performed on the nanotube structure, fracture occurs inside the nanotubes.

Effects of micro- and nano-scale wave-like structures on fatigue strength of a beta-type titanium alloy developed as a biomaterial

Some newly developed β-type titanium alloys for biomedical applications exhibit distinctive heterogeneous structures. The formation mechanisms for these structures have not been completely revealed; however, understanding these mechanisms could lead to improving their properties. In this study, the heterogeneous structures of a Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), which is a candidate for next-generation metallic biomaterials, were analyzed. Furthermore, the effects of such heterogeneous structures on the mechanical strength of this alloy, including fatigue strength, were revealed by comparing its strength to that of homogenous TNTZ. The heterogeneous structures were characterized micro-, submicro- and nano-scale wave-like structures. The formation mechanisms of these wave-like structures are found to be different from each other even though their morphologies are similar. It is revealed that the micro-, submicro- and nano-scale wave-like structures are caused by elemental segregation, crystal distortion related to kink band and phase separation into β and β′, respectively. However, these structures have no significant effect on both tensile properties and fatigue strength comparison with homogeneous structure in this study.

Development of Changeable Young's Modulus with Good Mechanical Properties in β-Type Ti-Cr-O Alloys

A novel β-type titanium alloy with a changeable Youngs modulus, that is, with a low Young's modulus to prevent the stress-shielding effect for patients and a high Young's modulus to suppress springback for surgeons, should be developed in order to satisfy the conflicting requirements of both the patients and surgeons in spinal fixation operations. In this study, the oxygen content in ternary Ti-11Cr-O alloys was optimized in order to achieve a large changeable Young's modulus with good mechanical properties for spinal fixation applications. The increase in Youngs moduli of all the examined alloys by cold rolling is attributed to the deformation-induced ω-phase transformation which is suppressed by oxygen. Among the examined alloys, the Ti-11Cr-0.2O alloy exhibits the largest changeable Youngs modulus and a high tensile strength with an acceptable plasticity under both solution-treated (ST) and cold-rolled (CR) conditions. Therefore, the Ti-11Cr-0.2O alloy, which shows a good balance among a changeable Youngs modulus, high tensile strength and good plasticity, is considered a potential candidate for spinal fixation applications.