Kengo Narita

Predominant factor determining wear properties of β-type and (α+β)-type titanium alloys in metal-to-metal contact for biomedical applications

The predominant factor determining the wear properties of a new titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ) and a conventional titanium alloy, Ti-6Al-4V extra-low interstitial (Ti64) was investigated for TNTZ and Ti64 combinations in metal-to-metal contacting bio-implant applications. The worn surfaces, wear debris, and subsurface damages were analyzed using a scanning electron microscopy combined with energy-dispersive spectroscopy and electron-back scattered diffraction analysis. The volume loss of TNTZ is found to be larger than that of Ti64, regardless of the mating material. The wear track of TNTZ exhibits the galled regions and severe plastic deformation with large flake-like debris, indicative of delamination wear, which strongly suggests the occurrence of adhesive wear. Whereas, the wear track of Ti64 have a large number of regular grooves and microcuttings with cutting chip-like wear debris and microfragmentation of fine oxide debris, indicative of abrasive wear combined with oxidative wear. This difference in the wear type is caused by severe and mild subsurface deformations of TNTZ and Ti64, respectively. The lower resistance to plastic shearing for TNTZ compared to that of Ti64 induces delamination, resulting in a higher wear rate.

Development of thermo-mechanical processing for fabricating highly durable β-type Ti–Nb–Ta–Zr rod for use in spinal fixation devices

The mechanical strength of a beta titanium alloy such as Ti-Nb-Ta-Zr alloy (TNTZ) can be improved significantly by thermo-mechanical treatment. In this study, TNTZ was subjected to solution treatment, cold caliber rolling, and cold swaging before aging treatment to form a rod for spinal fixation. The {110}(β) are aligned parallel to the cross-section with two strong peaks approximately 180° apart, facing one another, in the TNTZ rods subjected to cold caliber rolling and six strong peaks at approximately 60° intervals, facing one another, in the TNTZ rods subjected to cold swaging. Therefore, the TNTZ rods subjected to cold swaging have a more uniform structure than those subjected to cold caliber rolling. The orientation relationship between the α and β phases is different. A [110](β)//[121](α), (112)(β)//(210)(α) orientation relationship is observed in the TNTZ rods subjected to aging treatment at 723 K after solution treatment and cold caliber rolling. On the other hand, a [110](β)//[001](α), (112)(β)//(200)(α) orientation relationship is observed in TNTZ rod subjected to aging treatment at 723 K after cold swaging. A high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa indicate that aging treatment at 723 K after cold swaging is the optimal thermo-mechanical process for a TNTZ rod.

Wear transition of solid-solution-strengthened Ti–29Nb–13Ta–4.6Zr alloys by interstitial oxygen for biomedical applications

In previous studies, it has been concluded that volume losses (V loss) of the Ti-29Nb-13Ta-4.6Zr (TNTZ) discs and balls are larger than those of the respective Ti-6Al-4V extra-low interstitial (Ti64) discs and balls, both in air and Ringers solution. These results are related to severe subsurface deformation of TNTZ, which is caused by the lower resistance to plastic shearing of TNTZ than that of Ti64. Therefore, it is necessary to further increase the wear resistance of TNTZ to satisfy the requirements as a biomedical implant. From this viewpoint, interstitial oxygen was added to TNTZ to improve the plastic shear resistance via solid-solution strengthening. Thus, the wear behaviors of combinations comprised of a new titanium alloy, TNTZ with high oxygen content of 0.89 mass% (89O) and a conventional titanium alloy, Ti64 were investigated in air and Ringers solution for biomedical implant applications. The worn surfaces, wear debris, and subsurface damage were analyzed using a scanning electron microscopy and an electron probe microanalysis. V loss of the 89O discs and balls are smaller than those of the respective TNTZ discs and balls in both air and Ringers solution. It can be concluded that the solid-solution strengthening by oxygen effectively improves the wear resistance for TNTZ materials. However, the 89O disc/ball combination still exhibits higher V loss than the Ti64 disc/ball combination in both air and Ringers solution. Moreover, V loss of the disc for the 89O disc/Ti64 ball combination significantly decreases in Ringers solution compared to that in air. This decrease for the 89O disc/Ti64 ball combination in Ringers solution can be explained by the transition in the wear mechanism from severe delamination wear to abrasive wear.

Enhancing Functionalities of Metallic Materials by Controlling Phase Stability for Use in Orthopedic Implants

This chapter aims to review the recent trends pertaining to the enhanced functionalities, including low Young’s modulus, self-tunable Young’s modulus, and low magnetic susceptibility, of titanium and zirconium alloys for use in orthopedic implants. These value-added functionalities can be realized by controlling the type of crystal structure and their lattice structure stabilities, which are related to the phase stability of titanium and zirconium alloys.

Bending Fatigue and Spring Back Properties of Implant Rods Made of β-Type Titanium Alloy for Spinal Fixture

Implanting a spinal fixture using metallic rods is one of the effective treatments for spinal diseases. Because cyclic bending stress is loaded on the implant rods when patients move their upper bodies in daily life, bending fatigue properties are important for the implant rod. Further, the implant rods are bended plastically into a curved shape of spine by hand in a surgical operation. In that case, keeping shape is important, namely bending spring back properties are important factors. On the other hand, a biomedical β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (mass %) alloy (TNTZ), has been developed by the authors. Currently, this alloy are investigated to be applied to the above mentioned implant rod practically. Therefore, four-point bending fatigue and three point-bending spring back properties of TNTZ subjected various heat treatments were examined in this study. TNTZ rods were subjected to solution treatment, and then some of them were subjected to aging treatment at 673 K or 723 K for 259.2 ks, followed by water quenching. Then, four-point bending fatigue and three-point bending spring back tests were carried out on TNTZ rods subjected to the various heat treatments mentioned above. The bending fatigue strength at 2.5 million cycles in the high cycle fatigue region are not much different among any TNTZ rod. However, the bending fatigue strength of the Ti-6Al-4V ELI (Ti64) rod exceeds the fatigue strengths of every TNTZ rods in both low and high cycle fatigue regions. On the other hand, the lower spring back, which is a favorable property, was obtained for some TNTZ rod than Ti64 rod.

Beta-Type Titanium Alloys for use as Rods in Spinal Fixation Devices

Ti-12Cr has been developed for use in various biomedical applications, in particular; it is expected to be used for the rods of the spinal fixation devices. When Ti-12Cr is deformed, its Young’s modulus increases because of deformation-induced ω phase transformation. If a spinal rod made of Ti-12Cr is bent during operation, only the Young’s modulus of the bent region will increase; this phenomenon decreases the springback of the rod so that the bent shape is maintained. The compression fatigue strength of Ti-12Cr obtained from compression fatigue tests performed according to ASTM F1717 can be significantly improved by cavitation peening. Details of the development of this Ti-Cr alloy for use as spinal rods are discussed.

Endurance of Low‐Modulus β‐Type Titanium Alloys for Spinal Fixation

Newly developed low-modulus (β-type titanium alloys such as Ti-29Nb-13Ta-4.6Zr (TNTZ) and Ti-Cr alloys are expected to be used for rods in spinal fixation devices. As the endurance, i.e., fatigue strength, is one of the most important factors for this application. Various types of fatigue strengths of TNTZ and Ti-Cr alloys as well as their developments were investigated. Compared to conventional Ti-6A1–4V extra low interstitials (ELI) for biomedical applications the uni-axial fatigue strength of TNTZ is increased significantly by thermomechanical processing, which includes solution treatment, severe cold swaging or rolling, and followed by aging treatment. The uni-axial fatigue strength of Ti-12Cr is significantly higher compared to that of Ti-6A1–4V ELI. The four point bending fatigue strengths of the alloys exhibit opposite trends.

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.

Optimization of Mo Content in Beta-Type Ti-Mo Alloys for Obtaining Larger Changeable Young’s Modulus during Deformation for Use in Spinal Fixation Applications

In order to meet the requirements of the patients and surgeons simultaneously for spinal fixation applications, beta (β) -type Ti-Mo alloys with self-tunable Young’s modulus due to deformation have been developed to prevent the stress-shielding effect for patients and to suppress springback for surgeons. In this study, the effects of Mo on the deformation-induced omega-phase transformation were investigated and then the Mo content in binary Ti-Mo alloys was optimized in order to achieve a large increase in Young’s modulus via deformation-induced omega-phase transformation, leading to suppression of springback.

Effect of subsurface deformation on sliding wear behavior of Ti-29Nb-13Ta-4.6Zr alloys for biomedical applications

The wear mechanisms of conventional Ti–6Al–4V extra-low interstitial (Ti64) and the new Ti–29Nb–13Ta–4.6Zr (TNTZ) were studied to investigate the wear properties of Ti64/TNTZ for application in spinal fixation devices. Ti64 and TNTZ balls and discs were first prepared as wear-test specimens. A ball-on-disc frictional wear-testing machine was used in air to perform the frictional wear tests of the Ti64 and TNTZ discs mated against Ti64 and TNTZ balls. The wear mechanisms were investigated using a scanning electron microscopy to analyze the worn surfaces and wear debris. The volume losses for the TNTZ discs were larger than those for the Ti64 ones, regardless of the mating ball material. Furthermore, the morphologies of the wear tracks and the debris of the Ti64 and TNTZ discs were different, suggesting that the wear mechanisms for the Ti64 and TNTZ discs were abrasive and delamination wear caused by mild and severe subsurface deformations of the Ti64 and TNTZ, respectively, regardless of the mating ball material.