On the Effects of Hot Forging and Hot Rolling on the Microstructural Development and Mechanical Response of a Biocompatible Ti Alloy

The Effect of the Forging Process on the Microstructure and Mechanical Properties of a New Low-Cost Ti-5Al-1.5Mo-1.8Fe Alloy

This paper presents results on the microstructure and mechanical properties of a new low-cost titanium alloy Ti-5Al-1.5Mo-1.8Fe after different forging processes. The β phase transformation temperature of this alloy was 950 °C. In this study, the forging temperatures were designed at 920 °C and 980 °C, and the deformation degree ranged from 20% to 60%, with an interval of 20%. This study investigated the impact of the equiaxed α phase and shape of the lamellar microstructure on the tensile characteristics and fracture toughness of an alloy. The research employed a microstructure analysis and static tensile testing to evaluate the effect of forging temperatures and degree of deformation on the microstructure features. The findings revealed that forging temperatures could modify the microstructure characteristics, and the degree of deformation also affected this microstructure. This study demonstrates that a bimodal structure with an equiaxed α phase can be utilized to balance high strength and high ductility, resulting in better overall mechanical properties.

Mechanical Performance of Metallic Bone Screws Evaluated Using Bone Models

To evaluate mechanical performance properties of various types of cortical bone screw, cancellous bone screw, and locking bolt, we conducted torsional breaking and durability tests, screw driving torque tests into bone models, and screw pullout tests (crosshead speed: 10 mm/min) after driving torque tests. The 2° proof and rupture torques of a screw, which were estimated from torque versus rotational angle curves, increased with increasing core diameter of the screw. The durability limit of metallic screws obtained by four-point bending durability tests increased with increasing core diameter. The compressive, tensile, and shear strengths of the bone models used for the mechanical testing of orthopedic devices increased with increasing density of the bone model. The strength and modulus obtained for solid rigid polyurethane foam (SRPF) and cellular rigid polyurethane foam (CRPF) lay on the same straight line. Among the three strengths, the rate of increase in compressive strength with the increase in density was the highest. The maximum torque obtained by screw driving torque tests for up to 8.3 rotations (3000°) into the bone models tended to increase with increasing core diameter. In particular, the maximum torque increased linearly with increasing effective surface area of the screw, as newly defined in this work. The maximum pullout load increased linearly with increasing number of rotations and mechanical strength of the bone model. Screws with low driving torque and high pullout load were considered to have excellent fixation and are a target for development.

Microstructures and Mechanical Properties of Laser-Sintered Commercially Pure Ti and Ti-6Al-4V Alloy for Dental Applications

To apply laser-sintered titanium (Ti) materials to dental prostheses with a three-dimensional structure such as partial dentures, we examined the microstructures and mechanical properties of commercially pure (CP) Ti grade (G) 2 annealed after laser sintering and laser-sintered (as-built) Ti-6Al-4V alloy. The tensile and fatigue properties of CP Ti G 2 annealed at 700 °C for 2 h after laser sintering were close to those of wrought CP Ti G 2 annealed at the same temperature after hot forging. The ultimate tensile strengths (σ_UTS) of 90°- and 0°-direction-built CP Ti G 2 rods after laser sintering 10 times were 553 and 576 MPa and the total elongations (TE) of these rods were 26% and 28%, respectively. The fatigue strengths (σ_FS) at 10⁷ cycles of the 90°- and 0°-direction-built CP Ti G 2 rods after laser sintering 10 times were ~320 and ~365 MPa, respectively. The ratio σ_FS/σ_UTS was in the range of 0.5-0.7. The changes in the chemical composition and mechanical properties after laser sintering 10 times were negligible. The fatigue strength of the laser-sintered Ti-6Al-4V alloy was ~600 MPa, which was close to that of wrought Ti-6Al-4V alloy. These findings indicate that the laser-sintered CP Ti and Ti-6Al-4V alloy can also be applied in dental prostheses similarly to laser-sintered Co-Cr-Mo alloy. In particular, it was clarified that laser sintering using CP Ti G 4 powder is useful for dental prostheses.

Characterization of Oxide Film of Implantable Metals by Electrochemical Impedance Spectroscopy

The oxide film resistance (R_P) and capacitance (C_CPE) diagrams of implantable metals (commercially pure Ti, four types of Ti alloys, Co–28Cr–6Mo alloy, and stainless steel) were investigated by electrochemical impedance spectroscopy (EIS). The thin oxide film formed on each implantable metal surface was observed in situ by field-emission transmission electron microscopy (FE-TEM). The Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys had higher oxygen concentrations in the oxide films than the Ti–6Al–4V alloy. The thickness (d) of the TiO₂ oxide films increased from approximately 3.5 to 7 nm with increasing anodic polarization potential from the open-circuit potential to a maximum of 0.5 V vs. a saturated calomel electrode (SCE) in 0.9% NaCl and Eagle’s minimum essential medium. R_P for the Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys was proportional to d obtained by FE-TEM. C_CPE was proportional to 1/d. R_P tended to decrease with increasing C_CPE. R_P was large (maximum: 13 MΩ·cm²) and C_CPE was small (minimum: 12 μF·cm⁻²·sⁿ⁻¹, n = 0.94) for the Ti–15Zr–4Nb–(0 to 4)Ta alloys. The relative dielectric constant (ε_r) and resistivity (k_OX) of the oxide films formed on these alloys were 136 and 2.4 × 10⁶–1.8 × 10⁷ (MΩ·cm), respectively. The Ta-free Ti–15Zr–4Nb alloy is expected to be employed as an implantable material for long-term use.

Strength⁻Durability Correlation of Osteosynthesis Devices Made by 3D Layer Manufacturing

To develop orthopedic implants that are optimized for each patient’s needs or skeletal structure (custom-made implants), evaluations of the bending strength, bending stiffness, and durability of various types of conventional osteosynthesis devices have become important. Four-point bending tests and compression bending tests of osteosynthesis devices (bone plates, intramedullary nail rods, spinal rods, compression hip screws (CHSs), short femoral nails, and metaphyseal plates) were carried out to measure their bending stiffness, bending strength, and durability. The bending stiffness of bone plates, intramedullary nails, spinal rods, CHSs, short femoral nails, and metaphyseal plates increased with increasing bending strength. The durability limit of various types of osteosynthesis devices linearly increased with increasing bending strength. The relationship (durability limit at 10⁶ cycles) = 0.67 × (bending strength) (N·m) (R² = 0.85) was obtained by regression. The relationship for the highly biocompatible Ti-15Zr-4Nb-4Ta alloy was also linear. The mechanical strength and ductility of specimens that were cut from various osteosynthesis devices were excellent and their microstructures consisted of fine structures, which were considered to be related to the excellent durability. These results are expected to be useful for the development of implants suitable for the skeletal structure of patients using three-dimensional (3D) layer manufacturing technologies.

Spatial stress distribution analysis by thermoelastic stress measurement and evaluation of effect of stress concentration on durability of various orthopedic implant devices

Toward the development of highly durable devices, we investigated the effect of the thermoelastic constants of implantable raw metals and the surface stress distribution on the durability of various types of implant device by thermoelastic stress measurement and by evaluating the effect of the stress concentration. Surface stress was dynamically calculated from the bending moment, and the modulus of a section of a device was found to be consistent with the surface stress obtained by thermoelastic stress measurement. The durability limits of various types of bone plate and compression hip screw (CHS) calculated from maximum load vs number of cycles data (L-N data) were close to the notch fatigue strength of the raw material. The concentration factor of an artificial hip stem surface was estimated by comparing the L-N data of the stem and the S-N curve of the raw material. The dynamic analysis of durability by thermoelastic stress measurement is useful for selecting the worst case (a product deteriorating to the most severe state) in medical device design.

Biocompatibility and Surface Properties of TiO₂ Thin Films Deposited by DC Magnetron Sputtering

We present the study of the biocompatibility and surface properties of titanium dioxide (TiO₂) thin films deposited by direct current magnetron sputtering. These films are deposited on a quartz substrate at room temperature and annealed with different temperatures (100, 300, 500, 800 and 1100 °C). The biocompatibility of the TiO₂ thin films is analyzed using primary cultures of dorsal root ganglion (DRG) of Wistar rats, whose neurons are incubated on the TiO₂ thin films and on a control substrate during 18 to 24 h. These neurons are activated by electrical stimuli and its ionic currents and action potential activity recorded. Through X-ray diffraction (XRD), the surface of TiO₂ thin films showed a good quality, homogeneity and roughness. The XRD results showed the anatase to rutile phase transition in TiO₂ thin films at temperatures between 500 and 1100 °C. This phase had a grain size from 15 to 38 nm, which allowed a suitable structural and crystal phase stability of the TiO₂ thin films for low and high temperature. The biocompatibility experiments of these films indicated that they were appropriated for culture of living neurons which displayed normal electrical behavior.

Comparison of Fatigue Properties and Fatigue Crack Growth Rates of Various Implantable Metals

The fatigue strength, effects of a notch on the fatigue strength, and fatigue crack growth rate of Ti-15Zr-4Nb-4Ta alloy were compared with those of other implantable metals. Zr, Nb, and Ta are important alloying elements for Ti alloys for attaining superior long-term corrosion resistance and biocompatibility. The highly biocompatible Ti-15Zr-4Nb-4Ta alloy exhibited an excellent balance between strength and ductility. Its notched tensile strength was much higher than that of a smooth specimen. The strength of 20% cold-worked commercially pure (C.P.) grade 4 Ti was close to that of Ti alloy. The tension-to-tension fatigue strength of an annealed Ti-15Zr-4Nb-4Ta rod at 10⁷ cycles was approximately 740 MPa. The fatigue strength of this alloy was much improved by aging treatment after solution treatment. The fatigue strengths of C.P. grade 4 Ti and stainless steel were markedly improved by 20% cold working. The fatigue strength of Co-Cr-Mo alloy was markedly increased by hot forging. The notch fatigue strengths of 20% cold-worked C.P. grade 4 Ti, and annealed and aged Ti-15Zr-4Nb-4Ta, and annealed Ti-6Al-4V alloys were less than those of the smooth specimens. The fatigue crack growth rate of Ti-15Zr-4Nb-4Ta was the same as that of Ti-6Al-4V. The fatigue crack growth rate in 0.9% NaCl was the same as that in air. Stainless steel and Co-Cr-Mo-Ni-Fe alloy had a larger stress-intensity factor range (ΔK) than Ti alloy.