Papers by Author: Masaaki Nakai

Abstract: The microstructural evolution and its effect on biocompatibility of TNTZ through HPT processing were investigated systematically in this study. TNTZ_AHPT shows an enhanced mechanical biocompatibility, which is characterized by a higher tensile strength (1375 MPa) and hardness (450 HV) than those of TNTZ_ST, TNTZ_AT, 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 TNTZ_AT consists of randomly distributed needle-like α precipitates in the equiaxed β grains with a diameter of approximately 40 m. The microstructure of TNTZ_AHPT 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. TNTZ_AHPT exhibits an enhanced combination of excellent corrosion performance and improved cellular response compared to TNTZ_ST, TNTZ_AT, and Ti64 ELI.

Abstract: 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 (TNTZ_AHPT) 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 TNTZ_AHPT at N = 20. TNTZ_AHPT a great fatigue strength, which is comparable to those of (Ti64) ELI.

Abstract: 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.

Abstract: The fatigue strength of the newly developed β-type titanium alloy Ti-29Nb-13Ta-4.6Zr (referred to as TNTZ) is important for its applications in spinal fixation rods. Enhancements in the fatigue strength of TNTZ upon solution treatmnet and thermo-mechanical processing (aging after severe cold-caliber rolling or sever cold-swaging) have been investigated. Aging the rods at 723 K for 259.2 ks after cold-swaging is shown to be the optimal thermo-mechanical processing method for the TNTZ rod , yeilding a high 0.2% proof stress of about 1200 MPa, high elongation of 18%, and high fatigue strength of 950 MPa. High springback is undesirable for application of TNTZ spinal-fixation rods. To reduce springback, a High Youngs modulus is required; on the other hand, a low Youngs modulus is beneficial to reduce bone absorption and obtain good bone remodeling. To achieve a balance between these two factors, TNTZ has been modified by adding Cr to obtain an alloy with a high Youngs modulus at the deformed region and a low Youngs modulus throughout the undeformed region. TNTZ-8Ti-2Cr and TNTZ-16Ti-4Cr are novel alloys whose β phase is more stable than that of TNTZ. Only the β phase is present in these alloys, which exhibit relatively low Youngs moduli of <65 GPa after solution treatment, and higher Youngs moduli after cold rolling, owing to a deformation-induced ω-phase transformation. These modified TNTZ alloys show significantly less springback than the original TNTZ, based on tensile and loadingunloading bending tests.

Abstract: The objective of this study was to evaluate the corrosion behavior of Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) with immersion in an acidic saline solution containing fluoride by investigating change in color and the surface structure of the oxide film. With immersion in fluoride-containing solution, TNTZ showed a less marked change in color than commercially pure titanium (TI), and a smaller decrease in glossiness. The outermost surface was covered with oxides from its constituent elements at before and after immersion in solution with or without fluoride. When immersed in fluoride-containing solution, the film consisted of larger niobium and tantalum oxides than that before or after immersion in solution without fluoride. In summary, TNTZ showed superior resistance to discoloration to TI after immersion in fluoride-containing solution. The results suggest that the subsequent increase in niobium and tantalum fractions in the oxide film in TNTZ improves resistance to corrosion.

Abstract: Change in the Microstructure of the L1₀-Type Ordered β' Phase Precipitated in Ag-20Pd-12Au-14.5Cu Alloy (mass%) Subjected to Solution Treatment with Varying Solution Treatment Time Was Investigated. The Size of the β' Phase Is Found to Decrease with Increasing Solution Treatment Time and the Vickers Hardness of the Alloy after Solution Treatment Decreases. Experimental Observations Show that the Microstructural Change of the β' Phase Strongly Contributes to the Change in Vickers Hardness. In Addition, the Formation and Growth of the β' Phase Are Concluded to Be Affected by the Distribution of Elements through Solution Treatment.

Abstract: In the Present Study, the Effects of the Microstructural Morphologies of a Ti-6Al-4V (Ti-64) Alloy on its Fatigue Behavior Were Investigated. Ti-64 Bars Were Subjected to Two Different Thermo-Mechanical Processing Methods. The First Sample, Referred to as Material-A, Had a Forged Microstructure with the Average Primary α Volume Fraction of 44%. The Second One, Referred to as Material-B, Had a Hot-Rolled Microstructure with the Average Primary α Volume Fraction of 43%. Fatigue Tests Were Performed on each Sample to Obtain S-N Curves. The Microstructure of each Sample Was Observed Using an Optical Microscopy in Order to Measure the Grain Sizes of the Primary α and Secondary α Phases. The Results of the Fatigue Tests Indicated that Material-B Demonstrates Better Fatigue Strength than Material-A. The Microstructure of the Longitudinal Section of each Material Was Also Observed to Analyze the Results of the Fatigue Tests. The Measured Diameters and Volume Fractions of the Primary α Phases of the Two Types of Materials Are Similar. On the other Hand, the Secondary α Width of each Material Is Different. It Is Found that Fatigue Strength Is Related to the Width of the Secondary α Phase.

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: Presently Metallic Rods that Are Used for Spinal Fixtures Cannot Meet the Requirements of both Surgeons and Patients; Surgeons Require the Material to Have a High Young’s Modulus to Suppress Springback during the Operation, whereas Patients Require the Material to Have a Low Young’s Modulus to Prevent the Stress-Shielding Effect. In Order to Develop a Novel Biomedical Titanium Alloy with a Changeable Young’s Modulus for Spinal Fixation Applications via Deformation-Induced ω Phase Transformation. The Effects of Deformation-Induced Phases on the Mechanical Properties of Metastable β-Type Ti-xCr Alloys Were Investigated. The Experimental Results Indicate that the Young’s Moduli, Tensile Strength, and Vickers Hardness of the Ti–(10–12)Cr Alloys Increase Remarkably by Cold Rolling. The Results of the Microstructural Observations of Ti–12Cr Alloys Using a Transmission Electron Microscopy (TEM) Show that Deformation-Induced ω Phase Transformation Occurs during Cold Rolling. Therefore, the Increase in Young’s Modulus of the Alloys Is Attributed to the Deformation-Induced ω Phase, which Is Formed in the Alloy during Cold Rolling at Room Temperature.

Abstract: A novel biomedical titanium alloy with the ability to undergo self-adjustment in its Young’s modulus was developed. In spinal fixation devices, the Young’s modulus of the metallic implant rod should be sufficiently high to suppress springback for the surgeon, but should also be sufficiently low to prevent stress shielding for the patient. Therefore, deformation-induced ω phase transformation was introduced into β-type titanium alloys so that the Young’s modulus of only the deformed part would increase during operation, while that of the non-deformed part would remain low. The increase in the Young’s modulus due to cold rolling was investigated for a binary Ti-12Cr alloy (mass%). Microstructural observation and Young’s modulus measurement reveal that the Ti-12Cr alloy undergoes deformation-induced ω phase transformation and exhibits the increase in the Young’s modulus by deformation.