Papers by Author: Toshikazu Akahori

Abstract: Plain and notch fatigue properties of a β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (TNTZ), which was subjected to various thermomechanical treatments, were investigated in order to judge its potential for biomedical applications. Microstructures of TNTZ aged at 723 K for 259.2 ks after cold rolling and those aged at 723 K for 259.2 ks after solution treatment are composed of a precipitated α phase in the β phase. However, microstructures of TNTZ aged at 598 and 673 K for 259.2 ks after cold rolling and aged at 598 K and 673 K for 259.2 ks after solution treatment are composed of a precipitated ω phase, and precipitated α and ω phases in the β phase, respectively. Futher, plain fatigue strengths of TNTZ aged after solution treatment and those of TNTZ aged after cold rolling increase with the aging temperature. In particular, TNTZ aged at 723 K after cold rolling exhibits the highest fatigue strength in both the low- and high-cycle fatigue life regions. Futher, the run-out, which is about 770 MPa, is nearly equal to that of hot-rolled Ti-6Al-4V ELI conducted with aging, which is one of the representative α+β-type titanium alloys for biomedical applications. The notch fatigue strengths of TNTZ aged at stress concentration factors of 2 and 6 decrease by 30% – 40% and 50% – 60%, respectively, as compared with the plain fatigue strengths in the low-cycle fatigue life region. Futher, the notch run-out range from 450 to 490 MPa and from 220 to 300 MPa, respecitvely; an exception to this is TNTZ aged at 598 K after cold rolling, which has a high volume fraction of the ω phase. Single- and multi- fatigue cracks initiate at the bottom of the notch at stress concentration factors of 2 and 6, respectively.

Abstract: Synchrotron X-ray microtomography has been utilized for the 3D characterisation of microstructure in the cell materials of aluminium foams. Tomographs, consisting of about 109 isotropic voxels with a maximum of 1.0µm edge, were collected at the SPring-8 in Japan. A combination of high-resolution phase contrast imaging technique and several state-of–the-art application techniques has enabled the quantitative image analyses of micro-pore, intermetallic particles and grain boundary as well as the assessment of their effects on compressive deformation and fracture behaviours in two kinds of aluminium foams.

Abstract: Recently, more attention has been focused on the research and development of biomaterials such as an artificial bone because of the rapid-increasing demand for biomaterials from the aging society in the world. However, the study on the bone itself is not enough. In particular, there is little information about the fracture toughness, and fatigue property of bones, and effect of the microstructure on the fracture characteristic. In this study, the fracture toughness of the bovine and swine humerus and femur as well as the fatigue property of the bovine humerus and femur was investigated with relating microstructures. The fracture toughness of plexiform bone is greater than that of haversian bone, and the fatigue strength of the plexiform bone taken from the humerus and femur is a little greater than that of the haversian bone taken from the humerus and femur in particular in high cycle fatigue life regions.

Abstract: In the present study, the effects of Ta content on the dynamic Young’s modulus and tensile properties of Ti−Ta alloys were investigated in order to find a Ti−Ta alloy that gives low modulus and high strength for biomedical applications. For this purpose, the ingots of Ti−Ta alloys with Ta contents from 10 to 50 mass % were melted, and then rolled into the plate of 3 mm thick. All the specimens were solution treated at 1223 K in the b field for 3.6 ks and then quenched in ice water. Subsequently, some of them were aged at 773 K for 259.2 ks followed by a rapid quenching in ice water. The corrosion capacity and biocompatibility of typical Ti−Ta alloy were also evaluated. The experimental results indicate that the Ti−30% Ta alloy has better mechanical biocompatibility, corrosion capacity and cyto-toxicity than Ti−6Al−4V alloy used as a standard biomaterial, and thus it will be of considerable development for biomedical applications.

Abstract: Duplex calcia coating method where firstly fine calcia was coated on wax pattern and then fine calcia reinforced silica fiber was coated on the fine calcia coated wax pattern, that is, duplex calcia coating method, was applied for making mold. The surface of Ti-29Nb-13Ta-4.6Zr cast using the duplex calcia coating method show very fine metallic lust. The thickness of alpha case formed on the surface of Ti-29Nb-13Ta-4.6Zr is much thinner comparing with the case where the commercial magnesia mold for casting conventional titanium and its alloys for dental applications is used. The duplex calcia coating method is highly expected to be put into practical use for dental precision casting of Ti-29Nb-13Ta-4.6Zr. The dental crown of Ti-29Nb-13Ta-4.6Zr is successfully fabricated using duplex calcia coating method.

Abstract: The effects of microstructure and contact pressure on fretting fatigue characteristics of Ti-4.5Al-3V-2Mo-2Fe conducted with annealing at 1123 K and 1223 K were investigated in this study. Fretting fatigue tests in low and high cycle fatigue life regions of the alloys with equiaxed α and acicular α structures were carried out at each contact pressure of 10, 15, 30, 45, 75, 105 and 153 MPa. In the alloy with equiaxed α structure, fretting fatigue strength tends to be very low at contact pressures of 10 MPa and 15 MPa in low and high cycle fatigue life regions, respectively. Furthermore, fretting fatigue strength tends to be nearly constant at the contact pressure over 45 MPa in each fatigue life region. On the other hand, in the alloy with acicular α structure, fretting fatigue strength tends to be very low at contact pressures of 15 MPa and 30 MPa in low and high cycle fatigue life regions, respectively. Furthermore, fretting fatigue strength tends to be nearly constant at contact pressures of 45 MPa and over 30 MPa in low and high fatigue life regions, respectively.

Abstract: Effects of alloying elements on elastic moduli (Young’s moduli) of Ti-Nb-Ta-Zr system alloys for biomedical applications were investigated. In this case, Ti-30Nb-10Ta-5Zr which is the simplified chemical compositional alloy of Ti-29Nb-13Ta-4.6Zr for biomedical applications has been selected as the basic alloy composition. Each alloying element content of Ti-Nb-Ta-Zr quaternary alloys was varied independently. Nb content was varied from 0 through 40 mass % by eight levels. Ta content was varied from 0 through 20 mass % by five levels. Zr content was varied from 0 through 10 mass % by five levels. Every alloy was fabricated by powder metallurgy processing, followed by forging. Ti-30Nb-10Ta-5Zr with single
phase, which is a basic alloy composition, shows the lowest Young’s modulus.  phase precipitation in
phase has much greater effect on increasing Young’s modulus than  phase precipitation in Ti-Nb-Ta-Zr quaternary alloys.

Abstract: Microstructures of Ti-29Nb-13Ta-4.6Zr (TNTZ) aged at temperatures between 573 and 723 K after solution treatment at 1063 K have super fine omega phase, or􀀂 both super fine alpha and omega phases, respectively in beta phase with an average grain diameter of 20 µm. Plain fatigue strength of TNTZ aged after solution treatment is much greater than that of as-solutionized TNTZ in both low cycle fatigue and high cycle fatigue life regions. This is due to the improvement of the balance of strength and ductility by the precipitation of alpha phase. Fretting fatigue strength of TNTZ conducted with various heat treatments decreases dramatically as compared with their plain fatigue strength in both low cycle fatigue and high cycle fatigue life regions. In this case, the decreasing ratio of fretting fatigue life increases with increasing the small crack propagation area where both the tangential force and frictional force at the contact plane of pad exist. In fretting fatigue in air, the ratio of fretting damage (Pf/Ff), where Pf and Ff stand for plain fatigue limit and fretting fatigue limit, respectively, increases with increasing elastic modulus. In fretting fatigue in Ringer’s solution, the passive film on specimen surface is broken by fretting action in TNTZ, which have excellent corrosion resistance, and, as a result, corrosion pits that lead to decreasing fretting fatigue strength especially in high cycle fatigue life region, are formed on its surface.

Abstract: The effect of Ta content on the mechanical properties of the quenched binary Ti
Ta alloys with different mass percentage of Ta from 10 to 80% was investigated in order to find a Ta content that gives a good balance of low modulus and high strength for biomedical applications. The mechanical properties of binary Ti
Ta alloys depend strongly on the microstructures caused by Ta content. Among all the studied Ti–Ta alloys, Ti
30 mass % Ta alloy with martensite ” and Ti
70 mass % Ta alloy with metastable  phase have the potential to be the new candidates for biomedical applications