Papers by Author: Hisao Fukui

Abstract: Silver alloy is one of semi-precious alloys for dental prosthesis, which has been applied remarkably in Japanese dental field. Ag-20Pd-17.7Cu-12Au alloy (G12), which was newly developed for commercial dental silver alloy, shows the unique hardening mechanism after a simple solution treatment (ST) at relatively high temperature. However, the relationship between ST at various temperature and the mechanical strength of G12 has not been investigated fully. Therefore, the relationship between ST and mechanical properties of G12 after ST at various temperatures was investigated systematically in this study. G12 subjected to ST at relatively high temperature of 1173 K is composed of three kinds of phases with Ag-rich α₂ phase, Cu-rich α₁ phase and β phase of Pd-Cu intermetallic compound. Although nominal melting point of G12 was around 1233 K, the high Cu concentration area like α₁ phase was partially dissolved and then the α₁, α₂ and partial β phases re-precipitated during cooling. On the other hand, the microstructure of G12 aged at 673 K after ST at 1023 K (STA) was mainly composed of two kinds of phases with α₂ and α₁ phases with a small amount of β phase, which was a typical microstructure of STA. The Vickers hardness of G12 subjected to ST at 1173 K was identical to that of the same alloy subjected to STA although the tensile strength, fatigue limit and ductility deteriorated to some degrees. The corrosion potential of G12 changed drastically with the temperature of ST. The potential may have been increased by the decrease in the volume fraction of the α₁ phase with relatively high Cu content.

Abstract: Ti-29Nb-13Ta-4.6Zr (TNTZ), which is one of metastable beta-type Ti alloys, has developed as one of representative biomedical and dental Ti alloys in Japan. TNTZ subjected to solution treatment shows Young’s modulus of 60 GPa, which is close to that of cortical bone. In addition, TNTZ has very low cytotoxicity and good bone biocompatibility as well. Heat treatment like solution treatment and aging (STA) is mainly used for improving the mechanical properties of metastable beta-type Ti alloys because of alpha precipitates, while Young’s modulus also rises drastically. This study was investigated the effects of mechanical surface modifications such as fine particle bombarding (FPB) with steel and hydroxyapatite particles or friction stir processing (FSP) on the mechanical strength of TNTZ in order to maintain low Young’s modulus. The relative bone contact ratios between the cancellous bones of Japanese white rabbits and column-shaped TNTZ subjected to FPB of steel particles were also evaluated. Vickers hardness (HV) of TNTZ subjected to FPB with fine particles of steel and hydroxyapatite particles increased by HV30 to 200 at the edge of the specimen surface to around 100 to 300 mm in depth as compared with that of TNTZ subjected to solution treatment. The hydroxyapatite layer was formed on the specimen surface by FPB with fine particles of hydroxyapatite particles, although the trend was not significant by FPB with steel particles. Furthermore, the fatigue strength in high cycle fatigue region of TNTZ subjected to FPB with steel particles was improved and the fatigue limit showed around 400 MPa, although that of TNTZ subjected to FPB with fine particles of hydroxyapatite particles were around 60 MPa higher than that to TNTZ subjected to solution treatment (230 MPa). TNTZ with a rough surface texture (Ra: 0.65 μm) showed a relative bone contact ratio of more than 80% after undergoing FPB with fine particles of steel particles; this value was significantly higher than that of TNTZ with a surface texture (Ra: 0.07 μm). Lastly, the microstructure of TNTZ subjected to FSP showed the recrystallization area by the frictional heating with very fine equiaxed beta phase with an average grain diameter of 3.0 μm. The change in Vickers hardness of TNTZ subjected to FSP was almost identical to that of Young’s modulus and showed the almost same trend of FPB.

Abstract: Beta-type Ti-29Nb-13Ta-4.6Zr (TNTZ) was recently developed as a representative biomedical Ti alloy. As-solutionized TNTZ has a low Young’s modulus less than 60 GPa close to that of cortical bone along with very low cytotoxicity and good bone biocompatibility. Solution treatment and aging (STA) is a typical heat treatment for improving the mechanical properties of beta-type titanium alloys. However, STA also drastically increases the Young’s modulus. Therefore, this study investigated the effects of surface modification, micro-shot peening, on the mechanical properties of TNTZ subjected to severe thermomechanical treatment in order to maintain a relatively low Young’s modulus. The bone contact characteristics of TNTZ samples subjected to surface modification and cancellous bone were also compared. The Vickers hardness of cold-swaged TNTZ (TNTZ_SW) subjected to micro-shot peening was significantly increased within 20 mm from the very edge of the specimen surface. The fatigue strength of TNTZ_SW subjected to micro-shot peening increased especially in the high cycle fatigue life region. The fatigue limit was around 400 MPa. The bone formations on TNTZ_SW subjected to micro-shot peening and TNTZ_SW with the mirror surface as comparison material were almost identical to each other. However, the relative bone contact ratio of TNTZ_SW subjected to micro-shot peening was better than that of TNTZ_SW with the mirror surface.

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: Hardness of Ag-20Pd-12Au-14.5Cu (mass%) subjected to a solution treatment (ST) at a temperature over 1073 K followed by water quenching increases drastically. This unique hardening behavior is not clarified becasue of their complex microsturucures. In this study, the relationship between the unique hardening behaviour and the microstructure of dental Ag-20Pd-12Au-xCu subjected to ST with different Cu/Ag ratios was investigated. The Vickers hardness of Ag-20Pd-12Au-14.5Cu increases remarkably from 192 to 286 Hv after ST whereas that of Ag-20Pd-12Au-6.5Cu decreases and that of Ag-20Pd-12Au-20Cu increases slightly after ST, respectively. The spotty regions are observed in only certain areas of Ag-20Pd-12Au-14.5Cu subject to ST. It is considered that the appearance of the spotty regions affects mainly to the unique hardening behaviour in Ag-20Pd-12Au-xCu.

Abstract: A new -type Ti alloy composed of non-toxic and allergy-free elements like Nb, Ta, and Zr, Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) proposed by present authors, has been developed in order to achieve relatively low Young’s modulus and excellent mechanical performance. On the other hand, Zr has been also paid attention as metallic biomaterial for the next generation because of good biocompatibility nearly equal to Ti or a few GPa smaller Young’s modulus as compared to one. In this study, mechanical performances such as tensile properties and Young's modulus of TNTZ subjected to thermo-mechanical treatments or severe deformation, and the mechanical properties and biocompatibility of Zr-Nb system alloys were investigated in order to judge their potential for biomedical applications. Young’s modulus of as-solutionized TNTZ, which is around 63 GPa, is pretty similar to that of as-cold-rolled TNTZ. The Young’s moduli of hot-rolled Ti-6Al-4V ELI alloy are respective around 110 GPa. The Young’s moduli of as-solutionized and as-cold-rolled TNTZ are around a half of those, and are twice as large as that of the cortical bone. The tensile strengths of TNTZ aged after solution treatment and those aged after cold rolling decrease with an increase in the aging temperature, although the elongation shows the reverse trend. The tensile strength of as-cold-rolled TNTZ is improved drastically through severe deformation such as high pressure torsion and shows more than 1000 MPa. Zr-XNb system alloy (X: 5-30mass%) shows the smallest value of Young’s modulus (around 58 GPa) at Nb content of 20mass%. In the case of implantation of the bars made of Zr-XNb system alloys into the lateral femoral condyles of Japanese white rabbits, the tendency of contact between the cancellous bone and the bar becomes remarkably at 24 weeks after the implantation according to increasing with Nb content.

Abstract: The surface of Ti-29Nb-13Ta-4.6Zr (TNTZ) subjected to gas nitriding at 1023–1223 K was investigated in comparison with the conventional biomedical titanium alloy, Ti-6Al-4V ELI (Ti64). After gas nitriding, the microstructures near the surface of these alloys were observed by optical microscopy, X-ray diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. In both alloys, two titanium nitrides (TiN and Ti2N) are formed and the α phase precipitated by gas nitriding. Furthermore, oxygen impurity in the gas nitriding atmosphere reacts with the titanium nitrides; thus, TiO2 is formed at the outermost titanium nitride layer. The surface hardening was also evaluated by Vickers hardness measurement. The Vickers hardness near the surface of TNTZ and Ti64 increases significantly by gas nitriding.

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: An intensive amplitude arrangement for reciprocal tribocontact of biomedical alloys, Ag-20.0Pd-14.5Cu-12.0Au, Au-5.0Pt-2.0Pd-9.2Ag-15.0Cu and Ti-29Nb-13Ta-4.6Zr was conducted in order to check the details of friction and wear alterations around the transition from fretting to sliding. It is shown that the friction force exhibits stagnation for the Ag alloy and decrease for the Au alloy and the Ti alloy in certain domain of oscillating amplitude. Beyond the domain the friction force increases with the amplitude, and below the lower threshold and above the upper threshold triboevent is complete fretting and sliding respectively. Observation of friction-force waveform and SEM topography found, a hybrid mechanism of fretting and sliding is dominant in the amplitude domain, and the strong adhesion between the self-mated alloys is responsible to this exhibition.

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.