High temperature oxidation of a pure iron, Fe 2Cr alloy, Fe 10Cr alloys and an Fe 10Cr 0.08C steel was examined in both air and steam at 923 K. In case of pure iron, the thickness of the oxide scale formed in steam at 923 K for 360 ks was comparable to that of the scale formed in air. On the other hand, in case of the Fe Cr binary alloys and the ternary martensitic steel, the oxide scale was much thicker in steam than in air. The amount of hydrogen dissolved into the pure iron and the steels during exposure to the high temperature steam was measured with thermal desorption spectroscopy (TDS). It was found that the amount of the dissolved hydrogen in the oxide scale was much larger in both the binary alloys and the ternary ferritic steel than in pure iron, and then it leads to the more accelerated oxidation rate of the binary alloys and the ternary steel in the steam. Furthermore, the martensite structure of the ternary steel exhibited an excellent oxidation resistance in air compared to the ferrite phase in the same steel. This is probably because the martensite phase has a lot of plane defects such as block boundaries for the fast diffusion path of Cr element.
Microstructural evolution accompanying mechanical properties change by aging treatment at 723 K in cold swaged Ti 29Nb 13Ta 4.6Zr were investigated in order to obtain an optimized aging condition for getting excellent mechanical properties.The precipitate free zone exists in Ti 29Nb 13Ta 4.6Zr after aging treatment for various time periods. Such the precipitate free zone corresponds to Nb and Ta enriched region according to the result of element distribution analysis. With the increase in aging time, the area of the precipitate free zone decreases.Two types of a phase, acicular and ellipsoidal ones, are observed in Ti 29Nb 13Ta 4.6Zr after aging treatment for various time periods. The acicular a phase is precipitated in the Tiand Zr enriched region. While the size increases, the area fraction barely changes with aging time for acicular a phase, On the other hand, the ellipsoidal a phase precipitates at b grain boundary and its subgrain boundary. Both the size and area fraction increase with the increase in aging time for the ellipsoidal a phase.The lattice correspondence between acicular a phase and b matrix is Tensile strength of Ti 29Nb 13Ta 4.6Zr subjected to aging treatment after cold swaging is around 1200 GPa, regardless of aging time. On the other hand, elongation of those are decreased by the aging treatment up to 86.4 ks, increased at ones over 259.2 ks.
As a new type of metallic biomaterial, porous pure titanium filled with a medical polymer has been developed for obtaining a low Young's modulus similar to that of bone. This type of biomaterials will inhibit the deterioration of mechanical properties due to the presence of pores, and provide biofunctionalities that are intrinsically possessed in certain polymers. However, the inhibition of the deterioration of mechanical properties is not satisfactory because of the poor interfacial adhesiveness between the titanium particles and the medical polymer. Therefore, in the present study, silane coupling treatment is employed in order to improve the interfacial adhesiveness, and silane coupling treated (Si treated) porous pure titanium (pTi) filled with polymethylmethacrylate (PMMA) is fabricated. Subsequently, the effect of the silane coupling treatment on the mechanical properties of the pTi filled with PMMA is investigated.The tensile strengths of the Si treated pTi filled with PMMA are higher than those of pTi and non Si treated pTi filled with PMMA. In the fractographs of non Si treated pTi filled with PMMA obtained after the tensile test, the detachment of titanium particles from PMMA is observed; this occurs because of poor interfacial adhesiveness between titanium particles and PMMA. However, in the case of the Si treated pTi filled with PMMA, the interfacial adhesiveness between titanium particles and PMMA is improved by the silane coupling treatment. This leads to the dispersion of the stress concentration at necks between particles, resulting in an improvement in the tensile strength of pTi. On the other hand, PMMA filling hardly affects Young's modulus of pTi because Young's modulus of PMMA is lower than that of pTi.
Formation of the reaction product layer on the surface of biomedical titanium alloys, Ti 29Nb 13Ta 4.6Zr (TNTZ) and Ti 6Al 4V ELI (Ti64), during gas nitriding was investigated. These alloys were exposed to nitrogen atmosphere at 1023, 1073, 1123 and 1223 K. After the gas nitriding, a reaction product layer was observed on the surface of both alloys, and was analyzed using an X ray diffraction (XRD), Auger electron spectroscopy (AES) and X ray Photoelectron spectroscopy (XPS). The layer was comprised of two phases, which were outer oxide layer (mainly TiO 2 ) and inner nitride layer (mainly TiN or Ti 2 N). In these layers, the thickness of the oxide layer particularly depended on the kinds of alloys. According to the thermodynamics and point defect theory, the growth rate of oxide layer is expected to be increased by the presence of Al in TiO 2 . Namely, the dissolution of Al into TiO 2 may increase the number of oxygen vacancies, resulting in acceleration of oxygen diffusion inward. On the other hand, the elements that accelerate the growth of the oxide layer are not contained in TNTZ. Thus, the oxide layer formed on Ti64 was thicker than that of TNTZ. In a similar way, the elements that accelerate the growth of the nitride layer are not contained in both TNTZ and Ti64. Thus, the nitride layers with similar thicknesses may be formed on TNTZ and Ti64 during gas nitriding.
Installing a spinal fixture using implant rods is one of the effective operations for spinal diseases. Most of the implant rods are made of Ti 6Al 4V ELI alloy (Ti64). However, some problems regarding the Ti64 rod have been pointed out; it contains vanadium, which is considerably toxic to the human body, and exhibits a much higher Young's modulus than that of the cortical bone. Ti 29Nb 13Ta 4.6Zr alloy (TNTZ) developed by the authors exhibits good biocompatibility due to its nontoxicand allergy free elements, and has a lower Young's modulus than that of Ti64. In addition, the mechanical properties of TNTZ can be changed drastically according to various heat treatments. The mechanical properties of TNTZ rods subjected to various heat treatments have been investigated in this study.The as solutionized TNTZ rod consists of a single b phase. Isothermal v phase and a phase precipitate in the b phase of the TNTZ rod aged at 673 K for 259.2 ks. Only a phase precipitates in the b phase of the TNTZ rod aged at 723 K for 259.2 ks. Tensile strength and 0.2 proof stress become larger in order of as solutionized TNTZ rod, TNTZ rod aged at 723 K for 259.2 ks and TNTZ rod aged at 673 K for 259.2 ks, while elongation is smaller in order of as solutionized TNTZ rod, TNTZ rod aged at 723 K for 259.2 ks and TNTZ rod aged at 673 K for 259.2 ks. Young's modulus become larger in order of as solutionized TNTZ rod, TNTZ rod aged at 723 K for 259.2 ks and TNTZ rod aged at 673 K for 259.2 ks.
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