Youji Taneda
National Defense Academy of Japan
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Featured researches published by Youji Taneda.
Journal of Alloys and Compounds | 1995
Akito Takasaki; Yoshio Furuya; Kozo Ojima; Youji Taneda
Abstract A commercially pure α-titanium was electrochemically charged with hydrogen in a 5% H2SO4 solution at a current density of 5 kA m−2 for 14.4 ks (4 h), and the dissociation process of the electrochemically formed hydride and the evolution behavior of hydrogen from the samples were investigated by means of high temperature X-ray diffractometry, thermal desorption spectroscopy (TDS) and differential thermal analysis (DTA). The electrochemical charging produced δ-titanium hydride; this dissociated completely at temperatures around 600 K; (α + β) titanium then appeared, indicating that the hydride formed eutectoidally. The DTA detected the dissociation of the hydride (or (α + δ)−(α + β) boundary in the titanium-hydrogen system) as an endothermic peak. The TDS analysis, however, revealed that the accelerated hydrogen evolution could not be found at the dissociation temperature of the hydride but could be at higher temperatures. It was suggested that the hydride dissociation, (α + δ), into (α + β) two-phase region was not accompanied by hydrogen evolution from the samples, but the free hydrogen owing to the hydride dissociation was diffused into the samples. The peak temperatures of both DTA and TDS analyses shifted to lower temperatures with decreasing heating rate. The Kissinger plots fitted these results fairly well and indicated that the apparent activation energies for δ-hydride dissociation and hydrogen evolution were estimated to be about 106 kJ mol−1 and about 49 kJ mol−1 respectively.
Scripta Metallurgica Et Materialia | 1995
Akito Takasaki; Yoshio Furuya; Kozo Ojima; Youji Taneda
In this study, two-phase ({alpha}{sub 2} + {gamma}) titanium aluminides were thermally charged with hydrogen, and the hydrogen solubility and the hydrogen evolution behavior were investigated by means of thermal desorption spectroscopy (TDS). Hydrogen solubility of two-phase (Ti{sub 3}Al + TiAl) titanium aluminides occurred endothermically. A heat of solution for hydrogen dissolution in a Ti-50Al alloy was estimated to be 36.4 kJ/mol and that for a Ti-45 Al alloy was 58.3 kJ/mol in the temperatures range of 723 K to 843 K. At higher temperature, hydrogen solubility was not fitted well with Arrhenius type plots mainly because of oxidation. It was suggested from thermal desorption spectrums that there were three kinds of dissolution states for hydrogen in the alloys.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing | 1997
Akito Takasaki; Yoshio Furuya; Youji Taneda
Abstract The weight gains of Ti–25Al, Ti–45Al and Ti–53Al alloys, with typical single-phase Ti 3 Al, two-phase Ti 3 Al/TiAl (fully lamellar) and single-phase TiAl microstructures, respectively, were measured at temperatures up to 923 K in high pressure hydrogen, up to 10 MPa. The total hydrogen uptakes during heating to 923 K at constant hydrogen pressures and during increasing the hydrogen pressure to 10 MPa at constant temperatures increased with increasing amounts of the Ti 3 Al in the alloys. The Ti–25Al alloy cracked and then spontaneously disintegrated at high-hydrogen pressures. A ternary (Ti–Al–H) hydride then formed, whose crystal structure is the same as that of the γ hydride (f.c.c.), known in the titanium–hydrogen binary system. No hydride could be detected in the Ti–45Al and Ti–53Al alloys. Most of the hydrogen taken up in the Ti–45Al and Ti–53Al alloys during heating and during pressure increase was released during cooling to room temperature or during pressure decrease to 0 MPa.
Journal of Alloys and Compounds | 1994
Akito Takasaki; Kozo Ojima; Youji Taneda
Abstract A (TiAl)H x hydride which has a tetragonal crystal structure with lattice parameters a = 0.452 nm and c = 0.326 nm ( c / a = 0.721) has been observed in Ti42Al, Ti45Al and Ti50Al (at.%) two-phase (Ti 3 Al ( α 2 ) + TiAl ( γ )) titanium aluminides by cathodic charging in a 5% H 2 SO 4 solution. Cracks or pits are also observed within the γ phase regions in the two-phase ( α 2 + γ ) coexisting grains (such as lamellar grains) but not within the α 2 phase or single γ grains. Weights of the samples decrease with increasing charging time owing to the crack or pit formation, and this is more drastic in the Ti—50Al alloy than in the Ti—42Al and Ti—45Al alloys. The hydride is thermally stable at temperatures up to about 550 K (277 °C) and dissociates completely at temperatures between 673 K (400 °C) and 723 K (450 °C).
Scripta Metallurgica Et Materialia | 1993
Akito Takasaki; Kozo Ojima; Youji Taneda
Titanium aluminides have attractive properties at elevated temperatures. Recently, considerable effort has been directed toward determining the hydrogen susceptibility of Ti[sub 3]Al ([alpha][sub 2]) and TiAl ([gamma]) intermetallic compounds. It is known that the [alpha]-titanium, which is the low temperature phase of pure titanium, has a very low solubility for hydrogen and precipitates a hydride when the solubility limit is reached. From this aspect, the [alpha][sub 2] phase, whose titanium content is much more than that of the [gamma] phase, is considered to precipitate a hydride easily. This means that hydrogen embrittlement in [alpha][sub 2] based alloys occurs more easily than in [gamma] based alloys. There have been a number of recent reports on hydride in [alpha][sub 2] or [gamma] based alloys, but less in two-phase ([alpha][sub 2] + [gamma]) alloys. This report presents a preliminary result of the effect of hydrogen charging on the microstructures in two-phase ([alpha][sub 2] + [gamma]) titanium aluminides by cathodic charging.
Journal of Alloys and Compounds | 1995
Akito Takasaki; Yoshio Furuya; Kozo Ojima; Youji Taneda
Abstract Ti-45Al and Ti-50Al (at.%) titanium aluminides, whose microstructures consisted of Ti 3 Al ( α 2 ) and TiAl (γ), were cathodically hydrogen-charged in a 5% H 2 SO 4 solution for charging times up to 14.4 ks (4 h), and the dissociation process of a hydride and the hydrogen evolution process during heating were investigated by thermal analyses (differential thermal analysis and thermal desorption spectroscopy). The hydride formed during cathodic charging dissociated at the temperature of about 700 K (427 °C), and corresponding to the hydride dissociation, hydrogen gas was evolved from the alloys at the dissociation temperature. In both alloys, accelerated hydrogen evolutions were observed at the lower temperatures than that for hydride dissociation. The evolution of hydrogen in the Ti-50Al alloy was extremely accelerated at about 523 K (250 °C) and the Ti-45Al at about 600 K (323 °C). The difference in the accelerated evolution temperatures was strongly dependent on the microstructures, in which structural imperfections, such as microvoids or internal cracks, could be formed during cathodic charging. The Ti-45Al alloy picked up about 1.5-times as much hydrogen as the Ti-50Al alloy, and more than 80% of the hydrogen was concentrated at the surface layer up to 20 μm in depth from the surface of the sample.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science | 1994
Akito Takasaki; Kozo Ojima; Youji Taneda
Ti-42A1, Ti-45A1, and Ti-5OA1 (at. pct) titanium aluminides, which were cathodically hydrogen charged in a 5 pct H2SO4 solution for charging times between 1.8 ks (0.5 hours) and 14.4 ks (4 hours), were oxidized in a static air under atmospheric pressure at temperatures between 1170 K (897 °C) and 1350 K (1077 °C). All the hydrogen-charged alloys, as well as alloys without hydrogen charging, followed parabolic oxidation kinetics. The weight gains of the alloys after hydrogen charging for normally less than 3.6 ks (1 hour) were 20 to 30 pct less than those without hydrogen charging. In the alloys charged with hydrogen for more than 7.2 ks (2 hours), the weight gains increased with increasing the charging time. The activation energies of oxidation indicated that the oxidation-controlling factor would change after a charging time of 7.2 ks (2 hours) in all the alloys. The decrease in the activation energies with charging time was more drastic in the Ti-5OA1 alloy, which suggested that hydrogen damage, such as cracking, was more severe in the Ti-50Al alloy than in the Ti-42A1 or Ti-45A1 alloys. The formation of cracks during hydrogen charging provides titanium-diffusion paths and accelerates formation of rutile (TiO2) scale on the surface of the alloys. The TiO2 on the alloys after hydrogen charging formed at a comparatively lower temperature than that on the alloys without charging.
Japanese Journal of Applied Physics | 1995
Junji Sunada; Ken–ichi Fukuchi; Youji Taneda; Masami Hirose
Taking account of the high photoabsorption coefficient of Ge for 290 nm light, we investigate the thermal growth of thin oxide in Ge films by utilizing a double-beam UV spectrometer, and discuss the results in connection with photoconduction accompanied by oxidation. Rapid growth in the initial stage of oxidation up to 18 A thickness follows the kinetics of slow linear rates. Oxidation at temperatures lower than 400° C induces oxygen bond defects with high density at the Ge-GeO2 interface.
Scripta Metallurgica Et Materialia | 1994
Akito Takasaki; Kozo Ojima; Youji Taneda
A chemical etching technique is widely used for metallographic observation. Because this technique is based on a local corrosion phenomenon on a sample, the etching mechanism, particularly for two-phase alloys, can be understood by electrochemical consideration. This paper describes formation of a new phase in a Ti-45Al (at.%) titanium aluminide during chemical etching, and the experimental results are discussed electrochemically.
Japanese Journal of Applied Physics | 1989
Kozo Ojima; Youji Taneda
The core structures of end-on dislocations at a low angle tilt boundary in white tin have been observed using high-resolution electron microscopy (HREM). HREM observation shows that the tilt boundary is constructed by alternate arrangements of a 1/2 perfect dislocation and two 1/2 imperfect dislocations.