Kozo Ojima

Hydride dissociation and hydrogen evolution behavior of electrochemically charged pure titanium

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.

Hydrogen solubility of two-phase (Ti3Al + TiAl) titanium aluminides

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.

Hydride formation in two-phase (Ti3Al + TiAl) titanium aluminides during cathodic charging and its dissociation

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 Ti42Al, Ti45Al and Ti50Al (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).

Hydride formation in two-phase (Ti3Al+TiAl) titanium aluminides by cathodic charging

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.

Hydrogen evolution from cathodically charged two-phase (Ti3Al + TiAl) titanium aluminides

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.

High-Temperature oxidation of cathodically hydrogen-charged two-phase (Ti3Al, TiAl) titanium aluminides

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.

High-resolution electron microscopy of the grey-white-tin interface

Abstract The grey-white phase interfaces of tin have been studied using high-resolution electron microscopy (HREM). The lattice fringe images revealed the interfacial regions of the transitional atomic arrangement from grey tin (α-phase) into white tin β-phase). The orientation relationship obtained from the lattice fringe image between α- and β-phases is that the (011) plane of grey tin is parallel to the (001) plane of white tin and the [211] direction of grey tin is nearly parallel to the [010] direction of white tin. A model of the α → β transformation mechanism is proposed on the basis of the atomic arrangement observed by HREM. The model indicates that the α → β transformation of tin is accomplished partly by martensitic transformation and partly by atomic motion that is massive in nature.

New phase formation in titanium aluminide during chemical etching

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.

An Arrangement of Boundary Dislocations in White Tin Observed by High-Resolution Electron Microscopy

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.

Observations of Dislocations in White Tin Using High-Resolution Electron Microscopy

End-on dislocations in white tin crystals have been examined using high-resolution electron microscopy. The experimental lattice images indicated the dissociation of a perfect dislocation lying in the (100) plane. The imperfect dislocation formed by the dissociation had a Burgers vector ½[010].