Yoshio Furuya

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.

Mechanical alloying of the Ti-Al system in atmosphere of hydrogen and argon

Abstract Three kinds of Ti-Al powders, Ti 72 Al 28 , Ti 57 Al 43 and Ti 48 Al 52 , were mechanically alloyed by a planetary ball mill in atmosphere of argon or hydrogen gases (0.1 MPa) with alloying times up to 30 h. The mechanical alloying (MA) process as well as the phase variations of each powder after subsequent heating at 1173 K were investigated. About 5000 wppm hydrogen, which could be easily removed by a heat treatment at 800 K (heating rate was 20 K/min), was occluded in all powders during MA in the hydrogen atmosphere, whereas the mechanically alloyed powders in the argon atmosphere occluded about 1000 wppm hydrogen. In the hydrogen atmosphere, the titanium powder easily crumbled into finer particles, assisting the diffusion of aluminum into titanium (solid-solid reaction) at an early stage of the MA process and accelerating the formation of an amorphous-like phase at a longer MA process. The phase formation after heat treatment of MA powders at 1173 K could be estimated by the Ti-Al binary phase diagram without the effect of the gas atmosphere.

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.

Hydrogen uptake in titanium aluminides in high pressure hydrogen

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.

Hydride formation and thermal desorption spectra of hydrogen of cathodically charged single-phase gamma titanium aluminide

The authors have previously reported thermal desorption spectra of hydrogen obtained from cathodically charged two-phase (Ti{sub 3}Al ({alpha}{sub 2}) + TiAl ({gamma})) titanium aluminides by means of thermal desorption spectroscopy (TDS), in which hydrogen ion current (H{sub 2}{sup +}) corresponding to hydrogen evolution rate during heating was measured by a quadrupole mass spectrometer in an ultra-high vacuum condition. Several accelerated hydrogen evolutions (TDS peak temperatures) have been observed in a series of TDS measurement, and then the authors have suggested that these peaks were dependent on the microstructures ({alpha}{sub 2} and {gamma} phases) as well as dissociation of the hydride phase which formed during cathodic charging. A comparison with the TDS spectra from other series of titanium aluminides, such as a single-phase {gamma} alloy, might give clearer views of the microstructural dependence on hydrogen evolution kinetics. In this paper, hydride formation, hydrogen uptake and hydrogen evolution kinetic of a cathodically charged single-phase {gamma} titanium aluminide are investigated, and these results are compared with the previous ones obtained in two-phase ({alpha}{sub 2} + {gamma}) titanium aluminides.

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.

Hydrogen evolution from cathodically charged Ti3Al-based titanium aluminium alloy

Abstract A single-phase Ti 3 Al-based titanium aluminium alloy, Ti–25Al, was cathodically charged with hydrogen in a 5% H 2 SO 4 aqueous solution for charging times up to 43.2 ks, and the formation and dissociation of a hydride, hydrogen evolution behavior and total hydrogen uptake were investigated by means of X-ray diffractometry and thermal desorption spectroscopy (TDS). After cathodic hydrogen charging, hydrogen concentrated mainly at the surface of the sample as a hydride phase, whose crystal structure probably is of a hexagonal type with the lattice parameters of a =0.60 nm, c =0.47 nm. Hydrogen also induced cracks and pits at grain boundaries or within grains. Three kinds of TDS peak, for which one corresponded to the hydride dissociation and the others presumably corresponded to trapping of hydrogen, could be found after longer hydrogen charging. The apparent hydrogen uptake in the Ti–25Al alloy increased with increasing charging time and the uptake level after charging for 43.2 ks was estimated to be about 700 wppm.

Hydrogen evolution from chemically etched titanium aluminides

Abstract Ti45Al (Ti 3 Al ( α 2 ) and TiAl (γ)) and Ti53Al (single γ) titanium aluminides were chemically etched in a solution of 10 ml HF + 5 ml HNO 3 + 85 ml H 2 O for etching times between 10 and 120 s at room temperature, and hydrogen evolution behavior during heating and microstructural change were investigated by means of thermal desorption spectroscopy (TDS), X-ray diffractometry and scanning electron microscopy. The TDS spectra indicated that hydrogen was dissolved in the alloy matrix at an early stage of etching for both alloys and an additional etching enhanced the formation of a hydride phase in the Ti45Al alloy and the formation of pit in the Ti53Al alloy. No hydride formation was observed in the Ti53Al alloy. The solubility limit of hydrogen during chemical etching was estimated to be about 8 wppm in the Ti45Al alloy and about 3 wppm in the Ti53Al alloy. The activation energy of hydrogen evolution, which could be determined from TDS peak temperatures at different heating rates, resulting from hydrogen dissolution in the α 2 phase and the γ phase, hydride dissociation in the Ti45Al alloy and hydrogen trapping at pit sites were determined to be 1029, 144, 37 and 30 kJ mol −1 respectively.

Hydrogen Permeation through Nickel

The time dependence of the rates of permeation of hydrogen through annealed and as-rolled nickel foils was measured in the temperature ranges 120–430°C and 80–200°C, respectively. Annealed specimens exhibited permeabilities, diffusivities and solubilities that were in fairly good agreement with previous results, while the permeation characteristics of an as-rolled specimen differed from those of the annealed ones owing to the trapping of hydrogen at sites probably associated with vacancies. The trap binding energy was about 0.26 eV. Furthermore, anomalous permeation due to an oxide barrier was observed after specimens were kept under low vacuum for a long time. This anomaly exhibited a characteristic behavior around the Curie point.

Effect of surface on hydrogen permeation through pure aluminum

The permeation behavior of hydrogen through pure aluminum has been investigated in the temperature range of 625 K to 773 K by a gas permeation technique. The permeation rate of hydrogen was strongly affected by the conditions of the surface where the hydrogen was introduced into the sample. The presence of the oxide layer on the surface made the hydrogen be hard to permeate through the sample. The oxide layer was reduced by hydrogen during the permeation run at a higher temperature above 700K, which resulted in the larger permeation rate of hydrogen in the subsequent permeation runs. On the other hand, the permeation rate of hydrogen was decreased by the temporary poisoning of the surface. The poisoned surface was cleaned by keeping the sample under high vacuum at a higher temperature above 700K.