Papers by Keyword: Hydrogen Diffusion Coefficient

Abstract: For a unique microstructure creation, thermo-hydrogen treatment (THT), using hydrogen as a temporary alloying element within the heat treatment, is applied. This advanced heat treatment requires reliable data about the hydrogen diffusion coefficient (D_H) for understanding diffusion kinetics and its effect on the mechanical behavior of the resulted phases. In this research, three different homogeneous microstructures were established for the investigation using different homogenization parameters. After that, the concentration of hydrogen, charged in the half-length of thin titanium rods via electrochemical hydrogenation, is specified. Then, a diffusion annealing heat treatment was carried out at different temperatures, leading to hydrogen diffusion in the hydrogenated specimens. Furthermore, D_H was systematically determined using two methods including the explicit finite difference method (EFDM) and Matano technique (MT). For this purpose, Abaqus software was employed for modeling the hydrogen gradient established in the specimens. Additionally, scanning electron microscopy (SEM) was used for the microstructure examination in order to specify the influence of different hydrogen concentrations on the hydrogenated specimens. The experimental outcomes reveal a substantial effect of the β phase stability and grains sizes of the β and α phases on the hydrogen diffusion. Correspondingly, the results confirm that D_H was independent of the hydrogen concentration, and obeys an Arrhenius-type temperature dependence. Furthermore, hydrogen diffusion in the α+β titanium alloys Ti-6Al-4V was slower in comparison to the hydrogen diffusion in the metastable β titanium alloys Ti-10V-2Fe-3Al. In conclusion, it was observed that D_H is influenced by the previously performed heat treatments that determine the resulted microstructure types, and a slight influence of the volume fraction of the α phase on D_H was observed as well.

Abstract: The concept for alloy design of Nbbased hydrogen permeable membrane is applied to NbWMo ternary system. The alloying effects of tungsten and molybdenum on the solubility of hydrogen, the resistance to hydrogen embrittlement, the hydrogen permeability and diffusivity are investigated in a fundamental manner. It is found that the addition of tungsten and molybdenum into niobium decreases the hydrogen solubility. As a result, the resistance to hydrogen embrittlement improves and higher hydrogen pressures can be applied to the NbWMo alloy membrane. It is shown that the designed Nb5mol%W5mol%Mo alloy membrane with single solid solution phase exhibits excellent hydrogen permeability together with strong resistance to hydrogen embrittlement. In addition, it is found that the alloying of tungsten and molybdenum with niobium enhances the hydrogen diffusivity. In fact, the activation energy for hydrogen diffusion decreases in the order, pure Nb > Nb5mol%W > Nb5mol%W5mol%Mo.

Abstract: The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, , and the difference of hydrogen concentration, C, between the inlet and the outlet sides of the membrane. It is found that the hydrogen diffusion coefficient during the hydrogen permeation is increased by alloying ruthenium or tungsten into niobium. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion decreases by the addition of ruthenium or tungsten into niobium.

Abstract: The prediction of the applicability range of beta titanium alloys in hydrogen containing environments and the systematic study of hydrogen effects on the microstructure during heat treatment require reliable information about the hydrogen diffusion coefficient DH in the respective titanium alloy. Up to now the little information available on hydrogen diffusivity in commercial titanium alloys indicates a higher hydrogen diffusion coefficient in beta titanium alloys as compared to alpha and alpha + beta titanium alloys. In the present study, the hydrogen diffusion coefficients were determined systematically by means of electrochemically charging the half length of thin titanium rods and subsequent annealing, thereby enabling hydrogen diffusion. The Matano technique was applied in order to identify any effect of hydrogen concentration on DH. The hydrogen diffusion coefficients determined were correlated with results from microstructure examination applying optical and electron microscopy. Since molybdenum and vanadium are the most important beta-stabilizing alloying elements, binary titanium alloys of the Ti–Mo and the Ti–V systems at various contents of the respective alloying element were systematically studied in addition to commerical beta titanium alloys. The results of the experiments revealed the strong effect of beta stability and phase composition on hydrogen diffusion.

Abstract: The hydrogen diffusion coefficients are investigated during the hydrogen permeation through Nb-based hydrogen permeable membranes at high temperature. It is found that the hydrogen diffusion coefficient for pure niobium under practical conditions is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.

Abstract: β–titanium alloys are very attractive materials for many applications because they combine low density, high strength and excellent corrosion resistance. The available data indicate a much higher hydrogen diffusion coefficient in β–titanium alloys as compared to α and α + β alloys. In order to predict the range of applicability of β–titanium alloys in environments, which release hydrogen, the hydrogen diffusion coefficient (DH) needs to be known quantitatively. In the framework of this study the value of DH was determinated on samples, which were electrochemically hydrogen charged. Long thin rods were used as samples and charged in such a way that high hydrogen concentrations were obtained in one half of the length of the specimens, while the other half was kept virtually unaffected. After charging, the rods were annealed enabling hydrogen to diffuse. Hydrogen concentration profiles were experimentally determined and evaluated on the basis of the Matano technique, in order to reveal any effect of concentration on DH. The experiments were carried out on β–titanium alloys of the binary Ti–V system. The concentration range of vanadium in the alloys studied was selected in such a way that it represents the compositions commonly found in commercial alloys. The results show that the effect of hydrogen concentration on DH is negligible and that DH increases with the vanadium concentration.