Phase Diagram and Superabundant Vacancy Formation in Cr-H Alloys

Abstract

X-ray diffraction measurements on the Cr-H system were made using synchrotron radiation at high hydrogen pressures and high temperatures, and the phase diagram was determined up to p(H 2 ) = 5.5 GPa and T 1400 K. Three solid phases were found to exist; a bcc phase (α) of low hydrogen concentrations, x = [H]/[Cr] 0.03 existing at low hydrogen pressures ( 4.4 GPa), and two high-pressure phases, an hcp (ε) phase at lower temperatures and an fcc (γ ) phase at higher temperatures, both having high hydrogen concentrations x ∼ 1. A drastic reduction of the melting point is caused by dissolution of hydrogen. A gradual lattice contraction observed in the fcc phase indicates the formation of superabundant Cr-atom vacancies (vacancy-hydrogen clusters). Thermal desorption measurements after recovery from high p(H 2 ), T treatments revealed several desorption stages including those due to the release from vacancy-hydrogen clusters and from hydrogen-gas bubbles, and allowed determination of relevant trapping energies.

The Phase Diagram of Mo-H Alloys under High Hydrogen Pressures

“…5) divided by the relaxation volume of a vacancy calculated for a hypothetical fcc Mo v R = ¼ À0:40 18) leads to the concentration of vacancy-hydrogen clusters x cl ¼ 0:02. This value is smaller than the equilibrium concentrations observed in other fcc hydrides; x cl ¼ 0:09 for Cr-H, 4) 0.1 for Mn-H, 13) 0.19 for Fe-H, 6) 0.10 for Co-H, 13) 0.30 for Ni-H 24) and 0.10 for Pd-H. 25) Considering the extreme sluggishness of atomic processes experienced in phase determinations, it appears very probable that the equilibrium cluster concentration was not yet achieved in the experiment shown in Fig. 5.…”

“…2 is very similar to that of the Cr-H system. 4) In both cases, the bcc structure transforms to monohydrides of hcp and fcc structure at high hydrogen pressures, with the triple point located at 500 C and 4.5 GPa in the Mo-H system, and 900 C and 3.7 GPa in the Cr-H system. The appearance of the fcc monohydride as a hightemperature phase at high hydrogen pressures has in fact been found in many other transition metal-hydrogen systems as well (Mn-H, 5) Fe-H, 6) V-H, Nb-H and Ta-H 22) ), and is believed to be one of the common trends in the hydride phase relations.…”

“…1) A systematic investigation of the phase diagram of transition metalhydrogen (M-H) alloys under extended pressure-temperature conditions (p(H 2 ) 6 GPa, T 400 C) was performed by a Russian group, by combination of in situ resistometry and Xray diffraction (XRD) of recovered specimens, and the results were described in their review papers. 2,3) We have succeeded in taking a step forward by developing techniques of in situ XRD under more extended p(H 2 )-T conditions, and reported the phase diagrams for Cr-H, 4) Mn-H, 5) Fe-H 6) and Ni-H 7) systems. For the Mo-H system at high hydrogen pressures, the Russian group showed that an hcp hydride of a nearly stoichiometric composition [H]/[Mo] % 1 was formed, 8,9) and the bcc ()-hcp (") boundary was located at 250 C, 2 GPa increasing to 450 C, 5 GPa.…”

Hydrogen Dissolution and Structural Changes in Electrodeposited Cr Films

“…The situation is very similar to what was observed in highpressure experiments. 13) Thermal desorption spectra measured on samples of the three different structures are shown in Figs. 8(a)-(c).…”

Effects of Hydrogen on the Vacancy Formation in Magnesium

“…Also, if the difference, 49.3 kJ/ mol (0.51 eV), between two activation energies, 40.5 kJ/mol (0.42 eV) and 89.8 kJ/mol (0.93 eV), obtained from the results of B is taken as the order of the binding energy of hydrogen to the vacancies (to be discussed in the next section and, as a result, reduced to a smaller value), it is not appropriate to consider hydrogen to be stably bound to the vacancies at temperatures above 500 C; in fact, several examples of other metals exhibiting the same order of binding energy show that hydrogen in metals is released from vacancies at high temperatures. [23][24][25] From the above, it is reasonable to conclude that model (b), stating that hydrogen at high temperatures coexists with the vacancies without interactions, is adequate for magnesium. Figure 6 represents again the result of 0 C quenching in Fig.…”