J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1355-1356 FARADAY COMMUNICATIONS Solubility of Hydrogen and Deuterium in Ti,AI Masao Kimura Advanced Materials & Technology Research Laboratories, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki 211, Japan Tatsuo Tsuchiyama, Shizuo Naito* and Masahiro Yamamoto Institute of Atomic Energy, Kyoto University, Uji, Kyoto 61I,Japan The solubility of hydrogen and deuterium in Ti,AI has been measured and found to obey Sieverts law. The difference in the solubility between hydrogen and deuterium can be explained by a model in which hydrogen and deuterium atoms perform harmonic oscillations in the tetrahedral site of the Ti,AI crystal lattice. The intermetallic compound Ti,Al has attracted much research interest owing to its excellent mechanical properties and corrosion resistance at high temperatures.Ti3Al, however, has a potential problem of hydrogen embrittlement when used as a structural material. A quantity of prime importance to this problem is the solubility of hydrogen in Ti3A1. Knowledge about the sites occupied by hydrogen atoms in the Ti,Al crystal lattice' is no less important because it underlies an understanding of the solubility. Only a limited number of studies have been reported on the solu- bility of hydrogen in Ti,A12v3 and its alloy^,^ there are no reports on the sites for hydrogen occupation although there are some discussion^^*^ about the sites in intermetallic com- pounds other than Ti,Al. In this study we measure the solu- bility of hydrogen and deuterium in Ti,Al at temperatures between 823 and 1323 K and at concentrations, 8 [H/(Ti3A1/4) and D/(Ti,A1/4)] between 0.001 and 0.1, and determine the likely sites for hydrogen occupation from the difference in the measured solubility of hydrogen and deute- rium.The solubility was obtained by measuring the amount of hydrogen and deuterium gases absorbed by a Ti,Al sample. The apparatus and procedure for measurements were essen- tially the same as those reported previo~sly.~.~ The sample was a 512 mm3 cube of polycrystalline Ti,Al. Chemical analysis showed that its purity was better than 99.9%, the main impurities being oxygen, carbon and nitrogen. X-Ray diffraction measurement showed that the sample had exactly the DOl9 structure.' To ensure homogeneity of the structure of the sample we heat-treated the sample in a vacuum of ca.5 x lo-' Pa at 1323 K for 48 h. Temperature was measured with a calibrated W5%Re-W26%Re thermocouple spot-welded to the sample. Pressure was measured with ionization gauges calibrated for hydrogen and deuterium against a cali- brated diaphragm gauge. The time for hydrogen to be distrib- uted uniformly over the sample was longer than in Ti, e.g. 24 h at 823 K because the diffusivity of hydrogen in Ti3Al is less than in Ti; this will be discussed elsewhere. Fig. 1 shows plots of the measured equilibrium hydrogen pressure p us. 8 at various temperatures. The solid lines are the computed result, which will be discussed later. The plots for hydrogen and deuterium both lay on straight lines with a slope of ca.2. This shows that Sieverts law can be applied to the relationship between p and 8 at small 8 in the form: p = ke' (1) where k is a function of temperature and independent of 8. Deviations from Sieverts law have been reported for the Ti,Al-hydrogen system and explained on the basis of block- ing of the sites by substitutional A1 atoms.' Small deviations from Sieverts law observed in this study (Fig. 1) may be due to this blocking factor. From the Arrhenius plot of the values of k obtained graphically in Fig. 1, the heat of solution per hydrogen atom was found to be 0.57 eV for hydrogen and 0.56 eV for deuterium. The value 0.57 eV for hydrogen is in agreement with the reported value' and, within experimental error, the same as that for a-Ti.' A possible cause may be the similarity of their electronic structures: the structure of the Ti d partial density of states (DOS) below the Fermi level is similar to that of the total DOS of a-Ti,' which is mainly contributed by the Ti d states.On hydrogen absorption the hydrogen s states couple with the Ti d states as in the Ti- hydrogen system" if hydrogen atoms occupy tetrahedral (T) sites, and the resulting DOSs for the Ti,Al-hydrogen and Ti- hydrogen systems are unlikely to be much different, implying a similar behaviour in the heat of solution. We now consider the ratio of k for hydrogen and deute- rium, kD/k,, in order to find the states of hydrogen and deu- terium in the Ti,Al crystal lattice.Fig. 2 shows the temperature dependence of kD/k, obtained from measured p 10-'/0 I 1 I ' 1 I"' I I I I I Ill. 1356 TIK 1400 2.01 I 1200 I 1000 1 800 1.o 0.7 0.9 I .1 1.3 lo3 KIT Fig. 2 Temperature dependence of kdk,. (0)Present study. The solid lines show eqn. (3) computed for haHci,= ,/(2)hoD(,,= (a) 0.07, (b)0.10, (c)0.13 and (d)0.16 eV. and 8. Values of kD/kH were found to decrease as the tem- perature increased and to be in the range of values observed for some other hcp metals.8~” Since k in eqn. (1) may be written, using the partition func- tions for hydr0genfH(,) in the gas phase andfH(i, in the metal, as7*8 for hydrogen and similarly for deuterium, the expression for kD/kH can easily be obtained.Here k’ is a constant and E, is the heat of solution per hydrogen atom, which is assumed to be the same for deuterium. Using the explicit form for the partition function^^.^ we have (3) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 fourth, sixth and seventh terms in the right-hand side of eqn. (3) are corrections for the anharmonicity. The solid lines in Fig. 2 show eqn. (3) computed for the values of = ,/(2)h~,(~,shown in the figure caption and =for 4H(i)24D(i)= 0. The values used for fH!g) and fD(g) have been taken from standard tables. The experimental result is best reproduced for h~~(~)= 0.13 eV, which is consistent with haHci,= 0.141 eV obtained for a-Ti.I2 It can be seen from numerical calculations that the increase in 4H(i)has the same effect on the values of kD/kH as the increase in ho,(,,.We have assumed that c#I~(~)= 24D(i)= 0 because it is difficult to determine and 4H(i)simultaneously. The solid lines in Fig. 1 have been computed for =. ,/(2)ho,(, = 0.13 eV and 4H(i)24D(i)= 0. The heat of solution has been found to = be E, = 0.58 eV. It should be noted that E, does not include contributions from the partition functions and therefore does not coincide with the heat of solution obtained graphically from Fig. 1, i.e. 0.57 eV for hydrogen and 0.56 eV for deute- rium. The sites occupied by hydrogen atoms in Ti,Al can now be discussed. The similar values of the heat of solution observed for Ti,A1 and Ti suggest hydrogen occupation of tetrahedral (T) sites, which is the case for Ti.12 The fact that the value of ~oI~(~).for Ti,Al estimated in this study is consistent with that for Ti also suggests the T-site occupation.Octahedral (0)-site occupation, for example, would result in a smaller value of Aco,(~)because a larger space is available for the oscillation of the hydrogen atom in the 0 site. For example, = 0.069 eV for palladium,6 in which the 0 sites are preferentially occupied, but this value of AmH(,, can only poorly reproduce the measured values of kD/kH, as can be seen in Fig. 2. In conclusion, the solubility of hydrogen and deuterium in Ti,Al at high temperatures and at small 8 has been found to obey Sieverts law.It is suggested from the measured values of kD/kH and the energy of the hydrogen atoms in Ti,Al that hydrogen atoms occupy the T sites, in which they perform harmonic oscillations. References 1 J. L. Murray, Phase Diagrams of Binary Titanium Alloys, ASM, Metals Park, 1987, p. 12. 2 P. S. Rudman, J. J. Reilly and R. H. Wiswall, Ber. Bunsenges. Phys. Chem., 1977,81,76. 3 P. S. Rudman, J. J. Reilly and R. H. Wiswall, J. Less-Common Met., 1978,58,231. 4 W. Y. Chu, A. W. Thompson and J. C. Wiliams, Acta Metall. Mater., 1992,40,455. 5 K. Yvon and P. Fischer, in Hydrogen in Intermetallic Com- pounds, ed. L. Schlapbach, Springer, Berlin, 1988, vol. I, p. 87. 6 D. Richter, R. Hempelmann and R. C. Bowman Jr., in Hydrogen in Intermetallic Compounds, ed.L. Schlapbach, Springer, Berlin, 1992, vol. 11, p. 97. 7 S. Naito, J. Chem. Phys., 1983,79,3113. 8 T. Maeda, S. Naito, M. Yamamoto, M. Mabuchi and T. Hashino, J. Chem. Soc., Faraday Trans., 1993,89,4375. 9 T. Hong, T. J. Watson-Yang, X. Q. Guo, A. J. Freeman and T. Oguchi, Phys. Rev. B, 1991,43,1940. M. Gupta and L. Schlapbach, in Hydrogen in Intermetallic Com- 10where mH is the mass of the hydrogen atom, o~(~)and o~(~) are the angular frequencies of hydrogen atoms in the hydro- gen molecule and in Ti,Al, respectively, and 4H(g)and 4H(i) are the magnitudes of anharmonicity in the oscillation of hydrogen atoms in the hydrogen molecule and in Ti,Al, respectively. The subscript D indicates deuterium. The third, pounds, ed. L. Schlapbach, Springer, Berlin, 1988, vol. I, p. 139. 11 S. Naito, T. Hashino and T. Kawai, J. Chem. Phys., 1984, 81, 3489. 12 R. Khoda-Bakhsh and D. K. Ross, J. Phys. F: Met. Phys., 1982, 12, 15. Communication 4/01233J; Received 28th February, 1994