J. MATER. CHEM., 1994, 4(8), 1309-1311 Inelastic Neutron Scattering Study of Hydrogen Embrittlement in Titanium Alloys Peter J. Branton,a Gary Burnell: Peter G. Hall*" and John Tomkinson" a Department of Chemistry, Exeter University, Exeter, Devon UK EX4 4QD Atomic Weapons Establishment, Aldermaston, Reading, Berkshire, UK " Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, UK OX17 OQX Inelastic neutron scattering studies have been carried out on the mixed CI and p phase of the titanium alloy IMI T318 for hydrogen loadings ranging from 400 to 15 500 ppm (a hydrogen loading of 80 ppm gave a spectrum too weak for interpretation). For a hydrogen content of 15 500 ppm, metal subsurface tetrahedral interstitial sites were occupied. As the hydrogen content was decreased, it became evident that an additional site was occupied, this was assigned to hydrogen in surface threefold symmetry binding sites. Titanium metal is of great industrial importance because of its simultaneous properties of low density (less dense than iron), high strength (stronger than aluminium), toughness and corrosion resistance.' These mechanical and chemical proper- ties can be enhanced by addition of alloying metals, such as aluminium, molybdenum, manganese and iron to name but a few.The IMI T318 alloy used in the present work comprises 90% Ti, 6% A1 and 4% V by weight.2 The mechanical properties of titanium (and also zirconium and hafnium in the same group) are greatly affected by traces of impurities such as 0,N, C and H, which have an embrittling effect on the metals making them difficult to fabricate.The embrittle- ment (significant reductions in strength, strain to failure and toughness) caused by hydrogen can be ascribed to the forma- tion of intergranular hydride. At lower hydrogen concen-trations, the materials are embrittled but there is no conclusive evidence of hydride f~rmation.~ As a result, this work aims to identify the site of the hydrogen in the metal lattice using inelastic neutron scattering (INS) spectroscopy. This tech- nique is ideal for the study of the atomic and molecular vibrations of hydrogenous materials where a good resolution is required. The IMI T318 alloy is a two-phase structure containing an hcp IXphase and a bcc fi phase in approximately equal volume fractions.The phase distribution consists of primary SI islands surrounded by a necklace of b. Experimental Hydrogen charged titanium alloy specimens were prepared by the Atomic Weapons Establishment, as plates with dimen- sions 1200 mmx27 mmx 1 mm using a thermal charging technique based on the Sieverts principle. This involved heating the specimen to 630°C and introducing hydrogen at reduced pressure. A thickness of 1mm was chosen so as to minimize the amount of multiple ~cattering.~ The nominal hydrogen contents of the specimens used were 80, 400, 1800 and 15 500 ppm. (A specimen free of hydrogen was analysed and the results showed a weight percentage of C, 0.01; N, 0.013; Fe, 0.2; Al, 6.2; V, 3.8; Y, 50ppm and 0, 0.120, Y (0.005; Ti, balance).The INS spectra were obtained at 30 f5 K using the time- focused crystal analyser (TFXA) spectrometer at the spallation neutron source, ISIS, Rutherford Appleton lab~ratory.~ This is an inverted geometry spectrometer with a fixed analyser neutron energy of approximately 4meV. The energy range covered is typically the molecular vibrational range from ca. 2 to 500meV with a good resolution, at some 2% in AEIE. The specimen containing no hydrogen was used as a back- ground and subtracted from the data, which were tranlsformed by standard programs to S(Q,o) against neutron energy transfer over the range 2-500 meV. Before the experiments, the specimens were quenched from room temperature to 77 K, but it is considered unlikely that this would have any signifi- cant effect in relation to the location of the hydrogen Results and Discussion INS Spectrum of 15 500 ppm Hydrogen-loaded Alloy The spectrum with a hydrogen loading of 15 500 ppm (where 1ppm= 1x lop4wt.%) is shown in Fig.1 and is dorninated by an intense peak at 148meV with a full-width at half maximum (fwhm) of 40meV. This has been assigned to hydrogen vibrations in the interstitial tetrahedral sites of the metal lattice. The frequencies of protons on tetrahedral sites are typically around 150meV, whereas for octahedral sites values around 60 meV are common. Kolesnikov et a1 made this assignment to a peak at 160meV for TiH0.74 in the 6 phase and Hempelmann et aL7 observed only one vibrational mode at 150.5meV due to hydrogen in tetrahedral sites for the a and b mixed phase of TiHo.07.The site chosen by hydrogen is highly dependent on the phase. For example, Kolesnikov et aL8 revealed that in the E phase of rfiH0.71, hydrogen principally occupied the octahedral interstices. Khoda-Bakhsh and Ross' have measured the INS spectra for the cc and b phase of TiHo.05 and TiH0,14, respectively, and concluded that the hydrogen occupied the tetrahedral sites in r neutron energy transferlmev Fig. 1 INS spectrum of 15 500 ppm hydrogen-loaded alloy the titanium hcp a phase and the distorted tetrahedral site in the titanium bcc fi phase. Broadening overtones at 297 and 445 meV indicate that there is no anharmonicity present; fwhm values were 60 and 120meV, respectively.The less intense sharp features at low energy transfer can be attributed to the lattice vibrations of titanium. There are characteristic peaks at 12.2 and 21 meV before the spectrum cuts off at about 40 meV. Similar to the 6 phase: these have been assigned to transverse acoustic modes near the edge of the Brillouin zone. INS Spectra of 1800 ppm Hydrogen-loaded Alloy The spectrum of titanium alloy with hydrogen content 1800 ppm is far more complex as can be seen in Fig. 2. Two peaks at 117 and 154 meV now dominate the spectrum and it appears that the hydrogen occupies two different sites. There is a very broad feature, which peaks at 297 meV, that can be attributed to the first overtone of hydrogen in tetra- hedral sites (the second overtone is too weak to be observed).The new site has been assigned to hydrogen in threefold symmetry binding sites on the surface. Binding in a threefold symmetric site predicts an intensity ratio of 2: 1 for the observed spectral features. Three Gaussian curves were suc- cessfully fitted to the spectrum using the interactive least- squares fitting package, FR1LLS.l' A peak fitted at 151 meV (fwhm =40 meV) was assumed to be from hydrogen in tetra- hedral sites and two peaks with intensities of 2: 1 were fitted giving energy transfer values of 105 (fwhm=25 meV) and 124 meV (fwhm =25 meV) respectively, due to the hydrogen in threefold sites on titanium. The location of hydrogen in threefold sites is not uncommon, for example, using spectro- scopic measurements for Ti(0001)-H( 1x 1), it was calculated that for a monolayer of hydrogen on titanium, the hydrogens were found to be in threefold sites, 0.8 au outside the outer titanium layer, under the site where the next titanium atoms would be if the crystal continued." It is known that adsorption in adjacent threefold sites is less stable than in separated sites sharing only one surface atom and no two hydrogen atoms form a tightly bound molecule in titanium.12 This is consistent with the fact that titanium is known for its strong surface catalytic reaction in dissociating a H, molecule into two adjacent hydrogen atoms.Hydrogen has also been shown to occupy threefold sites on other metals, e.g.raney nickel13 and pa1ladi~m.l~ INS Spectra of 400 and 80 ppm Hydrogen-loaded Alloys The spectrum with 400ppm hydrogen, together with fitted Gaussian curves are shown in Fig. 3 over the energy transfer range 50-200 meV. It is very weak, making interpretation 0.16r I 1 0 100 200 300 400 500 neutron energy transfedme\/ Fig. 2 INS spectrum of 1800ppm hydrogen-loaded alloy J. MATER. CHEM., 1994, VOL. 4 I I II50 100 150 200 neutron energy transferlmev Fig. 3 INS spectrum of 400 ppm hydrogen-loaded alloy difficult (hydrogen found in impurity sites could contribute significantly to the spectrum). Error bars are shown to give an estimate of the accuracy involved. The spectrum reaches a maximum at 119 meV, which slowly tails off.A peak at about 150meV is still present but now it is not the dominant peak in the spectrum, i.e. it 'appears' as if the hydrogen occupies the surface threefold sites in preference to the subsurface tetrahedral sites. Gaussian curves were fitted to the spectrum using the following assumptions; hydrogen occupies surface threefold sites and subsurface tetrahedral sites only; the pos- ition and fwhm of the peak due to hydrogen in tetrahedral sites is 148 and 40 meV, respectively (from the spectrum with 15 500 ppm H); and the two spectral features for hydrogen in surface threefold sites have an intensity ratio of 2 : 1 and the same fwhm values. A best fit of this data gave values of 105 (fwhm=26 meV) and 124 meV (fwhm =26 meV) for hydrogen in surface threefold sites and 148 meV (fwhm=40 meV) for hydrogen in subsurface tetrahedral sites.By calculating the areas under the Gaussian curves, it appears as if the two sites are populated in roughly equal proportions, due to the broadness of the peak from hydrogen in tetrahedral sites compared with that in surface threefold sites. The spectrum with 80ppm hydrogen was too weak for interpretation although one peak at 48.4 meV was clearly visible. This peak was also visible on the spectra with 400 and 1800ppm H although it was less intense relative to the rest of the spectrum. This has been interpreted as being due to bending and stretching vibrations of -OH groups bound to near-surface titanium atoms (0.12 wt.% of oxygen was present in our alloys).Comparison of this work with that of Renouprez et ~21.~'on the characteristic frequencies of the -OH vibrations of water chemisorbed on nickel, and that by Albers et all6 on oxygen impurity in TiMn,.,H,, suggests that the particular vibration being observed is probably due to a Ti-0-H stretch. Conclusions The INS spectra of the titanium alloy IMI T318 were meas- ured as a function of hydrogen content. At very low hydrogen loadings, the hydrogen was shown to occupy surface threefold symmetry sites and subsurface tetrahedral sites in roughly equal proportions. As the hydrogen concentration was increased, the spectra became dominated by hydrogen vibrations in the subsurface tetrahedral sites and for the highest hydrogen loading used (15 500 ppm), no surface scat- tering was visible.However, owing to the scattering from the surface structures being very much weaker than that from the bulk hydrogen, it cannot be ruled out that underlying the big peak at 148 meV in Fig. 1 there is also surface scattering as J. MATER. CHEM., 1994, VOL. 4 1311 revealed by Fig.2 when the bulk hydrogen was very much reduced. 7 Malyshev and E. G. Ponyatovskii, J. Phys: Condens. Mlrtter, 1991, 3, 5927. R. Hempelmann, D. Richter and B. Stritzker, J. Phy.5. F, 1982, 12, 79. We thank the SERC for the provision of neutron beam time, the AWE for the preparation of our hydrogen-loaded alloys and Professor D. K. Ross (Salford University) for helpful discussions. 8 9 10 11 A.I. Kolesnikov, V. K. Fedotov, I. Natkanets, S. Khabrylo, I. 0.Bashkin and E. G. Ponyatovskii, JEPT Lett., 1986,44, 509. R. Khoda-Bakhsh and D. K. Ross, J.Phys. F, 1982,12,15. R. Osborn, internal Report No. RAL91-01 I, Rutherford Appleton Laboratory, 1991. P. J. Feibelman and D. R. Hamann, Phys. Rev: B Condetrs. Matter. 1980,21, 1385. References 12 Myung-Ho Kang and J. W. Wilkins, Phys. Rev: B Condens. Matter, 1990,41, 10182. A. D. McQuillan and M. K. McQuillan, Titanium, Butterworth Scientific, London, 1956. IMI Titanium Ltd., P.O. Box 704, Witton, Birmingham, B6 7UR. 13 14 R. R. Cavanagh, R. D. Kelley and J. J. Rush, J. Chem. Phys., 1982, 77, 1540. J. M. Nicol, T. J. Udovic, J. J. Rush and R. D. Kelley, d,ungmuir, 1988, 4,294. N. S. Clarke, AWE, Aldermaston, Personal Communication. 15 A. J. Renouprez, B. Fouilloux and J. P. Candy, Surf. Sci., 1979, C. G. Windsor, Pulsed Neutron Scattering, Taylor and Francis, London, 1981. 16 83, 285. P. W. Albers, G. H. Sicking and D. K. Ross, J. Phys. Condens. J. Penfold and J. Tomkinson, Internal Report No. RAL86-019, Matter, 1989, 1,6025. Rutherford Appleton Laboratory, 1986. A. 1. Kolesnikov, M. Prager, J. Tomkinson, I. 0.Bashkin, V. Yu. Paper 3/07321A; Received 12th December, 1993