Gettering of hydrogen in iron and nickel caused by the implantation of titanium ions

Materials Science and Engineering, 69 (1985) 421-428

421

Gettering of Hydrogen in Iron and Nickel caused by the Implantation of Titanium Ions* H. BAUMANN, TH. LENZ and F. RAUCH

Institut fiir Kernphysik, Universita't Frankfurt, D-6000 Frankfurt am Main (F.R.G.) (Received September 17, 1984)

ABSTRACT

Large amounts o f hydrogen were f o u n d to be gettered in the near-surface region o f iron and nickel after titanium implantation. This effect was studied as a function o f various implantation parameters. Hydrogen and titanium depth profiles were measured using the nuclear reactions IH(15N, (rt)12C and 4STi(p, 7)49V. The gettering starts at a titanium fluence o f about 1 × 1017 ions cm -2 for iron and about 2 × 1017 ions cm -2 for nickel (Eion = 30 keV). The a m o u n t o f gettered hydrogen increases with increasing titanium fluence, and concentrations o f up to 30 at.% are reached. A rough correlation exists between the shape o f the hydrogen profile and that o f the titanium profile. Release temperatures for the gettered hydrogen were found to be in the range 4 2 0 - 6 1 0 K in annealing experiments. The structure o f the hydrogen traps and the gettering mechanism are discussed. 1. INTRODUCTION

The interaction of hydrogen with impurities in metals introduced by ion implantation has attracted increasing attention in the last decade, mainly in connection with the radiation effects in the first wall of fusion reactors and the hydrogen embrittlement of steels and other metals [ 1-5 ]. It was found that the mobility and solubility of hydrogen can be markedly changed compared with the corresponding values for the pure host material. Apart from the technological importance of these effects, they are also of basic interest. *Paper presented at the International C o n f e r e n c e on Surface Modification of Metals by Ion Beams, Heidelberg, F.R.G., September 17-21, 1984. 0025-5416/85/$3.30

The experiments reported here belong to a research programme on the influence of ionimplanted elements on the behaviour of hydrogen in transition metals, mainly titanium, iron and nickel [6, 7]. One objective of the programme was to find out whether implanted titanium, which as a bulk metal has a strong affinity for hydrogen, would form traps in iron and nickel as host metals, both elements having a low solubility for hydrogen. The normal procedure which has also been used by other workers [1-5] was to implant the titanium ions into the host material; this was followed by ion implantation of hydrogen; finally, the samples were profiled for trapped hydrogen using a nuclear reaction. During these investigations, we observed that iron and nickel samples implanted with a titanium fluence of 1018 ions cm -2 showed hydrogen concentrations of up to several tens of atomic per cent in the surface layer, w i t h o u t the implantation of hydrogen ions. The hydrogen concentration distributions were found to remain stable at room temperature for several weeks. Apparently, hydrogen had been gettered, either from the bulk of the samples or from the residual gas of the implantation chamber, by deep traps in the layer modified by the implanted titanium. In order to obtain more information on this gettering effect which has to our knowledge not been reported in the literature, we performed a series of experiments in which several implantation parameters were varied. As well as the measurement of the stationary hydrogen depth profiles, the annealing behaviour of the samples was determined and, furthermore, the titanium depth profiles were measured since theoretical predictions are not reliable for the high titanium fluences in our experiment. © Elsevier Sequoia/Printed in The Netherlands

422 2. EXPERIMENTAL DETAILS

PolycrystaUine foils of iron and nickel (purity, 99.99%, supplied by Ventron G.m.b.H.) with a thickness of 0.13 m m were used for the samples. They were cleaned with methanol before the implantation process. The implantations were carried out at room temperature, using the 50 kV ion implanter at the Institut fiir Kernphysik [8, 9]. The ion beam (4STi ions) was swept electrostatically over the sample to obtain a uniformly implanted area of 6 m m diameter. During the implantations the pressure in the target chamber was about 5 × 10 -5 mbar because the implanter was evacuated with only one oil diffusion pump located between the ion source and the analysing magnet. The standard implantation parameters were as follows: ion energy, 30 keV; ion flux, 1 8 g A cm-2; titanium fluence, 4 × 1017 ions cm -2. These values were changed separately to test their influence on the gettering effect. The hydrogen concentration profiles were determined using the 15N m e t h o d which is based on the reaction 1H(15N, ~7)12C. This m e t h o d has been described in detail in the literature [2, 6]. The profiling measurements were performed with a 15N2+ beam delivered by the 7 MV Van de Graaff accelerator at the Institut ffir Kernphysik. The beam was collimated to a diameter of 2 mm. The samples were m o u n t e d on a holder which can be heated to 900 K. Its temperature can be measured with a 100 ~ platinum resistor. The pressure in the target chamber was about 10 -5 mbar. The 7 rays from the reaction were measured with two 8 in X 4 in NaI detectors. All profiling measurements were performed at room temperature. The analysing beam was found to have a negligible effect on the hydrogen distributions in all measurements. As calibration standards we employed silicon crystals implanted with a known a m o u n t of hydrogen ions: To calculate the depth scales the densities of the pure host metals were used. Stopping power data were taken from ref. 10. Since the dE/dx value of titanium is n o t very different from those of iron and nickel, a correction for the presence of titanium in the samples was n o t needed. The uncertainty of the hydrogen concentration values in the samples is estimated to be less than 10%.

To determine the titanium concentration profiles, we used a method developed recently [11]. It is based on a resonance (F ~ 50 eV; Ep -- 1362 keV) in the cross section of the reaction 48Ti(p, 7)49V. By measuring the 7 ray yield (E~ = 7.94 MeV) as a function of the proton energy the titanium depth distribution is obtained. The measurements were performed with the proton beam of the 2.5 MV Van de Graaff accelerator at the Institut ffir Kernphysik. The 7 rays were measured with a 5 in × 5 in NaI detector. The energy spread of the proton beam implied a depth resolution of about 15 nm at the sample surface. To calculate the depth scales, the proton stopping power values of ref. 12 were used. The stopping powers of the host metals and also their densities were corrected for the presence of titanium in the samples. Pure titanium metal was used as calibration standard. The uncertainty of the titanium concentration values is estimated to be about 10%. For the annealing experiments the 15N beam energy was kept fixed; for each sample it was chosen to correspond to the maximum of the hydrogen depth distribution. The sample temperature was increased linearly at a rate of 0.1 K s-1 from room temperature to a temperature at which all the gettered hydrogen had been released. The counting rate of the 4.43 MeV ~/rays, the temperature and the time were recorded using a PDP-15 computer in a multiscaling mode. The signals for increasing the channel number were delivered by the 15N beam current integrator.

3. RESULTS

In the following we shall denote titaniumimplanted iron samples as F e - T i (and titanium-implanted nickel saniples as Ni-Ti). The dependence of the gettering effect on the titanium fluence was investigated for a series of F e - T i samples (1X 1017, 2 X 1017, 4 × 1017, 8 X 1017, 10 × 1017 and 16 × 1017 Ti + ions cm -2) and a series of N i - T i samples (1X 1017, 2 X 1017, 4 × 1017 and 8 X 1017 Ti + ions cm-2). The hydrogen profiles of the F e - T i samples are shown in Fig. 1, and the titanium profiles in Fig. 2. The titanium profiles of the N i - T i samples were very similar to those of the corresponding F e - T i samples; their hydrogen profiles were similar in shape, but the a m o u n t

423

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o f gettered hydrogen was smaller. The hydrogen profiles o f non-implanted samples showed, as well as the surface peak due to adsorbed hydrocarbons, a hydrogen c o n t e n t of

1 - 2 at.% in a surface layer of about 200 nm. The maximum concentration and the total a m o u n t of hydrogen derived from the depth profiles are shown in Fig. 3. It can be seen

424

from the figures that the gettering starts at a fluence of about l X 1017 Ti + ions c m -2 for F e - T i and about 2 X 1017 Ti + ions cm -2 for Ni-Ti; at about 10 is Ti + ions cm -2, saturation seems to occur, probably because the retained titanium dose does n o t increase further. It should be noted t h a t the titanium distributions extend much deeper than the theoretical

range (about 11 nm); however, this fact will not be discussed here. The hydrogen profiles roughly correspond in shape to the titanium profiles but do n o t extend as deep. The concentration ratio does n o t show a simple dependence on depth or fluence. However, it appears that gettered hydrogen is only found when c~ is higher than about 10-15 at.%. A variation in the ion energy (20 and 80 keV instead of 30 keV) and the ion flux ( 6 p A cm -2 instead of 1 8 p A cm -2) did n o t have much influence on the hydrogen profiles (nor on the titanium profiles), as can be seen in Figs. 4 and 5. Also shown in Fig. 5 is the profile of a sample which was implanted under worse vacuum conditions with the idea that more hydrogen would be available. A pressure of about 5 X 1 0 - 4 mbar was achieved with an air inlet using a needle valve. It can be seen that the a m o u n t of gettered hydrogen is indeed higher. Some F e - T i and Ni-Ti samples were analysed again several weeks after the titanium implantation. The hydrogen profiles were found to be almost unchanged. An example of an annealing curve is shown in Fig. 6. The gettered hydrogen is retained up to about 580 K and is then released in a narrow temperature interval centred at about 610 K. The annealing curves for other F e - T i

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426 and Ni-Ti samples had very similar shapes, except that the release o~curred at other temperatures. F o r F e - T i samples with fluences between 2 × 1017 and 16 × 10 iv Ti + ions cm -2, the release temperature increased from 484 to 610 K; for Ni-Ti samples with fluences between 2 X 1017 and 8 X 1017 Ti ÷ ions cm -2 it increased from 423 to 543 K. The full curve in Fig. 6 was calculated using a simple model [13] in which it is assumed that hydrogen atoms detrapped by thermal activation can diffuse out of the sample witho u t retrapping. Then the decrease in the hydrogen' concentration with temperature follows the first-order equation

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where v ~ 1013 s-1 is the Debye frequency, # is the heating rate and Ed is the detrapping energy. Ed can be determined b y fitting the experimental annealing curve with a theoretical curve obtained by numerical integration of the above equation, with E d a s the fit parameter. The E d values determined in this way were between 1.46 and 1.86 eV for F e Ti and between 1.28 and 1.65 eV for Ni-Ti. The rather good agreement between the theoretical curve and the experimental curve in Fig. 6 can be improved when a distribution of detrapping energies with a width of a b o u t 0.1 eV around a mean value is assumed, as was found for all samples. The detrapping energies determined using this model are certainly t o o high because of the neglect of retrapping. A comparison with the results of Myers e t a l . [3], who used a model which takes retrapping into account, shows that the E d values should be reduced by a b o u t 0.4 eV to obtain more realistic trap depths. These axe then in the range 1.1-1.5 eV. Profiling measurements of the annealed samples showed that the hydrogen concentration over the whole depth had fallen to a value of 1-2 at.%, which means that all the trapped hydrogen had been released in the annealing meaurements. Although in these measurements only one depth for each sample was probed, the annealing temperatures and trap depths are probably constant over the whole hydrogen profile for each sample. This assumption is based on the observation that in an Ni-Ti sample (4 × 1017 Ti + ions cm-2), which was analysed again after 20 months,

the average hydrogen concentration had fallen to a b o u t one-quarter; however, the shape of the hydrogen profile was almost the same as that of the original profile. It is interesting to note that the original hydrogen profile could be almost completely restored b y putting the sample into hydrogen gas.

4. DISCUSSION The results presented in Section 3 show that the titanium fluence is the main parameter influencing the a m o u n t of gettered hydrogen, and probably the vacuum conditions during the implantation are also of importance. The ion energy and flux have little influence, at least in the ranges covered in our measurements. These observations suggest that the strong chemical affinity of titanium for hydrogen is the decisive factor in the gettering effect. This assumption is supported by experiments with F e - F e , F e - N i and Ni-Ni samples implanted to a fluence of 10 is ions cm -2. In these samples, no gettered hydrogen was found, and even implanted hydrogen ions were not trapped [14]. Since titanium, iron and nickel have similar masses, we can exclude the possibility that radiation damage contributes to the deep traps responsible for the gettering effect. However, it would be expected that the implantation of other elements with a strong hydrogen affinity, such as zirconium, hafnium, scandium and yttrium, would also cause hydrogen gettering. Indeed, we observed an even stronger gettering in preliminary experiments on the system F e - Z r than for Fe-Ti. For a discussion of the rather strong binding of the gettered hydrogen, we may compare our results with those of Myers e t al. [3]. They found that implanted deuterium ions were trapped in yttrium-implanted iron (4 × 1016 Y+ ions cm -2) and were released only at a b o u t 550 K, corresponding to a trap depth of 1.27 eV. The strong binding was attributed to the high affinity of yttrium for hydrogen and to the formation of y t t r i u m - v a c a n c y complexes because yttrium atoms are oversized in an iron matrix. For titanium-implanted iron, much lower detrapping temperatures were found. The difference was explained b y the lower hydrogen affinity and the smaller atomic radius compared with that

427 of yttrium. Besenbacher et al. [4] found that implanted deuterium ions were released from traps in yttrium-implanted nickel (3 × 1016 Y+ ions cm -2) even at 278 K. The main difference in our work seems to be the higher fluences and the ensuing higher titanium concentrations of several tens of atomic per cent. For such concentrations, it is expected from the F e - T i and Ni-Ti phase diagrams [14] that FeeTi and Ni3Ti precipitates respectively are formed. However, neither of these phases reacts with hydrogen [15]. Even if the phase FelTil which is known to react with hydrogen and to result in a ternary hydride is formed, the fact that hydrogen is retained in the F e - T i samples to high temperatures could n o t be explained since the hydrogen decomposition pressure for this hydride even at room temperature is a b o u t 5 bar [15]; a similar consideration holds for NilTil [15]. We speculate therefore that some part of the implanted titanium exists as titanium precipitates in which the gettered hydrogen is b o u n d as titanium hydride (TiHx). This hydride is thermodynamically very stable [15] ; hence it is conceivable that, when hydrogen is present in the samples, the formation of TiHx m a y c o m p e t e with the formation of Fe2Ti or Ni3Ti precipitates. From this assumption, the high retention temperatures observed could be explained qualitatively. Hydrogen atoms to be detrapped would have to overcome the potential energy difference between TiHx and the surrounding medium before they can diffuse out of the sample (or are retrapped in between). In annealing experiments on samples having thin TiHx layers it was found [16] that a temperature of a b o u t 420 K is required to bring hydrogen into solution in titanium. Since the solubility of hydrogen in iron and nickel is much lower than in titanium [17], correspondingly higher annealing temperatures would result. The increase in annealing temperature with the increasing titanium fluence could be understood within this framework as resulting from enhanced retrapping due to an increased density of titanium precipitates. However, the structure of the implanted layer might be more complex. Several research groups [ 1 8 - 2 0 ] have found that, when titanium is implanted into iron and steel, carbon is incorporated from the vacuum system and

an amorphous F e - T i - C surface layer develops, with the carbon b o u n d as carbide. The layer had a thickness of 60 nm for a fluence of 2 × 1017 Ti + ions cm -2 (Eio n -- 9 0 - 1 8 0 keV). A measurement using Auger electron spectroscopy on one of our F e - T i samples showed that carbon b o u n d as carbide was present in the titanium-implanted layer at a concentration of several atomic per cent. This makes it likely that in all our samples the surface layer is an amorphous F e - T i - C or N i - T i - C alloy which contains TiC (perhaps also titanium) precipitates. We plan to examine this by profiling the samples for carbon and by investigating their structure using Xray diffraction. The final explanation of the strong hydrogen binding, which may be influenced by carbon in the implanted layer, must await the o u t c o m e of these measurements. The gettering mechanism is another interesting point. As indicated above, the hydrogen is incorporated in the samples during the titanium implantation; therefore its origin must be in the hydrocarbon and water molecules which are the main components of the residual gas of our vacuum chamber. These molecules strike the sample surface at a rate of about 1016 cm -2 s-1 at a pressure of 5 × 10 -5 mbar, and a certain fraction of them will be adsorbed. Since successive atomic layers of the sample are removed by sputtering during the ion implantation, titanium atoms are exposed from a certain fluence onwards, so that the instantaneous surface contains chemically reactive sites. At those sites, adsorbed molecules can dissociate and hydrogen atoms can be generated; this is perhaps assisted by beam-induced disintegration. Hydrogen atoms can then diffuse into the sample, where the traps formed during the implantation provide a sink for the hydrogen. In order to obtain more experimental support for this model, we are planning experiments in which during the implantations the partial pressure of different gases containing hydrogen, deuterium and carbon is varied.

5. CONCLUSION The hydrogen gettering effect reported here is expected to occur whenever ions of an element with a strong affinity for hydrogen

428

are implanted into iron, nickel or steels. The presence of hydrogen in the implanted layer may influence its mechanical and other properties, in addition to the changes caused by the implanted element. Therefore, it should be interesting to analyse such samples for gettered hydrogen because this may help us to understand the properties of the modified surface layer. With respect to the field of hydrogen-impurity interactions, the possibility that some of the deep traps are filled by gettered hydrogen (1H) should be taken into account in experiments in which the trapping of implanted deuterium ions is investigated. Altogether, the hydrogen gettering effect seems interesting not only as a basic process but also from the application point of view; therefore a deeper understanding appears desirable.

ACKNOWLEDGMENTS

The authors acknowledge interesting discussions with K. Bethge. Thanks are due to S. Blhmner, P. March and J. Soennecken for their help in some of the experiments. Technical assistance with the annealing equipment by H. Kreyling and R. Staudte is acknowledged. We are indebted to H. Schmiedel, Technische Hochschule, Darmstadt, for performing the Auger analysis. The work reported here was made possible by a grant from the Deutsche Forschungsgemeinschaft.

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