Investigation of H+ implanted Fe-Al alloys

In the present work hydrogen interaction with vacancies was investigated in Fe-Al alloys with various concentration of vacancies. The Fe-Al samples were implanted with low energy H+ ions (100 keV). This procedure introduced high hydrogen concentration into relatively narrow sub-surface region in the depth of ∼500 nm. Variable energy positron annihilation spectroscopy (VEPAS) was employed for investigation of hydrogen interaction with vacancies in the sub-surface region. This study revealed formation of vacancy hydrogen complexes in the sub-surface region. Thermal stability of vacancy-hydrogen complexes was investigated as well.


Introduction
Iron aluminides are widely considered as prospective materials due to high specic mechanical strength and enhanced corrosion resistance at elevated temperatures. However, work machinability of these alloys suers from poor ductility at room temperature. Since it has been shown that the ductility of Fe-Al alloys is remarkably improved in the absence of hydrogen and water vapor [1], environmental hydrogen embrittlement was suggested to take place in Fe-Al alloys. In addition to this, Fe-Al alloys are well known for a low vacancy formation enthalpy. As a consequence, the equilibrium concentration of vacancies in Fe-Al alloys is substantially high compared to pure metals. Thermal vacancies formed at elevated temperatures in Fe-Al alloys can be relatively easily quenched to room temperature. Hydrogen interaction with vacancies could play very important role in the embrittlement process. The importance of hydrogen interaction with vacancies is further amplied by high mobility of hydrogen in Fe-Al lattice.
In this work we employed VEPAS for investigation of defects introduced by H + implantation and interaction of implanted hydrogen with vacancies in Fe-Al alloys with various composition.

Experimental
Fe-Al alloys of Al concentration c Al 27 and 35 at.% were prepared by arc melting from high purity Fe (99.99%) and Al (99.99%) in Ti-gettered Ar atmosphere. As-cast alloys exhibited coarse grains with the mean diameter of a few mm. Specimens were quenched to room temperature after one hour of annealing at 1000 • C. The Fe 73 Al 27 sample was subsequently annealed at 520 • C for one hour. The samples were implanted by H + ions with the energy of 100 keV up to a uence of 3 × 10 18 at./cm 2 . The implantation was performed at room temperature on a cascade accelerator using the accelerating voltage of 100 kV. Variable energy positron annihilation spectroscopy (VEPAS) measurements were performed using magnetically guided energy variable positron beam "SPONSOR" [2] with positron energy adjustable in the range from 30 eV to 35 keV. Energy spectra of annihilation γ rays were measured by HPGe detector having an energy resolution of (1.06 ± 0.01) keV (FWHM at 511 keV). The Doppler broadening of annihilation prole was evaluated using the line-shape S-parameter.

Results and discussion
The aim of this study was to investigate the interaction of hydrogen with vacancies in Fe-Al alloys. For this purpose the Fe-Al alloys with known vacancy concentrations were prepared, Fe 73 Al 27 sample exhibits low concentration of vacancies c V ≈ 4 × 10 −6 while Fe 65 Al 35 contains high vacancy concentration of c V ≈ 5 × 10 −3 [3], consistent with c V ≈ 3 × 10 −3 for Fe 64 Al 36 [4]. The implantation prole of H + ions calculated by SRIM code [5] is plotted in Fig. 1 as gray histograms. The mean H + penetration depth isz H + ≈ 520 nm and the width of the implantation prole is characterized by FWHM of ≈ 130 nm.
From the H + implantation prole one can calculate that 75% of implanted hydrogen is stopped in a sub-surface layer located at a depth from 470 to 600 nm below the surface. Using the total uence of H + implantation one can easily estimate that hydrogen concentration in this layer is as high as 2.5 H per one host metal atom (Fe, Al). Hence, the hydrogen concentration in the sub-surface region 470-600 nm is much higher than the concentration of vacancies.
The VEPAS results for Fe 73 Al 27 alloy are plotted in Fig. 1(a). In the virgin Fe 73 Al 27 sample the S-parameter decreases with increasing positron energy from the surface value to the bulk value. Fitting of the S(E) dependence resulted in positron diusion length L + = (130 ± 7) nm which is in agreement with low concentration of defects in this sample. One can see in Fig. 1(a) that Fe 73 Al 27 alloy implanted with H + exhibits a bump with signicantly enhanced S parameter in the sub-surface region inuenced by hydrogen implantation. This is obviously due to new defects created in the sample by H + implantation. Protons with energy 100 keV implanted into the sample lose their kinetic energy in collisions with Fe and Al atoms and produce Frenkel pairs in the host lattice. Some of them recombine, i.e. vacancy is lled by interstitial atom, but other ones remain in the sample since the displaced atom was kicked out to sucient distance from the vacancy. The increase of the S parameter in the sub-surface region in Fe 73 Al 27 sample is due to positron trapping at such vacancies created by H + implantation. Since it is known that an attractive interaction exists between hydrogen and vacancies [6], vacancies introduced by H + implantation are likely associated with hydrogen. However, since the concentration of implanted hydrogen in the sub-surface region exceeds the concentration of vacancies by several orders of magnitude the majority of hydrogen atoms are located in the interstitial sites of Fe-Al lattice. Fig.1 Al 27 but at the same time hydrogen implanted into the sub-surface region is trapped at vacancies existing already in the sample. The former process increases the S parameter while the latter leads to a decrease of S due to the fact that the electron density in vacancy is enhanced by the presence of hydrogen and therefore reduced positron localization. One can see in Fig. 1(b) that the latter eect prevailed and H + implanted Fe 73 Al 27 exhibits lowered S parameter in the sub-surface region inuenced by implantation. Similar eect of decrease of the S parameter caused by H + implantation was recently observed in Fe 52 Al 48 [7] which presumably contains very high concentration of vacancies in orders of 10 −2 due to its high Al content [3].
The implanted samples were subjected to annealing for 1 h in vacuum at various temperatures in order to examine the thermal stability of defects. The S(E) curves for annealed Fe 73 Al 27 and Fe 65 Al 35 alloys are plotted in Figs. 1(a) and 1(b), respectively.
Annealing at 400 • C activates a diusion of hydrogen. The implanted hydrogen located in interstitial sites diuses out of the narrow sub-surface region further into the sample, i.e. the hydrogen concentration prole becomes broader and vacancies located in larger distance from the maximum of the hydrogen implantation prole can be lled with hydrogen. Driving force for this process is a gradient of the hydrogen concentration. This process leads to a decrease of S in a broader sub-surface region which can be seen for Fe 73 Al 27 and Fe 65 Al 35 alloys annealed at 400 • C in Fig. 1(a) and 1(b), respectively. Positrons with kinetic energy of 16 keV exhibit the mean penetration depth of ∼ 510 nm which is close to the maximum of the implantation prole of hydrogen. The implantation proles for positrons and hydrogen are compared in Fig. 2a. Obviously, the positron implantation prole is signicantly wider which testies that the S parameter value measured at positron energy of 16 keV contains a superposition of a contribution from positrons annihilated inside the region with high hydrogen concentration and also a contribution of positrons annihilated outside this region. The broadening of the hydrogen concentration prole extends the spatial range in which vacancies are lled with hydrogen and thereby decreases the S parameter. An additional mechanism which occurs at higher temperatures is recovery of vacancies which disappear by diusion into sinks at grain boundaries and on the surface. This process is clearly visible in Fig. 1(b) as a drop of the bulk S parameter of both alloys annealed at 500 • C. Note that enhanced S parameter at low energies in the Fe 73 Al 27 alloy annealed 600 • C is due to the formation of a thin oxide layer during annealing.
It is instructive to compare the temperature dependence of the bulk S parameter S(35 keV) determined at the positron energy of 35 keV and the S parameter S(16 keV) determined at energy of 16 keV where the contribution of positrons annihilated in the sub-surface region with high hydrogen concentration was maximal. The behavior of the S parameters S(35 keV) and S(16 keV) for the Fe 73 Al 27 and Fe 65 Al 35 alloys is plotted in Fig. 3b and 3c, respectively. The S parameter S(16 keV), which characterizes the sub-surface region inuenced

Conclusions
Fe-Al alloys implanted by H + ions with the energy of 100 keV were studied by the VEPAS technique. It was suggested that H + implantation creates vacancy-hydrogen complexes in a sub-surface region. Moreover, trapping of hydrogen at vacancies existing already in sample was observed. The investigation of the thermal stability of defects in H + implanted alloys revealed that hydrogen diusion is activated by annealing at 400 • C leading to a spread of the hydrogen concentration prole. Vacancies are annealed out by further annealing at 500 • C.