<p> DIFFUSION AND PERMEABILITY OF HYDROGEN IN AMORPHOUS FE-NI-SI-B-C-P ALLOY</p><p>E.A. Pastuchov, N.I.Sidorov, V.P.Chentsov. Institute of metallurgy, Russian Academy of Sciences, Ur Dep.. 101 Amundsen St., 620016 Yekaterinburg, Russia.</p><p>Abstract The basic concepts of hydrogen permeability (HP) mechanism for pure metals in crystal state are already formulated. Enough adequate theoretical models and numerous experimental researches are published. As far as disorder systems are concerned, such works, particularly devoted to interaction of hydrogen with amorphous structures are published rather recently. Deficiency of similar researches is explained by temporary and thermal instability of structure and properties of the given class materials. It leads to great difficulties of vacuum-dense membranes construction subjected to thermal affects in the experiments devoted to the hydrogen transportation mechanism in amorphous materials matrix. An attempt to perform complex researches of diffusion mechanism and hydrogen permeability through amorphous materials using uniform positions is presented in this paper. Such developed theoretical models and experimental methods as molecular dynamics, stationary flux and x-ray diffraction are used. Computer calculations Geometrical correlations of spatial distribution and form of the inter-node cavities are central problem of computer modelling. Developed molecular-dynamic (MD) model of the diffusion movement mechanism of hydrogen atoms through amorphous materials took into account interaction of hydrogen only with the equi distance nearest atoms of metal. Detailed description of the presented MD-model mathematical concepts and computer</p><p>85 calculation results are published in [1,2]. The MD-calculations analysis confirmed its high efficiency in definition of the neighbor ordering details in interaction of hydrogen with 3d-metals. Analysis also revealed possible hydrogen permeability mechanism in cyclic sorption-desorption of hydrogen. According to the obtained computer experiment data, stable life time of hydrogen in the inter-node cavities is not exponent form as it is predicted by analytical theories. This divergence is explained by essential contribution of short hydrogen atoms stable life times due to high probability of consecutive transitions through several inter nodes space without delay. Essentially such transitions between cavities are correlated. Neighbor ordering in amorphous alloys is more stable in comparison with melt. However all (Bernal) cavities are deformed by the statistical deformations that also decrease potential barriers on a way of hydrogen atoms output from such cavities (hydrogen delocalization). As a consequence, transitions among cavities in amorphous metals are high degree correlated. Stationary states contribution to hydrogen movement is negligible. Stable life times function is not potential form therefore. Amorphising elements (Si, B, C, etc.) inducing in amorphous metals decrease large cavities amount providing most energetically available migration ways for hydrogen. It decreases absorbing ability of metal, as well as hydrogen diffusion movement, decreasing hydrogen permeability. Thus, permeability and solubility of hydrogen in amorphous materials are structure-sensitive characteristics that actually demonstrate MD-model of amorphous Fe-H alloy. Experiment Experimental researches of hydrogen absorption influence on structural and physical and chemical properties of alloys based on transition metals (palladium and iron) are presented in [1]. This works show, that absorption of hydrogen causes essential shift of the starting and</p><p>86 the ending of structural relaxation to higher heating temperatures. It also turns to significant modification of amorphous materials toughness properties on the iron basis, causing increased fragility. All mentioned changes are adequately presented by atoms distribution curves changes, had been obtained by diffraction method. Samples for x-ray graphic researches were prepared from short (1.5÷2mm) band attached on nickel substrate. Experiments were carried out by Dron UM1 type diffractometer (Mo-Kα -radiation, pyrographite monochromator in the reflected beam mode and angle interval from 50 up to 120o). For preliminary diffraction graphs processing we created computer program including dispersion, polarization, incoherent scattering by heavy atoms and normalization corrections formulas. Amorphous iron (hydrogen and non hydrogen) alloys neighbor ordering was analyzed using this technique. Hydrogen induction was performed 2 by electrolysis in 5% H2SO4 alcohol solution at 10mA/sm current density during 3 hours. The effects found out by the molecular-dynamic researches related to hydrogen presence in metal glasses were confirmed by the x-rays structural analysis. X-ray analysis results allow assume amorphous samples, both initial, and hydrogenated, to be inhomogeneous. Different structure and components distribution are possible. There are nuclear microgroupings with various interatomic distances, coordination numbers and components concentration in samples. Thus, separate cluster like structures of local atoms packings are possible in an amorphous matrix. This process tends to oscillations of atoms radial distribution curves g (R) (Fig. 1). That oscillations are revealed as different diffraction maxima positions, heights of g (R), "multiple" peaks and its asymmetry in the case of hydrogenated samples. Stationary flux method was used for hydrogen permeability researches of 25 microns thickness membranes made from amorphous and recrystallized Fe-alloy</p><p>87 (Fe77.333Ni1.117Si7.697B13.622C0.202P0.009). Recrystallized alloy has been made from amorphous sample by vacuum annealing at 400oС. Molecular hydrogen admission to the entrance side of degassed sample at the maximal allowed temperature (300°С for amorphous and 400°С for recrystallized one) did not result in observable increase of output flux: hydrogen flux achieved 3.8∙1012 sm-2/с at 10 Torr hydrogen pressure. Glow discharge in hydrogen atmosphere for overcoming of amorphous sample passivating surface layer was used. The hydrogen ions, generated in discharge area, easily penetrate into the bulk of sample [3]. Thus we observed noticeable penetrating flux. All researches were carried out at input hydrogen pressure 2 Torr at which the glow discharge is most stable. Stationary hydrogen flux temperature dependences for amorphous and recrystallized samples were obtained. Lower limit of researched temperature interval was defined by an opportunity of reliable flux registration and was registered as 125 oC for amorphous and 200oC for crystal samples. The most important difference in properties of two researched alloy states consists in non-monotonous increase of output flux at temperature increase in amorphous material. Flux increases in temperature interval from 125oС up to 200°С, achieves maximal value of 3.3∙1013 sm-2с-1 at 200°С and shows abnormal decreasing at further heating. Second sample obeys classic Arrenius dependence with 17.9 kJ/mole activation energy with maximal flux value 2.7∙1013 sm- 2с-1 at 375 oС. (fig. 2)</p><p>2 3 4 5 6 7 8 9 10 Stationary14 flux of hydrogen in samples has been realized differently.g(r) Fast output flux increasing had been developed in 12 575<h > both cases in all a 2 550 10 500 8 480 475<h > 2 6 450 425<h > 2 4 425</p><p>2 300<h2> 275 0 88 275</p><p>-2 2 3 4 5 6 7 8 9 10 r, A Fig. 1. Atoms radial distribution curves for amorphous. </p><p>Fe77.3 Ni1.1Si7.7B13.6C0.2P0.009 alloy with hydrogen and without it range of researched temperatures with stabilizing characteristic times about 30 ÷ 60с. After fast increasing hydrogen output, very “prolonged” time-depended output flux has been observed at 125 oС up to 225°С temperatures for amorphous membrane with stabilization times 6000с. Temperature increasing results in disappearance of slow penetrating flux component and general flux decreasing. Thus, reciprocal temperature dependence of flux is not monotonous, but has maximum in 200°С area (see fig. 2). Results and discussion Obtained results analysis of the computer and experimental researches enables to make assumption about mechanism of diffusion and permeability of hydrogen in amorphous metal - metalloid alloys. It had been shown by the MD-calculations, that the inter-atom interaction potentials with deep and broad minima should be related to the amorphous alloys characterized by increased hydrogen permeability. Such conditions are necessary to simultaneous achievement of strong structure and good inter-</p><p>89 node pores deformability for the better hydrogen atoms passing through the pores. </p><p>Fig. 2. Temperature dependence of stationary hydrogen flux. (● - amorphous alloy, ▲- recrystallized alloy)</p><p>Alloys amorphization results in significant increase of free volume, increasing HP, solubility and diffusion. It should be especially noted to effect of sharp (by order) HP change at relatively low increase solubility. This effect is caused by competition from the amorphizing elements also occupying the largest polyhedral Bernal pores. This effect first of all takes place in the high concentration field of amorphizing elements. Carried out HP experimental researches of the iron based amorphous alloy membranes reflect the general laws of structure formation noted in the MD-method. "Prolonged" flux transition to stationary value is obviously related to reversible diffusant trapping [4]. Thus hydrogen release probability from the traps increases faster than probability of trapping during temperature increase. It is experimentally shown, that at temperature increase</p><p>90 up to 200oС slow hydrogen flux growth is observed and then its decrease begins above 200oС. Such dependence is typical for traps with activation energies of releasing and trapping Еrel >Etrap. Penetrating flux value decrease in amorphous sample in temperature interval 200oС up to 300oС is most probably related to surface processes. As the penetrating flux is three orders less 16 -2 -1 than incident one, falling to input surface (Vf ≈ 10 cm c ), flux balance can be written </p><p>Vf = VT + Vr (1) where - Vr = VfCi/Cmax and VT = biexp (-Ei/RT) Ci (2) are fluxes of ionic-induced reemission and thermodesorption at the input side. Hydrogen concentration in not damaged structure of an alloy near input surface is Ci and bi is pre-exponent factor. Maximum concentration in pre-surface layer Сmax at room temperature (when thermodesorption is negligible) is estimated as 18 3 10 at/sm [5,6]. Considering, that С2 concentration at the output side is much less than Ci, for a stationary penetrating flux we obtain following expression</p><p>J = Aexp (-Ed/RT) / (1 + Bexp (-Ei/RT)) (3) where Ed - diffusion activation energy. The equation does not include parameters of hydrogen interaction with traps since diffusant trapping and release in stationary condition go with equal speeds and do not affect stationary flux value. Approximation results are represented on fig.3 by continuous lines. Good enough approach to experimental data am am gives energy values Ed = 40.8, Ei = 86.7kJ/moll for amorphous cr cr and Еd = 71.2, Ei =51.7kJ/moll for crystal samples. Calculation of concentration at the input membrane side using formulas (1) and (2) and obtained thermodesorption activation energies is</p><p>91 presented on fig. 4. Parameter Сmax used in calculations does not affect activation energy, but only preexponents. Surface processes and Ed Ei values correlation determine temperature dependence of stationary flux. In case of amorphous alloy, concentration Ci takes value Сmax up to to 175°С temperature, and speed</p><p>Fig 3. Stationary hydrogen flux through amorphous membrane. of hydrogen penetration is determined by diffusion, the flux thus increases. Concentration Ci decreases exponentially with the further temperature increase, and Ed < Ei correlation turns to flux decreasing. Input concentration for recrystallised alloy decreases in all temperature range (fig. 3, curve 2), and Еd > Ei correlation results to classic Arrenius dependence </p><p>J ≈ exp (-Ef/RT) (4)</p><p> where Ea= Ed - Ei ~ 19.6 kJ/moll by our calculations that is close to experimentally obtained value Eа = 17.9 kJ/moll.</p><p>92 Activation energies difference is obviously related to structure of samples. Excess free volume presence in amorphous alloy provides smaller energy expenses at atom skips of hydrogen from one internodes to another. Besides the part of the internodes can be deformed and not correct Bernal cavities form. Thermodesorption activation energy in</p><p>Fig 4. Hydrogen concentration on the input membrane side. (1-amorphous alloy, 2-recrystallised alloy)</p><p> cr am recrystallized alloy is less, than in amorphous one (Ei <Ei ). It can be explained by surface reorganization and change of passivation layer influence to hydrogen desorption. 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