LI Shou-Ying ZHAO Wei-Min WANG Yong

a (Schools of Materials Science and Engineering, China University of Petroleum, Qingdao 266580, China)

b (Engineering Research Center for New Metallic Functional Materials, Qingdao Binhai University, Qingdao 266555, China)

ABSTRACT Hydrogen is known to play a negative role in mechanical properties of steel due to hydrogen embrittlement. Surface strain modifies the surface reactivity. In this paper, we employed spin-polarized periodic density functional to study the atomic H adsorption and diffusion on the biaxial strained Fe(110) surface. The result shows that the adsorption of H at the Tf site is the most stable on compressive surface and tensile surface. And H atom on the top site relaxes to Tf site on the strained surface. The adsorbed hydrogen atom at all calculated adsorption sites relaxes towards the surface due to the tensile strain. Lattice compression makes the bonding strength weaker between H atom and the surface. The analysis of the partial density of states shows that H 1s orbital hybridizes with the Fe 4s orbital. The result of charge density difference shows electrons are transferred from Fe to H atom. Compressive strain reduces the transferred electrons and decreases the Mulliken electrons of Fe 4s orbital, which weaken the bonding interaction between H and Fe atoms. H atom diffuses into subsurface through a distorted tetrahedron. Surface strain does not change diffusion path but affects the diffusion barrier energy. Tetrahedron gap volume in the transition state of compressive system decreases to increase the diffusion barrier. This suggests compressive strain impedes H penetrating into the Fe subsurface. The present results indicate that it is a way to control adsorption and diffusion of hydrogen on the Fe surface by surface strain.

Keywords: hydrogen embrittlement, adsorption, first principles, surface strain;

1 INTRODUCTION

Decreasing the emission of CO2is currently a global concerned serious problem. Scientists continually explore a new energy to solve the pro- blem. Hydrogen is a clean-burning fuel to replace fossil fuel[1]. Hydrogen mainly exists in the form of H2. Transporting H2through pipelines is the most effective way relative to other transportation methods[2,3]. Hydrogen embrittlement of pipeline steels is an extremely important topic which reduces the mechanical properties of pipeline steel[4]. The gas pipelines always require welding to assemble each other for long distance transmission. Hydrogen embrittlement on welded joint and welding heat affected zone becomes complex because of the welding residual stress and microstructure hetero- geneity. Welding residual stress on the surface has been shown to change surface reactivity in a significant way[5,6]. Adsorption of hydrogen is sensitive to surface stress. The ductility decrease of X80, X100 steel during tensile test in hydrogen environment mainly occurs in necking stage[7]. Some scientists attributed this phenomenon to surface stress state turning into biaxial stress[8]. Abundant dislocations slip along Fe(110) during plastic deformation. New Fe(110) surface appears on the surface.

Hydrogen adsorption is the first step to penetrate into steel. For the past few years, adsorption of hydrogen on Fe(110) surface has been extensively investigated by means of experimental and theore- tical methods. Some information has been con- firmed, such as adsorption energy, adsorption site and adsorption configuration[9-13]. Jiang first studied H atom adsorption on Fe(110) by spinpolarized density functional theory calculation[14]. He con- firmed that the adsorption structure and adsorption energy agree with experiment. Urslaan found that quasi-threefold site is the most stable adsorption position with adsorption energy of 3.0 eV per hydrogen atom[15]. The surface layer space increases after the adsorption of hydrogen, which facilitates hydrogen atoms diffusing into the Fe bulk.

Adsorbed H atom penetrating into Fe bulk involves three steps. First, H atom diffuses between the adsorption sites on the surface. Second, H atom enters the gap between surface atoms and the second layer atoms. Third, H atom diffuses between gaps in bulk[16]. H atom diffuses on Fe surface with a barrier of 1.9 kcal/mol. The energy barrier from surface to subsurface is 9.5 kcal/mol[14,17]. The diffusion energy barriers from the second to the third layer decreases, which is almost identical to the barrier in bulk Fe. The step that hydrogen atom penetrates into the subsurface is the most difficult. It is the key step in determining the velocity of hydrogen atom penetrating into Fe bulk[18,19]. Some scientists studied hydrogen diffusion in strained Fe bulk[20,21]. But the effect of surface strain on hydrogen diffusion into subsurface has not been reported.

The above-mentioned work focused on the unstrained Fe surface. It is a new way to control adsorption of gas on many metal surfaces by changing surface strain[22-24]. Tensile strain increases the binding strength of CO on the Pt surface[25]. Oxygen atom adsorbs preferentially in the regions of expanded Cu(111) surface[26]. However, there is no theoretical study on hydrogen atom adsorption and diffusion on the strained Fe surface. The theoretical calculations on hydrogen adsorption and diffusion at strained Fe(110) surface are of great significance to investigate the relationship between hydrogen embrittlement and surface strain. H adsorption and diffusion on the strained Fe(110) surface were studied by employing density function theory to explore the mechanism biaxial strain hydrogen embrittlement in this paper.

2 COMPUTATION DETAILS AND MODELS

Our first principles calculation was carried out by the Cambridge Serial Total Energy Package (CASTEP) based on density functional theory (DFT). The Kohn-Sham equation with a plane wave basis set was solved by employing three-dimen- sional periodic boundary conditions[27]. Generalized gradient approximation (GGA) of revised Perdew Burke Ernzerhof (rPBE) functional was applied for the treatment electron exchange-correlation term. All calculations were done in the spin polarized. We tested kinetic energy cutoff andk-point sampling for the calculated principle cells. The kinetic energy cut off energy of 425 eV was used. A uniformk-point sampling of a 19 × 19 × 19 k-mesh for Fe principle cell was employed[28]. The equilibrium lattice constanta= 2.845 ? was obtained. It agrees well with the experiment value (2.866 ?)[29]and previous calculations of 2.834 ?[30].

The Fe(110) surface has been studied in this work, since it is the new surface during plastic deforma- tion. The Fe(110) surface model was represented by a seven layers slab model and a 2× 2 unit cell which has been used in other articles[14,31]. A 12 ? depth vacuum was placed on the model to ensure separation. Uniform k point was used on the Monkhorst-Pack and grid size of 7 × 7 × 1. BFGS algorithm was used to relax the slab model. The upper three layers of Fe atoms were allowed to relax, and the other bottom layers were fixed to represent bulk under the surface.

There are four different coordinated sites on Fe(110). They are short bridge (Sb), on-top (Top), long bridge (Lb) and pseudo three-fold hollow (Tf), as present in Fig. 1. Hydrogen atom was then adsorbed on the four coordinated sites. The H atom and the upper three layers of Fe atoms were allowed to relax while the other bottom Fe atoms were constrained. The adsorption energyEadsfor the four sites was calculated by Equation 1.

WhereEHis the energy of a single H atom,EslabandEH+slabare the total energy of the slab without and with an adsorbed hydrogen atom.EHis calculated by putting a H atom in a cubic box with 12 ? side and carrying out 1 × 1 × 1kpoint calculation. The negative value ofEadsmeans that the adsorption of H atom releases heat. The more negativeEadsillustrates more binding strength between the slab and H. The surface strain varies from -5% to 5%. Lattice constants of 2.702 ?, 2.788 ?, 2.845 ? and 2.902 ?, 2.987 ? correspond to 5% compressive strain, 2% compressive strain, no strain, 2% tensile strain, and 5% tensile strain, respectively.

Complete LST/QST calculation was used to find the minimum energy path for H atom diffusion from preferentially adsorbed site to the gap between surface and subsurface. Linear synchronous transit (LST) maximization was performed, followed by repeated conjugate gradient minimizations and quadratic synchronous transit (QST) maximizations until a transition state was found. The force acting on atoms was minimized to 0.03 eV/? for the geometry optimization in the LST/QST calculation.

3 RESULTS AND DISCUSSION

3. 1 Hydrogen adsorption on the Fe(110) surface

The H-surface distance, the H atom adsorption energy, and the distance from H atom to its nearest Fe atom neighbor are shown in Table 1. It is shown that the adsorption energy of hydrogen is negative. This illustrates the process of H adsorption is exothermic whether the Fe(110) surface is strained or not. The adsorption energy for the unstrained surface coincides well with the previous calcula- tions[32]. Tf site is found to be the most negative for all strained and unstrained surfaces. The adsorption energy of the Lb site is similar to that of the Tf site, which agrees well with the experiment[33]. Hydro- gen atom prefers to be adsorbed at the high-coor- dinated site. Other researchers confirmed that the Tf site is a true minimum, whereas all the other sites exhibit imaginary frequency[15]. The distance of 0.95 ? between H and the surface at Tf agrees well with previous calculations (0.91 ?)[34]. The hydrogen atom on Tf site binds with the largest number of Fe atoms. The hydrogen atom on Tf site maintains the maximum distance to the nearest Fe atoms, but keeps the shortest distance to the Fe(110) surface. Lb site is virtually a low coordinated site because Lb site at the Fe(110) surface matches to top site of the second layer Fe atom. Therefore, the adsorption energy on Lb site is not the most negative.

Table 1. Adsorption Energy of Hydrogen (Eads), Adsorption Height (h), and Distance from Hydrogen to the Nearest Neighboring Fe (dFe-H)

H atom on the top site of the strained surface moves to the tf site after configuration is relaxed. H atom can not be adsorbed on top site when surface is strained. Chohan[15]found that top site was a rank-two saddle point for two imaginary frequen- cies. H atom on top site lives the farthest away from Fe(110) surface, which indicates the instability of H adsorption on the Top site.

The adsorbed hydrogen atoms relax towards the surface on Tf, Sb and Lb sites when lattice is expanded. It demonstrates tensile strain reduces the distance between adsorbed H atom and surface. In the studied strain range, the binding strength of H atom at Tf site is stronger than other sites with the most negativeEads. The adsorption energy becomes more negative with increasing the tensile strain. This demonstrates that lattice expansion enhances the binding strength between the H atom and Fe(110) surface. However, compressive strain makes the adsorption energy less negative. This means the binding strength between H atom and the surface becomes weaker. Lattice compression is expected to be a way to suppress hydrogen adsorp- tion on the Fe surface.

3. 2 Hydrogen diffusion into the Fe(110) subsurface

Complete LST/QST calculation is a reliable method to find the diffusion path. It was performed to find transition state and the minimum energy pathways of hydrogen diffusion from surface to subsurface. As calculated above, H prefers to be adsorbed at the Tf site on Fe(110) which was defined as the initial state. Hydrogen atom prefers to be in the tetrahedral interstice site of Fe bulk[30]. Finial state can be considered as hydrogen atom occupying tetrahedral interstice site between the surface and subsurface Fe atoms, as shown in the inset of Fig. 2.

The diffusion activation energyEais defined as the energy deference between the transition state and initial state. It is calculated by Eq. 2 whereETSis the energy of transition state andEISis that of the initial state.

Diffusion paths of H atom on the unstrained and strained Fe(110) surfaces were calculated. Fig. 2 shows the diffusion path of H atom on the unstrained Fe(110) surface. Transition state con- figuration of unstrained system is presented in the inset of Fig. 2. Transition state is found that H atom is in a distorted tetrahedron which is almost identical as the final state. Diffusion path does not change with respect to strain variation, and it is similar to that of unstrained Fe(110). Transition state configuration is not sensitive to surface strain. Fig. 3 shows the diffusion activation energy as the function of strain. Surface strain varies the diffusion barrier. H atom has to overcome 0.85, 1.05 and 1.27 eV into subsurface for 5% tensile strained, 0 strained, and -5% strained surface respectively. The diffusion energy barrier for 0 strain in this calcula- tion coincides with Jiang’s calculation 1.02 eV[30].

Fig. 2. Minimum energy pathways for hydrogen diffusion from surface to subsurface on the unstrained surface

Fig. 3. Strain dependence of diffusion activation energy

Tensile strain decreases diffusion energy barrier and accelerates H diffusion into Fe bulk. On the contrary, compressive strain suppresses H diffusion into subsurface. Tetrahedron gap volume in the transition state was calculated based on the coordinates of four Fe atoms, as shown in Fig. 4. There is more tetrahedron gap volume in the transition state of expanded surface. It is responsible for the lower diffusion energy barrier. Smaller tetrahedron gap volume of compressive Fe(110) surface increases the diffusion energy barrier.

Fig. 4. Strain dependence of the tetrahedral gap volume

3. 3 Electronic structure of H/Fe(110) system

Partial density of states (PDOS) of H atom and the nearest neighbor Fe atom were calculated to investigate the orbital interaction for H adsorbed on the Tf site. Fig. 5 shows projected density of states of H and Fe atoms on the unstrained surface before and after hydrogen adsorption. Adsorption of H atom obviously changes the PDOS profiles. The orbital of H 1shybridizes with Fe 4s, 3pand 3dstates[28]. There is a sharp peak to clean H atom around 0 eV. H 1sshifts down to -6.1 eV and becomes broader after adsorption due to the strong H 1sand Fe 4shybridization and a little hybridization with Fe 3pand 3d.

Fig. 5. Projected density of states before and after H adsorption on the surface. (a) is for the clean H atom, (b) for the clean Fe atom on the first layer, and (c) for the nearest Fe with H atom

Fig. 6 shows PDOS evolutions of Fe 4s, Fe 3p, Fe 3dand H 1sorbitals with respect to strain variation. The orbital hybridization between Fe 4sand H 1sis still dominant on different strained surface, whereas H 1sorbital hybridizes a little with Fe 3pand Fe 3dorbitals. The PDOS of H 1sand Fe 4sshift right to higher energy, which results in strong binding between H and Fe atoms with strain increase. The similar feature of PDOS dependence on strain has been observed on the Mg[0001] surface[35].

Fig. 6. Strain dependence of projected density of states of (a) Fe 4s, (b) Fe 3p and (c) Fe 3d of the nearest neighboring Fe atom, and (d) H 1s on the Tf site

Mulliken populations of the adsorbed H atom on Tf site and the nearest neighboring Fe atom with respect to the strain variation were calculated to further analyze the orbital hybridization, with the results shown in Table 2. Surface strain alters the orbital occupation of clean surface and H adsorbed system. When the surface is compressed, the Mulliken electrons of Fe 4sand Fe 3dorbitals decrease, and Mulliken electrons of Fe 3pincrease. When surface strain changes from 5% to -5%, the Mulliken electrons of Fe 4sorbital are decreased by 0.1 e on the clean surface and 0.08 e on the H adsorbed surface. The loss of Fe 4sis due to the gain of Fe 3p. It demonstrates that electrons transfer from Fe 4sto Fe 3pwhen surface strain varies from elongation to compression. Previous PDOS analysis shows the binding interaction between Fe and H results from the hybridization of H 1sand Fe 4s. Electrons transfer fromstodon compressive surface, which weakens the binding strength between H and Fe atoms. The electron donation from Fe to H atom is observed. The population analysis shows that the charges transfer to H from the Fe atom on 5%, 0 and -5 strained surfaces are 0.31, 030, and 0.29 e, respectively. More electrons are donated when the surface is expanded.

Table 2. Mulliken Population for H 1s, Fe 4s, Fe 3p and Fe 3d Orbitals for the Tf and Clean Surface as the Function of Strain

The charge transfer between Fe surface and H on tf site was investigated by charge density difference. The charge density difference is defined as ΔρFe+H(ε)=ρFe+H(ε) -ρFe(ε) -ρHwhereρFe+H(ε) is the total charge density of H adsorbed on Fe surface, andρHandρFe(ε) are the charge density of isolated H atom and clean Fe surface, respectively. Red region in Fig. 7 represents negative charge center, and blue region shows the positive charge center. H atom gains extra electrons (in red), wheras Fe surface loses electrons (in blue). The region of H atom whenε= 5% is more red than others. It demonstrates more eletrons are transferred from Fe to the H atom, which concides well with the above results of Mulliken population analysis.

Fig. 7. Charge density difference of H adsorption on the Tf site when ε = 5% (a), ε = 0 (b), ε = -5% (c)

4 CONCLUSION

Employing the first principles techniques, atomic H adsorption and diffusion on biaxial strain strained Fe(110) surface were studied. We have found that the Tf site is the most stable on compressive and tensile surfaces. H atom can not adsorb on the top site of the strained surface. Our calculation reveals that H atom becomes less adsorptive when compressive strain is applied to the Fe(110) surface. The analysis of the projected density of states shows that H 1shybridizes with Fe 4sorbital. Electrons are transferred from the Fe to H atom. Compres- sive strain reduces the transferred electrons and decreases the Mulliken electrons of Fe 4sorbital, which weakens the binding interaction between H and Fe atoms. H atom diffuses from Tf site into subsurface through a distorted tetrahedron. Surface strain does not change diffusion path but affect the diffusion barrier energy. Tetrahedron gap volume in the transition state of -5% compressive strain system reduces, which increases the diffusion barrier. This suggests compressive strain impedes H penetrating into the Fe subsurface.

The present results indicate that the surface strain is a factor to control adsorption and diffusion of hydrogen on Fe(110) surface. There are residual and working stresses in welded joint and welding heat affected the zone of pipeline steel. Tensile strain decreases the distance between adsorbed hydrogen atom and surface, and increases the binding strength, and accelerates hydrogen atom penetrating into Fe(110). However, compressive strain has the opposite effect on adsorption and diffusion of hydrogen atom. To decrease hydrogen embrittle- ment of pipeline, it is necessary to take measures to decrease surface tensile stress and apply com- pressive stress on the surface. This is meaningful to further study the effect of surface stress on hydro- gen embrittlement.