| Due to its excellent mechanical properties,high-strength steels have been widely used in critical fields,such as aerospace,transportation,and nuclear power.However,hydrogen embrittlement has been a severe obstacle for the application of high-strength steels.Generating nano precipitates as hydrogen traps to capture hydrogen atoms could improve the resistance to hydrogen embrittlement of steels,but the microscopic mechanism is unclear.First-principles calculations have been carried out to study the hydrogen trap characteristics of nano precipitates and interfaces formed with iron.The hydrogen diffusion behaviors inside the NbC,TiC,and VC phases and coherent interfaces were calculated.The adsorption behavior of hydrogen atoms and water molecules on the surface of nickel hydroxide with nanonickel clusters and nickel vacancies were also analyzed,which helps to clarify the hydrogen evolution reaction mechanism.(1)Vacancy-strain coordination effects on hydrogen traps.Density functional theory(DFT)calculations of the hydrogen trap characteristics of a serial of nanoprecipitates(VC,VN,TiN,NbN,NbC,and NiAl)in ?-Fe have been carried out.Calculation results indicate that the Hydrogen solution energies of different gaps in ideal nano-precipitates are too high so that those gaps could not be effective hydrogen traps.Introducing vacancies into nano-precipitates could remarkably reduce the H atom solution energies.Besides,tetrahedral sites at the coherent interfaces between iron and nano-precipitates are possible hydrogen traps.Volume evolutions and Bader charge analyses have been performed,and it was found that the strain at the coherent interfaces benefitted the hydrogen trap effects.In addition,introducing vacancies near the coherent interfaces could improve the hydrogen trap characteristics attributed to the combined effect of strain and vacancies.(2)The influence of different types of vacancies on the energy barrier of hydrogen diffusion.The DFT calculations for the hydrogen diffusion characteristics of the three nano precipitates(VC,TiC,and NbC)in ?-Fe have been carried out.When H atoms diffuse between adjacent tetrahedrons with the same carbon atom as the apex in the defect-free nano precipitate,the energy barrier of the three nano precipitates are all about 0.2 eV which is the lowest energy barrier.The energy barriers for hydrogen diffusion from the nearest neighboring tetrahedral gap into the metal atom vacancies of nano precipitates are only 0.03 eV(VC),0.04 eV(TiC)and 0.05 eV(NbC),but H atoms need to overcome high energy barriers as 1.62 eV(VC),1.49 eV(TiC)and 1.94 eV(NbC)to diffuse out of the vacancies.Carbon atom vacancies can lower the energy barrier for hydrogen atoms to diffuse from the tetrahedral gap of nano precipitates to the interior of the vacancies,while metal atom vacancies are the opposite.For a double-carbon vacancy system,adjacent carbon vacancies could reduce the energy barrier of hydrogen diffusion between carbon vacancies.It is challenging for H atoms to diffuse from the iron matrix into the defect-free nano precipitates.Hydrogen atoms can only diffuse into the nano precipitates at high temperatures.When introducing vacancies at coherent interfaces,the diffusion energy barrier for H atoms to diffuse from the iron matrix to the vacancies is shallow.The introduction of metal atom vacancies at the coherent interface can reduce the energy barrier for hydrogen atoms to diffuse from the iron matrix into the nano precipitates.In contrast,carbon atom vacancies do not have similar functions.(3)The synergistic effect of the vacancies and interfaces lay on the hydrogen trap.The models of ?-Fe/NbC semi-coherent interface of single misfit dislocations and the Periodic structure of the misfit dislocations intersecting regions formed by?-Fe and nano precipitates(VC,VN,TiN,NbN,and NbC)were constructed,respectively.The DFT calculations of the semi-coherent interface hydrogen trap characteristics were carried out.The tetrahedral gaps of ?-Fe at the ?-Fe/NbC semicoherent interface with single misfit dislocations can form hydrogen trap sites,while the tetrahedral gaps of ?-Fe at the intersecting area of the misfit dislocations could be deeper hydrogen trap sites.The strain field plays a leading role in the hydrogen trapping effect of the ?-Fe tetrahedral gaps at?-Fe/NbC semi-coherent interface with single misfit dislocation.Besides,the structural change after hydrogen dissolution plays the dominant role in the hydrogen trapping effect at the misfit dislocation intersection area.The introduction of non-metal atom vacancies in the two semi-coherent interface structures could form deep hydrogen traps.The metal atom vacancies of nano precipitates at the intersecting area of coordination dislocations cannot trap hydrogen atoms.However,the metal atom vacancies at the semi-coherent interface with single misfit dislocation are good hydrogen traps.(4)The adsorption behaviour of hydrogen atoms and water molecules on the surface of perfect nickel hydroxide,defective nickel hydroxide with nickel vacancies,and nickel clusters have been investigated.Due to the influence of hydroxyl groups,hydrogen atoms cannot be adsorbed on a defect-free nickel hydroxide surface.Besides,hydrogen atoms can be stably adsorbed at the bridge and hollow sites of the nickel clusters.The Gibbs free energy correction was introduced to obtain the change of Gibbs free energy at room temperature.The calculations show that the free energy change of hydrogen atoms in the bridge and hollow positions of the nano-nickel clusters are close to 0 eV.These sites were hydrogen active sites.Water molecules can be stably adsorbed on the defective nickel hydroxide surface near the nickel vacancies.The nickel vacancies could enhance the adsorption of water molecules.DFT calculations proved that defective Ni(OH)2 could enhance H2O adsorption at Ni vacancies and H activation at Ni0 clusters.The synergistic effect between the Nivacancy and Ni0 cluster significantly accelerates the kinetics of hydrogen evolution reaction(HER). |
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