Study On The Behavior Of Hydrogen Diffusion And Segregation In Nickel By The Scanning Kelvin Probe Force Microscopy

Hydrogen atoms will be inevitably introduced into metallic materials in material smelting,preparation,processing and subsequent service process.In the presence of an applied load,hydrogen atoms in the metal materials can reduce the plasticity of materials and cause delayed fracture,which is commonly referred to as hydrogen embrittlement(HE).The HE damage of metal materials seriously restricts their application.Unfortunately,although the study of HE has a history more than a century,the understanding about the mechanism of hydrogen induced delayed fracture is still controversial.It is generally accepted that the hydrogen atoms can segregate at the defects and interact with them,which will cause the HE failure of materials.Therefore,it is very important to study the diffusion and segregation behavior of hydrogen atoms at the defects for the further understanding of HE mechanism.However,due to the limitations of traditional hydrogen measurement methods,there are still many controversies about the diffusion and segregation behavior of hydrogen atoms at different defects.In this thesis,the hydrogen diffusion and segregation behavior at the grain boundaries and dislocations in pure nickel were studied in depth by combining the scanning Kelvin probe force microscopy(SKPFM)technique and the Electron Back Scatter Diffraction(EBSD)analysis methods.The main conclusions are as follows:(1)By combining the SKPFM technique and EBSD analysis method,the hydrogen segregation behavior at different individual grain boundaries was visually mapped.By comparing the Volta potential before and after hydrogen charging,it was found that the segregation degree of hydrogen atoms at different grain boundaries strongly depended on the structure of grain boundaries.For the low-∑special grain boundaries,the sequence of hydrogen trapping capacity was ∑5≈∑11≈∑27a>∑7≈∑9≈∑27b>∑3.The ∑3 GB didn’t trap any hydrogen atoms at all.In addition,for several random grain boundaries with different misorientation,it was found that the evolution of the degree of hydrogen segregation at the grain boundaries was in accord with the evolution of the grain boundary structure,and the low angle GBs did not trap hydrogen.(2)In directionally solidified pure nickel,the diffusion behavior of hydrogen along the random grain boundary with 42.5°misorientation and the coherent ∑3 twin boundary has been studied by combining the SKPFM technique and EBSD analysis method.It was found that the diffusion of hydrogen atoms along grain boundaries strongly depended on the grain boundary structure.Different from the random grain boundaries with the misorientation of 42.5°,the coherent ∑3 twin boundary could significantly promote hydrogen diffusion.The recombination of lattice structure at the twin boundary could explain the promotion of hydrogen diffusion.In addition,the "short circuit" effect of hydrogen diffusion along grain boundaries was studied by the Comsol software.It was found that the effect of grain boundaries on hydrogen diffusion was determined by the hydrogen segregation factor and the hydrogen diffusion coefficient at grain boundaries.In the absence of hydrogen segregation at the grain boundaries,the promotion of hydrogen diffusion could be visualized only when the hydrogen diffusion coefficient at the grain boundaries was very large.The segregation factor could significantly increase the"short circuit" effect of hydrogen atoms diffusion along grain boundaries,but this effect was restricted by the diffusion coefficient.(3)Combining the SKPFM technique and slow tensile tests,the behavior of hydrogen transport by moving dislocations in the pure nickel single crystal was observed directly,and the theoretical analyses were given.It was found that the critical velocity of dislocation carrying hydrogen was very high.In the slow tensile tests,dislocations could carry hydrogen and realize the uphill transport of hydrogen atoms.In general application materials,the motion rate of dislocations was very low,resulting in a very short distance of hydrogen transport.Therefore,the role of dislocations in hydrogen transport could not be reflected in accelerating hydrogen transport.At the same time,the diffusion coefficient of hydrogen atoms in the material was measured by the SKPFM technique,and ideal results were obtained.(4)The quantitative relationship between the Volta potential measured by SKPFM and the hydrogen concentration was calibrated by combining the diffusion equation and electrochemical hydrogen permeation experiments.It was found that the Volta potential measured by the SKPFM was more sensitive to the change of hydrogen concentration at the range of low hydrogen concentration.The quantitative relationship between Volta potential and the hydrogen concentration could be described by the modified Nernst equation.Moreover,the Volta potential was linearly dependent on the hydrogen concentration over a wide range of potential change that hydrogen atoms could cause.