The Study Of Interactions Among Hydrogen And Defects Using Synchrotron X-ray Nano Diffraction

Hydrogen embrittlement(HE)has long been a key deleterious issue restricting the application of high strength steels.Normally,real materials contain large number of defects,such as grain boundaries,dislocations,alloy elements etc.Hydrogen will accumulate at and interact with these defect sites,which will eventually cause hydrogen damage or embrittlement there.Understanding the mechanism of how hydrogen interact with the intrinsic defects is of vital importance in preventing HE and designing anti HE materials.Intensive researches have been conducted addressing the HE phenomenon of steels.However,it's hard to exclude the interference of the different studied steel types.the complexity of the environment,multiple existing defects,and external applied stress.Moreover,the small size of hydrogen and the defects require higher demands in the experimental method.In this work,the BCC iron was used as a model system for BCC structure steels.Synchrotron X-ray Laue Nano-diffraction(XND)technique combining the atomistic simulation methods were adopted to conduct systemic and in-depth investigation of the intrinsic interaction mechanism among hydrogen and defects,the following conclusions are drawn:(1)Through the XND technique,the interaction between hydrogen and dislocation in fully annealed external stress-free ?-iron was investigated.Results indicated that hydrogen could induce dislocation multiplication in iron,the dislocation density after hydrogen charging increased nearly one order of magnitude.Hydrogen elevated the elastic strain and shear stress level,and the increment was directly related to the pre-existing dislocation stress field in the specimen.Higher dislocation region would endure larger impact by hydrogen which resulted in significant disparity in the distribution of dislocations over time,suggesting that hydrogen could enhance the localization of the dislocations.(2)Through the XND technique,hydrogen induced misorientation change in pure iron under different stress conditions were investigated.Results showed that after in-situ hydrogen charging,new small angle sub-grain boundaries were generated near some grain boundaries,indicating that hydrogen could promote the reorganization of the random distributed into dislocation walls.Misorientation angle between sub-grains were also enlarged by the presence of hydrogen.H-induced misorientation change increased by the increasing stress level in the material.When an external stress is applied,hydrogen could facilitate the formation of micro scaled twin zones on sub-grain boundaries.(3)Through the XND technique combing the Molecular Dynamics simulation to investigate the impact of grain boundary types on the initiation of hydrogen blisters.Results showed that hydrogen would accumulate at and nucleated preferentially at grain boundaries.Compared with coincide site lattice grain boundaries,hydrogen induced higher stress/strain level at random grain boundaries,and could combine with the vacancy-like defects there,attracted more hydrogen atoms thus eventually generated the nuclei of the blisters.(4)Through the combination of hydrogen permeation tests,electrochemical impedance spectroscopy tests,and the first principle calculations,the influence of carbon doping on iron surface of the hydrogen adsorption/absorption and diffusion was investigated.Results showed that carbon on the surface could alter the structure of the surface,making it unstable for hydrogen to adsorb on the surface,thus lower the hydrogen coverage.In the meantime,carbon addition could lower the initial reaction distance between the electrical double layer and hydrogen,lower the energy barrier for hydrogen to penetrate from surface to subsurface and from subsurface to 2nd-subsurface,which means that carbon doping on iron surface would make it more facile for hydrogen to diffuse into the bulk.