Atomic Scale Study Of The Effect Of Hydrogen On The Tensile Deformation Behavior Of Single Crystal And Bicrystal α-Fe

The microscopic mechanism of hydrogen embrittlement of metal materials and the design of anti-hydrogen embrittlement metal materials(especially steel)have always been the focus of material science research.At present,the microscopic mechanism of hydrogen embrittlement of steel materials has not reached a unified understanding.For metal materials that do not form hydrides,the most widely concerned hydrogen embrittlement mechanisms include Hydrogen Enhanced Decohesion Mechanism(HEDE),Hydrogen Enhanced Local Plasticity(HELP)and Hydrogen-Enhanced and Strain-Induced Vacancies(HESIV),etc.The interaction between hydrogen and cracks and the interaction between hydrogen/vacancies and grain boundaries is the key to understanding the mechanism of hydrogen embrittlement in steel materials.At present,multi-scale theoretical simulation methods such as first-principle calculation,molecular dynamics simulation,phase field simulation and finite element analysis have been increasingly used to analyze the formation and propagation mechanisms of hydrogen-induced cracks.In this paper,based on molecular dynamics simulation,the effect of hydrogen on the tensile deformation behavior of pure iron single crystals with pre-set cracks in different orientations has been investigated.Meanwhile,the effect of hydrogen,vacancies,and combined additions of hydrogen and vacancies on the tensile deformation behavior of two pure iron bicrystal models has been also investigated.The results of this work reveal that although hydrogen reduces the yield stress of the single crystal model with pre-cracks and gathers in the high stress region of the crack tip under the action of the stress gradient,the mechanism of the influence of hydrogen on the fracture behavior of the single crystal model with cracks with different orientations exists significant differences.Meanwhile,hydrogen tends to segregate the grain boundary and reduce the grain boundary bonding strength,thereby reducing the yield stress of the bicrystal model.With the increase of hydrogen concentration,the fracture mode of pure iron bicrystal model with low energy grain boundary gradually transitions from transgranular fracture to intergranular fracture;the bicrystal model with high-energy grain boundary itself exhibits the characteristics of intergranular brittle fracture,and hydrogen further strengthens its tendency of intergranular fracture.For the bicrystal model with low-energy grain boundary,the recombination effect of vacancies and hydrogen is stronger than the weakening effect of hydrogen or vacancies alone on grain boundary;for high-energy grain boundary,vacancies reduce the weakening behavior of hydrogen on grain boundary.In addition,this paper further explains the simulation results from the perspective of the effect of hydrogen on the generalized stacking fault energy and grain boundary bonding strength.