Atomistic Study Of Hydrogen-Induced Intergranular Fracture In Nickel

As the smallest atom in nature,hydrogen can easily penetrate the surface of metals.After entering polycrystalline materials in Nickle,H atoms generally segregate at grain boundaries,resulting in obvious intergranular feature of fracture surfaces.With the progress of experimental techniques and advanced characterization methods,the morphology of fracture surface and microstructure beneath fracture surface have been thoroughly studied in recent years,revealing three characteristics of H-induced intergranular fracture:(1)crack tip can remain sharp during propagation along grain boundaries;(2)significant plastic evolution dominated by dislocation occurs below intergranular fracture surface;(3)prominent nanoscale dimples arise on intergranular fracture surface.These typical features indicate that the hydrogen-induced intergranular fracture,which exhibits brittleness at the macroscale,presents remarkable ductility at the microscale.This further implies that Hinduced intergranular fracture involves a synergetic effect of multiple micro-mechanisms,for which deep and systematic investigation is however still lacking.Therefore,it is of great significance to study the influence of hydrogen segregating at grain boundaries on the mechanical behavior of grain boundaries,which is indispensable for revealing the microscopic mechanism of hydrogen-induced intergranular fracture in polycrystalline materials in Nickle.By analyzing the atomic structure of defects,atomic-scale simulations can effectively establish the relationship between the macroscopic elastoplastic response and the microstructure evolution of materials under different loading conditions and H segregation.Through atomic-scale simulations,the effect of hydrogen segregating at grain boundaries on the mechanical behavior of grain boundaries is deeply investigated in this dissertation.The main research content of this work is as follows:(1)The atomistic simulation method is proposed to simulate the H segregation at grain boundaries,and H segregation at different kinds of grain boundaries is studied.The simulations indicate that after H segregation at grain boundaries,local hydrides tend to form on grain boundaries,leading to a significant distortion of the grain boundary structure.Moreover,almost no H atoms can be found in the region only several atomic planes away from the grain boundary.H segregation at grain boundary can significantly change the mechanical properties of the grain boundary via decreasing the potential energy of the grain boundary atoms.This part lays the foundation for the following study of the influence of hydrogen segregation on the mechanical behavior of grain boundaries.(2)By building symmetric tilt grain boundaries incorporating characteristic grain boundary structures,the influence of H segregation on the short-range interaction between dislocation and symmetric tilt grain boundary is studied via molecular dynamics simulations.The simulations indicate that for the cases without H segregation at grain boundaries,dislocation nucleation from the grain boundary and dislocation gliding on the grain boundary are the fundamental mechanisms governing the short-range interaction between dislocation and grain boundary;for the cases with H segregation at grain boundaries,hydrogen can inhibit grain boundary dislocation emission,dislocation transmission across grain boundary and dislocation gliding on grain boundary,which significantly changes the short-range interaction between dislocation and grain boundary.(3)Based on the symmetric tilt grain boundaries incorporating characteristic grain boundary structures,the influence of hydrogen segregation at grain boundaries on the dislocation nucleation from the symmetric tilt grain boundary is studied by using molecular dynamics simulations.The results indicate that the mechanisms and the critical stress of grain boundary dislocation nucleation highly depends on both the characteristics of grain boundary structures and tensile directions;under different conditions of grain boundary structures and tensile directions,there are two kinds of grain boundary dislocation nucleation mechanisms: dislocation dissociation nucleation mechanism and heterogeneous dislocation nucleation mechanism;the segregated hydrogen presents a significant influence on these two kinds of grain boundary dislocation nucleation mechanisms,thus resulting in an important difference in the critical stress of grain boundary dislocation nucleation.(4)Through a bicrystal model containing an edge crack at grain boundary,the influence of H segregation at grain boundaries on the mode I crack propagation behavior along grain boundaries is studied by using molecular static simulations.The results indicate that in the absence of hydrogen,during crack propagation,significant crack tip blunting is observed and the crack turns to propagate into the grain interior,implying a transition toward ductile transgranular fracture;In the case of H segregation,during crack propagation,hydrogen can enhance dislocation emission from the grain boundary,and also inhibit grain boundary migration,resulting in the alternate occurrence of grain boundary dislocation emission and grain boundary cleavage,implying a ductile-brittle alternating intergranular fracture feature.