Atomic Scale Study Of Effect Of Hydrogen On Metallic Cohesion And Grain Boundaries Cleavage

Hydrogen embrittlement(HE)is a pervasive but harmful seriously natural phenomenon,which means that hydrogen dissolved in the metal worsens the plasticity performance of metallic materials.For about a century and a half,several mechanisms for the HE phenomenon found in engineering have been proposed through experimentals and observations,theoretical analysis,and numerical simulations.However,the existing mechanisms for HE are limited to only the specific metallic systems for which they are targeted,and they can neither reveal the HE universal essence nor formulate the HE discipline by the uniform physical-based models.It is a significant and urgent scientific challenge to reveal the physical essence of the HE phenomenon and establish the quantitative models applicable to various metallic systems.This dissertation focuses on the universal discipline of the HE at the atomic scale for promoting solving the mentioned problems.Based on the relationship of atomselectrons interaction and the principle of minimum energy for the equilibrium crystals,the consideration of HE essence is proposed at first that the H atoms occupied in the metallic lattices would change the electronic structure of systems,enhance the local interaction potential,and lead the metallic atoms to offset the equilibrium position to re-reach the new equilibrium state with the minimum energy,induce the lattice distortion and then metallic cohesion diminishment.Based on the mentioned consideration,the lattice distortion induced by the trapped H atoms is regarded as the intermediate link between the electronic structure variation and the mechanical properties degradation.On the one hand,lattice distortion is used to characterize the electronic structure variation.On the other hand,it is regarded as the origin of the HE in metallic systems.And thus,the theoretical models of the lattice distortion induced the point defects,the metallic cohesion variation caused by the trapped atoms in lattice interstices,the grain boundaries(GBs)cleavage,and the cleavage strength variation caused by the trapped H atoms in the GBs interstices are established,respectively.Furthermore,the theoretical calculation based on the mentioned models and the first principles simulation for various metallic systems is conducted to study the effect of the interstitial H atoms in the metal on the metallic cohesion and the GB’s cleavage strength.The researches reveal that the essence of HE at the atomic scale is that the H atoms occupied in the metals change the electronic structure of systems,cause lattice distortion,then induce metallic cohesion diminishment,induce the GBs cleavage strength,then enhance the GBs cleavage of the metallic systems containing GBs.Specifically,four theoretical studies are carried out in this dissertation as follows:1.The universal discipline of lattice distortion induced by the substitution atoms(point defect)is systematically studied for quantifying lattice distortion induced by the interstitial H atoms.The theoretical models to describe the whole-field distortion of the metallic substitution solid solutions are established,and their accuracy and universality are validated by the first principles simulation.Moreover,the present models are extended to describe the distortion of crystals with various lattice spaces(fcc,bcc,and hcp structure)and different bond types(metallic,ionic,and covalent bond)induced by the general point defects(substitution,vacancy,interstitial and coexist of multiple kinds of points defects).The study reveals the universal discipline of distortion and lays the foundation for exploring the effect of the occupied H atoms in the lattices and at the defects on the metallic mechanical properties.2.The effect of the occupied H atoms at the interstices of the perfect metallic lattices on the metallic cohesion is studied.Based on the internal energy equation of crystals,the relationship between the system binding energy variation and the volume changing of the equilibrium lattice is established.Based on the study of the lattice distortion induced by the point defects,the method to quantify the lattice distortion induced by the trapped H atoms in the lattice interstices is developed from the view of the force balance of the atoms in the equilibrium lattices.Combining the above,the theoretical model to formulate the effect of the occupied H atoms at the lattice interstices on the metallic cohesion is established.And its accuracy and universality is validated by the first principles simulation.The theoretical calculation results of various metallic systems indicate that trapped H-induced metallic cohesion reduction is the universal essence of HE.The present study supplements and improves the H-enhanced decohesion mechanism and provides theoretical support for exploring the interaction between H atoms and GBs.3.The physical processes of the GBs cleavage is studied for extending the theoretical model of the effect on metallic cohesion of the occupied H atoms at the perfect lattices to the more general lattices with GBs(face defects).Based on the universal discipline of GBs structure,the theoretical model to describe the physical processes of the GBs cleavage are proposed by expressing the interaction from atoms pair to the two parallel crystal planes and then to the surfaces at the GBs.And its accuracy and universality is validated by the first principles simulation.The present study provides theoretical support for exploring the effect of the occupied H atoms at the GBs interstices on the GBs cleavage.4.The effect of the occupied H atoms at the GBs interstices on the GBs cleavage is studied.Based on the study of the lattice distortion induced by the point defects and the metallic cohesion variation caused by the trapped atoms in lattice interstices,the method to describe uniformly the H-induced GBs distortion is proposed.And on this basis,the theoretical model to formulate the effect of the trapped H atoms on the GBs cleavage is developed by introducing the effect of trapped H into the GBs cleavage formulas.And its accuracy and universality is validated by the first principles simulation of systems of various GBs trapping various H atoms.The systematic theoretical calculation indicates that the trapped H-induced GBs cleavage strength reduction is the universal essence of HE of the metallic systems with GBs defects.Besides,the reduction degree increases as the number of trapped H atoms increases.The present study provides quantification the of the effect of the trapped H atoms on the GBs cleavage strength at the atomic scale.Based on the universal discipline of the HE at the atomic scale,we establishes a series of theoretical models universally applicable to various metallic crystals,and reveals the physical essence of HE.The theoretical results provide the support for the quantification study of HE at the mesoscale and macroscale and lay the foundation for the prediction of the mechanical properties of the metals and the structural integrity and service life of the bearing structure in the environment containing H.More significantly,the theoretical considerations and conceptions provide a new methodology and perspective for the theoretical study of materials,physics,and mechanics of micro/nano discrete atomic systems.