Influence of hydrogen and helium nucleation on the mechanical properties of beryllium by first-principles calculations

Abstract

A systematic investigation has been conducted to clearly elucidate the physical pictures of H, He, and hybrid bubbles nucleation and their effects on the mechanical properties of beryllium for the application in extreme environment. The simulation is carried out by first-principles calculations based on the density functional theory as implemented in VASP. The change of the configurational entropy has been considered throughout the simulation. In the nucleation of H bubble H m -2V B e along the direction of 〈 0001 〉 , the structures of a triangle and a tetragon in the plane of ( 11 2 ̄ 0 ), a hexahedron and two symmetrically distributed hexahedron have been successively observed as more H atoms are captured. The deformation degree increases with the increase of number of H atoms. For the nucleation of He bubble He n -2V B e along the direction of 〈 1 ̄ 102 〉 , the structures of an isosceles triangle in the plane of ( 11 2 ̄ 0 ), an octahedron and a regular polyhedron have also been observed as more and more He atoms are captured. The total nucleation free energy change always remains negative when fifteen He atoms are captured due to the production of new vacancies induced by self-trapping mechanism. The degree of distortion is more serious compared with that induced by H atoms. For the nucleation of hybrid bubble, the separate nucleation of H and He bubbles begin to interact with each other by exchanging atoms with the trapping of eight He atoms. The influence of retained H and He atoms on the mechanical properties of bulk Be has been characterized by first-principles uniaxial tensile test and the essence of weakening effect has been elucidated by charge density. The Young’s modulus of pure Be is consistent with experimental result. Both the nucleation of H, He, and hybrid bubbles could cause the reduction of the Young’s modulus and the tensile strength of Be. The degradation induced by He bubble is more severe than that of H bubble. • Some interesting structures are observed during the nucleation of H/He bubbles. • The configurational entropy is considered to estimate the nucleation of bubbles. • New vacancies are induced by the self-trapping mechanism. • The effect of bubbles nucleation on the mechanical properties of Be is elucidated.