First-principles Study On The Nucleation Of Helium And Hydrogen Bubbles In Metals

The solubilities of helium and hydrogen in metals are low,and they tend to aggregate at grain boundaries,phase boundaries,vacancies and dislocations forming helium bubbles and hydrogen bubbles.These bubbles will lead to microstructure changes of surface and bulk phases,as well as a decrease in mechanical properties,which seriously affect the service life of materials.Limited by experimental techniques,some basic problems for the growth of helium bubbles and hydrogen bubbles in metals are not clear,for instance,when to form hydrogen molecules,how do helium clusters migrate,and nucleation mechanisms.With the improvement of computer hardware and parallel computing methods,massive parallel first-principles calculation based on quantum mechanics has become a reliable tool to solve these problems.We have carried out a first-principles method investigation into the self-trapping and migration of He pair in body centered cubic(bcc)iron,molybdenum,tungsten,and face-centered cubic(fcc)copper,palladium,and platinum.The calculations show that the migration barriers are determined predominantly by low-energy He-pair configurations which depend mainly on the energy state of a single He in different interstices.The migration barrier for a He pair in bcc metals are similar,always slightly higher than those for a single He.Due to the existence of metastable bridge interstitial position,migrations of a He-pair in fcc metals are very complicated.The migration barrier for a He-pair is slightly lower than(in Cu),or similar to(in Pd and Pt)a single He.Our results do not only provide a starting point for future explorations on the calculations of larger He clusters,serve as touchstones to test the rationality of pairpotentials in molecular dynamics,but also provide accurate parameters for larger scale simulation calculations.To illustrate the interaction of helium bubbles,hydrogen bubbles and alloy elements,we take the second phase of beryllium-tungsten alloy Be22W as an example to study the trapping of alloy phase for helium and hydrogen,and its effect on the initial nucleation of helium and hydrogen bubble nucleation.Calculations show that compared to pure beryllium the solution energies of interstitial H and He atoms are lower,migration barriers are higher in Be22W,thus it will trap H and He atoms,and impede their migration.The interaction of Be and W in Be22W improved the formation energy of W,Be16C and Be16d vacancies,reduced those of Be48f and Be96g.On the other hand,the self-trapping for helium is weaker,trapping energies of beryllium vacances for H is lower,so the trend of helium and hydrogen clusters is decreasing.We predict that dispersive Be22W particles in hcp-Be might serve as the hydrogen trapping centers,hinder hydrogen and helium bubble nucleation and growth,and improve the resistance to irradiation void swelling.The accumulation of hydrogen is the prerequisite for the formation of hydrogen bubbles,and the formation of hydrogen molecules is the sign and starting point for the formation of hydrogen bubbles.Hydrogen molecules can not fonn in a single vacancy in bcc metal,requiring a larger free volume.Taking bcc Cr and W as examples,we studied the critical size of vacancy clusters for molecular hydrogen formation.The smallest H bubble in Cr is 27-vacancy spherical nanovoids.After the surfaces being saturated by 54 atomic H atoms,9 H2 molecules can form in the center of the void.The smallest H bubble in W is also 27-vacancy spherical nanovoids.After the surfaces being saturated by 66 atomic H atoms,9 H2 molecules can form in the center of the void.The trapping H and encapsulated H2 can promote the growth of void.Our first-principles results reveal that in atomic scale H bubble nucleation and growth in Cr and W are not driven by self-trapping,in contrast to the helium bubble which can spontaneously produce a vacancy and a self-interstitial atom.Our calculation results can help us to understand the hydrogen embrittlement in other metal materials.