Simulation Of The Influence Of Helium On The Mechanical Properties Of RAFM Steel

Reduced activation ferritic/martensitic(RAFM)steel is the most promising candidate for testing the engineering feasibility of structural materials in fusion reactors.The high-flux neutron produced from the fusion reaction introduces transmutation helium inside the lattice,which might further evolve into helium bubbles,thus degrading the mechanical properties of RAFM steel.Meanwhile,the complicated strain,periodically changing thermal stress and the high temperature environment might superimpose with the degradation effect of helium,making it more difficult to interpret the interaction between helium and RAFM steel.To better predict the influence of helium on the mechanical properties of RAFM steel,and reveal the mechanism from the microscopic view,interaction between the helium atoms and the bcc-Fe matrix have been simulated.First principles calculation has been used to simulate the solution and migration behaviors,and molecular dynamics(MD)has been used to simulate the pinning effect of the helium atoms or clusters on the dislocations.By first principles calculation,it is found that whether there is a strain or not,the preferential site for interstitial helium atom is always tetrahedral site.The helium atom at the octahedral interstitial site has more charge transfer than the tetrahedral site,but has weaker bonding and thus higher solution energy.Ionic bonding is formed between the helium atom and iron atoms,which weakens the metallic bonds in the bcc-Fe matrix.When strain is applied,the migration barriers of different migration directions show different linear trends.Under certain strain,the migration barriers for some directions rise up,while others decrease,which makes it easier for helium atoms to migrate along certain directions.This can break the isotropic distribution of helium atoms under 0 strain,making it easier for helium atoms to segregate into clusters and form bubbles,thus deteriorating the mechanical properties of the material.MD has been used to simulate the interaction of helium clusters and dislocations.The critical shear stress for a dislocation to unpin for the helium atoms or clusters is found to increase with the increase of the size of cluster,the decrease of the inter–cluster spacing,and the decrease of the distance away from the glide plane.A linear trend is found between the critical shear stress and the reciprocal of the inter-cluster spacing.The stress field induced by dislocation itself can influence the pinning effect of the helium clusters.Meanwhile,the rising of the temperature can also lead to the decrease of shear stress.Our results have shown that the microstructural interactions of helium in iron can cause changes in the electronic structure of iron matrix,and influence the movements of dislocations.Meanwhile,the existence of strain can make it easier for helium atoms to segregate into clusters,and the pinning effect of the clusters can also be influenced by the different stress fields.The rise of the temperature will make it easier for dislocations to move,thus offsetting part of the strengthening and hardening effect caused by helium atoms or clusters.The microstructural mechanisms revealed by our simulations can help understand the change of mechanical properties of RAFM steel under service conditions.