Infoscience

Thesis

Molecular Dynamics Simulations of Helium Atoms Clustering in bcc Iron

Helium atoms are known to have a significant impact on materials used in fission and fusion reactors. In particular, the presence of helium atoms can change the mechanical properties and degrade the lifetime of reactors. In order to develop the helium-resistance materials, the underlying interactions between helium atoms and matrix atoms have to be understood. In the past, atomistic simulations have contributed a lot to the basic energy states of helium atoms in different defects of materials. Especially, insights into the formation energies of helium atoms at simple defects have been calculated by ab initio and molecular dynamics. The substitional helium has lower formation energy than the tetrahedral and octahedral interstitial helium atom. Additionally, the binding energies of helium with small HenVm cluster have also been found to be all positive. However, all these simulations have been performed without showing the kinetic formation process of such helium defects. The interactions of defect with larger number of helium atoms are also not explored in detail. In the first part of this thesis, a new relaxation-fitting method has been developed for cross potential of Fe-He system based on more energy data of helium-defect clusters from ab initio calculations and by including the migration energy of single free helium atom in the fitting process. Besides, the recently fitted magnetic potentials of Fe have been used to describe the Fe-Fe interactions for such cross potential. The new fitted Fe-He potentials have predicted well not only the properties of simple helium defect, but also the properties of helium clusters and diffusion process of helium atoms. Based on the new Fe-He potentials, the interactions between helium atoms and Fe with the effect of single vacancy have been simulated utilizing the classical molecular dynamics (MD) method in the single bcc Fe lattice at room temperatures by helium atoms up to 18. The binding energies of helium atom with HenV cluster are computed. By increasing number of helium atoms, the binding energy decreases first, then slowly increases up to n = 4. It is then almost constant between n = 5 to 15 but increases strongly at n = 16 to form a peak. This last increase is related to the athermal self-interstitial atom (SIA) emission in the form of <110> dumbbell which relaxes the restrained system to He16V2 cluster. The second binding energy peak occurs at n = 18 by the formation of the second SIA to form He18V3 cluster. Close inspection of the local configuration shows the formed SIA combines with the helium-defect cluster. Calculation of the pressure of the helium-defect cluster shows local peak normal stress and shear stress values up to 9 GPa and 4 GPa, respectively. The local configurations of helium-defect cluster suggest that with increasing helium content, some symmetrical structures can be formed. By increasing the number of helium atoms inserted in single bcc Fe lattice, the clustering process of the formed SIAs with presence of helium cluster has been simulated with MD methods at room temperatures with the new fitted Fe-He potential. The first SIA is formed after 6 He atoms being inserted without vacancy. The simulations indicate that the SIA has mainly two configurations as either a <110> dumbbell or a <111> crowdion, which can be exchanged in the interface between helium bubble and matrix atoms. The SIA cluster can change direction and absorb more SIAs to grow. Small dislocation loops have been observed to form with Burgers vector b = 1/2 a0<111> on a {111} habit plane. The kinetic process of formation and growth of dislocation loop explored by MD simulations includes: (1) the formation of a self-interstitial atom by the high pressure of the helium cluster/bubble; (2) the diffusion of these self-interstitial atoms on the interface between helium bubble and matrix; (3) the clustering of the self-interstitial atoms in the form of a dislocation loop and its diffusion away from bubble when the number of SIAs in loop reaches a critical number. Moreover, the effect of grain boundary (GB) on the formation of helium clusters has been explored with MD simulations in symmetrical GBs and normal nano-grain boundary samples. Helium atoms can be trapped by GBs at free spaces. The local atomic excess free volume (LAEFV) is found to the one of the main parameters not only related with the energy state of GB but also with the nucleation and growth of He atoms in the GB.

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