Developing The Interatomic Potentials Of Typical Tungsten-based Systems And Their Applications To Radiation Damage

Although some future energy sources such as nuclear fusion energy are promising energy alternatives,developing applicable materials for fusion reactors still pose as one of the major challenges in fusion energy development.Metal tungsten(W),with a high melting point and thermal conductivity and a high sputtering resistance to energetic particles,is currently considered to be one of the leading candidates for plasma-facing materials(PFMs).Under high energy(14.1 Me V)neutrons irradiation,a large number of defects will be created,and will lead to W transmutation of several elements,such as Re,Os,Ta,and Hf.Among these elements,Re is a main product and could precipitate to form?and?phases.As a PFM,W is also subjected to high-flux and low-energy helium(He)plasma bombardment during its lifetime.Because of the low solubility and migration energy barrier of He in W,He can easily diffuse and accumulate,ultimately leading to the formation of bubbles and fuzzy nanostructures,and hence will lead to the deterioration of the properties of W material,causing a serious reduction in the lifetime of reactors.Due to the difficulty in performing experiments,computational simulation methods have become increasingly widespread as ways of understanding the material properties under neutron irradiation and explaining or extending experimental observations.Among these methods,large-scale molecular dynamics and Monte Carlo simulations have been widely used.However,they rely on the interatomic potentials to describe the interactions between atoms.Due to lack of appropriate interatomic potentials,large-scale computational simulations can hardly be performed for W-based alloys.In this dissertation,the elemental potentials for W,Ta,V,Mo,and Re,and the alloy potentials for W-Re,W-Ta,W-V,W-Mo,and W-Ta-He systems were constructed,and the accuracy of these potentials tested.For pure elements,the potentials reproduced the lattice constant,cohesive energy,bulk modulus,elastic constants,equilibrium state equation,formation energies of vacancy,and the most stable interstitial atom.In addition to the fitting properties,the potentials accurately predicted the vacancy migration energy,interstitial atom migration and rotation energies,stacking fault energies and relative stability of the<100>and1/2<111>interstitial dislocation loops.For alloy potentials,some major properties were well reproduced,such as the formation energies of substitutional solute atoms,binding energies between solute atoms and point defects,formation energies of a single He atom at different sites,binding energies of an additional interstitial He atom to existing He-V clusters in W/Ta,and the formation energies and lattice constants of artificial ordered alloys.The developed potentials were based on the Finnis–Sinclair formalism,which has high computational efficiency.For use in collision cascade simulations,the pair functions were then connected to the ZBL universal function.Based on the test results,the developed potentials were suitable for studying point defect and dislocation loop properties and can be further used to explore displacement cascade simulations.Furthermore,the present potentials also provided a basis for fitting interatomic potentials for tungsten-based ternary alloys and BCC refractory high-entropy alloys.By using the constructed potentials,displacement cascades in W-based alloys,interactions between Re and interstitial-type defects in bulk W,and the effect of solute Ta on He behavior in W were studied.Cascade collision process is analyzed in detail from the aspects of defect generation,number and structure.The defect database from displacement cascades provides a basis for understanding the primary damage of the W-based system and the long-term evolution simulation of defects in the subsequent annealing process.An interstitial W atom has a high binding energy with a substitutional Re atom,resulting in the formation of a Re-W dumbbell.It would migrate in 3-dimension due to the low migration and rotation energies,and the small W interstitial cluster will combine with solute Re to form a stable cluster,causing its mobility to be obviously weakened.The strong attractive interaction between a Re atom and a 1/2<111>interstitial dislocation loop occurs when the Re atom is located at the core of the loop.The mobility of the 1/2<111>interstitial dislocation loop decreases progressively with increasing Re concentration.These results were helpful to understand the interactions between Re and interstitial-type defects and the initial nucleation mechanism of Re atom clusters in the neutron-irradiated W.The small He atom clusters(N_He?4)were easy to diffuse in pure W,and their diffusion activation energies were less than 0.3 e V.However,the binding energies between Ta and these clusters were between 0.5-0.9 e V,which has a pinning effect for the He cluster diffusion.At high temperature,the solute Ta cannot qualitatively hinder the agglomeration of He atoms,but due to the pinning effect,compared with pure W,solute Ta in W has a certain delay effect on the agglomeration of He atoms in time.These results provide a basis for understanding the behavior of He in W-Ta alloy.