Multiscale Simulations of Irradiation Defects—From the Electronic Scale to Continuum

Defects can be produced, accumulated, and annealed by irradiation, thermal treatment, or even mechanical deformation. Frenkel pairs, dislocation loops and voids are examples of spatially localised defects that one can readily identify in simulations and experiment. Being non-linear distortions of the local lattice structure, defects generate spatially varying stress and strain fields in the surrounding lattice. These fields, together with thermal fluctuations, diffusion and interactions between defects, drive microstructure evolution.<br/>The long-range elastic interaction between localised defects can be described by elastic dipole tensor and elastic Green’s function formulae. The elastic dipole tensor fully defines elastic properties of a localized defect in the asymptotic far-field limit. One can compute the elastic dipole tensor of a defect and the corresponding relaxation volume tensor from density function theory (DFT) or molecular statics calculations. The relaxation volume of a defect can be readily evaluated if we know the elastic dipole tensor and elastic constants. Such quantity is a key to the treatment of radiation swelling of materials under irradiation.<br/>We have computed dipole tensors and volumes of point defects in all the BCC and FCC elemental metals. DFT calculations also enable the assessment of how the diffusion coefficient of a defect varies under external stress. We have performed large scale DFT simulations of dislocation loops in tungsten. The size of the loops transcends the microscopic scale and reaches the mesoscopic scale where asymptotic elasticity treatment applies. A detailed analysis of metastable configurations of closed and open vacancy loops performed using molecular statics simulations shows that vacancy loop configurations are not unique, and significant fluctuations of defect structures may occur during microstructural evolution under irradiation.<br/>Using the information obtained from electronic and atomistic scale calculations, we can develop a dynamic model for simulating the evolution of an ensemble of hundreds of interacting irradiation-induced mobile nanoscale defects in a micrometre size sample. The model uses a Langevin defect dynamics approach coupled to a finite element model, treated using the superposition method. We explore the dynamic evolution of an ensemble of interacting dislocation loops of various sizes and with different Burgers vectors, proving the feasibility of simulating defect evolution at a macroscopic scale.<br/>This work has been carried out within the framework of the EUROFusion Consortium and has received funding from the Euratom research and training programmes 2014-2018 and 2019-2020 under grant agreement No. 633053 and from the RCUK Energy Programme [grant No. EP/T012250/1]. To obtain further information on the data and models underlying the paper please contact [email protected]. The views and opinions expressed herein do not necessarily reflect those of the European Commission.