multi-scale modelling of radiation damage
In materials under neutron irradiation, neutrons may hit lattice-atoms which are called Primary Knock-on Atoms (PKAs). Under the impact, each hit atom may leave its site, it is then called interstitial and its empty site is called vacancy (both vacancy and interstitial are called point defects). This moving atom may hit other atoms, which in turn do the same with other atoms, and so on. The result is an atomic displacement cascade, which leads to the formation point defects. When all the energy given to the PKA by the neutron during the initial collision is dissipated, the series of collisions stop and most of the interstitials go back on vacancies (they annihilate with each other). At the end of the process, only a few residual interstitials and vacancies (residual point defects) survive in the cascade region.
The residual interstitials and vacancies produced by all the cascades diffuse and may undergo different fates: i) annihilate with each other; ii) be absorbed in sinks (dislocations, interface...); iii) form clusters of vacancies or interstitials; iv) induce the formation of clusters of solute atoms, induce the modification of chemical composition in the grain-boundary (the phenomena is called segregation). All the formed clusters are obstacles for the dislocations and then harden the materials, they are called hardening defects.
A reliable prediction of irradiation effects via numerical simulation requires a multi-scale approach based on the integration of several computational codes. Each of the latter has to deal with appropriate space or time scales where the physics can be described with well-defined concepts and parameters and experimentally validated. The main steps which have to be reproduced are the following:
- the production of point defects by displacement cascades (a few pico-seconds) and their short term evolution (a few seconds)
- the long-term evolution of the irradiation-induced hardening defects and segregations (several tens of years). This evolution is controlled by diffusion of the involved species (interstitials, vacancies, solute atoms)
- the collective behavior of dislocations (glide, interactions,…) that is controlled by i) the crystalline structure and chemistry of the material, ii) the applied strain rate and temperature, and iii) the irradiation-induced hardening defects
For each step, several models, tools and associated assumptions can be used. The selection of the most effective approaches is one of the key point of the project.