Christophe Domain

Atomic displacement cascade distributions in iron

Abstract Full molecular dynamics (MD) and its binary collision approximation (BCA) are used in a complementary way in order to study displacement cascade distributions in iron. Frenkel pair distributions are particularly narrow and symmetrical. They are thus well described by their first moment. Therefore, quantitative estimates by MD are possible. The comparison between the dependence of Frenkel pair production on primary energy predicted by both computational techniques suggests a post-cascade recombination model. Its physical grounds are discussed. The variability between spatial distributions of individual cascades is particularly large as a consequence of instability, which takes place in the early stage of the cascade development. The subsequent loss of correlation with initial conditions is statistically demonstrated on the basis of BCA simulations of 5000–15xa0000 cascades. Sufficient statistics can be reached by MD in order to characterise spatial distributions within cascades. It comes out of systematic comparison between MD and its BCA that, after the ballistic phase, the spatial extent of both vacancies and interstitials tends to increase. This phenomenon correlates with atomic mixing in the cascade core. This mixing is not predicted in the BCA. It is suggested to be responsible for the fragmentation of vacancy clusters formed during the ballistic phase of the cascades.

Ab initio calculations of some atomic and point defect interactions involving C and N in Fe

The critical importance of interstitial impurities in bcc metals has been well established over many years. Ab initio calculations based on the density functional theory offer a powerful means to revisit this subject and the present article is focused on the interactions of C and N atoms with point defects in Fe. The structures and relative stabilities of different configurations as well as their formation and binding energies have been determined. The consequences of the obtained results are discussed.

Comparison of algorithms for multiscale modelling of radiation damage in Fe–Cu alloys

A key issue for the simulation of radiation effects in reactor pressure vessel (RPV) steels is the kinetics of formation of Cu–vacancy complexes (Cu–VC) in a ferritic matrix, starting from displacement cascade debris. In the present work the evolution of molecular dynamics (MD) and corresponding binary collision approximation (BCA) displacement cascades has been studied using two different kinetic Monte Carlo (KMC) techniques in Fe–0.2% Cu. This exercise allows an assessment of the cascade debris features that are likely to influence their long-term evolution in interaction with the solute atoms, as well as the differences between simulation techniques. The results show that, at the current level of approximation of KMC methods, the use of BCA as damage input in KMC simulations does not introduce major biases, the difference with respect to the use of an MD source being a second-order effect. This justifies the use of BCA cascade debris as input damage for KMC parametric studies of Cu precipitation in Fe under irradiation, with a view to increasing the statistical representativity of the results. The main open question remains the mobility and dissociation rate of small Cu–VC, as described with different KMC techniques.

Copper precipitation in iron: a comparison between metropolis Monte Carlo and lattice kinetic Monte Carlo methods

Several variants are possible in the suite of programs forming multiscale predictive tools to estimate the yield strength increase caused by irradiation in RPV steels. For instance, at the atomic scale, both the Metropolis and the lattice kinetic Monte Carlo methods (MMC and LKMC respectively) allow predicting copper precipitation under irradiation conditions. Since these methods are based on different physical models, the present contribution discusses their consistency on the basis of a realistic case study. A cascade debris in iron containing 0.2% of copper was modelled by molecular dynamics with the DYMOKA code, which is part of the REVE suite. We use this debris as input for both the MMC and the LKMC simulations. Thermal motion and lattice relaxation can be avoided in the MMC, making the model closer to the LKMC (LMMC method). The predictions and the complementarity of the three methods for modelling the same phenomenon are then discussed.

Recent radiation damage studies and developments of the Marlowe code

Radiation damage in materials relevant to applications evolves over time scales spanning from the femtosecond – the characteristic time for an atomic collision – to decades – the aging time expected for nuclear materials. The relevant kinetic energies of atoms span from thermal motion to the MeV range.The question motivating this contribution is to identify the relationship between elementary atomic displacements triggered by irradiation and the subsequent microstructural evolution of metals in the long term. The Marlowe code, based on the binary collision approximation (BCA) is used to simulate the sequences of atomic displacements generated by energetic primary recoils and the Object Kinetic Monte Carlo code LAKIMOCA, parameterized on a range of ab initio calculations, is used to predict the subsequent long-term evolution of point defect and clusters thereof. In agreement with full Molecular Dynamics, BCA displacement cascades in body-centered cubic (BCC) Fe and a face-centered cubic (FCC) Febond Nibond Cr alloy display recursive properties that are found useful for predictions in the long term.The case of defects evolution in W due to external irradiation with energetic H and He is also discussed. To this purpose, it was useful to extend the inelastic energy loss model available in Marlowe up to the Bethe regime. The last version of the Marlowe code (version 15) was delivered before message passing instructions softwares (such as MPI) were available but the structure of the code was designed in such a way to permit parallel executions within a distributed memory environment. This makes possible to obtain N different cascades simultaneously using N independent nodes without any communication between processors. The parallelization of the code using MPI was recently achieved by one author of this report (C.J.O.). Typically, the parallelized version of Marlowe allows simulating millions of displacement cascades using a limited number of processors (<64) within only few hours of CPU time.