Molecular dynamics simulations of high energy cascade in ordered alloys: Defect production and subcascade division

Abstract

Displacement cascades have been calculated in two ordered alloys (Ni 3 Al and UO 2 ) in the molecular dynamics framework using the CMDC (Cell Molecular Dynamics for Cascade) code (J.-P. Crocombette and T. Jourdan, Nucl. Instrum. Meth. B 352, 9 (2015)) for energies ranking between 0.1 and 580keV. The defect production has been compared to the prediction of the NRT (Norgett, Robinson and Torrens) standard. One observes a decrease with energy of the number of defects compared to the NRT prediction at intermediate energies but, unlike what is commonly observed in elemental solids, the number of produced defects does not always turn to a linear variation with ballistic energy at high energies. The fragmentation of the cascade into subcascades has been studied through the analysis of surviving defect pockets. It appears that the common knowledge equivalence of linearity of defect production and subcascades division does not hold in general for alloys. We calculate the average number of subcascades and average number of defects per subcascades as a function of ballistic energy. We find an unexpected variety of behaviors for these two average quantities above the threshold for subcascade formation.

Improved model for atomic displacement calculation

“…The above efficiency function tends to 0 by increasing the damage energy. However, the defect production is a linear function of the damage energy after the formation of displacement sub-cascade at high energy collisions [8,9]. The efficiency should thus be a constant after the subthreshold energy, which defines the threshold of subcascade formation.…”

Analysis of strain and disordering kinetics based on combined RBS-channeling and X-ray diffraction atomic-scale modelling

“…The former corresponds to displaced atoms calculated in SRIM, whereas the latter is related to the actual number of defects introduced in the MD cells, which fully contribute to the evolution of defects. It is known that the SRIM results tend to overestimate the actual defect numbers, i.e., the remaining displacements after potential recombination [60,61].…”

“…As shown in ref. [61], the discrepancy between BCA models and MD calculations intensies with increasing the primary knock-on atoms (PKA) energy, with much less surviving defects in MD (than for BCA) at high energy. In the current case, the median PKA energy of 500 keV Ce ions is obviously larger than that of 20 keV He ions.…”

Quick calculation of damage for ion irradiation: implementation in Iradina and comparisons to SRIM

“…W [31], where a re-increase of the damage production seems to take place at high PKA energies. In the same way, it has been shown that the arc-dpa formula is not applicable to alloys or multi-elemental solids [32]. Its utility and field of application appears therefore quite restricted.…”