We compare systematically the threshold displacement energy surface of 11 interatomic potentials in Fe. We discuss in detail different possible definitions of threshold displacement energies, and how they relate to different kinds of experimental threshold displacement energies. We compare the threshold results to experiments, and find that none of the 11 tested potentials agrees fully with experiments. However, all the potentials predict some qualitative features in the same way, most importantly that the threshold energy surface close to the 100 crystal direction is flat and that the largest threshold energies occur around very roughly the 123 crystal direction. [All rights reserved Elsevier]
Molecular dynamics computer simulations have proven to be an extremely useful method to examine the basic mechanisms of ion-induced damage production in materials. In the particular case of insulators with a high degree of ionic bonding, there are some complications in the simulations associated with how the charges on atoms should be handled. In this article I review different approaches of handling the ionicity and discuss the limits of their applicability. I also present some of our recent simulation results in GaN and hydrogenated amorphous carbon. Comparison of defect production in collision cascades in GaN simulated with two different models, one with explicit ionic interactions and another without, gives the surprising result that the primary damage production does not seem to be affected by explicit ionic interactions. This indicates that at least in GaN it is justified to use fast short-range interaction models in simulations of the primary states of radiation damage
Recent work on the sizes of craters produced by ion impacts of solids has shown that the size of the crater scales with the inverse square of the cohesive energy. This observation is in contrast to the size of craters produced in macroscopic impacts, which scale directly with the inverse of the cohesive energy. It has relied on the assumption that the melting temperature is proportional to the cohesive energy. Using computer simulations, we now show that the size scales in fact with the inverse of the product of the melting temperature and cohesive energy. This provides direct proof that the reason to the different behavior of macroscopic and ion-induced cratering is flow of the liquid produced by the ions
K. Nordlund
P. Partyka
Y. Zhong
I.K. Robinson
R.S. Averback
P. Ehrhart
Diffuse X-ray scattering (DXS) at glancing incidence is a potentially powerful means for elucidating damage structures in irradiated solids. Fundamental to the analysis of diffuse X-ray scattering data is a knowledge of the atomic displacement field around defects, which for implantation damage in crystals like Si has been difficult to obtain using analytical solutions of elastic continuum theory. We present a method for predicting the diffuse scattering pattern by calculating the displacement field around a defect using fully atomistic simulations and performing discrete sums for the scattering intensity. We apply the method to analyze experimental DXS results of defects produced by 4.5 keV He and 20 keV Ga irradiations of Si at temperatures of 100-300 K. The results show that the self-interstitial in ion-irradiated Si becomes mobile around 150 K, and that amorphization of silicon by light and medium-heavy projectiles occurs homogeneously through the buildup of interstitial clusters, and not within single cascade events
Molecular dynamics (MD) computer simulations, while very extensively used in chemistry and materials physics, have largely been absent in the theoretical treatment of ion beam analysis. Instead the computationally more efficient binary collision approximation (BCA) methods are widely used. In this paper I compare the two methods regarding the level of physical approximation versus accuracy, using a simple model study as an illustrative example. I then show, based on results in the literature, that although in most cases BCA methods are well sufficient for ion beam analysis, there are special conditions where MD methods are required even for keV and MeV kinetic energy processes.
Molecular dynamics (MD) computer simulations, while very extensively used in chemistry and materials physics, have largely been absent in the theoretical treatment of ion beam analysis. Instead the computationally more efficient binary collision approximation (BCA) methods are widely used. In this paper I compare the two methods regarding the level of physical approximation versus accuracy, using a simple model study as an illustrative example. I then show, based on results in the literature, that although in most cases BCA methods are well sufficient for ion beam analysis, there are special conditions where MD methods are required even for keV and MeV kinetic energy processes. [All rights reserved Elsevier].
K. Nordlund
J. Tarus
J. Keinonen
M. Ghaly
R.S. Averback
Although ion beam mixing has been studied intensively over the last 20 years, many questions about the fundamental mechanisms involved during mixing remain unresolved. We review here recent simulation and experimental work which provides answers to some of the lingering questions about mixing in elemental materials. The results make clear the specific role which thermodynamic material properties, the nature of atomic bonding and electron-phonon coupling can have on ion beam mixing. Agreement obtained by direct comparison of simulated and experimental mixing coefficients gives confidence in our results, indicating that the experimental mixing values in heavy metals can be understood predominantly on the basis of atomic motion in liquid-like zones, and that the role of the electron-phonon coupling on ion beam mixing is much smaller than previously thought
The repulsive part of the interatomic potential affects the outcome of computer simulations of many irradiation processes of practical interest, like sputtering and ion irradiation range distributions. The accuracy of repulsive potentials is studied by comparing potentials calculated using commonly available density-functional theory (DFI) and Hartree-Fock (HF) methods to highly accurate fully numerical HF and Hartree-Fock-Slater (HFS) calculations. We find that DFT calculations utilizing numerical basis sets and HF calculations using decontracted standard basis sets provide repulsive potentials which are significantly improved compared to the standard universal ZBL potential. The accuracy of the calculated potentials is almost totally governed by the quality of the one-particle basis set. The use of reliable repulsive potentials open up new avenues for analysis of ion irradiation experiments
R.L. Brotzman
N.B. Cook
K. Nordlund
T.B. Bennett
A. Gomez Rivas
D. Döpfer
Abstract Principal component analysis (PCA) is a variable reduction method used on over-parameterized data sets with a vast number of variables and a limited number of observations, such as Dairy Herd Improvement (DHI) data, to select subsets of variables that describe the largest amount of variance. Cluster analysis (CA) segregates objects, in this case dairy herds, into groups based upon similarity in multiple characteristics simultaneously. This project aimed to apply PCA to discover the subset of most meaningful DHI variables and to discover groupings of dairy herds with similar performance characteristics. Year 2011 DHI data was obtained for 557 Upper Midwest herds with test-day mean ≥200 cows (assumed mostly freestall housed), that remained on test for the entire year. The PCA reduced an initial list of 22 variables to 16. The average distance method of CA grouped farms based on best goodness of fit determined by the minimum cophenetic distance. Six groupings provided the optimal fitting number of clusters. Descriptive statistics for the 16 variables were computed per group. On observations of means, groups 1, 2, and 6 demonstrated the best performances in most variables, including energy-corrected milk, linear somatic cell score (log of somatic cell count), dry period intramammary infection cure rate, new intramammary infection risk, risk of subclinical intramammary infection at first test, age at first calving, days in milk, and Transition Cow Index. Groups 3, 4, and 5 demonstrated the worst mean performances in most the PCA-selected variables, including DIM, age at first calving, risk of subclinical intramammary infection at first test, and dry period intramammary infection cure rate. Groups 4 and 5 also had the worst mean herd performances in energy-corrected milk, Transition Cow Index, linear somatic cell score, and new intramammary infection risk. Further investigation will be conducted to reveal patterns of management associated with herd categorization. The PCA and CA should be used when describing the multivariate performance of dairy herds and whenever working with over-parameterized data sets, such as DHI databases.
A.E. Sand
J. Dequeker
C.S. Becquart
C. Domain
K. Nordlund
Abstract The reliability of atomistic simulations of primary radiation damage hinges on the quality of the interatomic potential. However, irradiation induced collision cascades involve strongly non-equilibrium processes, and thus depend on properties of potentials not usually included in the potential fitting. Here, we compare the predictions of five interatomic potentials for tungsten in cascade simulations with primary knock-on energies ranging from threshold energies for defect production, up to 200 keV. The highest energies represent the energetic recoils induced by the 14 MeV fusion neutron irradiation. We further compare properties related to dynamic collisions predicted by the different potentials to DFT calculations, to assess the accuracy of these predictions. We also present two hardened versions of a recent EAM-type potential, and demonstrate explicitly the importance of carefully adjusting the range of the potential at interaction distances smaller than those included in the fitting of potentials to equilibrium properties.
H. Vázquez
E.H. Åhlgren
O. Ochedowski
A.A. Leino
R. Mirzayev
R. Kozubek
H. Lebius
M. Karlušic
M. Jakšic
A.V. Krasheninnikov
J. Kotakoski
M. Schleberger
K. Nordlund
F. Djurabekova
E. Safi
C. Björkas
A. Lasa
K. Nordlund
I. Sukuba
M. Probst
Abstract Beryllium (Be) is the main plasma-facing material in the present day fusion reactor JET as well as in the upcoming ITER. Thus, the Be erosion plays a key role in predicting the life-time and viability of the reactors. In this work, Be surface erosion and morphology changes due to deuterium (D) irradiation are studied by using molecular dynamics simulations, varying key parameters such as particle flux, surface temperature and impact energy. At low temperatures, the main molecular species among the sputtered particles is BeD due to a low D surface concentration, as the incoming D projectiles cluster beneath the surface. At higher temperatures, the D surface concentration increases and larger species (BeD 2 , BeD 3 ) dominate the molecular erosion, lowering the BeD to Be ratio. When approaching the Be melting point, D desorbs from the surface, increasing the fraction of Be eroded as BeD. The larger molecules will dissociate as soon as entering the edge plasma, with only a minor contribution to the BeD formation. These findings correlate well with observations at JET. The effect of the incoming D flux on the results is negligible.
Abstract Recent work has shown that the repulsive part of the interatomic potential at intermediate atomic separations strongly affects the extent and morphology of the damage produced by collision cascades in molecular dynamics simulations. Here, we modify an existing embedded atom method interatomic potential for iron to more accurately reproduce the threshold displacement energy surface as well as the many-body repulsion at intermediate and short interatomic distances. Using the modified potential, we explore the effects of an improved repulsive potential on the primary damage production and the cumulative damage accumulation in iron. We find that the extent of the damage produced by single cascades, in terms of surviving Frenkel pairs, directly correlates with the change in threshold displacement energies. On the other hand, the damage evolution at higher doses is more dependent on the formation and stability of different defect clusters, defined by the near-equilibrium part of the interatomic potential.