Browsing by Author "Sand, Andrea E."

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Graph theory based approach to characterize self interstitial defect morphology
(2021-07) Bhardwaj, Utkarsh; Sand, Andrea E.; Warrier, Manoj
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The defect morphology is an essential aspect of the evolution of crystal microstructure and its response to stress. While reliable and efficient standard computational algorithms exist for finding defect concentration and size distribution in a crystal, defect morphology identification is still nascent. The need for an efficient and comprehensive algorithm to study defects is becoming more evident with the increase in the amount of simulation data and improvements in data-driven algorithms. We present a method to characterize a defect's morphology precisely by reducing the problem into graph theoretical concepts of finding connected components and cycles. The algorithm can identify the different homogenous components within a defect cluster having mixed morphology. We apply the method to classify morphologies of over a thousand point defect clusters formed in high energy W collision cascades. We highlight our method's comparative advantage for its completeness, computational speed, and quantitative details.
Local electronic excitations induced by low-velocity light ion stopping in tungsten
(2024-04-15) Ponomareva, Evgeniia; Pitthan, Eduardo; Holeňák, Radek; Shams-Latifi, Jila; Kiely, Glen Pádraig; Primetzhofer, Daniel; Sand, Andrea E.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Accurately predicting the electronic energy deposition of ions in materials is an important challenge in both fundamental and applied research. While employing ab initio simulations to investigate electronic stopping of ions in matter holds promise, its combined use with experimental measurements paves the way for obtaining reliable data. In this paper, we present a collaborative study using real-time time-dependent density functional theory and experimental methods to determine the electronic stopping power of hydrogen and helium ions in tungsten, a primary candidate material for future nuclear fusion devices. While calculated stopping powers in hyperchanneling trajectories are significantly lower than the experimental data, off-center and random geometries demonstrate a better agreement. We show that the deviation from velocity proportionality for both projectiles traversing the hyperchanneling directions can be explained through the existence of a threshold velocity leading to the activation of semicore states. Additionally, we analyze the pseudopotential and the trajectory dependence of computed electronic energy losses. It is demonstrated that the role of including semicore electrons varies depending on the velocity range. While these states play a crucial role at high projectile velocities by introducing additional dissipation channels, their impact diminishes in the low-velocity range. Finally, we introduce a simple expression that links electronic energy losses in different trajectories to local electron density, and we show that utilizing this formula allows for quite accurate predictions of stopping powers around the Bragg peak.
Microstructure of a heavily irradiated metal exposed to a spectrum of atomic recoils
(2023-01-30) Boleininger, Max; Mason, Daniel R.; Sand, Andrea E.; Dudarev, Sergei L.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
At temperatures below the onset of vacancy migration, metals exposed to energetic ions develop dynamically fluctuating steady-state microstructures. Statistical properties of these microstructures in the asymptotic high exposure limit are not universal and vary depending on the energy and mass of the incident ions. We develop a model for the microstructure of an ion-irradiated metal under athermal conditions, where internal stress fluctuations dominate the kinetics of structural evolution. The balance between defect production and recombination depends sensitively not only on the total exposure to irradiation, defined by the fluence, but also on the energy of the incident particles. The model predicts the defect content in the high dose limit as an integral of the spectrum of primary knock-on atom energies, with the finding that low energy ions produce a significantly higher amount of damage than high energy ions at comparable levels of exposure to radiation.
Stability of 〈100〉 dislocations formed in W collision cascades
(2022-10) Bhardwaj, Utkarsh; Sand, Andrea E.; Warrier, Manoj
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Experiments and simulations both have verified the presence of 〈100〉 dislocations in irradiated W. It is essential to know the properties and behavior of these defects to study the evolution of microstructures at higher scales. We study the thermal stability and transition mechanism of various 〈100〉 dislocations formed in a molecular dynamics (MD) database of 230 collision cascades using three different interatomic potentials. The activation energy to transition to more stable 〈111〉 dislocations is found for various 〈100〉 dislocation defects that transition within the 100 nanosecond time scale that is readily accessible to MD. The stability of 〈100〉 dislocations increases with size, but the trend is not strict. The reasons for irregularities are the aspects of internal configuration such as (i) the arrangement of 〈100〉 directed crowdions within the defect, (ii) the presence and arrangement of non-〈100〉 crowdions on the fringes of the defect. We show the typical pathways of transitions and discuss the sources of instability in the defect configurations. We also discuss the similarities and differences in stability found across different interatomic potentials. Understanding transition mechanisms and internal morphology gives insights into the stability of 〈100〉 dislocations, useful in higher scale models such as Kinetic Monte Carlo (KMC).