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Osetskiy, Yury N

Publications and source records attributed to Osetskiy, Yury N.

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Atomic-level level dislocation dynamics in irradiated metals

Primary damage and microstructure evolution in structural nuclear materials operating under conditions of a high flux of energetic atomic particles and high temperature and stress lead to formation of a high concentration, non-homogeneous distribution of defect clusters in the form of dislocation loops, voids, gas-filled bubbles and radiation-induced precipitates of nanometer scale. They cause changes in many material properties. Being obstacles to dislocation glide, they strongly affect mechanical properties in particular. This gives rise to an increase in yield and flow stress and a reduction in ductility. Atomic-scale computer simulation can provide details of how these effects are influenced by obstacle structure, applied stress, strain rate and temperature. Processes such as obstacle cutting, transformation, absorption and drag are observed. Some recent results for body-centered and face-centered cubic metals are described in this review and, where appropriate, comparisons drawn with predictions based on elasticity theory of crystal defects.

dislocations↗
DOE OSTI2013

Cascade Defect Evolution Processes: Comparison of Atomistic Methods

Determining the defect evolution beyond the molecular dynamics (MD) time scale is critical in bridging the gap between atomistic simulations and experiments. The recently developed self-evolving atomistic kinetic Monte Carlo (SEAKMC) method provides new opportunities to simulate long-term defect evolution with MD-like fidelity. In this study, SEAKMC is applied to investigate the cascade defect evolution in bcc iron. First, the evolution of a vacancy rich region is simulated and compared with results obtained using autonomous basin climbing (ABC) +KMC and kinetic activation-relaxation technique (kART) simulations. Previously, it is found the results from kART are orders of magnitude faster than ABC+KMC. The results obtained from SEAKMC are similar to kART but the time predicted is about one order of magnitude faster than kART. The fidelity of SEAKMC is confirmed by statistically relevant MD simulations at multiple higher temperatures, which proves that the saddle point sampling is close to complete in SEAKMC. The second is the irradiation-induced formation of C15 Laves phase nano-size defect clusters. In contrast to previous studies, which claim the defects can grow by capturing self-interstitials, we found these highly stable clusters can transform to <111> glissile configuration on a much longer time scale. Finally, cascade-annealing simulations using SEAKMC is compared with traditional object KMC (OKMC) method. SEAKMC predicts substantially fewer surviving defects compared with OKMC. The possible origin of this difference is discussed and a possible way to improve the accuracy of OKMC based on SEAKMC results is outlined. These studies demonstrate the atomistic fidelity of SEAKMC in comparison with other on-the-fly KMC methods and provide new information on long-term defect evolution in iron.

Xu, Haixuan↗
DOE OSTI2013

Mapping Strain-rate Dependent Dislocation-Defect Interactions by Atomistic Simulations

Probing the mechanisms of defect-defect interactions at strain rates lower than 106 s-1 is an unresolved challenge to date to molecular dynamics (MD) techniques. Here we propose a novel atomistic approach based on transition state theory and the concept of a strain-dependent effective activation barrier that is capable of simulating the kinetics of dislocation-defect interactions at virtually any strain rate, exemplified within 10-7 to 107 s-1. We apply this approach to the problem of an edge dislocation colliding with a cluster of self-interstitial atoms (SIA) under shear deformation. Using an activation-relaxation algorithm (1), we uncover a unique strain-rate dependent trigger mechanism that allows the SIA cluster to be absorbed during the process leading to dislocation climb. Guided by this finding, we determine the activation barrier of the trigger mechanism as a function of shear strain, and use that in a coarse-graining rate equation formulation for constructing a mechanism map in the phase space of strain-rate and temperature. Our predictions of a crossover from a defect recovery at the low strain rate regime to defect absorption behavior in the high strain-rate regime are validated against our own independent, direct MD simulations at 105 to 107 s-1. Implications of the present approach for probing molecular-level mechanisms in strain-rate regimes previously considered inaccessible to atomistic simulations are discussed.

activation mecanism↗
DOE OSTI2013

Self-interstitial configurations in hcp Zr: a first principles analysis

Analysis of microstructure evolution in irradiated Zr and Zr alloys based on the modern radiation damage theory suggests some important features of self-interstitial atoms (SIAs). Alignment of vacancy loops and voids along basal planes requires anisotropic interstitial transport with a dominant contribution along the basal plane. Under neutron irradiation this can be explained by one-dimensional mobility of SIA clusters but experiments under electron irradiation indicate unambiguously that even the single SIA should exhibit anisotropic diffusion. No experimental information is available on SIA properties in Zr and the data obtained by ab initio calculations within the last decade reported stable SIA configurations that should provide essentially three-dimensional diffusion. To clarify this issue, an extensive investigation of SIAs in Zr has been performed from first principles using two different codes. It was demonstrated that simulation cell size, type of pseudopotential, the exchange-correlation functional, and the c/a ratio are crucially important for determining the properties of interstitials in hcp Zr. The most stable SIA configurations lie in the basal plane, which should lead to SIA diffusion mainly along basal planes. The results provide a confirmation of basic mechanisms for microstructural evolution under irradiation.

Samolyuk, German D↗
DOE OSTI2013

The influence of transition metal solutes on dislocation core structure and values of Peierls stress and barrier in tungsten.

Several transition metals were examined to evaluate their potential for improving the ductility of tungsten. We investigate the dislocation core structure, Peierls stress and barrier of $1/2\langle111\rangle$ screw dislocations in binary tungsten-transition metal alloys (W$_{1-x}$TM$_{x}$) based on first principles electronic structure calculations. The periodic quadrupole approach was applied to model the structure of $1/2\langle111\rangle$ dislocation. The alloying with transition metals was modeled using the virtual crystal approximation. In order to verify the applicability of this approach, the equilibrium lattice parameter and elastic constants were calculated for tungsten alloyed with the set of transition metals. Reasonable agreement was obtained between results using the virtual crystal approximation and those using both a conventional super-cell approach and existing experimental data. Increasing the concentration of a transition metal from the VIIIA group leads to reduction of the $C^\prime$ elastic constant and increase of elastic anisotropy A=$C_{44}/C^\prime$. It was demonstrated that alloying W with a group VIIIA transition metal changes the structure of the dislocation core from symmetric to asymmetric, similar to results obtained for W$_{1-x}$Re$_{x}$ alloys in the earlier work of Romaner {\it et al} (Phys. Rev. Lett. 104, 195503 (2010)). Following the core symmetry change, the values of the Peierls stress and barrier are reduced. This combination of two effects could lead to increased ductility in a tungsten-based alloy\comments. Our results demonstrate that similar effects could be achieved with any of the transition metals from the VIIIA group.

Samolyuk, German D↗
DOE OSTI2012

Cascade annealing simulations of bcc iron using object kinetic Monte Carlo

Simulations of displacement cascade annealing were carried out using object kinetic Monte Carlo based on an extensive MD database including various primary knock-on atom energies and directions. The sensitivity of the results to a broad range of material and model parameters was examined. The diffusion mechanism of interstitial clusters has been identified to have the most significant impact on the fraction of stable interstitials that escape the cascade region. The maximum level of recombination was observed for the limiting case in which all interstitial clusters exhibit 3D random walk diffusion. The OKMC model was parameterized using two alternative sets of defect migration and binding energies, one from ab initio calculations and the second from an empirical potential. The two sets of data predict essentially the same fraction of surviving defects but different times associated with the defect escape processes. This study provides a comprehensive picture of the first phase of long-term defect evolution in bcc iron and generates information that can be used as input data for mean field rate theory (MFRT) to predict the microstructure evolution of materials under irradiation. In addition, the limitations of the current OKMC model are discussed and a potential way to overcome these limitations is outlined.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Twist angle effect on anisotropic mobility of hexagonal dislocation networks in {110} of alpha-iron

Atomistic studies of anisotropic mobility of hexagonal dislocation networks (HDNs) in a series of twist boundaries (1 -1 0) has been performed in alpha-iron. In contrast with previous work that neglected the twist angle effect, we find when the twist angle approaches to 0, the resistance to the HDN motion could become much lower than Peierls stress of edge dislocations <1 1 1>/2 when the HDN moves along [0 0 1], but beyond Peierls stress of screw dislocations <1 1 1>/2 when the HDN moves along other directions. Vector form of Orowan equation and differential displacement map of dislocation core are used to analyse the behaviour of these boundary dislocations. This work seems favourable for understanding the absence of anomalous slip in alpha-iron.

36 MATERIALS SCIENCE↗