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Warrier M.,Computational Analysis Division | Bhardwaj U.,Computational Analysis Division | Hemani H.,Computational Analysis Division | Schneider R.,University of Greifswald | And 2 more authors.
Journal of Nuclear Materials | Year: 2015

We report on molecular Dynamics (MD) simulations carried out in fcc Cu and bcc W using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code to study (i) the statistical variations in the number of interstitials and vacancies produced by energetic primary knock-on atoms (PKA) (0.1-5 keV) directed in random directions and (ii) the in-cascade cluster size distributions. It is seen that around 60-80 random directions have to be explored for the average number of displaced atoms to become steady in the case of fcc Cu, whereas for bcc W around 50-60 random directions need to be explored. The number of Frenkel pairs produced in the MD simulations are compared with that from the Binary Collision Approximation Monte Carlo (BCA-MC) code SDTRIM-SP and the results from the NRT model. It is seen that a proper choice of the damage energy, i.e. the energy required to create a stable interstitial, is essential for the BCA-MC results to match the MD results. On the computational front it is seen that in-situ processing saves the need to input/output (I/O) atomic position data of several tera-bytes when exploring a large number of random directions and there is no difference in run-time because the extra run-time in processing data is offset by the time saved in I/O. © 2015 Elsevier B.V. Source

Bhardwaj U.,Computational Analysis Division | Bukkuru S.,Andhra University | Warrier M.,Computational Analysis Division
Journal of Computational Physics | Year: 2016

An algorithm to rigorously trace the interstitialcy diffusion trajectory in crystals is developed. The algorithm incorporates unsupervised learning and graph optimization which obviate the need to input extra domain specific information depending on crystal or temperature of the simulation. The algorithm is implemented in a flexible framework as a post-processor to molecular dynamics (MD) simulations. We describe in detail the reduction of interstitialcy diffusion into known computational problems of unsupervised clustering and graph optimization. We also discuss the steps, computational efficiency and key components of the algorithm. Using the algorithm, thermal interstitialcy diffusion from low to near-melting point temperatures is studied. We encapsulate the algorithms in a modular framework with functionality to calculate diffusion coefficients, migration energies and other trajectory properties. The study validates the algorithm by establishing the conformity of output parameters with experimental values and provides detailed insights for the interstitialcy diffusion mechanism. The algorithm along with the help of supporting visualizations and analysis gives convincing details and a new approach to quantifying diffusion jumps, jump-lengths, time between jumps and to identify interstitials from lattice atoms. © 2015 Elsevier Inc. Source

Abhishek A.,Indian Institute for Plasma Research | Warrier M.,Computational Analysis Division | Ganesh R.,Indian Institute for Plasma Research | Caro A.,Los Alamos National Laboratory
Journal of Nuclear Materials | Year: 2016

Helium(He) produced by transmutation process inside structural material due to neutron irradiation plays a vital role in the degradation of material properties. We have carried out Molecular dynamics(MD) simulations to study the growth of He bubble in Iron-Chromium alloy. Simulations are carried out at two different temperatures, viz. 0.1 K and 800 K, upto He bubble radius of 2.5 nm. An equation for variation of volume of He bubbles with the number of He atoms is obtained at both the temperatures. Bubble pressure and potential energy variation is obtained with increasing bubble radius. Dislocations are also found to be emitted after the bubble reaches a critical radius of 0.39 nm at 800 K. Separate MD simulations of He with pre-created voids are also carried out to study the binding energies of He and Vacancy (V) to Hem-Vn cluster. Binding energies are found to be in the range of 1-5.5 eV. © 2016 Elsevier B.V. Source

Abhishek A.,Indian Institute for Plasma Research | Warrier M.,Computational Analysis Division | Ganesh R.,Indian Institute for Plasma Research
Transactions of the Indian Institute of Metals | Year: 2015

Helium has very low solubility in metals leading to formation of helium clusters and complexes with vacancies/interstitials. Clusters and complexes then diffuse and coalesce to form helium bubbles of nanometric size or higher. These bubbles are one of the reasons for the brittle failure of materials. In order to study the diffusion process, molecular dynamics simulations of helium diffusion in FeCr grain boundaries are carried out for different orientations of the bicrystal. A total of eight different configurations of the bicrystal, have been studied in the temperature range of 700–1,000 K. Potential energy analysis of host and grain boundary (GB) atoms predicts average GB potential energy in the range of $$-$$-3.7 to $$-$$-3.6 eV, which is 0.2 eV higher than that in the host matrix. The migration energy of helium in grain boundary is found to be   0.3–0.7 eV, which is an order of magnitude larger than that in the bulk crystal. The grain boundary width calculated for all the bi-crystals lie within 11–14 Å and the cage distance of helium atom is of the order of the bond length (2.87 Å) of the host atoms. For $$\sum 9 \langle 110 \rangle \{1 \ 1 \ 4\}$$∑9⟨110⟩{114} orientation more than 100 helium trajectories have been analysed to measure the statistical variation of migration energy. © 2015, The Indian Institute of Metals - IIM. Source

Warrier M.,Computational Analysis Division | Pahari P.,Computational Analysis Division | Chaturvedi S.,Computational Analysis Division
Journal of Molecular Modeling | Year: 2015

Molecular dynamics (MD) simulations of high velocity impact (1–6 km/s) of RDX crystal with a nanometer-sized void, has been carried out to understand the mechanism of increase in temperature at void locations under shock loading. Similar simulations are then carried out on single-crystal copper for better interpretation of the results. A reactive potential that can simulate chemical reactions (ReaxFF) has been used for RDX, whereas an EAM potential has been used for Cu. Increased temperature at the void locations are observed under shock loading. The atomic motion, temperature, average potential energy per atom (PE), and average kinetic energy per atom (KE) in and around the voids are closely monitored in order to understand the reason for temperature increase. We compare our results with existing proposed mechanisms and show that some of the proposed mechanisms are not necessary for increased temperature at a void location. It is shown that the directed particle velocity is efficiently is converted into randomized velocity due to the presence of voids thereby increasing the local temperature transiently. In this initial stage (few picoseconds) of the shock, chemical reactions of energetic materials do not play a part in the temperature rise. © 2015, Springer-Verlag Berlin Heidelberg. Source

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