Competing Kinetics and He Bubble Morphology in W

Luis Sandoval, Danny Perez, Blas P. Uberuaga, and Arthur F. Voter

Phys. Rev. Lett. 114, 105502 – Published 11 March 2015

Abstract

The growth process of He bubbles in W is investigated using molecular dynamics and parallel replica dynamics for growth rates spanning 6 orders of magnitude. Fast and slow growth regimes are defined relative to typical diffusion hopping times of W interstitials around the He bubble. Slow growth rates allow the diffusion of interstitials around the bubble, favoring the biased growth of the bubble towards the surface. In contrast, at fast growth rates interstitials do not have time to diffuse around the bubble, leading to a more isotropic growth and increasing the surface damage.

Received 23 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.105502

Authors & Affiliations

Luis Sandoval¹, Danny Perez¹, Blas P. Uberuaga², and Arthur F. Voter¹

¹Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
²Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA

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Vol. 114, Iss. 10 — 13 March 2015

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Images

Figure 1
Growth rate dependence of bubble growth. (a) Average He content in the bubble at the time of the first detected event. Points with no error bars (red squares) are obtained from a single simulation. Points corresponding to rates $\geq 1 0^{11} He s^{- 1}$ were obtained via direct MD simulations. (b) Average pressure in the He bubble vs He content and growth rate. The limited number of simulations for the slowest rates significantly affects the average for He content larger than 25 atoms (blue circles line). (c) Average position of the center of mass of the He bubble. (d) Average position of the deepest He atom. (e) Average He content in the bubble at the bursting point as a function of growth rate. (f) Average number of W vacancies at the bursting point as a function of growth rate.
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Figure 2
(a) Diffusion hopping time as a function of the number of He atoms per W vacancy for two cases: one W interstitial on a 2-vacancy bubble, and one W interstitial on a 15-vacancy bubble. Insets: yellow (grey) and blue (black) spheres denote the W interstitial and W vacancies, respectively. (b) Snapshots showing the diffusion of a W interstitial to the top of the bubble, the subsequent nucleation of additional Frenkel pairs, and the tearing off process of adatom nucleation. Green (medium grey) spheres denote adatoms on the W surface. (c) Final vacancy configuration for the 15-vacancy bubble (after adatoms are formed) from 8 independent ParRep simulations. The cyan (grey) spheres indicate the position of the additional vacancies. Top and bottom snapshots correspond to views on the planes ${101}$ and ${\bar{1} 01}$ , respectively.
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Figure 3
(a) Average number of interstitial clusters around the He bubble after a new Frenkel pair is nucleated, when at least one interstitial is already present, as a function of the growth rate, for two values of the cutoff distance defining the clusters. (b) Average number of Frenkel pairs per event as a function of the growth rate. (c) Spatial probability for the nucleation of new vacancies with respect to the center of the current vacancy cluster [see (d)], as a function of the growth rate. (d) Histogram of the location of new vacancies in the direction perpendicular to the surface for all the simulations. As described in the inset, the location is defined as the distance between the new vacancy and the center of the current vacancy cluster, binned into $d y = 1 Å$ bins. In order to compare bubbles with different sizes this distance has been normalized by $\sqrt[3]{N_{vac}}$ , with $N_{vac}$ the number of vacancies already existing in the bubble.
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