Grain boundary motion assisted via radiation cascades in bcc Fe

Molecular-dynamic simulations were performed to study the influence of displacement cascades on grain boundary (GB) structure and stability in bcc Fe. A $\Sigma =5$, (310)[001] symmetric tilt boundary with a tilt angle of $\theta =36.9°$ was used for the simulations. We find that GB motion, either sliding or migration, is activated under the influence of displacement cascades at lower internal stresses as compared to the unirradiated GBs. We postulate that radiation-induced GB damage aids the nucleation mechanisms that trigger GB motion. Furthermore, radiation-induced GB sliding significantly relaxes internal stress and may provide a viable mechanism for promoting irradiation creep via GB accommodation processes.

Figure 1

(Color online) Configurations of a bcc Fe $\Sigma =5$, (310)[001] symmetric tilt bicrystal strained along $xy$. The top configuration represents the initial bicrystal. The bottom configuration shows the bicrystal 20 ps after a GB displacement cascade, initiated with a PKA energy of 1 keV. GB migration can be identified by comparing the initial and final positions of the GB (tilted straight line).

Figure 2

(Color online) Evolution of the number of vacancies $N_v\left(t\right)$ over time for irradiated bulk samples (lines) and unstressed Fe bicrystals (symbols). For a given PKA energy, the transient $t_B$ that characterizes the time and the meltlike region that lasts within the bulk is always smaller than the GB transient $t_{\text{GB}}$. The inset of the figure is a log-lin plot of the tails in order to better visualize the number of vacancies generated.

Figure 3

(Color online) Stress relaxation in a symmetric tilt Fe bicrystal after its GB has been bombarded with a 1 keV PKA energy cascade. Initially, only the nondiagonal component in the plane of the applied strain $xy$ is different from zero.

Figure 4

(Color online) Visualization of the symmetric tilt bcc Fe bicrystal (a) before and (b) during GB sliding event under the action of a 2 keV GB displacement cascade. Several layers of marker atoms along the $z$ axis were colored on both sides of the GB. After the cascade takes place, GB sliding can be identified by comparing the relative displacements of the markers.

Figure 5

(Color online) Stress relaxation for a Fe $\Sigma =5$, (310)[001] symmetric tilt bicrystal after being bombarded with a 1 keV GB cascade. The initial applied strain was ${\gamma }_{xz}=0.031$. The time evolution of the center of mass on both sides of the GB is shown in the inset plot.

Figure 6

(Color online) Stress relaxation for a Fe $\Sigma =5$, (310)[001] symmetric tilt bicrystal after being bombarded with a 1 keV GB cascade. The initial applied strain was ${\gamma }_{xz}=0.040$. The time evolution of the center of mass on both sides of the GB is shown in the inset plot.

Figure 7

(Color online) Relaxation of the stress component ${\sigma }_{xz}$ of a tilt Fe bicrystal as a function of the size of the GB. Stress relaxes following a linear law via dislocation motion. Increasing the size of the boundary reduces the stress relaxation rate, conversely, increasing the PKA energy has no influence on such a variable (see inset). The inset plot relates to the case in which $L_z=428.30\text{Å}$.

Figure 8

(Color online) Superposition of the stress plots included within Fig. 7 by rescaling the time axis such that $t=t_0+t{L}_{z_{\text{ref}}}/L_z$. $t_0$ represents a time shift employed to synchronize the starting of the stress relaxation in all cases and $L_{z_{\text{ref}}}$ corresponds to the length of a reference GB.

Figure 9

(Color online) Relaxation of the stress component ${\sigma }_{xz}$ of a Fe bicrystal after the action of a secondary GB cascade of 2 keV. The initial status of the system corresponds to two of the relaxed configurations shown in Fig. 7. The inset plot shows the corresponding sliding due to the second cascade. The coms are centered about the origin as we have subtracted from both the average initial coms because the GBs considered within this set of calculations had different lengths.

Figure 10

(Color online) The time evolution of the ratio $f_c$ of atoms belonging to each type of lattice structure according to the algorithm of Ackland and Jones (Ref. 36) is shown in the figure. The algorithm proved to be successful when identifying crystal structures on slightly strained (sheared) simulation boxes.

Figure 11

(Color online) An algorithm proposed by Ackland to identify crystalline structures is used to track the migration of the GB from Fig. 1. Only the atoms labeled as hcp coordinated are plotted.

Figure 12

(Color online) Temperature effects on the stress relaxation in a symmetric tilt Fe bicrystal after its GB has been bombarded with a 1 keV PKA energy cascade. Increasing the temperature of the bicrystal reduces its intrinsic stress threshold ${\sigma }^{\ast }$. Higher temperatures also ensure lower values of the equilibrium stress after the relaxation process has happened.