*3.4. Dynamic Interaction between an IDL and a Vacancy Cluster*

A previous study showed that the IDL exhibits a fast one-dimensional motion along the <111> direction in W[16]. The existence of a vacancy inside the IDL can inhibit its motion, which also closely depends on the IDL size and temperature [24]. Besides, the number of the vacancies can also have a large influence on the movement of the IDL or even absorb it [27]. Therefore, the vacancy cluster plays an important role in the evolution of the IDL and needs to be investigated systemically.

We calculate the movement distance of the IDL (10.0Å) along the Burgers vector with a vacancy cluster (containing eight vacancies) by varying its position and temperature. The movement distances of the IDL along the Burgers vector as a function of the simulation time at different temperatures are shown in Figure 8. The inset pictures show the movement distance of the IDL at the initial stage (0–150 ps). The interaction between the IDL and the vacancy cluster largely depends on the temperature.

**Figure 7.** The distribution of binding energies of the different sized IDLs to a vacancy cluster (containing eight vacancies) as a function of their distance when the vacancy clusters is placed along the (**a**) Direction I and (**b**) Direction II, respectively. The inset figure shows the binding energies at a small energy scale.

**Figure 8.** The movement distance of the IDL along the Burgers vector when a vacancy cluster is placed at (**a**) Position I and (**b**) Position II, respectively.

Figure 8a shows the movement distance of the IDL with the vacancy cluster placed at the position I as a function of the simulation time at different temperatures. We can see that the IDL will diffuse towards the vacancy cluster at the initial stage as shown in the insert figure. Then the IDL is captured by the vacancy cluster for all temperatures. At a temperature below 1200 K, the simulation results show the IDL is pinned by the vacancy cluster and immobile. At a temperature above 1500 K, the IDL can move after 400 ps and 800 ps, respectively. Based on the analysis of the evolution of the microstructure, the vacancy cluster first attracts the IDL to drive it to the position of the vacancy cluster, then the vacancies inside the cluster will be absorbed by IDL. However, not all vacancies in the vacancy cluster will be absorbed. The left vacancies will inhibit the movement of the IDL. At a temperature above 1200 K, the vacancy captured inside the IDL can migrate towards the edge of the IDL, and then the vacancy can be annihilated. Finally, the loop mobility is recovered. Figure 8b shows the calculated movement distance of the IDL with a vacancy cluster placed at Position II at several selected times. It is shown that the interaction behavior is very similar to that with a vacancy cluster placed at Position I. While the IDL requires a higher temperature of 1800K than that at Position I to unpin the vacancy cluster.

In conclusion, the movement distance of the IDL depends on the position of the vacancy cluster and temperature. At low temperature, the IDL is pinned by the vacancy cluster, while it can move at high temperature because it can promote the recombination of the IDL and the vacancy. Besides, the attracting capability of the IDL by the vacancy cluster placed at Position II is stronger than it placed at Position I.
