Mechanical properties of crystals, like hardness and toughness are related to the generation and motion of dislocations—crystal defects form due to local displacement in the atomic lattice—in response to the application of stress.

Recent theoretical studies suggest that dislocation loops of only a few nanometers in size that form in metals and alloys can undergo one-dimensional diffusion along the loop axis—even without external stress—due to thermal excitation. Further, it has been theoretically predicated that the diffusive motion of dislocation loops is responsible for the degradation of materials for nuclear fission and fusion. A deeper understanding of loop motion is therefore important for accurately predicting the lifetime of these materials.

Now, Kazuto Arakawa and colleagues from Osaka University, Shimane University and Tohoku University in Japan have made the first experimental observation of the motion of dislocation loops in thin films of α-Fe (or ferrite)1 without application of external stress. “The dislocation loops that we observed are clusters of point defects. By shrinking the dislocation loops to extremely small sizes, they will finally become single point defects [such as] vacancy or self-interstitial atoms,” says Arakawa.

Fig. 1: Sequential images of a dislocation loop in a thin slice of α-Fe showing the motion in one dimension. The loop has a diameter of 5.9 ± 0.2 nm. The observation axis is along the [011] direction while the motion occurs in a direction parallel to the <111> orientation vector.

The researchers used a transmission electron microscope (TEM) to follow the movement of the projection of the centres of mass of several isolated dislocation loops on a screen. The motion was slow enough to be followed with frames taken every 1/30 s (Fig. 1). The dislocation loops moved backwards and forwards, resulting in an average displacement of zero. But the mean square displacement increased linearly with time, which is typical of diffusion based processes. Further investigation of the dependence of the diffusion constant on temperature and dislocation loop size allowed the researchers to gain more insight into the origin of the diffusive motion.

The TEM experiments on the ferrite films are a new approach for investigating the dynamics of nanometer-sized dislocations. Arakawa says, “The precise understanding of the behavior of point defects is fundamental for several fields of materials science. In spite of the great amount of research to-date, the structure and dynamics of self-interstitial atoms is still a mystery; even in simple metals. We are now aiming at the first direct observation of self-interstitial atoms by developing in-situ TEM techniques”.