**4.2. Annihilation of micropipes**

MP merging may lead to the annihilation of initial MPs at the growing surface. We considered the motion of subsurface segments of MPs under the action of elastic forces due to their interaction and some effective friction [9]. The latter accounted for some extra surface energy related to the steps appearing on the pipe cylindrical surfaces during lateral displacements of the pipe segments. As a result, various reactions between the subsurface pipe segments were observed. In particular, it has been shown that the reaction of MP coalescence can lead to the annihilation of initial MPs. Some typical defect configurations in a 3D space are displayed in Fig. 11. They may be subdivided into planar and twisted pipe configurations. The planar configurations arise when the interacting pair of MPs is located far from other MPs; and the coalescing MPs come to each other along the shortest way [Fig 11(a)]. The twisted configurations like double spirals form if the interacting MPs are located within dense groups of other MPs. In this case, the coalescing MPs twist [Fig 11(b)]. When the magnitudes of Burgers vectors are the same, the initial defect configuration of a dipole is transformed into a new configuration of a semiloop [Fig. 11(c)]. As a result, we expect that the density of MPs would diminish during the crystal growth. The smaller the surface energy is the faster this process would be.
