'Non-vacancy' mechanisms of self-diffusion in two-dimensional metals

Full text: It is known that vacancy mechanism is the most wide-spread mechanism of self-diffusion in metals. But there are 'non-vacancy' mechanisms having higher activation energy. They also take part in the process of diffusion. 'Non-vacancy' mechanisms of self-diffusion are understood as mechanisms, which are not connected with vacancies, formed by Schottky method. They are the following: interchange two-atomic, interchange cyclic, crowdion, migration of atoms at interstitial and vacancy mechanisms, forming the vacancies by Frenkel method. The paper is concerned with the research of leading non-vocational mechanisms of self-diffusion in two-dimensional metals. Similar study of three-dimensional metals is connected with the difficulty of the visualization of the atomic structure dynamics inside the calculated block. The research was made for four FCC metals: Ni, Al, Cu, Au. The packing of two-dimensional metals under study was corresponded to the plane [111] of FCC lattice. To describe interatomic interactions, pair Morse potentials were used. The metals were heated in the interval of temperatures 0.7-0.95 from the melting temperature. Periodical boundary conditions were applied on the boundaries of the calculated block. It is found that self-diffusion mechanisms in an ideal two-dimensional crystal are interchange cyclic mechanisms. They contain from three to several dozens of atoms in their cycle. It is concluded that main non-vacancy mechanism is formation and annihilation of Frenkel pairs. It is proved that the reason of their formation is thermal collective atomic displacements (TCAD). They have been discovered earlier studying vacancy self-diffusion mechanism. The TCADs are mainly oriented along closely-packed directions, and they are the reason of crowdion displacement of atoms. Frenkel pair forms in the result of crossing of two crowdion displacements. The first crowdion makes the area with the excess of free volume behind itself. The second crowdion brings the atom to this area crossing the first one. It prevents the reverse recoil of the first crowdion. Thus, the atom in the interstitial appears near the edge of the first crowdion the vacancy appears at the end of the second crowdion. The interchange cyclic mechanism finishes by the annihilation of these two defects. The calculations of the migration energy and the diffusion velocity of separate defects of Frenkel pair shows that interstitial atoms migrate considerably more rapid than the vacancy