18.104.22.168 Defect dynamics, migration energies and jump frequencies
This chapter discusses the defect dynamics, migration energies and jump frequencies. Theoretical considerations show that defects, especially interstitials, constitute a strong perturbation of the host lattice. The local vibrational properties of the defect atoms and their neighbours will differ markedly from the average lattice behaviour. In body centered cubic lattice (BCC), computer calculations indicate a softening of the vibrational spectrum for the neighbours of a vacancy and the migration of the vacancy occurs by a nearest neighbour jump. The vibrational behaviour of the (110)-dumbbell should in principle be similar to that of the (100)-dumbbell in the face centered cubic lattice (FCC) materials. In FCC, calculations for a (100)-dumbbell in Cu show that the vibration of the dumbbell atoms is mainly by resonant and localized modes. Also, the neighbours of the dumbbell show some resonant and localized modes. In hexagonal close packed lattice (HCP), the vibrational spectra for the neighbours of vacancies and divacancies were investigated. In an empirical approach the migration energies of self interstitial atom (SIA) have been correlated to the deviation from the ideal value of the c/a -ratio of (8/3)1/2. Experimental information on the dynamics of the jump process of the SIA has been obtained by relaxation techniques and n-scattering. Most reliable data for vacancy migration energies have been obtained from the analysis of the annealing kinetics of stage III after electron irradiation. Additional information on migration energies has been obtained by the nuclear methods from the temperature, where defect trapping is observed.