Dislocation Loop Formation and Growth under In Situ Laser and/or Electron Irradiation
Vacancies and interstitial atoms are primary lattice (point) defects that cause observable microstructural changes, such as the formation of dislocation loops and voids in crystalline solids. These defects' diffusion properties determine the phase stability and environmental resistibility of macroscopic materials under ambient conditions. Although in situ methods have been proposed for measuring the diffusion energy of point defects, direct measurement has been limited. In this study, we propose an alternative in situ method to measure the activation energy for vacancy migration under laser irradiation using a pulsed laser beam from a laser-equipped high-voltage electron microscope (laser-HVEM). We made in situ observations that revealed the formation and growth of vacancy dislocation loops in an austenitic stainless steel during laser irradiation. These loops continued to grow when thermal annealing was performed after laser irradiation at the same temperature. We anticipate that laser-HVEM will provide a new method for investigating lattice defects.
Vacancy migration and void formation during annealing of electron irradiated molybdenum
WE have used a combination of three techniques—resistivity, positron annihilation and transmission electron microscopy (TEM)—to examine the recovery on annealing of point defect damage introduced into high purity molybdenum by 10 MeV electron irradiation. The work produced two points of particular interest, both arising directly from the sensitivity of the positron annihilation technique to vacancy defects.
Computer simulation of fundamental behaviors of interstitial clusters in Fe and Ni
Atomistic features of small interstitial clusters in Fe and Ni have been investigated by computer simulation. The gradual change from an interstitial cluster to a dislocation loop was observed in detail from three points of view, (i) strain distribution along crowdion axis for each crowdion in a cluster, (ii) distribution of Burgers vector of peripheral dislocation line of a cluster, and (iii) recombination behavior of a vacancy of with a cluster. It is found that strain distribution was spread into two opposite directions on a crowdion axis with the increase of a cluster size. The irregularity on the curve of the distribution of Burgers vector of a smaller loop gradually disappears with the increase of a loop size. Recombination between a crowdion and a vacancy occurs on the peripheral position of smaller clusters, but this does not occur in larger clusters. These aspects show that the gradual change from an interstitial cluster to a dislocation loop occurs at a certain width of cluster size. Dynamic behavior was also investigated under cylindrical shear stress and the Peierls stress was obtained as a function of loop size. The results show that the Peierls stress decreases with increasing loop size down to the value of a straight edge dislocation. Activation energies of one atomic jump of these small dislocation loops were also calculated and small values of about 0.2 eV were found for loops of about 200 crowdions for both Fe and Ni.