Showing papers in "Materials Science Forum in 1987"

Energetic Displacement Cascades and Their Roles in Radiation Effects

Abstract: The roles of energetic displacement cascades are ubiquitous in the fields of radiation damage and ion beam modifications of materials. These roles can be described on two time scales. For the first, which lasts less than approx. = 10/sup -11/ - 10/sup -10/ s, small cascade volumes are characterized by large supersaturations of point defects and energy densities of the order of eVs per atom. During this period, the system can be driven far from equilibrium with significant rearrangement of target atoms and the production of Frenkel pairs. Studies of ion beam mixing and the primary state of damage, in conjunction with molecular dynamics computer simulations, have contributed largely toward understanding these dynamic cascade processes. It is shown that in cascades, ''thermal spike'' diffusion is primarily responsible for atomic rearrangements, and that vacancy radial distribution functions provide a natural description of the primary state of damage. At later times, the microstructure of the material evolves as cascades begin to overlap, or at elevated temperatures, point defects migrate away from their nascent cascades. It is shown how the primary state of damage in cascades influences this microstructural development. 90 refs., 10 figs., 4 tabs.

Production of Freely-Migrating Defects during Irradiation

Abstract: During irradiation at elevated temperatures, vacancy and interstitial defects that escape can produce several different types of microstructural changes. Hence the production rate of freely-migrating defects must be known as a function of irradiating particle species and energy before quantitative correlations can be made between microstructural changes. Our fundamental knowledge of freely-migrating defect production has increased substantially in recent years. Critical experimental findings that led to the improved understanding are reviewed in this paper. A strong similarity is found for the dependence of freely-migrating defect production on primary recoil energy as measured in a variety of metals and alloys by different authors. The efficiency for producing freely-migrating defects decreases much more strongly with increasing primary recoil energy than does the efficiency for creating stable defects at liquid helium temperatures. The stronger decrease can be understood in terms of additional intracascade recombination that results from the nonrandom distribution of defects existing in the primary damage state for high primary recoil energies. Although the existing data base is limited to fcc materials, the strong similarity in the reported investigations suggests that the same dependence of freely-migrating defect production on primary recoil energy may be characteristic of a wide variety of other alloy systems as well. 52 refs., 4 figs.