Effects of the primary recoil spectrum on microstructural evolution

Abstract

For quantitative predictions and comparisons of microstructures that evolve during exposure to different radiation environments at elevated temperature one needs to develop methods that go beyond those based on the number of displacements per atom. The number of freely migrating defects that contribute to the microstructural development is far less than the total number of defects produced, as has been recognized for some time from measurements of radiation-induced segregation and of radiation-enhanced diffusion. One major reason for the small amount of defects available for long range migration is the high concentration and close spatial correlation of vacancies and, to a somewhat lesser degree, of interstitials in cascades produced by high energy knock-ons. As a consequence, many defects either recombine or form immobile defect clusters during the defect formation and cooling phases of the cascades. After doses exceeding a few tenths of a displacement per atom, the residue of small clusters and dislocation loops of vacancy type remaining in the central portions of energetic cascades and subcascades, is the second major reason for the reduction of the mean free path of defects between creation and annihilation. Defect production in various neutron and ion irradiation environments is discussed in light of these facts. A method to calculate the fraction of freely migrating defects from the cluster size distribution of defects produced in cascades is suggested. The results are in good agreement with available data.