Radiation-Induced Segregation

Abstract

A profound consequence of irradiation at elevated temperature is the spatial redistribution of solute and impurity elements in the metal. This phenomenon leads to the enrichment or depletion of alloying elements in regions near surfaces, dis-locations, voids, grain boundaries, and phase boundaries. Figure 6.1 shows a plot of solute element profiles across a grain boundary in stainless steel irradiated in a reactor to a dose of several dpa at about 300 °C. There is significant depletion of chromium, molybdenum, and iron and enrichment of nickel and silicon. Such drastic changes at the grain boundary will cause changes in the local properties of the solid and may induce susceptibility to a host of processes that can degrade the integrity of the component. For this reason, understanding radiation-induced segregation (RIS) is of great importance for reactor performance. First postulated by Anthony in 1972 [2], and observed by Okamoto and Weidersich in 1973 [3], RIS has its origin in the coupling between defect fluxes and fluxes of alloying elements. Irradiation produces point defects and defect clusters with an approximately random distribution throughout the material. Those defects that are mobile and escape recombination are reincorporated into the crystal structure at dislocations, grain boundaries, and other defect sinks. As shown in Chap. 5, point defects flow to spatially discrete sinks. Since the motion of atoms is by way of defects, atom fluxes are associated with defect fluxes. Any preferential association of defects with a particular alloying component and/or preferential participation of a component in defect diffusion will couple a net flux of the alloying element to the defect fluxes. The flux of an element causes its buildup or depletion in the vicinity of defect sinks and, therefore, concentration gradients in initially homogeneous alloy phases. The concentration gradients induce back diffusion of the segregating elements, and a quasi-steady state may be set up during irradiation whenever the defect-driven alloying element fluxes are balanced by diffusion-driven back diffusion. Figure 6.2 presents a schematic of processes driving segregation in a binary, 50 % A-50 %B alloy, under irradiation at an elevated temperature. As described in Chap. 5, vacancies and interstitials flow to the grain boundary causing a concentration profile to develop. In case of vacancies (Fig. 6.2(a)), the flux of vacancies to the grain boundary is balanced by an equal flux of atoms in the opposite direction. However, if the participation of A atoms in the vacancy flux is larger than the atom