RIS can be observed for very small irradiation doses; an enrichment of ~10% of Si has been measured, for example, at the surface of an Ni—1%Si alloy, after a dose of 0.05 dpa at 525 °C.32 Such doses are much lower than those required for radiation swelling5 or ballistic disordering effects.42

Increasing the radiation flux, or dose rate, directly results in higher point defect concentrations and fluxes towards sinks. The transition between RIS regimes is then shifted toward a higher temperature. But because point defect concentrations slowly evolve with the radiation flux (typically, proportional to its square root43 in the temperature range where RIS occurs), a high increase is needed to get a signif­icant temperature shift.

Radiation dose and dose rate are usually estimated in dpa and dpas~ , respectively, using the Norgett, Robinson, and Torrens model,44 especially when a comparison between different irradiation conditions is desired. It is then worth noting that the amount of RIS observed for a given dpa is usually larger during irradiation by light particles (electrons or light ions) than by heavy ones (neutrons or heavy ions). In the latter case, point defects are created by displacement cascades in a highly localized area, and a large frac­tion of vacancies and interstitials recombine or form

0. 8 0.6

0.4

P:

0.2 0.0

10-6 10-5 10-4 10-3 10-2

K0 (dpas-1)

Figure 3 Temperature and dose rate effect on the radiation-induced segregation.

point defect clusters. The fraction of the initially produced point defects that migrate over long dis­tances and could contribute to RIS is decreased. On the contrary, during irradiation by light particles, Frenkel pairs are created more or less homo­geneously in the material, and a larger fraction sur­vive to migrate (Figure 3).45

1.18.2.3.3 Impurity effects

The addition of impurities has been considered as a possible way to control the RIS in alloys, for example, in austenitic steels. The most common method is the addition of an oversized impurity, such as Hf and Zr, in stainless steels,46 which should trap the vacancies (and, in some cases, the interstitials), thus increasing the recombination and decreasing the fluxes of defects towards the sinks.