Segregation concentration profiles induced by irradi­ation display some specific features. They can spread over large distances — a few tens of nanometers (see examples in Russell1 and Okamoto and Rehn29) — while equilibrium segregation is usually limited to a few angstroms. This is due to the fact that they result from a dynamic equilibrium between RIS fluxes and the back diffusion created by the concentration gra­dient at the sinks, while the scale of equilibrium segregation profiles is determined by the range of atomic interactions. Equilibrium profiles are usually monotonic, except for the oscillations, which can appear — with atomic wavelengths — in alloys with ordering tendencies.35 Segregation profiles observed in transient regimes are often nonmonotonic because of the complex interaction between concentration gradients of point defects and solutes. A typical example is shown in Section 1.18.5.3, where an enrichment of solute is observed near a point defect sink, followed by a smaller solute depletion between the vicinity of sink and the bulk. In this particular case, the depletion is due to a local increase in vacancy concentration, which results from the lower interstitial concentration and recombination rate.

Other kinds of nonmonotonic profiles are some­times observed, with typical ‘W-shapes.’ In some austenitic or ferritic steels, a local enrichment of Cr at grain boundaries survives during the Cr depletion induced by irradiation (see below). This could result from a competition between opposite equilibrium and RIS tendencies. However, the extent of the Cr enrichment often seems too wide to be simply due to an equilibrium property (around 5 nm, see, e. g., Sections 1.18.2.5 and 1.18.5.3).

RIS profiles at grain boundaries are sometimes asymmetrical, which has been related to the migra­tion of boundaries resulting from the fluxes of point defects under irradiation.37,38 The segregation is affected by the atomic structure and the nature of the sinks. It has been clearly shown that RIS in

austenitic steels is much smaller at low angles and special grain boundaries than at large misorientation angles,39’40 the latter being much more efficient point defect sinks than the former.

1.18.2.3.2 Temperature effects

RIS can occur only when significant fluxes of defects towards sinks are sustained, which typically happens only at temperatures between 0.3 and 0.6 times the melting point. At lower temperatures, vacancies are immobile and point defects annihilate, mainly by mutual recombination. At higher temperatures, the equilib­rium vacancy concentration is too high; back diffusion and a lower vacancy supersaturation completely sup­press the segregation. Temperature can also modify the direction ofthe RIS by changing the relative weight of the competing mechanisms, which do not have the same activation energy. In Ni—Ti alloys, for exam­ple, the enrichment of Ti at the surface below 400 °C has been attributed to the migration of Ti—V com­plexes, and the depletion observed at higher tempera­tures should result from a vacancy IK effect.41