Many experimental studies ofRIS were carried out in the 1970s in model binary or ternary alloys, as well as in more complex and technological alloys (especially in stainless steels). It became apparent quite early on that RIS was a pervasive phenomenon, occurring in many alloys and with any kind of irradiating particle (ions, neutrons, or electrons). Extensive reviews can be found in Russell,1 Holland et al.,2 Nolfi,3 Ardell,4 and Was5: here, we present only the general conclu­sions that can be drawn from these studies.

1.18.2.3.1 Segregating elements

From the previous discussion, it is clear that it is difficult to predict the segregating element in a given alloy because of the competition between sev­eral mechanisms and the lack ofprecise diffusion data (especially concerning interstitial defects). As will be shown in Section 1.18.3, only the knowledge of the phenomenological coefficients Lj provides a reliable prediction of RIS. Nevertheless, on the basis of the body of RIS experimental studies, several general rules have been proposed. In dilute binary AB alloys, thermal self-diffusion coefficients D^ and impurity diffusion coefficients are generally well known, at least at high temperatures. Tracer diffusion or intrinsic diffusion coefficients in some concentrated alloys are also available.34 RIS experiments do not reveal a sys­tematic depletion of the fast-diffusing and enrichment of the slow-diffusing elements near the point defect sinks4,29: this suggests that the IK effect by vacancy diffusion is usually not the dominant mechanism. On the other hand, it seems that a clear correlation exists between RIS and the size effect33; undersized atoms usually segregate at point defect sinks, oversized atoms usually do not. This suggests that interstitial diffusion could control the RIS, at least for atoms with a significant size effect. There are some excep­tions: in Ni-Ge and Al-Ge alloys, the segregation of oversized solute atoms has been observed. Neverthe­less, as pointed out by Rehn and Okamoto,33 no case of depletion of undersized solute atoms in dilute alloys has ever been reported. According to Ardell,4 this holds true even today.