We have already observed the effect of fluence in Figure 6.18 on the void swelling behavior of austenitic stainless steels like 316 type. It has been observed that the swelling extent increases with increasing neutron fluence (or dpa) and empirical relations have also been developed. It has been noted that there exists an incubation period that represents the neutron dose needed to produce enough helium and point defect concentration to make void nuclea — tion possible. The incubation period may also be needed to develop enough number of interstitial dislocation loops to allow the preferred absorption of interstitials by dislocations to sufficiently bias the point defect population in the material in favor of vacancies as to permit vacancy agglomeration into voids. However, incubation period does depend on temperature and material/ microstructure. Figure 6.20 shows a generalized form of void swelling behav­ior as a function of dose or fluence. The initial transient period is followed by a steady-state swelling period. This steady-state period for FCC-based materials continues to proceed with no sign of saturation. However, void swelling in BCC-based metals/alloys (like F-M steels) shows saturation effect. In these systems, at higher radiation doses, voids/bubbles self-organize following the crystallography of the metal leading to void/bubble lattices.

6.1.3

Radiation-Induced Segregation

Radiation-induced segregation (RIS) involves segregation of alloying elements under fast particle irradiation to certain microstructural locations leading to a situa­tion where otherwise homogeneous alloys become heterogeneous. During fast par­ticle irradiation, significant diffusion fluxes of point defects (vacancies and interstitials) can be set up in the vicinity of the defect sinks like surfaces or internal grain boundaries. Generally, we have come to know from Section 2.3 that the differ­ent atomic species in an alloy migrate at different rates in response to the already set up point defect fluxes so that some species travel toward the said defect sinks while others move away. Thus, RIS can lead to significant alterations in the local composition near the sinks like grain boundaries, and this can have substantial bearing on the macroscopic properties of the materials. This phenomenon has been studied in some detail, particularly in structural materials used in nuclear reactor components. Understanding RIS is of particular importance in chromium containing austenitic stainless steels or nickel base superalloys because these alloys are used in commercial power reactors and potential candidate materials for advanced reactors. RIS can potentially lead to irradiation-induced stress corrosion cracking as chromium segregates away from the grain boundaries where it is most needed. At low temperatures, defect concentration builds up and rather than going to the sinks, point defects tend to recombine. At higher temperatures, thermal dif­fusion dominates and composition becomes equilibrated or homogeneous. At intermediate temperatures, RIS becomes acute due to the operation of a process known as “inverse Kirkendall” effect. Figure 6.21 shows the composition profile in an irradiated 300 series stainless steel, analyzed by energy dispersive spectroscopic measurement conducted with a JEOL 2010F high-resolution transmission electron microscope, showing depletion of Cr and enrichment of Ni, Si, P at the grain boundary due to RIS.

6.1.4