Solute atoms of importance include elements origi­nally added to the material during fabrication and species produced by nuclear transmutation reactions (e. g., He and H, and a range of other elements). Solute atoms may exhibit preferential coupling with point defects created during irradiation, leading to either enhancement or depletion of solutes at point defect sink structures such as dislocations, grain boundaries, preexisting precipitates, and voids.193-198 The solute-defect coupling can modify the kinetics for point defect diffusion, and the resultant solute enrichment or depletion may sufficiently modify the local composition to induce the formation of new phases. There are three general categories of precip­itation associated with radiation-induced segregation processes10 , : radiation-induced (phases that form due to irradiation-induced nonequilibrium solute segregation and dissolve during postirradiation annealing), radiation-enhanced (precipitate forma­tion accelerated or occurring at lower temperatures due to irradiation, but are thermally stable after formation), and radiation-modified (different chemi­cal composition of precipitates compared to thermo­dynamically stable composition). In some materials,
radiation-retarded precipitation (phase formation shifted to higher temperatures or longer exposure times) has been reported.2

A phenomenon that is uniquely associated with ion irradiation is the potential for the ions from the irra­diating beam to modify the microstructural evolution by perturbing the relative balance of SIAs compared to vacancies flowing to defect sinks. The injected ions act as a source of additional interstitial atoms and can significantly suppress void nucleation and growth.149,154,201,202 The peak concentration of the injected ions occurs near the displacement damage peak for ion irradiation, and therefore considerable care must be exercised when evaluating the void swelling data obtained near the peak damage region in ion-irradiated materials.15 ,201,202 Figure 22 shows an example ofthe dramatic changes in microstructure that can occur in the injected ion region.203 In this example, void formation in ion-irradiated nickel at 400 °C is completely suppressed in the regions with the injected interstitials and the void microstructure is replaced with an aligned array of small interstitial — type dislocation loops.

Numerous studies have observed that the precipita­tion behavior during irradiation can strongly influence microstructural evolution, for example, the swelling behavior of austenitic stainless steels.103,106,204-206

_i_________________ і________________ і_________________ і———————— >————————- 1——————— — j— ———————- 1—

0 1 2 3 a) Voids

Depth (pm) b) Random loops

and voids
c) Ordered loops

Figure 22 Cross-section TEM microstructure of nickel irradiated at 400 °C with 14 MeVCu ionstoafluenceof 5 x 1020 ions m~2 which produced a peak damage level of about 55 dpa at a depth near 2 pm. Void formation is completely suppressed in the injected interstitial regime (~1.3-2.8 pm) and the void microstructure is replaced with an array of small interstitial-type dislocation loops aligned along {100} planes. Reproduced from Whitley, J. B. Depth dependent damage in heavy ion irradiated nickel. University of Wisconsin, Madison, 1978.

0.4dpa/0.2 appm He/675 0C 109dpa/2000 appm He/675 0C

Figure 23 Comparison of the cavity microstructures for a pure Fe-13Cr-15Ni austenitic alloy (left panel) and the same alloy with P, Si, Ti, and C additions that produced dense radiation-induced phosphide precipitation (center and right panels) following dual beam Ni + He irradiation at 675 °C. The irradiation conditions were 0.4 dpa and 0.2 appm He for the left panel (70 dpa and 35 appm for the inset figure), and 109 dpa and 2000 appm He for the other two figures. Reproduced from Mansur, L. K.; Lee, E. H. J. Nucl. Mater. 1991, 179-181, 105-110.

In extreme cases, large-scale phase transformations can occur such as the g (austenite, fcc) to a (ferritic, bcc) transformation in austenitic stainless steel following high dose neutron irradiation.106,207 Depending on the type of precipitation, either enhanced or suppressed swelling can occur. Void swelling enhancement has generally been attributed to a point defect collector mechanism and typically occurs for moderate densities of relative coarse precipitates such as G phase in austenitic stainless steels, whereas void swelling suppression is generally observed for high densities of finely dispersed precipi­tates and is usually attributed to high sink strength effects.103,1 1,208 Figure 22 shows an example of the strong void swelling suppression associated with forma­tion of radiation-induced Si — and Ti-rich phosphide precipitates compared to a simple Fe-Cr-Ni ternary austenitic alloy.208 Similarly, the He/dpa ratio can influence the types and magnitude ofpoint defect clus­ters and precipitation due to modifications in the point defect evolution under irradiation (Figure 23).106