Several different cavity geometries are created in irradiated materials. For helium-filled bubbles, the cavity shape is typically spherical. For voids, faceted cavities with faces corresponding to low-index crys­tallographic planes are often created, e. g. truncated {111} octahedra or {001} cubes in fcc materials, truncated {110} dodecahedra or {001} cubes in bcc materials, and more complex shapes in HCP materials,21,22,340-342 although nearly spherical shapes are also sometimes observed for voids. When helium is generated during irradiation (due to neutron — induced transmutation reactions, etc.), a bimodal cav­ity distribution is usually observed with the small cavities corresponding to helium-filled bubbles and the large cavities corresponding to underpressurized voids. The critical radius transition between bubbles and voids is determined by a balance between disloca­tion bias-induced vacancy influx and pressure-modified thermal emission of vacancies.120,151,208,274,343 Figure 38
shows an example of large faceted voids and small helium-filled spherical bubbles in a neutron — irradiated copper-boron alloy.107 The visible cavity density usually increases rapidly at low doses, and approaches a constant value for damage levels above ~ 1—50 dpa. The void size tends to increase continu­ously with increasing dose.

Well-developed periodic void lattices have been observed in several irradiated materials.297,344,345 Void lattice formation has most frequently been observed in bcc materials, but periodically aligned void struc­tures have also been observed in HCP272,346,347 and fcc303,348-351 materials. Aligned voids have been observed in both metals and ceramic insulators. The aligned cavities in HCP materials are usually mani­fested as one — or two-dimensional arrays perpendicu­lar or parallel to the basal plane, respectively.297,346 The void lattices in bcc and fcc materials adopts the same three-dimensional crystallographic symmetry as the host lattice.297 The swelling levels in bcc metals with well-developed void lattices are typically a few percent, which has led to hypotheses that void lattice formation may coincide with a cessation in steady — state swelling.117,352 The saturation in void swelling is associated with achieving a constant average void size. Figure 39 shows an example of a well-developed bcc void lattice in ion-irradiated Nb—1Zr.353 In the study by Loomis et al. it was reported that void lattice formation did not occur unless a threshold level of oxygen was present (60—2700 appm oxygen, depending on the irradiated material).

1.03.5 Summary

Radiation-induced microstructural modifications can create large changes in the physical and mechanical properties of materials, as detailed in accompanying chapters in this book. The two most important extrin­sic variables that influence microstructural evolution under irradiation are the radiation damage level and temperature. Many similarities are observed for diverse materials and irradiation spectra if the com­parisons are performed at comparable damage levels and defect mobility regimes (defect recovery stages). The PKA energy often exerts a significant influence on the microstructural evolution, in particular by inducing direct cascade amorphization or creation of defect clusters within displacement cascades when the PKA energy exceeds a threshold energy value. Numerous other parameters such as dose rate, crystal structure, and atomic weight typically exert less pronounced influence on microstructural evolu­tion, although very large qualitative and quantitative effects can be observed under some circumstances.

Acknowledgments

Much of the author’s work discussed in this chapter was sponsored by the U. S. Department of Energy, Office of Fusion