MD simulations23 predict the absolute level of defect production is not strongly affected by crystal struc­ture. Conversely, electrical resistivity studies of fis­sion neutron-irradiated metals suggest that the overall defect production is highest in HCP metals, intermediate in bcc metals, and lowest in fcc metals,1 1 which suggests that the anisotropic nature of HCP crystals might inhibit defect recombination within displacement cascades. TEM measurements of defect cluster yield (number of visible cascades per incident ion) in ion-irradiated metals have found that the relatively few visible defect clusters are formed directly in displacement cascades in bcc metals,122 whereas cluster formation is relatively efficient in fcc metals and variable behavior is observed for HCP metals.123 Faulted dislocation loops are often observed in irradiated fcc and HCP metals, but due to their high stacking fault energies most studies on irradiated bcc metals have only observed perfect loops.8,16,21,47,124 Since perfect loops are glissile, this can lead to more efficient sweeping up of radiation defects and accelerate the development of dislocation loop rafts or network dislocation structures in bcc materials. Figure 16 shows examples of the disloca­tion loop microstructures in bcc, fcc, and HCP metals with similar atomic weight following electron irradiation at temperatures above recovery Stage III.47 All of the loops are interstitial type with com­parable size for the same irradiation dose. However, significant differences exist in the loop configura­tions, in particular habit planes and faulted (Ni, Zn) versus perfect (Fe) loops. One significant aspect of loop formation in HCP materials is that differential loop evolution on basal and prism planes can lead to significant anisotropic growth.125-129

In general, defect accumulation in the form of void swelling is significantly lower in bcc materials com­pared to fcc materials, although there are notable exceptions where very high swelling rates (approach­ing 3% per dpa)130,131 have been observed in some bcc alloys. Pronounced elastic and point defect diffu­sion anisotropy12 can also suppress void swelling in HCP materials, although high swelling has been observed in some HCP materials such as graphite.1 It has long been recognized that ferritic/martensitic steels exhibit significantly lower void swelling than

austenitic stainless steels.109,133,134Figure 17 com­pares the microstructure of austenitic stainless steel and 9%Cr ferritic/martensitic steel after dual beam ion irradiation at 650 °C to 50 dpa and 260appm He.135 Substantial void formation is evident in the Type 316 austenitic stainless steel, whereas cavity swelling is very limited in the 9%Cr ferritic/marten — sitic steel for the same irradiation conditions. Several mechanisms have been proposed to explain the lower swelling in ferritic/martensitic steel, including lower dislocation bias for SIA absorption, larger critical radii for conversion of helium bubbles to voids, and higher point defect sink strength.