Mechanical deformation of metals and alloys after irradiation at temperatures below recovery Stage V produces deformation microstructures that typically evolve from predominantly dislocation cell micro­structures in the unirradiated and low-dose irradiated conditions to a variety of localized deformation microstructures above a threshold damage level including twinning, planar dislocation deformation, and formation of dislocation channels.314-316 Forma­tion of cleared dislocation channels has been sug­gested to be the cause of low uniform elongations observed in tensile tests of metals and alloys irradiated at temperatures below recovery Stage V,221,317 and dislocation channeling is frequently observed follow­ing deformation of irradiated materials that exhibit low uniform elongation.95,96,100,312,316’318-321 An alter­native mechanism for the low uniform elongations in irradiated materials, based on a material-specific threshold stress for plastic instability, has also been proposed.216,322-324 The spacing between dislocation channels is typically on the order of 1 pm, and the width of the individual channels ranges from ^20 to 200 nm. Localized deformation visible as surface slip steps in irradiated copper following tensile straining has been directly correlated with cleared dislocation channels.325 The matrix regions between the cleared channels do not exhibit evidence of substantial dislo­cation activity, suggesting that all of the dislocation motion associated with deformation is restricted to the dislocation channel regions. Figure 35 shows an example of cleared dislocation channels observed in austenitic stainless steel following fission neutron

irradiation to 0.78 dpa near 80 °C and subsequent uniaxial tensile deformation to 32% strain.326

The mechanisms responsible for annihilation of SFTs by gliding dislocations within the dislocation channel include stress-induced collapse to triangle loops, multiple shear, partial annihilation with a rem­nant apex, collapse to a triangle loop or complete annihilation with multiple super jogs, and complete annihilation by screw dislocations followed by cross slip.327-329 Computer simulations of dislocation loop interactions with gliding dislocations suggest multi­ple potential mechanisms that could lead to defect — cleared dislocation channels, including absorption, unfaulting, and shear of the loops.330-333 Detailed experimental confirmation of these annihilation mechanisms is still needed.