4.04.4.1 Dislocation Structures

Dislocation structures in irradiated pressurized tube samples were examined by Gelles et al.72 The materi­als which were examined included stressed and unstressed samples of ST PE16, and stressed samples of ST and STA Inconel 706. A subsequent paper by Gelles73 extended these investigations to the stressed samples of PE16 in STA and OA conditions. Further details of this work were also provided by Garner and Gelles74, and by Gelles.70

Examination of ST PE16, which was irradiated at 550 °C to 2 x 1026nm~2 (E> 0.1 MeV) at hoop stres­ses of 0 and 167 MPa, revealed that the distribution of Frank dislocation loops was similar on all the four {111} planes in the unstressed sample but was aniso­tropic in the stressed material. In the stressed sample, the loop density on any particular {111} plane increased with increasing magnitude of the normal stress component on that plane. A near-linear rela­tionship between the loop density and the normal component of the deviatoric stress tensor, OdN (= sN — Oh, where sN is the normal component of the applied stress on a particular plane and sH is the hydrostatic stress), was found for PE16. This result is in line with the SIPA loop growth model described by Garner eta/.75 No such correlation was found in the similarly irradiated and stressed Inconel 706 samples, however, possibly because the low creep rate of this material at 550 °C did not allow the relaxation of internal stresses.

Unfaulting of Frank dislocation loops with a/3 {111} Burgers vectors proceeds via interaction with a/6{112} Shockley partials to produce perfect a/2 {110} line dislocations. Gelles70 described how this occurs via a two-step process, with the necessary partial dislocations (two per interstitial loop) first being nucleated by an interaction of the faulted loop with a suitable perfect dislocation and then sweeping across the loop to reestablish the perfect dislocation. Gelles73 examined the distribution of Burgers vectors among the six possible a/2{110} perfect dislocation types in irradiated pressur­ized tube samples of PE16. The samples examined included the stressed ST condition irradiated at 550 °C, and STA and OA conditions which were both irradiated at 480 °C to a fluence of 8 x 1026nm—2 at a hoop stress of 331 MPa. The results showed highly anisotropic distributions in the Burgers vectors of perfect dislocations in all the three heat-treated conditions, with dislocation den­sities of the various types differing by factors of up to 10-40 in each sample. The level of anisotropy pro­duced in the population of perfect dislocations was significantly greater than in the dispersion of Frank loops. This is a feasible outcome since, in principle, all loops may be unfaulted by just two variants of the six a/2{110} perfect dislocation types. In effect, the development of anisotropic dislocation structures is a response of the material to produce the strain which is required to accommodate the applied stress. Furthermore, it was found that the perfect disloca­tions in the irradiation creep samples of PE16 were primarily of edge type lying on {100} planes rather than {111} slip planes, indicating that they could only contribute to the creep strain via climb (i. e., by the SIPA mechanism) and not by processes involving dislocation glide.