1.03.5.1 Dislocation Loop Formation

A common feature in many irradiated metals and nonmetals at temperatures between recovery Stage III and Stage V is dislocation loop formation (either perfect or faulted), with typical loop diameters rang­ing from ^2 to ^100 nm. Both vacancy (intrinsic) and interstitial (extrinsic) loops are frequently observed in irradiated materials. The dislocation loop shape is frequently circular (in order to minimize dislocation line length), but rhombus, square, hexagonal, or other shapes have been observed in some materials due to elastic energy considerations.21Figure 31 shows an example of circular faulted interstitial-type disloca­tion loop formation in MgAl2O4 due to ion irradia­tion at 650 °C. The parallel fringes visible in the loop

Figure 31 Faulted interstitial-type dislocation loop formation in MgAl2O4 irradiated with 2 MeV Al+ ions at 650°C to 14 dpa. The image was taken with a beam direction near [101] using weak beam dark field (g, 6g), g = 202 diffraction imaging conditions (data from

S. J. Zinkle, unpublished research).

500 nm

Figure 32 Defect cluster patterning into aligned {001} walls in single crystal copper irradiated with protons at 100 °C to 2 dpa. Reproduced from Jager, W.;

Trinkaus, H. J. Nucl. Mater. 1993, 205, 394-410.

interiors are a signature of the stacking fault and are visible in TEM by selecting the appropriate diffrac­tion imaging conditions. Faulted loop formation is energetically unfavorable in most bcc materials due to their high stacking fault energies, although there is some evidence for formation of small faulted loops in some cases.224 Experimental studies using energetic ion beams at cryogenic temperatures (where long range point defect migration does not occur) have obtained convincing evidence for direct formation of visible defect clusters directly within displacement cascades above a threshold energy value.294 Dislocation loop formation is usually ran­domly distributed on the relevant habit planes, with no pronounced spatial correlation. In some cases where mechanical or radiation-induced stresses are present, significant anisotropy occurs regarding the habit planes for loop formation.295,296 Within a limited temperature and damage rate regime, the dislocation loop microstructure in some materials also exhibits a tendency to self-organize into aligned walls.297-299 Figure 32 shows an example of well-developed defect cluster patterning in pure copper following proton irradiation to 2 dpa.298 The defect clusters within the walls consist of SFTs and small dislocation loops.