A wide range of defect cluster morphologies can be created by particle irradiation.8,21,22 The thermody­namic stability of these defect cluster geometries is dependent on the host material and defect cluster size as well as the potential presence of impurities. There are four common geometric configurations for clusters of vacancies and self-interstitial atoms (SIAs): two planar dislocation loop configurations (faulted and perfect loops) that occur for both vacancies and SIAs, and two three-dimensional configurations that occur only for vacancy clusters (the stacking fault tetrahedron, SFT, and cavities).

The faulted loop (also called Frank loop) is most easily visualized as either insertion or removal of a layer of atoms, creating a corresponding extrinsic or intrinsic stacking fault associated with condensation of a planar monolayer of vacancies and SIAs, respec­tively. The faulted loop generally forms on close packed planes, i. e., {111} habit planes with a Burgers vector of b = 1/3(111) for face-centered cubic (fcc) materials, {110} habit planes with b = 1/2(110) for body-centered cubic (bcc) metals, and {1010} habit planes with b = a/2 (1010) for hexagonal close packed (HCP) metals.23 Faulted loops with b = a/2 [0001] on the (0001) basal plane are also observed in many irradiated HCP materials. All of these faulted loops are immobile (sessile). The high stacking fault energy of bcc metals inhibits faulted loop nucle — ation and growth, and favors formation of perfect loops. There have been several observations of faulted loops consisting of multiple atomic layers.8,21

The perfect loop in fcc materials is typically created from initially formed faulted loops by nucleation of an a/6(112) Shockley partial dislocation that sweeps across the surface of the faulted loop and thereby restores perfect stacking order by this atomic shear of one layer of atoms. The resultant Burgers vector in fcc materials is a/2 (110), maintaining the {111} loop habit planes. After unfaulting, rotation on the glide cylinder gradually changes the habit plane of the fcc perfect loop from {111} to {110} to create a pure edge loop geometry. After the loop rotates to the {110} habit plane, the perfect loop is glissile. Experimental studies of irradiated fcc materials typically observe perfect loops on either {111} or {110} habit planes (or both), depending on the stage of the glide cylinder rotation process. The glissile perfect loop configurations for bcc materials consist of b = a/2 (111) loops on {111} habit planes and b = a( 100) loops on {100} habit planes. The typical corresponding HCP perfect loop configura — tionis b = a/3 (1120) on {1120} prismatic habit planes.

SFTs are only observed in close-packed cubic structures (i. e. fcc materials). The classic Silcox — Hirsch24 mechanism for SFT formation is based on dissociation of b = 1/3(111) faulted loops into a/6 (110) stair rod and a/6(121) Shockley partial dis­locations on the acute intersecting {111} planes. Interaction between the climbing Shockley partials creates a/6 (011) stair rod dislocations along the

tetrahedron edges. The Silcox-Hirsch mechanism has been verified during in situ transmission electron microscope (TEM) observation of vacancy loops in quenched gold.25 Evidence from molecular dynamics (MD) simulations26-29 and TEM observations12’19’30-32 during in situ or postirradiation studies indicate that SFT formation can occur directly within the vacancy — rich cascade core during the ‘thermal spike’ phase of energetic displacement cascades.

There is an important distinction between the defi­nitions for the terms void’ bubble’ and cavity’ all of which describe a three-dimensional vacancy cluster that is roughly spherical in shape. Void refers to an object whose stability is not dependent on the presence of internal pressurization from a gaseous species such as helium. Bubbles are defined as pressurized cavities. The term cavity can be used to refer to either voids or bubbles and is often used as a generic term for both cases. In many cases, voids exhibit facets (e. g. truncated octahedron for fcc metals) that correspond with close — packed planes of the host lattice’ whereas bubbles are generally spherical in shape. However, the absence of facets cannot be used as conclusive evidence to dis­criminate between a void and a bubble.

Figure 1 shows the calculated energy for different vacancy geometries in pure fcc copper.22 The SFT is calculated to be the most energetically favorable configuration in copper for small sizes (up to about 4 nm edge lengths). Faulted loops are calculated to be stable at intermediate sizes, and perfect loops are calculated to be most stable at larger sizes. In practice,
many metastable defect cluster geometries may occur. For example, it is well established that the transition from faulted to perfect loops is typically triggered by localized stress such as physical impin­gement of adjoining loops, and not simply by loop energies; the activation energy barrier for unfaulting may be on the order of 1 eV atom-1.8 Similarly, large activation energy barriers exist for the conversion between planar loops and voids.33