The fraction of defects produced in a cascade is between 20-40% of that that is predicted by the NRT model because of intracascade recombination. If the clusters are stable, they may migrate away from the cascade region and can be absorbed at various sinks such as dislocations and grain boundaries. In general, vacancy clus­ters and interstitial clusters should be treated separately. Interstitial clusters are sta­ble, whereas vacancy clusters are not. Interstitial clusters possess higher mobility than their vacancy counterparts.

Incascade clustering is important as it helps promote nucleation of extended defects. Interstitial cluster occurs either in the transition phase between the colli — sional and thermal spike stages or during the thermal spike stage. The probability of clustering is enhanced with increase in the PKA energy with interstitial cluster­ing being predominant, as shown in Figure 6.6.

The structure of clusters is generally a strong function of the crystal structure. In a-Fe (BCC crystal structure), the most stable configuration is small clusters (<10 SIAs), a set of (111) crowdions. The next in stability is the (110) crowdions. As the cluster size grows, only two configurations become stable: (111) and (110). These crowdions can act as the precursor for the formation of perfect interstitial loops. Figure 6.7 shows a few TEM weak beam dark field images of small interstitial type loops in fast neutron irradiated molybdenum (BCC).

In copper (FCC crystal structure), the (100) dumbbell configuration is the stable configuration of the SIA; the smallest cluster may contain only two such dumbbells (di-interstitials). Larger clusters could be a set of (100) dumbbells or a set of (110) crowdions each with {111} habit plane. During growth, the clusters change to faulted Frank loops with Burgers vector (1/3)(111) and to perfect loops with (1 /2)(110). Figure 6.7 shows a few TEM weak beam dark field images of small interstitial type loops in fast neutron irradiated molybdenum.

Clustering of the vacancies occurs within the core of the cascade and the extent of clustering varies with the host lattice. Based on size and density measurements of vacancy clusters, the fraction of vacancies in clusters is estimated to be less than 15%. The stability of vacancy clusters is low relative to the interstitial clusters.

Alpha-Fe (BCC): A set of divacancies on two adjacent {100} planes (that can transform to a dislocation loop of Burgers vector (100)) or a set of first nearest neighbor vacancies on a {110} plane (that can change to a perfect dislocation loop with (1/2)(111) Burgers vector).

Copper (FCC): The most stable configurations are the stacking fault tetrahedron (SFT) and faulted clusters on {111} planes that form Frank loops with Burgers vector (1/3)(111). The binding energy per defect in vacancy cluster is much less than that for interstitial clusters.

Figure 6.9(a), (b) and (c) show the defect structures (consisting of primarily SFTs) in fusion neutron-irradiated gold, silver and copper (all FCC), respectively. The majority of vacancy clusters in FCC metals and alloys have the shape of stacking fault tetrahedra and they appear as white triangles when viewed along the [110] direction with a proper weak beam dark field imaging condition. SFTs are created from Frank dislocation loops and subsequent dislocation reactions. An SFT consists of four triangular {111} planes as faces and 1/6<110> type stair-rod dislocations as six sides.

6.1.2