Medium temperature regime: mobile SIAs and vacancies (Stage III

At temperatures where both SIAs and vacancies are mobile, the defect cluster evolution is complex due to the wide range of defect cluster geometries that can be nucleated.8,47,92,93 The predominant visible fea­tures in this temperature regime are vacancy and interstitial loops and SFTs for irradiated fcc mate­rials and vacancy and interstitial loops and voids for irradiated bcc materials. For medium — to high- atomic number fcc metals exposed to energetic displacement cascades (e. g., fast neutron and heavy ion irradiation), most of the vacancies are tied up in

50 nm

Figure 8 Weak beam microstructure of dislocation loops in AlN after 2 MeV Si ion irradiation to ~5 dpa at 80 K. The TEM figure is based on irradiated specimens described in Zinkle etal.91

sessile vacancy clusters (SFTs, vacancy loops) that are formed directly in the displacement cascades. As a consequence, the majority of observed dislocation loops in fcc metals in this temperature and PKA regime are extrinsic (interstitial type), and void nucle — ation and growth is strongly suppressed. For bcc metals, the amount of in-cascade clustering into ses­sile defect clusters is less pronounced, and therefore, vacancy loop and void swelling are observed in addi­tion to interstitial dislocation loop evolution. Due to the typical high sink strength of interstitial clusters in this temperature regime, the magnitude of void swelling is generally very small (< 1% for doses up to 10 dpa or higher). The loop density and nature in bcc metals is strongly dependent on impurity content in this temperature regime.5,8,55 For example, the loop concentration in molybdenum irradiated with fission neutrons at 200 °C is much higher in low-purity Mo with ^99% of the loops identified as interstitial type, whereas ^90% of the loops were identified to be vacancy type in high-purity Mo irradiated under the same conditions.8

The dose dependence of defect cluster accumula­tion in this temperature regime is dependent on the material and defect cluster type. For dislocation loops and SFTs in fcc metals, the defect accumulation is initially linear and may exhibit an extended inter­mediate regime with square root kinetics before reaching a maximum concentration level. The maxi­mum defect cluster density is largely determined by displacement cascade annihilation of preexisting defect clusters. In fcc metals, the defect cluster density may approach 1024m~3, which corresponds to a defect cluster spacing of less than 10 nm and is approximately equal to the maximum diameter of subcascades during the collisional phase in neutron- irradiated metals. As with irradiation near recovery Stage II, the critical dose for transition in defect cluster accumulation kinetics is dependent on the overall defect sink strength. With continued irradia­tion, the loops may unfault and evolve into network dislocations, particularly if external stress is applied. Figure 9 summarizes the dose-dependent defect cluster densities in neutron-irradiated copper and nickel.94-96 In both of these materials, the predomi­nant visible defect cluster was the SFT over the entire investigated dose and temperature regime. Depending on the purity of the copper investigated, the transition from linear to square root accumula­tion behavior may or may not be evident (cf. the differing behavior for Cu in Figure 9). The visible defect cluster density in irradiated copper reaches a constant saturation value (attributed to displacement cascade overlap with preexisting clusters) for damage levels above ~0.1 dpa. The lower visible defect clus­ter density in Ni compared to Cu at doses up to 1 dpa has been attributed to a longer thermal spike lifetime of the Cu displacement cascades due to inefficient
coupling between electrons and phonons (thereby promoting more complete vacancy and interstitial clustering within the displacement cascade).97,98

Figure 10 compares the defect cluster accumula­tion behavior for two fcc metals (Cu, Ni) and two bcc metals (Fe, Mo) following fission neutron irra-

30,95,96,99-101

diation near room temperature. For all

four materials, the increase in visible defect cluster density is initially proportional to dose. The visible defect cluster density is highest in Cu over the

Figure 10 Defect cluster density in copper, nickel, molybdenum, and nickel following fission reactor and 14-MeV neutron irradiation near room temperature, as measured by TEM. Based on data reported by Kiritani30, Hashimotoefa/.95,96, Eldrup etal. 99, Zinkleand Singh100, and Li eta/.101

investigated damage range of 10~4-1dpa. The irra­diated Fe has the lowest visible density at low doses, whereas Ni and Mo have comparable visible cluster densities. At doses above ^0.01 dpa, the visible loop density in Mo decreases due to loop coalescence in connection with the formation of aligned ‘rafts’ of loops. Partial formation of aligned loop rafts has also been observed in neutron-irradiated Fe for doses near 0.8 dpa, as shown in Figure 11.100 The individual loops within the raft aggregations in neutron — irradiated Fe exhibited the same Burgers vector. The maximum visible cluster density in the fcc metals is about one order of magnitude higher than in the bcc metals (due in part to loop coalescence associated with raft formation). Positron annihilation spectros­copy analyses suggest that submicroscopic cavities are present in the two irradiated bcc metals, with cavity densities that are about two orders of magnitude higher than the visible loop densities.99-