The typical microstructural features that appear dur­ing irradiation at temperatures above recovery Stage V include dislocation loops (vacancy and interstitial type), network dislocations, and cavities. SFTs are thermally unstable in this temperature regime and therefore only SFTs created in the latter stages of the irradiation exposure are visible during postirradiation examination.94 A variety of precipitates may also be nucleated in irradiated alloys.11,103-106 Defect cluster accumulation in this temperature regime exhibits

several different trends. The visible SIA clusters evolve from a low density of small loops to a saturation density of larger loops after damage levels of ~1-10dpa. Upon continued irradiation, a moderate density of network dislocations is created due to loop unfault­ing and coalescence. The dislocation loop and net­work dislocation density monotonically decrease with increasing temperature above recovery Stage V,20,107 whereas the density of precipitates (if present) can either increase or decrease with increasing temperature.

The major microstructural difference from lower temperature irradiations in most materials is the emer­gence of significant levels of cavity swelling. After an initial transient regime associated with cavity nucle — ation, a prolonged linear accumulation of vacancies into voids is typically observed.10 ,109 The cavity den­sity monotonically decreases with increasing tempera­ture in this temperature regime. figure 12

summarizes the densities of voids and helium bubbles (associated with n, a transmutations) in austenitic stain­less steel as a function of fission reactor irradiation temperature for damage rates near 1 x 10~6dpas~12° The bubble and void densities exhibit similar tem­perature dependences in fission reactor-irradiated austenitic stainless steel, with the bubble density ap­proximately one order of magnitude higher than the void density between 400 and 650 °C. For neutron-
irradiated copper and Cu-B alloys, the bubble den­sity is similarly observed to be about one order of magnitude larger than the void density for tem­peratures between 200 and 400 °C.107,110 At higher temperatures, the void density in copper decreases rapidly and becomes several orders of magnitude smal­ler than the bubble density. The results from several studies suggest that the lower temperature limits for formation of visible voids111-113 and helium bubbles53 can each be reduced by 100 °C or more when the damage rate is decreased to 10~9-10~8 dpas-1, due to enhanced thermal annealing of sessile vacancy clusters during the time to achieve a given dose. Dose rate effects are discussed further in Section

1. 03.3.7.

The void swelling regime for fcc materials typically extends from 0.35 to 0.6 TM, where TM is the melting temperature, with maximum swelling occurring near 0.4-0.45 TM for typical fission reactor neutron damage rates of 10~6dpas~192,114 Figure 13 summarizes the temperature-dependent void swell­ing for neutron-irradiated copper.110 The results for a neutron-irradiated Cu-B alloy, where ^100 atomic parts per million (appm) He was produced during the 1 dpa irradiation due to thermal neutron trans­mutation reactions with the B solute, are also shown in this figure.107 For both materials the onset

0.7 0.6 0.5

g

0.4

c

03

-C

О

°.з

c

Ф

Q

0.2 0.1 0

150 200 250 300 350 400 450 500 550

Irradiation temperature (°C)

Figure 13 Temperature-dependent void swelling behavior in neutron-irradiated copper and Cu-B alloy after fission neutron irradiation to a dose near 1.1 dpa. Adapted from Zinkle, S. J.; Farrell, K.; Kanazawa, H. J. Nucl. Mater. 1991, 179-181, 994-997; Zinkle, S. J.; Farrell, K. J. Nucl. Mater. 1989, 168, 262-267.

of swelling occurs at temperatures near 180 °C, which corresponds to recovery Stage V in Cu for the 2 x 10~7dpa s-1 damage rates in this experiment.

The swelling in Cu was negligible for temperatures above ^500 °C, and maximum swelling was observed near 300 °C. The lower temperature limit for swelling in fcc materials is typically controlled by the high point defect sink strength of sessile defect clusters below recovery Stage V. The upper temperature limit is controlled by thermal stability of voids and a reduction in the vacancy supersaturation relative to the equilibrium vacancy concentration.

As noted by Singh and Evans,92 the temperature dependence of the void swelling behavior of bcc and fcc metals can be significantly different. In par­ticular, due to the lower amount of in-cascade forma­tion of large sessile vacancy clusters in medium-mass bcc metals compared to fcc metals, the recovery Stage V is much less pronounced in bcc metals. The presence of a high concentration of mobile vacancies at temperatures below recovery Stage V (and a concomi­tant reduction in the density of sessile vacancy-type defect cluster sinks) allows void swelling to occur in bcc metals for temperatures above recovery Stage III (onset of long-range vacancy migration). Figure 14 compares the temperature dependence of the void swelling behavior of Ni (fcc) and Fe (bcc) after high dose neutron irradiation.11 Whereas the peak

occur near 0.3-0.35 TM,116,117 which is much lower than the 0.4-0.45 TM peak swelling temperature ob­served for fcc metals.