Annealing of Voids in Aluminum.

Performance of Aluminum in Research Reactors

“…Another attractive feature is that pure aluminum has no long-lived radioisotopes. The major source of immediate radioactivity is from decay of 24 Na produced via 27 Al(n,a) 24 Na, decaying by g-emission with a half-life of 15 h. In alloys, long-lived radioactivity arises from decay of isotopes produced from alloying elements and residual impurity elements present in the aluminum, primarily 65 Zn, 51 Cr, 59 Fe, with half-lives of 250, 28, and 45 days respectively. So if low residual radioactivity is an objective it can be met to a large extent by avoiding alloys containing significant quantities of Zn, Cr, and Fe.…”

“…It probably marks the position of a grown-in dislocation present during implantation that has climbed away during irradiation. At higher irradiation temperatures 58 or during postirradiation anneals, 59 to estimate the levels of He and H produced in Al, Fe, and Zr during a 1-year exposure in the high flux peripheral target positions of the HFIR core. There, on the horizontal mid-plane at current operating conditions, the annual fast neutron fluence will be 2.4 Â 10 26 n m À2 , E > 0.11 MeV, and the thermal fluence will be 4.5 Â 10 26 n m À2 .…”

Positron lifetimes in voids and other defects in annealed, neutron-irradiated aluminum

“…1, where Al is very small -less than 1 ppm at the irradiation temperature of ^55°C. It has been observed by TEM that voids in neutron-irradiated aluminum contain silicon precipitates [12], and some silicon is believed to be spread over the surface of the void [7]. In the present case, the process of precipitation of supersaturated Si is complicated by its time-dependent production rate by thermal neutrons as well as by the continuous flux of excess vacancies and interstitials resulting from radiation effects.…”

“…is the total impurity concentration). to the known voids at which the large excess of Si is precipitated (both on the void surface [7] and in the void corners [5,12]), we would expect the soluble Si to be present mostly in Si-vacancy pairs and some larger clusters. We suggest that these clusters are the trapping sites for the 230 psec component.…”

Materials Selection for the HFIR Cold Neutron Source

“…We have considered combating this accumulative hardening process by introducing high temperature anneals at intervals to induce coarsening of the silicon precipitate. Annealing experiments with solutiontreated-and-quenched Al-S1 alloys [66,67] and neutron irradiated aluminum [68] have shown that temperatures of 200°C or higher are required to ensure the appropriate coarsening in short times. A temperature of 200°C could be achieved from decay heat by reducing the hydrogen flow rate or the degree of refrigerator cooling.…”