TEM observations of damage structure in Al and Al-Mg(N4) alloy after irradiation with 100 and 400 keV Al ions

A comparative TEM study has been made of ion irradiation damage structure in aluminium and in a modified N4 (Al-2.95% Mg) alloy of the type used in the construction of the calandria of the Winfrith SGHWR. Atom displacements equivalent to many years neutron irradiation were simulated by bombardment with 100 and 400 kev Al+ ions to doses of up to 200 dpa at temperatures between 30 and 2500C. Dynamic observations of damage formation were made during irradiation with 100 keV ions in a linked heavy-ion accelerator/200 keV electron microscope, and further results were obtained by 400 kev Al+ ion bombardment in a Cockroft-Walton accelerator. Dislocation structure and voids were seen in aluminium irradiated with 100 and 400 kev Al+ at temperatures between 30 and 2500C. Void swelling of 8.7% at 104 dpa was a maximum at approximately 1500C in type 1100 aluminium. No voids were found at temperatures >2500C. No voids were seen in the Al-2.95% Mg alloys after bombardments up to 200 dpa with 100 kev Al+, and 150 dpa with 400 kev Al+ at temperatures between 50 and 1700C. The void-resistant property is consistent with observations in the USA of neutron-irradiated 5052 Al alloy which has a similar Mg content to the modified N4 alloy. The absence of voids indicates an effective vacancy annihilation mechanism, which in the N4 alloy possibly occurs at small Mg2 Si precipitates which form during irradiation. (author)