VOID FORMATION IN NEUTRON-IRRADIATED CU AND CU ALLOYS

Pure copper and copper-aluminum alloys were neutron-irradiated at high temperatures in the as-received condition, and after being melted under high vacuum or in argon. Melting under high vacuum was done to reduce the residual gas amount in the specimens. The number density of voids in the vacuum-melted Cu was one tenth of that in as-received Cu after JMTR irradiation to 5.2 x 10(24) n/m2 at 603K Similarly, voids were also formed in an argon-melted Cu-1at%Al specimen but were not formed in a vacuum-melted one. Following higher dose irradiation in the JOYO reactor, nearly the same number density and size of voids were formed in both argon and vacuum-melted Cu. In Cu-5at%Al, many voids were formed in argon-melted specimens, whereas in vacuum-melted specimens voids were not formed. These results show that voids nucleate at vacancy clusters which trap gas atoms. In the JOYO irradiations, diffused-in gas atoms play an important role in the formation of voids in Cu. In Cu-5at%Al, diffused-in gas atoms were trapped by Al atoms, resulting in a difference of void formation between the two types of specimens.