Effect of solute atom concentration on vacancy cluster formation in neutron-irradiated Ni alloys

Abstract The dependence of microstructural evolution on solute atom concentration in Ni alloys was investigated by positron annihilation lifetime measurements. The positron annihilation lifetimes in pure Ni, Ni−0.05 at.%Si, Ni−0.05 at.%Sn, Ni−Cu, and Ni−Ge alloys were about 400 ps even at a low irradiation dose of 3 × 10 −4 dpa, indicating the presence of microvoids in these alloys. The size of vacancy clusters in Ni−Si and Ni−Sn alloys decreased with an increase in the solute atom concentration at irradiation doses less than 0.1 dpa; vacancy clusters started to grow at an irradiation dose of about 0.1 dpa. In Ni−2 at.%Si, irradiation-induced segregation was detected by positron annihilation coincidence Doppler broadening measurements. This segregation suppressed one-dimensional (1-D) motion of the interstitial clusters and promoted mutual annihilation of point defects. The frequency and mean free path of the 1-D motion depended on the solute atom concentration and the amount of segregation.