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

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.