Defect clusters near to Hf were studied by using the perturbed angular correlation technique and positron annihilation spectroscopy. By measuring the lattice parameter, the volume size-factor of Hf was deduced to be 61%. The positron annihilation mean lifetime of Ni-0.5at%Hf, after neutron irradiation at 473K to a dose of 0.0053dpa, was 132ps. The lifetime decreased with increasing annealing temperature, and recovery was finished by annealing at 723K. The perturbed angular correlation spectrum of 181Ta (←181Hf) after irradiation indicated that there were 2 components. The first component consisted of a Larmor frequency of 534Mrad/s. The second consisted of a very broad range of frequencies which suffered destructive interference, and made the contribution to the spectrum almost zero. This component disappeared upon annealing at 873K. It was concluded that the first and second components represented Hf in the normal substitutional site, and Hf with defect clusters, respectively. From the difference in annealing temperature of defect clusters, clusters which annihilated by 723K were suggested to be stacking-fault tetrahedra and those which annihilated by 873K were suggested to be interstitial clusters.

Detection of Interstitial Clusters in Neutron-Irradiated Ni–Hf Alloy by Perturbed Angular Correlation and Positron Annihilation Lifetime Measurements. H.Okazawa, T.Yoshiie, T.Ishizai, K.Sato, Q.Xu, Y.Satoh, Y.Ohkubo, Y.Kawase: Journal of Nuclear Materials, 2004, 329-333[2], 967-70