Study of Nano-Clustering Mechanism by Positron Annihilation and 3D Atom Probe

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The purpose of the present study is to analyze the formation mechanism of nano-clusters in materials, the formation of vacancy-solute complexes and the kinetics of the cluster formation through the migration of the complexes, never revealed by conventional methods, by using positron annihilation and three-dimensional atom probe (3DAP).We have obtained the following results :1) The specimens, Fe-Cu, Al-Ag, Al-Zn etc. single crystals and polycrystals, were prepared.2) Positron annihilation and 3DAP techniques are employed to investigate the pre-precipitation of solute clustering in Al-Cu-Mg, Al-Cu-Mg-Ag and Al-Li-Cu-Mg-Ag alloys. Not only by observing the solute clusters using 3DAP but also by detecting quenched-in vacancies and their vacancy-solute complexes using positron lifetime and coincidence Doppler broadening (CDB) of positron annihilation, we reveal the effect of vacancy-solute interactions in the solute clustering in the early stage of the precipitation in the Al alloys. The present success shows that the combination of positron annihilation and 3DAP is very useful to study the mechanism of the solute clustering.3) Two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) measurements revealed Fermi surface topology of embedded nano-clusters. By comparing experimental results with the first principles calculations, we proposed a new rule to determine the solute concentrations in the embedded clusters : band gap effect analogous of Peiers distortion and Hume-Rothery rule.4) New method to estimate number density of subnano-clusters was developed, combined with positron age-momentum correlation measurements and positron diffusion evaluation by using energy variable positron beam in Oarai Center, Tohoku University.