Size estimation of embedded Cu nanoprecipitates in Fe by using affinitively trapped positrons

By utilizing a unique characteristic of the affinitively trapped positrons, here we demonstrate a powerful method to estimate the sizes of the embedded Cu nanoprecipitates (and subnanoprecipitates) (ECNPs) in Fe. The positron-electron momentum density distributions in the ECNPs with various sizes in a thermally aged Fe-Cu dilute alloy were measured by using the two-dimensional angular correlation of positron annihilation radiation technique and the momentum smearing around the Fermi cutoff was clearly observed. It is clarified that this smearing effect is proportional to the inverse of the average radii of the ECNPs and thus it can be employed to estimate the ECNPs' sizes. Moreover, the ECNPs' sizes obtained by the present method are compared with those obtained by the three-dimensional atom probe technique and good agreement is observed, justifying the size estimation using the smearing effect. It is noteworthy that the present technique can detect the smearing due to the ECNPs with their radii even below about 0.5 nm and thus gives an important trend in the smearing reflecting the average size of the ECNPs in this range.