Atomic free volumes in mechanically alloyed nanocrystalline Pd1-xZrx, where x was equal to 0.16 or 0.23, were studied by means of positron lifetime measurements. A stable microstructure which was produced by Zr segregation at the grain boundaries served as a novel model system for the study of positron trapping at nanovoids at up to 1170K. In addition to an increase in the specific positron trapping rate of nanovoids, de-trapping of positrons from free volumes with a mean size that was slightly smaller than a missing atom in a grain boundary, contributed to a reversible lifetime increase of more than 60ps with temperature. No sign of thermal vacancy formation in grain boundaries was detected, and this was attributed to a segregation-induced reduction of the grain-boundary energy.

Structural Stability and High-Temperature Positron Lifetime Study of Mechanically Alloyed Nanocrystalline Pd-Zr. E.Shapiro, R.Würschum, H.E.Schaefer, H.Ehrhardt, C.E.Krill, R.Birringer: Journal of Metastable and Nanocrystalline Materials, 2000, 8, 726-31