Structural stability and high-temperature positron lifetime study of mechanically alloyed nanocrystalline Pd-Zr

Atomic free volumes in mechanically alloyed, nanocrystalline Pd100-xZr, (x = 16,23) were studied by means of positron lifetime measurements. The stable microstructure induced by Zr segregation at the grain boundaries serves as a novel model system for studying the positron trapping process at nanovaids up to high measuring temperatures of 1170 K. In addition to an increase of the specific positron frapping rate of nanovoids, detrapping of positrons from fi ee volumes with a mean size slightly smaller than one missing atom in the grain boundaries contributes to a reversible increase of the positron lifetime of more than 60 ps with measuring temperature. No indication of thermal vacancy formation in grain boundaries was found, which is attributed to the segregation-induced reduction of the grain-boundary energy.