High-temperature studies of grain boundaries in ultrafine grained alloys by means of positron lifetime

Atomic free volumes and vacancies in the ultrafine grained alloys Pd₈₄Zr₁₆, Cu 0.1 wt % ZrO₂, and Fe₉₁Zr₉ were studied by means of positron lifetime. The thermally stable microstructures serve as a novel type of model system for studying positron trapping and annihilation as well as the thermal behavior of vacancy-sized free volumes over a wide temperature range up to ca. 1200 K by making use of a metallic ⁵⁸Co positron source. In ultrafine grained Cu the thermal formation of lattice vacancies could be observed. In Pd₈₄Zr₁₆ an increase of the specific positron trapping rate of nanovoids and, in addition, detrapping of positrons from free 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. In Fe₉₁Zr₉ similar linear high-temperature increases of the positron lifetime are observed in the nanocrystalline and the amorphous state. The question of thermal vacancy formation in grain boundaries is addressed taking into account the different types of interface structures of the present alloys.