The atomic free volumes and vacancies in these ultra fine-grained alloys were studied by using positron lifetime techniques. Their thermally stable microstructures served as novel systems for the study of positron trapping and annihilation, as well as the thermal behavior of vacancy-sized free volumes over temperature ranges of up to 1200K. The thermal formation of lattice vacancies was observed in ultra fine-grained Cu. In the case of Pd84Zr16, an increase in the specific positron trapping rate of nano-voids, and the de-trapping of positrons from free volumes having a mean size that was slightly smaller than a missing atom in the grain boundaries, contributed to a reversible increase in the positron lifetime, with, temperature, of more than 60ps. In the case of Fe91Zr9, similar linear high-temperature increases in the positron lifetime were observed in the nanocrystalline and amorphous states. The question of thermal vacancy formation in grain boundaries was addressed by taking account of the various types of interface structure in the present alloys.

High-Temperature Studies of Grain Boundaries in Ultrafine Grained Alloys by Means of Positron Lifetime. R.Würschum, E.Shapiro, R.Dittmar, H.E.Schaefer: Physical Review B, 2000, 62[18], 12021-7