Thermal vacancy formation and diffusion in nanocrystalline materials

An overview is given of high-temperature studies of atomic free volumes and thermal vacancies in nanocrystalline (n-) alloys (Pd84Zr16, Cu-0.1 wt.% ZrO2, Fe91Zr9, (Fe3Si)95Nb5, Fe73.5Si13.5B9Nb3Cu1) with different types of microstructures and grain boundaries. The thermally stable microstructures enable positron lifetime measurements over a wide temperature range up to about 1200 K by making use of a metallic 58Co positron source. In Fe3Si-based nanocrystalline alloys and in ultrafine grained Cu, thermal lattice vacancies and interfacial free volumes act as competing positron traps at high temperatures. In the mechanically prepared n-alloys the variation of the positron lifetime prior to the onset of thermal vacancy formation is due to an increase of the specific positron trapping rate of nanovoids. In the crystallization-prepared n-alloys, which are free of nanovoids, an amorphous intergranular phase between the crystallites strongly affects the atomic diffusivity. The question of thermal vacancy formation in grain boundaries is addressed taking into account the different types of interface structures of the present alloys. The chemical environment of the interfacial free volumes is characterized by means of coincident Doppler broadening. (orig.)