CHARACTERIZATION OF FREE-VOLUME IN ATOMIC MODELS OF METALLIC GLASSES

An analysis of interatomic space is presented for atomic models of amo rphous Pd52Ni32P16 in the asquenched state and in an annealed state. I nterstitial voids are constructed as clusters of overlapping spheres t hat are placed in Delauney tetrahedra. It is found that the difference between as-quenched and annealed Pd52Ni32P16 predominantly lies in th e quantity of the relatively large voids. More specifically, the numbe r of voids surrounded by nine or less atoms (''intrinsic voids'') incr eases, whereas the number of voids surrounded by ten or more atoms ('' holes'') strongly decreases. The interpretation is that during structu ral relaxation the holes break up into two or more intrinsic voids. Si nce in the annealed state the diffusivity is a factor 25 smaller than in the as-quenched state, the diffusion process is explained in terms of the occurrence of holes. The analysis of the volumes and shapes of the holes shows that (1) the volume distribution becomes narrower on s tructural relaxation, and (2) although holes with a volume of one atom ic volume or more do occur, their shape is strongly nonspherical, and they certainly do not resemble crystalline vacancies. It is furthermor e argued that, because of the importance of holes for the atomic mobil ity, the hole volume is to be regarded as the free volume appearing in the well-known free-volume model by Turnbull and Cohen.