Atomic transfer through interfacial free volumes in Sn _65.4Bi_34.6 eutectic systems

Atomic transfer through the interfaces upon Bi precipitation is specifically investigated with respect to vacancy-sized free volumes for a Sn_65.4Bi_34.6 eutectic alloy of a highly heterogeneous system by making full use of backscattering electron imaging, small-angle x-ray scattering, x-ray diffraction, and positron annihilation spectroscopy. Bi-rich particles of ∼100nm with segregated nanocrystallites of ∼30nm are observed for the alloy cooled at a cooling rate of 10^-1Ks ^-1. Bi particles and nanocrystallites intergrow with each other up to ∼1νm and ∼55nm, respectively, by an extremely slow cooling of 10^-5Ks^-1. Regardless of the cooling rate, high concentrations of vacancy-sized free volumes are found to be located at interfaces among short-range ordered phases. For the alloy cooled at 10 ^-1Ks^-1, the free volumes are dominantly surrounded by Bi atoms. Decreasing the cooling rate down to 10^-5Ks^-1 changes the free volumes with Bi-rich chemical surroundings to an Sn-rich environment, which directly indicates atomic transfer through the interfaces upon Bi precipitation. The present results demonstrate that the kinetics of vacancy-sized free volumes in the interfaces plays an important role in understanding the precipitation mechanism in nanostructured heterogeneous materials.