Polymorphic bcc to fcc transformation of nanocrystalline niobium studied by positron annihilation - art. no. 054107

Positron lifetime and Doppler broadening measurements on nanocrystalline ni obium over a wide range of grain sizes (5-35 nm) were performed. Significan t changes in the structure and properties of grain boundaries and intercrys talline regions were observed when the grain size was reduced below 10 nm. While positron lifetime at the grain boundaries sharply increased owing to an increase in the excess free volume associated with the atoms, the atomic reordering brought in a remarkable redistribution of the electron momenta at the grain boundaries. A model-based calculation of the excess free volum e of atoms at the grain boundaries supported these findings. The calculated bulk modulus and the negative hydrostatic pressure predicted a lower size limit of similar to2 nm for the mechanical and thermodynamic stability of t he grain boundaries. Quantum confinement of the electron energy levels can be expected only below 20 K for grains of size above this limit.