Polymorphic bcc to fcc transformation of nanocrystalline niobium studied by positron annihilation

Positron lifetime and Doppler broadening measurements on nanocrystalline niobium over a wide range of grain sizes (5--35 nm) were performed. Significant changes in the structure and properties of grain boundaries and intercrystalline 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 volume of atoms at the grain boundaries supported these findings. The calculated bulk modulus and the negative hydrostatic pressure predicted a lower size limit of {~}2 nm for the mechanical and thermodynamic stability of the grain boundaries. Quantum confinement of the electron energy levels can be expected only below 20 K for grains of size above this limit.