Polymorphic transformation and lattice expansion in nanocrystalline niobium revealed by positron annihilation at grain boundaries

Positron lifetimes in nanocrystalline niobium were measured over the grain size range from 35 nm down to 5 nm. Two lifetimes were obtained indicating distinct annihilation sites at the grain surfaces and in the intercrystalline region. X-ray diffraction studies had indicated a bcc to fcc transformation and a concomitant lattice expansion. Positron lifetimes drastically varied below a grain size of 10 nm, coinciding with a sharp increase in the excess free volume of atoms at the grain boundaries as revealed in a model analysis. Doppler broadening measurements also indicated a redistribution of electron momentum, implying a restructuring of the grain surfaces. The occurrence of bcc to fcc transformation is also observed in binary Nb–Al and ternary Al–Nb–Cu alloys.