Lattice expansion and phase stability in nanocrystalline titanium thin films

Crystallite-size-dependent lattice expansion of the hcp Ti phase has been observed by X-ray diffraction of polycrystalline Ti thin films. X-ray line profile analysis (XLPA) revealed a systematic reduction of crystallite size in the hcp Ti phase with decreasing film thickness. Increase of specific volume (i.e. volume/atom) of the hcp Ti phase with decreasing crystallite size has confirmed such lattice expansion. The observed lattice expansion has been simulated using an existing theoretical model after appropriately incorporating a crystallite-size-dependent width of the grain boundaries. It is further revealed that decreasing crystallite size and accompanying lattice expansion leads to lattice instability of the hcp Ti phase and eventually to a hcp-fcc phase transformation of elemental Ti in these thin films as reported earlier by the author.