Mechanically induced fcc phase formation in nanocrystalline hafnium

A face-centered-cubic (fcc) phase was obtained in high-purity hafnium (Hf) metal powders subjected to mechanical milling in a high-energy SPEX shaker mill. X-ray diffraction and electron microscopy techniques were employed to evaluate the structural changes in the milled powder as a function of milling time. The effects of mechanical milling included a reduction in grain size, an increase in lattice strain, and formation of an fcc phase instead of an equilibrium hexagonal-close-packed (hcp) phase. During milling, the grain size of Hf decreased to below about 7 nm. Additionally, there was approximately 6% increase in atomic volume during the formation of the fcc phase. Chemical analysis of the milled powder indicated the presence of significant amounts of interstitial impurities. Even though any or all of the above factors could contribute to the formation of the fcc phase in the milled powder, it appears that the high level of interstitial impurities is at least partially responsible for the formation of the fcc phase.