Vibrational density of states of nanocrystalline iron and nickel

We present an experimental determination of the vibrational density of states in nanocrystalline Fe and Ni by inelastic neutron scattering. Nanocrystalline specimens with different grain size and microstrain have been prepared by ball milling and thermal annealing. The vibrational density of states has been extracted from neutron time-of-flight spectra obtained with the spectrometer IN6 at the Institute Laue-Langevin. In comparison with reference coarse-grained specimens measured in the same conditions the nanocrystalline specimens exhibit: (i) a modest increase in the population of low-frequency modes and (ii) a distinct broadening of the transverse and longitudinal phonon peaks. The former feature is related to the presence of interface modes and it is critically compared with other observations in pure nanocrystalline metals. The latter is discussed in terms of reduced phonon lifetime due to the nanometric size of the crystallites. (C) 2000 American Institute of Physics. [S0021-8979(00)01021-5].