The Snoek relaxation in bcc metals—From steel wire to meteorites

Heavy interstitial atoms, such as O, N, or C, when dissolved in bcc metals (Fe, Nb, Ta, V), induce elastic dipoles with tetragonal symmetry. The stress induced reorientation produces anelastic relaxation and is the elementary step of interstitial diffusion. Mechanical loss spectra indicate the presence of interstitial atoms by Snoek peaks at distinct, characteristic temperatures. The magnitude of the peaks is proportional to the concentration of interstitial atoms in solution. The atomic model and theory of the Snoek relaxation for ideal interstitial solutions are presented.Experiments with single crystals allow determination of the tensor components of the dipoles. Measurements of the peak position over a wide range of frequencies (6 decades) allow determination of the interstitial diffusion coefficient as a function of temperature. Nonlinearitities in the Arrhenius plots are discussed. In concentrated alloys a broadening of the Snoek peaks is observed, which can be described by a random cooperative strain interaction (RCSI) model. Measurements of the Snoek peak height can be applied as a non-destructive analytical method. This is demonstrated for O-doped Nb wires with a wide range of concentrations (20 up to 10,000 at. ppm O) and for Ta and Nb single crystals doped with O and N. Finally various experimental studies of the carbon Snoek peak in iron are presented. These include measurements of a low carbon steel wire, a Fe-Ni meteorite, and a recently (April 2002) fallen meteorite named Neuschwanstein.