Mechanical behaviour of nanocrystalline Iron and Nickel in the quasi-static and low frequency anelastic regime.

In this research, we made use of mechanical spectroscopy to study the anelastic behaviour of nanocrystalline Fe and Ni in quasi-static, low-frequency (0.01-10 Hz) regime. The elastic energy dissipation coefficient (Q - 1 ) and the stress relaxation have been measured as a function of frequency and temperature, in a range of temperatures where appreciable grain growth is not expected to occur. The use of such low frequency probes puts into evidence a very strong change in the material response, induced by low temperature annealing (T < 200 o C). In spite of the fact that the grain size was nearly the same after the annealing treatment, the as-prepared samples displayed much higher Q - 1 values and faster rates of stress relaxation than the annealed samples. The anelastic spectrum of annealed specimens has been analysed by a combination of quasi-static and dynamic measurements. The results of this study are discussed in terms of typical activation energies reported for lattice and grain boundary diffusion in coarse-grained polycrystalline and nanocrystalline metals.