Anelasticity of nanocrystalline metals

Nanocrystalline (n-) materials posses a characteristic ultrafine structure where small crystallites less than ∼100 nm in diameter are connected by highly disordered grain boundaries (GBs). Although the recent mechanical tests on the high-density n-metals suggest that the dynamic modulus is close to the polycrystalline metals, we have found a large anelastic strain comparable with the elastic strain in the quasi-static tests. The estimated activation energy as low as 0.2 eV suggests that certain cooperative motions of many atoms in the GBs are responsible for this large anelastic strain. The strain amplitude dependence (SAMD) in the resonant frequency, f, was measured by the vibrating reed technique, where f decreased by about 1% with increasing strain, ε, up to 10−4 and then turned to increase showing the saturation at ε=10−4–5×10−3. This strange SAMD in f showed no change by the low-temperature irradiation, in contrast, f showed a large increase due to the accumulation of the point defects probably in the GBs during the irradiation. We surmise that the (an)elastic properties of n-metals are governed by the several processes not only in the GBs but also in the crystallites.