Underlying mechanisms for unique elastic properties of nanocrystalline Au

In order to get an insight into the grain boundaries (GBs) in nanocrystalline (n-) metal, we prepared the high-density n-Au with ρ/ρ0>99% by the gas-deposition method and carried out the vibrating reed measurements, where ρ/ρ0 is the relative density referring to the bulk density. The strain amplitude dependence (SAMD) of the resonant frequency (f) and the internal friction (Q−1) was measured for the strain (ε) amplitude between 10−6 and 2×10−3 and for temperature between 5 and 300 K. No plastic deformations are detected for the present strain range, where f decreases for ε up to 10−4 and then turns to increase, showing saturation for ε between 10−4 and 2×10−3. The low temperature irradiation by 2 MeV electrons or 20 MeV protons causes an increase in the Young’s modulus at 6 K, which is surmised to reflect a modification of the anelastic process in the GB regions. In contrast, the SAMD of f is hardly modified by irradiation, suggesting that it is indicative of a collective motion of atoms in n-Au.