Characteristic creep behavior of nanocrystalline metals found for high-density gold

Nanocrystalline (n) Au specimens with a density of 19.4±0.2g/cm³ and a mean grain size of about 20 nm were prepared below 300 K by the gas deposition method, where two types of n-Au specimens were obtained as a function of a deposition rate, the type-H specimens above 800 nm/s and the type-L specimens below 800 nm/s. The anelastic and the plastic creep responses are similar qualitatively but different quantitatively between the type-H and type-L specimens. The anelastic strain ɛ_an,GB, associated with the grain boundary (GB) regions, increases linearly with (T-T_an1)(σ_ap-σ_an1), when the temperature T is higher than a threshold temperature T_an1 of 200 K and the applied stress σ_ap is higher than a threshold stress, σ_an1, of a few MPa. The ratio of ɛ_an,GB to the elastic strain is as large as 1.1 for the type-H specimens and 0.2 for the type-L specimens at 320 K for σ_ap≫σ_an1. The activation energy for the GB anelastic process is 0.2 eV. We surmise that cooperative motions of many atoms in the GB regions are responsible for ɛ_an,GB, and both T_an1 and σ_an1 show a distribution depending on the number of atoms associated. The plastic creep rate ɛ^′ vs σ_ap data show a letter S-like curve. We classified the creep response into three categories, region I for the linear creep rate region for σ_ap between σ_pc1 and σ_pc2, region II for the transient creep rate region for σ_ap between σ_pc2 and σ_pc3, and region III for the saturation creep rate region for σ_ap between σ_pc3 and σ_y. The threshold stresses σ_pc1 and σ_pc2 and the yield stress σ_y are about 30, 150, and 360 MPa for the type-H specimens, and about 60, 300, and 500 MPa for the type-L specimens, respectively. σ_pc3 is slightly lower than σ_y. From scanning tunneling microscopy images, we surmise that the localized GB slip takes place in region I, and the mean separation between the localized GB slips decreases with increasing σ_ap in region II and becomes comparable with the mean grain size in region III. The plastic creep in region III may be explained by the Ashby creep. The present view for the creep behavior explains the low-temperature creep behavior of fcc n metals.