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

Abstract

Nanocrystalline (n) Au specimens with a density of (formula presented) and a mean grain size of about 20 nm were prepared below 300 K by the gas deposition method, where two types of (formula presented) 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 (formula presented) associated with the grain boundary (GB) regions, increases linearly with (formula presented) when the temperature T is higher than a threshold temperature (formula presented) of 200 K and the applied stress (formula presented) is higher than a threshold stress, (formula presented) of a few MPa. The ratio of (formula presented) 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 (formula presented) 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 (formula presented) and both (formula presented) and (formula presented) show a distribution depending on the number of atoms associated. The plastic creep rate (formula presented) vs (formula presented) data show a letter S-like curve. We classified the creep response into three categories, region I for the linear creep rate region for (formula presented) between (formula presented) and (formula presented) region II for the transient creep rate region for (formula presented) between (formula presented) and (formula presented) and region III for the saturation creep rate region for (formula presented) between (formula presented) and (formula presented) The threshold stresses (formula presented) and (formula presented) and the yield stress (formula presented) 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. (formula presented) is slightly lower than (formula presented) 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 (formula presented) 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. © 2002 The American Physical Society.