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

Article

关键词: TEMPERATURE CREEP;    GRAIN-BOUNDARIES;    NICKEL;    DIFFUSION;    PALLADIUM;    COPPER;   

DOI  :  10.1103/PhysRevB.66.214106

来源: SCIE

【 摘 要 】

Nanocrystalline (n) Au specimens with a density of 19.4+/-0.2 g/cm(3) 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 epsilon(an,GB), associated with the grain boundary (GB) regions, increases linearly with (T-T-an1)(sigma(ap)-sigma(an1)), when the temperature T is higher than a threshold temperature T-an1 of 200 K and the applied stress sigma(ap) is higher than a threshold stress, sigma(an1), of a few MPa. The ratio of epsilon(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 sigma(ap)>>sigma(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 epsilon(an,GB), and both T-an1 and sigma(an1) show a distribution depending on the number of atoms associated. The plastic creep rate epsilon' vs sigma(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 sigma(ap) between sigma(pc 1) and sigma(pc2), region II for the transient creep rate region for sigma(ap) between sigma(pc2) and sigma(pc3), and region III for the saturation creep rate region for sigma(ap) between sigma(pc3) and sigma(y). The threshold stresses sigma(pc1) and sigma(pc2) and the yield stress sigma(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. sigma(pc3) is slightly lower than sigma(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 sigma(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.

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