Experimental tests of the elementary mechanism responsible for creep deformation in nanocrystalline gold

Creep deformation tests at room temperature were carried out on nanocrystalline (n-) Au prepared by the gas-deposition method, where the mean primary grain size was ~20 nm but most of the primary grains formed clusters with a mean size of ~50 nm (called secondary grains after this). X-ray (111) reflection was predominant in the n-Au specimens in the as-deposited state and the mean grain size estimated from this reflection had good agreement with that of the secondary grains. The secondary grains came out onto the specimen surfaces and sank into them in the in situ scanning tunneling microscope observations done during the creep tests. This indicated that secondary grains in n-Au can move as a whole without coalescence with neighboring grains and such secondary-grain motion cannot be explained by diffusion-mediated mechanisms. Further, a large decrease in the x-ray (111) reflection and no changes in either the mean size of the secondary grains or the x-ray diffuse scattering pattern were observed after creep deformations. These results indicate that the grain-boundary slips in n-Au are associated with rotations of the secondary grains. The transition in heterogeneous grain-boundary slips to homogeneous grain-boundary slips with increasing stress found in virgin n-Au specimens was again observed in predeformed n-Au specimens, indicating that the modified grain-boundary regions due to grain boundary slips can be recovered after unloading, i.e., grain-boundary regions have static and modified structures