HRTEM Study Of The Lattice Distortion In Nano-crystallites And The Strain Gradient Effect Of Bulk Nanocrystalline Copper

In the first section of this dissertation, we studied the lattice distortion in the central part of an ellipsoid-like nano-crystallite in the bulk nano-crystalline copper. The lattice distortions of the ellipsoid-like nano-crystallite were measured by means of the peak finding method in the central part of the HRTEM image. The contrast delocalization effects on the peak position induced by the amorphous layers on the specimen surface and the amorphous-crystal interface were measured by simulating HRTEM images. The results of simulation indicate that it is possible to measure the changes in the column spacing in the central region away from interface about 1.5 nm by using the peak finding method from a HRTEM image of nano-crystallites. We measured the lattice distortions in an ellipsoidal nano-crystallite by peak finding method, the results indicate that the ellipsoidal nano-crystallite is expanded in the short axis direction and compressed in the long axis direction. The bulk nano-crystallite was considered as the inhomogeneity which is the crystallite embedded in the amorphous matrix. We calculated the strain in the inhomogeneity by Eshelby's formalism incorporating coupled surface/interface-bulk elasticity. The results reveals that the strains in ellipsoidal nano-inclusions incorporating surface/interface tension are accord with the strains measured by peak finding method. This shows that the non-uniform distortion of the ellipsoid-like nano-crystallite is induced by the interface tension of the amorphous-crystal interface.In second section of this dissertation, we studied the size effect of film in nano-crystallite copper and tried to analyse the mechanics data of nano-crystalline copper by the theory of strain gradient plasticity. In order to obtain the relationship between non-dimensional bending moment and surface strain of samples with different grain size and thinkness of film, we measured the radius curvature after under-loading of the samples bent with different bending radiuses. The experimental results of the coarse grain copper indicate obviously the effect of size, and the the prediction of SG theory for CG copper is in accordance with the experiment results. However, the experimental results of nano-crystalline copper do not show the size effect, and the intrinsic length predicted by SG theory is negative which is unreasonable. This means that the size effect of materials disappears, and the theory of gradient strain plasticity is no longer applicable, when the grain size is reduced into nano-scale. The reason is that the deformation mechanics of the nano-crystalline materials cannot be explained by the theory of dislocation.