Observation and measurement of grain rotation and plastic strain in nanostructured metal thin films

The deformation behavior of nanostructured gold thin films, with grain diameters of 10 nm and film thicknesses of 10-20 nm, has been studied by means of in situ high resolution transmission electron microscopy. Grain rotation was observed by measuring the changes in the angular relationships between the lattice fringes of different grains during deformation at low strain rates. The strain tensor was calculated by measuring the relative displacements of three material points, and using an analysis similar to that for strain gage rosettes. Relative grain rotations of up to 15 degrees, along with effective plastic strains on the order of 30%, were measured. No evidence of dislocation activity was detected during or after straining. Identical experiments on coarser-grained silver thin films, with grain diameters around 110 nm, yielded clear evidence of dislocation activity. These results indicate that grain rotation and grain boundary sliding can make significant contributions to the deformation of nanostructured thin films at low homologous temperatures. © 1995