Direct observation of superplastic flow mechanisms in torsion

A Comparative Study of the Nanocrystalline Material Produced by Sliding Wear and Inert Gas Condensation

A High Resolution Electron Microscopy (HREM) study of grain boundaries in nanocrystalline copper produced by sliding wear and by inert gas condensation is described. The results in both cases are essentially the same and similar to those in conventional polycrystals. Contrary to reports in the literature, these results do not indicate the presence of a fundamentally different structure in nanocrystalline materials. Measurements of the mechanical properties (such as yield strength, elastic modulus and strain rate sensitivity) of these materials using nanoindentation are also presented. These are found to be in good agreement with values expected from a polycrystalline material with a very fine grain size.

High Resolution Surface Studies of Superplastic Deformation

Previous studies of superplasticity using surface markers have tended to use either diamond paste scratches, with which it is difficult to make quantitative measurements, or regular grids with spacing greater than the grain size, which prevents measurements of any deformation occurring within the grains. This paper reports the etching of regular marker grids with submicron line spacings using Focused Ion Beams (FIB), as well as mesoscopic grids with line spacings many grains across. These have enabled us to make quantitative measurements of deformation within the surface grains, and at length scales up to 0.5 millimeters. Further refinements used include the use of Interferometric Surface Profilometry to measure the out-of-surface displacement during deformation. Results on Sn-Pb eutectic alloy tested in tension in Regions I and II showed that there was almost no intragranular deformation anywhere in any of the surface grains. Grain boundary sliding took place, particularly at Sn-Sn grain boundaries, and the accommodation mechanism was separation of interfaces normal to the tensile axis where sliding was blocked, particularly Pb-Pb grain boundaries. It is argued that this is a surface artifact not readily available in the bulk thus the case is made for testing under a different strain condition.