In situ TEM studies of deformation mechanisms in nanoindentation of ultrafine-grained and nanocrystalline metals

The mechanical properties of ultrafine-grained and nanocrystalline materials have received a great deal of recent attention because of their unusual and promising values. However, some of the most important mechanisms of deformation remain unclear. To address this issue, an in situ nanoindentation stage has been used in a transmission electron microscope to explore the deformation behaviors of nanocrystalline aluminum, ultrafine-grained aluminum, and ultrafine-grained iron in real time.; The special in situ indentation stage contains a Berkovich-type diamond indenter which can be coarsely actuated by a 3-axis screw-positioner and by a piezoelectric ceramic crystal for fine positioning actual indentation. Two methods are used to fabricate samples that are electron transparent, accessible to the indenter and mechanically stable. In one method, polygranular aluminum films are deposited on wedge-shaped silicon substrates so that the film above the wedge tip is transparent. The grain size of the film can be controlled by adjusting the deposition conditions. Alternatively, thin plates of iron are machined from bulk specimens by FIB.; In situ studies of nanocrystalline Al films were carried out under various diffraction conditions. Although it is difficult to image individual nanosized grains, the results suggest that strain is accommodated by grain boundary movement or, more interestingly, by strain-induced grain coarsening. In the ultrafine-grained Al films, strain-induced grain coarsening is also frequently observed during deformation at room temperature. The results show that the strain-induced coarsening is by normal grain growth (that is, by grain boundary migration), which may lead to a dramatic enhancement of the ductility. Strain-induced coarsening is more difficult to achieve in ultrafine-grained iron because of the much lower mobility of the grain boundaries. The lack of grain boundary motion in Fe is attributed to the pinning effect of nano-sized particles at the Fe grain boundaries.