In situ transmission electron microscopy observations of toughening mechanisms in ultra-fine grained columnar aluminum thin films

A unique straining device, fabricated using microlithographic techniques, has been developed to permit real-time investigation in the transmission electron microscope (TEM) of the deformation and failure mechanisms in ultrafine-grained aluminum. The tensile specimen is a freestanding thin film with a columnar microstructure that has a uniform cross-section (100 x 0.125 mu m) and a gauge length of 300 mu m. In situ TEM straining experiments show the fracture mode is intergranular with no accompanying general plasticity. Propagating cracks were halted at large-grains, and crack blunting occurred through grain-boundary-mediated processes. The blunting process was accompanied by dislocation emission and deformation twinning in the grain responsible for arresting the crack. Voids or microcracks nucleated and grew on grain boundaries ahead of the arrested crack, and crack advance occurred through linkage of the microcracks and the primary crack.