Interplay of stresses, plasticity at crack tips and small sample dimensions revealed by in-situ microcantilever tests in tungsten

The evolution and distribution of stresses and geometrically necessary dislocation densities during microcantilever testing were studied in-situ in a scanning electron microscope using high-angular resolution electron back scatter diffraction. Focused ion beam milling was used to prepare the beams with dimensions of ca. 5-7 µm in height and width and 20 µm in length in a tungsten single crystal along a 〈100〉 axis. Un-notched cantilevers were tested to gain insight into dislocation mechanisms leading to significant strengthening during loading. Locally increased yield and flow stresses were linked to pronounced stress gradients on specific slip planes and dislocation pile-ups at regions of zero stress. The limited plastic deformation behaviour of tungsten at room temperature allowed also detailed observations of the interplay between crack initiation and the formation of a plastic zone at the crack tip. Due to the lack of dislocation sources in the crack tip vicinity, dislocations were initially emitted on {110} planes from the crack tip, which led to finite crack tip blunting. The initiated crack was stopped by compressive stresses and a significant plastic zone developed, which kept the crack from growing with ongoing loading. The combined experimental and analytical observations allow a mechanistic understanding, which helps in explaining the recently reported size-dependent plastic deformation and fracture behaviour of semi-brittle materials.