EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

We present quantitative in-situ Transmission Electron Microscopy (TEM) fracture experiments on single crystal Silicon at room temperature. Findings consist of a brittle bulk fracture behavior of large samples at a stress intensity K_IC~1 MPa.m^(1/2). However, below characteristic dimensions of about 250 nm, the fracture toughness strikingly increases inversely with size to at least triple.
Advanced in-situ TEM nanoscale strain mapping reveal the stresses at the crack tip approach the theoretical strength. At the same time, below this critical transition length, nucleation and propagation of dislocations was observed, shielding the crack tip and enabling the unprecedented rise in fracture toughness. The observed critical dimension at which the intriguing ductilization of Si commences, is rationalized by three-dimensional FEM simulations. These detail that the local changes of the ratio between shear and cleavage stresses, quantified by (σ_(Tresca (70°))/σ_(YY (0°))), (70° and 0°, angles to the notch opening direction) increases with decreasing specimens’ thicknesses, causing a room temperature brittle-to-ductile transition for Si and increasing fracture toughness below 250 nm dimensions.