Mechanical properties of micro-sized copper bending beams machined by the focused ion beam technique

Micro-sized bending beams with thicknesses, t, from 7.5 down to 1.0 (mu)m were fabricated with the focused ion beam technique from a copper single crystal with an {1 1 1}(left angle bracket)0 1 1(right angle bracket) orientation. The beams were loaded with a nano-indenter and the force vs. displacement curves were recorded. A strong size effect was found where the flow stress reaches almost 1 GPa for the thinnest beams. A common strain gradient plasticity approach was used to explain the size effect. However, the strong t^-1.14 dependence of the flow stress could not be explained by this model. Additionally, the combination of two other dislocation mechanisms is discussed: the limitation of available dislocation sources and a dislocation pile-up at the beam centre. The contribution of the pile-up stress to the flow stress gives a t^-1 dependence, which is in good agreement with the experimental results.