A search for evidence of strain gradient hardening in Au submicron pillars under uniaxial compression using synchrotron X-ray microdiffraction

When crystalline materials are mechanically deformed in small volumes, higher stresses are needed for plastic flow. This has been called the "smaller is stronger" phenomenon and has been widely observed. Various size-dependent strengthening mechanisms have been proposed to account for such effects, often involving strain gradients. Here we report on a search for strain gradients as a possible source of strength for single-crystal submicron pillars of gold subjected to uniform compression, using a submicron white-beam (Laue) X-ray diffraction technique. We have found, both before and after uniaxial compression, no evidence of either significant lattice curvature or subgrain structure. This is true even after 35% strain and a high flow stress of 300 MPa were achieved during deformation. These observations suggest that plasticity here is not controlled by strain gradients or substructure hardening, but rather by dislocation source starvation, wherein smaller volumes are stronger because fewer sources of dislocations are available.