Size dependence of mechanical properties of gold at the micron scale in the absence of strain gradients”, Acta Materialia 53

The classical laws of materials science and mechanics hold that the mechanical properties of materials are independent of sample size; however the results of both experiments and molecular dynamics simulations indicate that crystalline materials exhibit strong size effects at the micron scale and below. In experimental studies, the observed size effect can be explained in terms of plastic strain gradients and the hardening effects of the associated geometrically necessary dislocations. By contrast, the atomistic simulations suggest that plastic deformation is intrinsically inhomogeneous, that the yield strength depends on the sample size even in the absence of strain gradients, and that for small single crystals, the yield strength scales with the surface area-to-volume ratio of the sample. These different views call for studies of the mechanical properties of small crystals without significant strain gradients. Results of uniaxial compression experiments on gold at the micron scale, without significant stress/strain gradients, are presented. Two unique fabrication processes are used to create single-crystalline and polycrystalline Au free-standing cylinders with sub-micron dimensions. These tiny cylinders are then tested in uniaxial compression using the MTS Nanoindenter with a custom-machined flat punch diamond tip. Stress, strain, and instantaneous stiffness of the pillars are measured and the compressive stress and strain are determined from these quantities. Test results indicate a significant increase in flow stress, up to several GPa, as the pillars are compressed. Although there appears to be a significant increase in flow stress with decreasing sample size, a physical model for this has not been developed. At the time of this writing, we are not certain of the cause of the high strengths observed. Several possibilities, including the development of ultra-fine dislocation substructures, the creation of stacking faults and the strain hardening of small crystals by dislocation starvation are all briefly discussed.