Investigating the Plastic Deformation Kinetics in Ultrafine Grained and Nanocrystalline Metal Thin-Films Using In Situ TEM Nanomechanical Testing

The plastic deformation kinetics of ultrafine grained (ufg) and nanocrystalline (nc) metal thin films have been investigated using an <i>in situ</i> TEM nanomechanical testing technique. This technique allows for simultaneous observation of the active deformation mechanisms and measurement of stress relaxation and true activation volume. Experiments have been conducted on gold (Au) and aluminum (Al) thin films with varying thicknesses, grain sizes, and texture. Activation volume measurements range from 5 – 15 b³ for ufg Au specimens with an average grain size of 150 nm. Activation volume values for nc Al decreased from 25 – 35 b³to 5 – 10 b³ as the average grain size decreases from 74 to 47 nm. Experimentally measured activation volume values are compared with values determined from atomistic simulations for different unit dislocation processes, using a model relating activation volume and grain size. Based on these comparisons, it is determined that diffusive processes, such as grain boundary disconnection climb, are most likely the rate-controlling mechanisms that govern deformation of these nc/ufg metal thin films, as opposed to surface and grain boundary dislocation nucleation.