Paper SE-ThP12
Strain-Rate Sensitivity of Nanocrystalline Nanolaminates

Thursday, October 23, 2008, 6:00 pm, Room Hall D

The strain-rate sensitivity of strength is one of the key parameters to understand the deformation mode of nanocrystalline materials. It is widely reported that many nanocrystalline materials strength harden with increasing strain rate. Often, an increasing strain rate exponent is observed as the grain size decreases from the micro- to the nano- scale. This trend suggests that the mode of mechanical deformation is transitioning from within the grains to becoming a grain boundary effect. For nanocrystalline nanolaminates, another dimensional feature becomes significant. The layer pair spacing must now be accounted for in addition to the grain size. This additional nanoscale feature can become especially important when its size decreases below that associated with conventional dislocation-based strengthening mechanisms. We now investigate the hardness of transition metal nanolaminates for which both grain size and laminate dimensions are less than 10 nm. New data is acquired through the technique of nanoscratch testing. We report an increasing rate sensitivity of strength, as measured by the plastic flow associated with hardness, when dimensional features are further reduced to below 10 nm.