Shear softening of grain boundaries in nanocrystalline Pd

Nanocrystalline materials are polycrystals with grain sizes L of the order of 10 nm. Unlike conventional polycrystals, the volume fraction of matter in the core region of grain boundaries tends to approach the share of matter in the interior of the nanometer-sized grains since it scales as 1/L. The discovery of room-temperature grain growth in nanocrystalline Pd makes it possible to monitor the coarsening of the microstructure and to measure simultaneously, or even in situ, the concomitant evolution of properties. From the resulting scaling behavior of properties we deduced interface elastic moduli, using ultrasonic techniques. In particular, we find a 30% shear softening of grain boundaries and discuss its relation to the shear softening observed in bulk metallic glasses.