Length scales in crystal plasticity

Significant effects of sample dimension on the yield strength of metallic crystals have been known for more than 50 years when researchers identified this phenomenon in metallic whiskers. These sample-size effects are once again attracting great interest with the discovery of the indentation size effect and the enhanced yield strength found for sub-micrometer diameter focused ion beam (FIB)-machined metallic pillars. Here, we discuss these issues and suggest mechanisms that may be responsible for the observed behaviors. In the case of FIB-machined pillars we draw an analogy between the yield strength of these structures and the fracture strength of glass rods and suggest that the experimentally observed yield behavior in these pillars is consistent with that expected from extreme value statistics. Additionally, we revisit the topic of surface effects in crystal plasticity and suggest a new mechanism via which a free surface could act as a measurable source of hardening for a crystal that has a bulk interior free of defects such as dislocations or grain boundaries. Finally we suggest experimental approaches that can be used to test the ideas discussed herein.