Lecturer:  Dr. Ilya Svetlizky, Harvard University
Abstract:
Plastic (irreversible) deformation of crystals requires disrupting the crystalline order, which
happens through nucleation and motion of topological line defects called dislocations.
Interactions between dislocations lead to the formation of complex networks that, in turn,
dictate the mechanical response of the crystal. The severe difficulty in atomic systems to
simultaneously resolve the emerging macroscopic deformation and the evolution of these
networks impedes our understanding of crystal plasticity. To circumvent this difficulty, we
explore crystal plasticity by using colloidal crystals; the micrometer size of the particles allows us
to visualize the deformation process in real-time and on the single particle level.
In this talk, I will focus on two classical problems: instability of epitaxial growth and strain
hardening of single crystals. In direct analogy to epitaxially grown atomic thin films, we show
that colloidal crystals grown on mismatched templates to a critical thickness relax the imposed
strain by nucleation of dislocations. Our experiments reveal how interactions between
dislocations lead to an unexpectedly sharp relaxation process. I will then show that colloidal
crystals can be strain-hardened by plastic shear; the yield strength increases with the dislocation
density in excellent accord with the classical Taylor equation, originally developed for atomic
crystals. Our experiments reveal the underlying mechanism for Taylor hardening and the
conditions under which this mechanism fails.