Plastic Deformation and Work Hardening of Colloidal Crystals

Abstract

When a crystal is stressed beyond a certain value (the yield strength), it exhibits plastic flow, which causes an irreversible change in its shape. As plastic flow progresses, increasingly large flow stresses are needed, a phenomenon called work hardening. Although it has been established for decades that plastic flow in crystals is governed by the nucleation and motion of line defects (dislocations), a full understanding of plastic deformation and work hardening remains to be developed due to the complex nature of the networks formed by the dislocations. Our ability to observe the exact interaction mechanism experimentally is limited by the difficulty of visualizing dislocation dynamics. Electron microscopy is the tool of choice, but in situ observations of the collective dynamics of dislocations during work hardening have so far remained out of experimental reach.

Colloidal crystals are uniquely suited for such an experiment since the micrometer size of the particles allows imaging of the three-dimensional structure of the crystal in the single particle resolution and in real-time. We use high-speed confocal imaging to visualize the crystals in-situ during plastic deformation and record both stress and strain, as well as the evolution of the three-dimensional dislocation networks.

This thesis reports experimental studies of two deformation processes of colloidal crystals: shear deformation (Chapters 3), and plastic relaxation during epitaxial growth (Chapters 4 and 5).

In the shear deformation experiments, we show hard-sphere colloidal crystals exhibit work hardening. Remarkably, despite their softness, the normalized strength of colloidal crystals exceeds that of most metals and approaches the theoretical limit. The strength-dislocation density relationship is in line with the classic Taylor predictions. Our work has implications not only for colloidal crystals but also for crystals in general: we demonstrate work hardening can be caused by the formation and destruction of dislocation junctions. Although this mechanism is widely accepted, it has so far been supported only by numerical simulations and indirect experimental evidences. We observe the transient dislocation structure at the initial stage of work hardening, and glide of Lomer dislocations that mediate shear localization at the later stage of deformation.

In the epitaxial growth experiments, we reveal novel dislocation mechanisms during epitaxial growth of strained crystalline films. We show how attractive interactions between dislocations promote the formation of new dislocations that are more efficient in relaxing the strain. This interaction mechanism gives rise to a new, two-stage plastic relaxation mechanism. We identify dislocation interactions that form barriers to dislocation motion and lead to the formation of complex dislocation networks with fragmented structures. We propose an order parameter that describes the transition from a fragmented structure to an ordered and elongated array of dislocations.