  Mechanical testing of micropillars is a field that involves new physics, as
the behaviour of materials is non-deterministic at this scale. To better
understand their deformation mechanisms we applied 3-dimensional high angular
resolution electron backscatter diffraction (3D HR-EBSD) to reveal the
dislocation distribution in deformed single crystal copper micropillars.
Identical micropillars (6 um x 6 um x 18 um in size) were fabricated by focused
ion beam (FIB) and compressed at room temperature. The deformation process was
stopped at different strain levels (~1%, 4% and 10%) to study the evolution of
geometrically necessary dislocations (GNDs). Serial slicing with FIB and
consecutive HR-EBSD mapping on the (100) side was used to create and compare
3-dimensional maps of the deformed volumes. Average GND densities were
calculated for each deformation step. Total dislocation density calculation
based on X-ray synchrotron measurements were conducted on the $4\%$ pillar to
compare dislocation densities determined by the two complementary methods.
Scanning transmission electron microscopy (STEM) and transmission electron
microscopy (TEM) images were captured on the 10% pillar to visualize the actual
dislocation structure. With the 3D HR-EBSD technique we have studied the
geometrically necessary dislocations evolving during the deformation of
micropillars. An intermediate behaviour was found at the studied sample size
between bulk and nanoscale plasticity: A well-developed dislocation cell
structure built up upon deformation but with significantly lower GND density
than in bulk. This explains the simultaneous observation of strain hardening
and size effect at this scale.
