Deformation underneath low-load indentations in copper

The origin of the indentation size effect has been studied using convergent beam electron diffraction in the transmission electron microscope to map the crystal lattice rotations that occur underneath low-load indentations made at loads of 5 and 15mN in copper. It is shown that the measured rotations are greater underneath the larger indentation. This is inconsistent with the predictions of strain gradient plasticity, although there appear to be no other substantial differences in the deformation patterns. It is therefore suggested that the observed increase in hardness at the lower load is associated with the reduced probability of finding a suitable dislocation source in the loaded volume.