In-situ SEM micro-pillar compression tests provide a way to measure the uniaxial mechanical response of low volumes of materials and to directly correlate the stress-strain data to individual deformation events. It enables to quantify specific phases and particles or to study size effects, in terms of deformation behavior and strengthening mechanisms. In order to resolve individual deformation events, key requirements for the measurement system are high load and displacement resolution, as well as fast data acquisition rate.
After identification of a suitable location in terms of microstructure or crystal orientation using SEM and EBSD, micro-pillars are prepared by top-down milling with focused-ion beam (FIB). Decreasing ion currents are used from the machining of the structure to the final surface polishing in order to reduce FIB damage. During compression, linear elasticity is observed in the initial loading stage, before yielding and plasticity. In the plastic regime, a serrated plastic flow behavior with sudden stress drops followed by reloading periods is often characteristic of dislocation slip events. For example, a correlation is showed between stress drops in the stress-strain data and shear-band formation seen in SEM with a) elastic loading, b) nucleation of the first slip event, c) intersection with top surface and d) multiplication of slip events. It is worth noting that load-controlled testing systems show strain bursts (not stress drops) in the stress–strain curve, preventing the quantitative study of these mechanisms. A key requirement of the system is therefore true displacement control. In combination with an ultra-low load noise floor, the statistical analysis of even smaller magnitude stress drops is possible. This enables to gain new insight into the nature of the interactions between dislocations and various lattice defects.