Peter Julian Imrich

Advanced nanomechanics in the TEM: Effects of thermal annealing on FIB prepared Cu samples

The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surface-mediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength.

Investigation of reversible plasticity in a micron-sized, single crystalline copper bending beam by X-ray μLaue diffraction

The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ µLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ µLaue experiments are useable. A critical size is evidenced below which, depending on Youngs modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample.

Sample Preparation by Metallography and Focused Ion Beam for Nanomechanical Testing

Abstract Mechanical size effects in micron and submicron scale sample testing are of immense interest in materials science. In this work, we report on a combination of structured chemical etching and focused ion beam fabrication to allow site specific and time efficient fabrication of miniaturized specimens for mechanical testing. Further, we demonstrate the applicability of these samples for quantitative in situ experiments in the scanning and transmission electron microscopes.

Novel methods for the site specific preparation of micromechanical structures

Abstract The ongoing trend towards miniaturization in various fields of material science requires the capability to investigate the local mechanical properties of the concerned structures by miniaturized mechanical experiments. Besides nanoindentation, miniaturized experiments such as micro-compression, micro-tension, micro-bending, or micro-fracture tests were employed frequently in recent times. A major challenge for these experiments is the fabrication of specimens. Therefore, we present different approaches to prepare miniaturized testing objects in a site specific way, using strategies that employ chemical etching, broad beam ion milling, and focussed ion beam milling. Depending on the required sample size and precision, the typical strategies for sample fabrication will be outlined, and the benefits and drawbacks of the techniques are discussed. Finally, applications of specimens produced by the different procedures are presented.