Scientific Advisory Board - Erich Schmid Institute

ERICH SCHMID INSTITUTE OF MATERIALS SCIENCE
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 tensile and
compression tests of annealed micro- and nanometer-sized tensile and compression samples
performed in the Sem and tem (see fig. 9a-h). 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. Regarding the submicron samples shown here,
the annealed material shows a yield point where dislocation nucleation takes place close to
the theoretical strength of the material. Additionally, it sustains significantly higher stresses
throughout the deformation (fig. 9j) compared to fiB prepared samples of comparable
diameter. this is explained by the high stresses required for surface mediated dislocation
nucleation of the annealed material, whereas for the fiB affected samples the near surface
defects facilitated dislocation nucleation, thereby lowering the material strength.
these results were published recently in: d. Kiener, Z. Zhang, S. Sturm, S. cazottes, p. J.
imrich, c. Kirchlechner, and g. dehm: Advanced nanomechanics in the tem: effects of thermal
annealing on fiB prepared cu samples. phil. mag. A 92, 3269 (2012). notably, this manuscript
involving several eSi scientists was also awarded the fritz grasenick award 2012 from the
Austrian Society for electron microscopy (ASem).
Fig . 10 Peak shape evolution of a selected Laue peak after deformation along the length and
width of a) a single crystalline cantilever with a step size of 2 µm and b) a single crystalline
cantilever with a slit along the neutral axis exhibiting a step size of 1 µm. The compression side
is above, the tensile side below. The bright area corresponds to the sample volume.
Bending experiments on single crystalline and bicrystalline Copper
microcantilevers:
the sample size effect on strength is one of the most controversially discussed phenomena
in material science in the last decade. there are several explanations proposed. the most
prominent are strain gradient plasticity, dislocation starvation, dislocation source limiting
mechanisms and dislocation pile-up. microcantilevers are one of the model systems. due to
a stress and strain gradient, microcantilevers exhibit varying mechanical response compared
to microtensile and microcompression samples. thereby the dislocation structure, especially
the pile-up of geometrically necessary dislocations (gnds) near the neutral axis, plays a
predominant role. three cantilever geometries with different expansions of the dislocation pileup
were applied (fig. 11a). consequently, higher and lower magnitudes of long-range pile-up
stresses were generated during loading.
Scientific RepoRt 2012 page 25