J. Lohmiller, N.C. Woo, R. Spolenak

The new and fast emerging field of flexible electronic devices requires highly ductile materials. Deposition of thin metal films on flexible substrates is a suitable method to create highly ductile interconnects. In this study, thin films consisting of a graded composition of Au?Cu were co-deposited by direct-current magnetron sputtering on polyimide (Kapton®) substrate for in situ SEM tensile testing, while silicon wafer supported thin film spreads were characterized by nanoindentation, XRD and EDX. Substrate quality turned out to be extremely important for strain delocalization to allow for uniform deformation characterized by high ductility. No cracking was observed up to the maximal strain of 30% for films consisting of pure gold and alloys with a low copper content up to 10 at.%, while cracking was more prevalent in films with higher copper contents and with applied heat treatment. In the most ductile thin films shear bands are the precursors of ductile cracks.

Materials Science & Engineering A, 527, 7731-7740 (2010)

D.V. Bachurin, P. Gumbsch

Atomistic simulations of uniaxial tensile and compressive straining of three-dimensional nanocrystalline palladium were performed at room temperature and different strain rates. Detailed analysis revealed that initial plastic deformation is due to grain boundary sliding accommodated by localized bending inside the grains and the formation of dislocation embryos. Intergranular cracking in the absence of dislocation activity was found at later stages of tensile straining. During compressive straining the sample shows a plastic response which is brought about mainly by intergranular accommodation processes. The contribution of extended partial dislocations emitted from the grain boundaries as well as full dislocations and twinning at later stages of deformation to the total strain was found to be insignificant.

Acta Materialia, 58, 5491-5501 (2010)

D. Kiener, C. Motz, W. Grosinger, D. Weygand, R. Pippan

A new technique to perform in situ miniaturized bending fatigue experiments was applied to single crystal copper beams. Pronounced monotonic hardening and an increasing Bauschinger e?ect were observed with increasing normalized displacement. Remarkably, no cyclic hardening or softening was observed. Three-dimensional discrete dislocation dynamics simulations support these results and point out the importance of dislocation pile-ups and local hardening mechanisms. These lead to local strain redistribution and the activation of new glide planes as observed experimentally.

Scipta Materialia 63, 500-503 (2010)

J. Markmann, D. Bachurin, L. Shao, P. Gumbsch, J. Weissmüller

We propose a method for computing virtual x-ray diffractograms for nanocystalline materials under uniaxial load based on molecular dynamics simulation data. While the increase in diffraction microstrain during deformation is generally taken as evidence for the generation of lattice dislocations, our virtual diffraction data show extra microstrain even at small load, before the onset of lattice dislocation activity. We show that the microstrain data have a natural explanation in the elastic response of a heterogeneous medium. The results imply that the conclusions of previous experimental in-situ diffraction data for nanocystalline metals may deserve a critical examination.

EPL, Vol. 89, 66002 (2010)

The institutional director of the izbs and director of the Fraunhofer Institute for Mechanics of Materials in Freiburg and Halle is honoured with the Gottfried-Wilhelm-Leibniz prize for outstanding research achievements in the field of stress-strain relations and breakage processes in materials. The Leibniz prize is the most prestigious German reasearch award, awarding 2.5 million Euro. Read the press release.

J. Senger, D. Weygand, C. Motz, P. Gumbsch, O. Kraft

In small dimensions, the flow stress of metallic samples shows a size-dependence such that smaller is stronger, even in nominally strain gradient-free loading conditions...

Philosophical Magazine, Volume 90 Issue 5, 617 (2010)