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Small Scale Plasticity

Most of applied metallic materials are polycristalline with grain sizes in the order of several micrometers. It is now possible to process metallic materials with grain sizes below 1 micron (ultra-fine grain – UFG) or 100 nm (nanocrystals).

At this scale, new physical properties surface: from a mechanical perspective, metallic nanocrystals exhibit a yield stress that is up to 10 times higher than their micrometer-grained counterparts. This is the reason why these materials focus a great deal of interest, both from the fundamental and applied point of view. This increased strength finds its roots in the confinement of dislocations that usually carry out the plastic deformation of crystals in a wide range of stress and temperature.

A significant research enterprise has been undertaken to identify the specific mechanisms responsible for this confined plasticity. These mechanisms are mostly unknown, especially because current analysis methods fails to overcome the problems linked to the very small size of the grains. Another obstacle is the supposed variety of such mechanisms : individual atomic movements, intergranular diffusion, partial dislocation emission,…
For a few years, in situ Transmission Electron Microscopy (TEM) has proved to be a key tool to simultaneously trigger and analyse in real time the fine plastic mechanisms at work in a material. CEMES has been recognized for long as a world leader for in-situ TEM. We currently work on several low to high temperature straining experiments. Through the "Nanolab " activity of the ESTEEM European project, we are also designing new TEM holders for in-situ straining experiments (tensile loading, bending, nano-indenting,..).

Our research activities focus on various micro or nano-structured materials :
- Pure nanocrystalline metals (Al, Cu) obtained by electrodeposition – free-standing films that can be strained in dedicated microtesting machines (JHU) (http://pegasus.me.jhu.edu/~sharpe/index.html).
- Bulk ultra fine grain materials
- Thin films on substrate
- Metallic glasses

We are focusing in understanding :
- Plastic relaxation mechanisms related to grain boundary motion unders stress for nanocristals and UFG materials
- Dislocation mechanisms in UFG materials (Bauschinger effects, dislocation/grain boundary interaction,…)
- Hardening in thin films
- Size effect in metallic glasses

To learn more

References:

Legros, M., B. R. Elliott, M. N. Rittner, J. R. Weertman and K. J. Hemker (2000). "Microsample tensile testing of nanocrystalline metals." Philosophical Magazine A (Physics of Condensed Matter: Structure, Defects and Mechanical Properties) 80(4): 1017-1026.

Mompiou F, Legros, M., Caillard, D., “Stress Assisted Grain Growth Revealed By In Situ TEM”, MRS proceedings, 1086-U09-04, 2008

Dehm G, Legros M, Heiland B. in-situ TEM straining experiments of Al films on polyimide using a novel FIB design for specimen preparation. Journal of Materials Science 2006;41:4484.

Dehm G, Oh SH, Gruber P, Legros M, Fischer FD. Strain compensation by twinning in Au thin films: Experiment and model. Acta Materialia 2007;55:6659.

Legros M. Relaxation plastique des couches minces métalliques par dislocations et défauts étendus. In: Mouis M, editor. Contraintes en microelectronique. Hermes, 2006.

Legros M, Dehm G, Balk TJ. In-Situ TEM Study of Plastic Stress Relaxation Mechanisms and Interface Effects in Metallic Films. In: Buchheit TE, Minor AM, Spolenak R, Takashima K, editors. Materials Research Society, vol. 875, Thin Films: Stress and mechanical properties XI. San Francisco: Materials Research Society, 2005. p.237.

Legros M, Hemker KJ, Gouldstone A, Suresh S, Keller-Flaig RM, Arzt E. Microstructural evolution in passivated Al films on Si substrates during thermal cycling. Acta Materialia 2002;50:3435.

Collaborations:
K. Hemker, J. Hopkins Univ., H. Mughrabi, University of Erlangen, T. Pardoen, UCL Louvain, S. Gravier (SIMAP Grenoble)

Project:
ESTEEM project Nanolab
ANR MEGAPROSE

Key-words:
in situ TEM, nanostructured materials, UFG materials, grain boundaries, dislocations, freestanding thin films, micromechanical testing machines